I think I'm actually on the right track with this one as is, but I wanted to get some thoughts from anyone who has more experience than me.

I'm rebuilding a National SW-54 shortwave radio. It's a pretty standard restoration so far, but this set has a few major issues. The cheater cord on the back was physically melted, and the shunt resistor across the rectifier tube was burned in half. As near as I could tell, this was due to the set being run with dead electrolytics for long enough to cause a meltdown. There's a scorch mark on the underside of the cabinet where it failed.

Anyway, I've replaced most of the caps and rebuilt the power supply. I picked up a kit from Hayseed Hamfest and apparently my set is a later revision with some additional .002 uf caps between the antenna inputs and one of the coils. No biggy, I've done the rest and those won't impact a basic power up.

Well, anyway, I got it to come to life for a moment, but it died again just as quickly. Checked under the chassis, and there's two resistors in series. One goes to a ground connection through Pin 1 of the 50C5, and the other acts as a bridge between two of the filter cap sections. The one providing the bridge was smoking and hot to the touch (my finger proves it, oh well) so I measured both. Bridge resistor (looks to be a 1K from the schematic) looks to be in tolerance, but I'm getting some odd behavior from the one going to the 50C5. It measures 1K when the set is cold (should be 15K if I'm reading the schematic right), but I noticed that the resistance dropped to 0 when I applied power.

A dead short to ground would explain the smoking 1K resistor, and probably my dead reception as well. I mostly wanted to know if having a resistor become a dead short when power is applied sounded like something anyone has any experience with.

I'll attach the schematic as well, although it's not a perfect match to my set. On mine there's an additional resistor between the 1K bridge and the 60uf filter section that I can't seem to find on the the diagram, and the resistor that's marked as being in parallel with the C1D section coming off of Pin 1 on the 50C5 is in series with it instead. I'm assuming mine is a different revision since it also has a network that isn't listed here, but I'm no expert.

Since both resistors were getting hot, I have parts on order to replace them. I'm hoping removing that wonky 15K will get me back up and running. I have to believe the meltdown I mentioned previously took that out as well.

Okay, a couple corrections.

C1 D is in parallel with that resistor. It didn't register with me that that line is just a straight bridge to ground through the filter capacitor on the other end of the chassis.

The reproduction electrolytic can from Hayseed Hamfest used stranded wire that was too thick for the holes in the tub socket terminals. I ended up cutting half of the wires off to make it fit and feeding that through. In poking around tonight, I discovered that a single strand on the 60 uF section had separated from the rest and was hiding underneath, shorting that section to ground. I've removed that, but now I have to see what damage that caused.

I did try powering up since then, and it looks like the resistors are no longer overheating. I still plan to replace them, of course, but I'm now reasonably confident that that short was the source of my issue.

Basic symptom at the moment is I have faint hum through the speaker (not filter hum so much, more like volume up too high faint distortion) but no signal besides a faint chirping every once in awhile on all bands. So I'll have to figure that out. I plan to check my B+ voltage coming off the rectifier tomorrow (I think that's going to be 115VDC coming off of pin 8 on the 35Z5, but correct me if I'm wrong on that) and trace it up to the 50C5. It looks like it has to go through those resistors that were overheating, so I still have some hope that that's the source of my issue.

I'm still trying to wrap my head around the resistor pattern coming off of pin 8 on the 35Z5. It doesn't match what I'm understanding from looking at the chassis. Pin 8 has two resistors. One is definitely the 15K ohm going to Pin 1 on the 50C5 and connecting the the D section of C1. The other (the one that was burning hot) is connected to pin 8, but goes off to a terminal strip, hooks onto one of the 40 uF sections on the electrolytic can, and then runs through an additional resistor to the 60 uF section in the can mounted to another terminal on the strip. At least that's what I think is happening, I'll retrace it tomorrow to double check.

There's also some stuff wired to pin 1 of the 35Z5. I don't even see that pin listed on the schematic. I know it's factory, as far as I can tell the underside was untouched so there must be some reason, but I'm not seeing where all that is happening. Is Pin 1 on the 35Z5 a no connect internally? The metal is present but perhaps a look at the tube diagram will clear that up.

I'm just spitballing here. Hopefully some of this makes sense. I'm sure another glance over the layout will clear some of this up, but in the meantime any thoughts are appreciated.

Okay, yep. Pin 1 on the 35Z5 isn't connected to anything internally. That threw me off. Pins 4 and 6 are also unused as well, but they didn't include any physical pins for those. I figured that meant that any pin that was present was connected. That may very well clear up some of my confusion.

Wow that's a lot of info...

One thing that jumps out to me is your statement that resistance of a resistor changes when power is applied. How did you determine that? If you left your DMM in resistance mode connected across the resistor and powered up the set then you are making a BIG mistake....DMMs measure resistance by either sending a fixed current into the resistor and measuring voltage drop or sending a fixed voltage into the resistor and measuring current flow....If the radio is powered and sending power through the resistor the resistor then has an additional power source across it that changes current flow and voltage across the resistor (which the DMM don't expect)....Best case scenario that power the meter doesn't expect makes your reading substantially inaccurate and worst case scenario the voltage and current it sends into your meter destroys the DMM...
I've had the later happen by accident multiple times...I tend to switch between measuring resistance with the set off and voltage with the set on and sometimes forget to change mode on the DMM.
If you like your DMM and want accurate readings NEVER measure resistance of a powered resistor!

Okay, thanks for the heads up. I think that may be my issue. The multimeter is fine, at least. I've tested it since then and everything is behaving as far as I can tell.

I'm not seeing any voltage on my B+ (Pin 8 of the 35Z5 if I'm correct on that) and I think a few of my electrolytic sections have opened. I read that you can check electrolytics with a multimeter by reading resistance across them. If they spike once and go down to OL very quickly, they're good. If they retain value, they're bad. Two of my sections are retaining value and two are dropping, so I'll just go ahead and replace the whole thing and those two resistors.

Of course, if anyone else has any thoughts or that all sounds completely off the mark, please let me know.

I would also check continuity (measure the ohms resistance) of the pilot lamp, the 330ohm resistor and the heater pins of the 35Z5 2-3, 2-7 and 3-7.

jr

I read that you can check electrolytics with a multimeter by reading resistance across them. If they spike once and go down to OL very quickly, they're good. If they retain value, they're bad.

;) Absolutely 101% not true ....

The only infallible test for an electrolytic capacitor is to either see it work in it's circuit without it or it's other associated components getting hot , or to test it using a capacitor tester that tests it at the actual voltage it's gonna see when it's in use . A cap that won't break down at the tiny (less than 9 volts) voltage your meter applies to it for the test could very well break down at the 100 or so volts your 35Z5 is gonna feed it .

My personal policy is "take no prisoners:pistols: with electrolytics , if I'm gonna get a piece of vintage tube electronics running , the first thing I do is to replace them before the first power up .

;) My personal policy is "take no prisoners:pistols: with electrolytics , if I'm gonna get a piece of vintage tube electronics running , the first thing I do is to replace them before the first power up .

These are replacements. The concern is that the positive of the 60 uF section was shorted to ground, which also has a direct connection to the negative of the same side. I know it caused the 1K and 15K resistors to overheat, and I'm concerned it may have also damaged the electrolytics.

I've ordered a replacement set for safety's sake. I'll try substituting that so I know those and the new resistors I ordered are good. I'll have to start doing continuity tests from there.

I know the pilot lamp and heaters are good because the lamp is still lighting and the series string is all coming on. So the problem is somewhere further into the set.

Does anyone have any specific ideas on checking the B+? Am I accurate in measuring between pin 8 and ground to see if that's coming off of the 35Z5?

Okay, I've got no B+ coming off of pin 8 on the 35Z5. For sake of testing, I've gone ahead and ordered a spare tube. My heater should be good since my tubes are all lighting and the filament tested with continuity, but I checked it in my tester and it came back as no emission so perhaps that's my problem?

I want to verify that my voltmeter is good, though. I know the resistance checker part is fine, but to be safe I'm going to check the voltage on the heater pins. That should give me a good indication on what's going on.

Any other suggestions? I have a new filter cap set on the way to see if I burned out the old one, and the new resistors are installed and not getting hot. This has to be a power supply problem, whatever it is.

Okay, now I'm really confused.

I used the voltmeter function on my multimeter (on AC setting) with the positive lead on Pin 7 of the rectifier, and the negative lead on ground. 0 volts. I then moved it to the negative lead to pin 3 on the 50C5 and measured voltage between pin 7 on the rectifier to pin 3 on the 50C5. That should be filament. I know it's lighting. I get hum from the speaker and there's a pop when I turn the bandswitch, plus the dial light is lighting. But 0 volts on the filament!

Am I just using this wrong? Maybe my meter is dead after all? I can't believe I'm not getting any voltage there.

I would also check continuity (measure the ohms resistance) of the pilot lamp, the 330ohm resistor and the heater pins of the 35Z5 2-3, 2-7 and 3-7.

jr

Oh, and I almost forgot. I did check this. Each pin set showed continuity.

Try between pin 3 and 4 of the 50C5, those are the heater pins.

jr

Try between pin 3 and 4 of the 50C5, those are the heater pins.

jr

Voltage or continuity?

Also would that heater be bad if I'm getting sound from the speaker, even if it's just a very low hum?

Either one... continuity with the tube out of the radio, or voltage with the tube in the socket and the radio turned on.

not likely.

jr

Gotcha. I'll test both.

Series string rectifiers typically have a dial lamp heater tap...the heater can go open on either side of the tap. During WWII when replacement tubes were scarce if the section of rect heater in parallel with the dial lamp blew (likely because the dial lamp died first) instead of changing the rect they would install a higher current dial lamp to allow it to soldier on with a poor tube.

If it is AC voltage scale you want to check on your meter just stick the probes into the wall outlet and you should see somewhere between 110 and 130VAC...Just check your positive lead is in the voltage connector and not the current before jamming it in the outlet. The current jacks on a DMM typically have a short (thick 1ohm piece of wire) between them...I killed my first meter by measuring line voltage with the probe in the 10A current jack...the meter leads melted, but the breaker never tripped.

DC voltage I typically check with a 9V battery. Remember to have the meter set to AC voltage if checking heater and DC voltage if checking B+...If you get that backwards you'll get near 0 volt readings.....Some folks prefer to measure voltage with a DC coupled oscilloscope because it will measure AC and DC at the same time.

I'll have to pump the brakes on this. My multimeter isn't working properly.

I'm showing 0 VDC across multiple brand new 9V batteries, and nearly 0VAC across known good wall outlets.

I'll order a new one (eventually I need to get my Heathkit VTVM restored but it's nice to have a digital one as well) and wait for that to come in before moving on.

Okay, so I'm back at it with a new multimeter. This one has a capacitor tester, which will hopefully make my life a little easier.

So I did a few voltage tests, and my VDC on Pin 8 of the 35Z5 looks okay. It's actually a little higher than what they listed in the Sams. So as far as I can tell, I have B+, but my signal is getting lost somewhere.

Does anyone have any thoughts on where I should be looking next? I tested voltages around the 35Z5, and they all looked okay, so I'm thinking this isn't a voltage problem.

I did notice that my capacitance readings on C18 and C19 look off, but I'm also not sure if those can be reliably measured in circuit or not. This is all with power off, of course.

Where would you all start looking?

Voltage and resistance measurements at the tube sockets and compare to the charts ?

jr

Gotcha. I sort of started that but I'll go through and get readings for each pin on each tube.

I did notice that my set is a different revision from the SAMS. It has a network where the one SAMS listed doesn't and the position of the 60 and 40uf sections of the filter can are swapped. Mine came with a schematic that shows those revisions, but it doesn't list the pin measurements for reference.

Do you think, barring any other information, the SAMS info will be safe to go off of for this? I can list any variations, I know tube gear tends to have a pretty wide tolerance.

Likely the parts in the network are very similar to the discreet parts that the network replaced... I suspect that the Sams charts would be close.

A more basic question... as the set warms up, do you hear anything from the speaker? a slight hum would be normal also slight static as the volume control is turned up and down. :scratch2:

jr

Yes to the slight hum and maybe a little static, and it clicks when I switch bands, but that's about it.

So it would seem that the speaker and audio stages are at least alive... Have you tried a fairly long antenna wire, perhaps 20 feet or so, to perhaps get some signal through the converter and if stages?

jr

Okay, sorry for the delay. I work in healthcare research so things have been crazy.

Anyway, I grabbed voltage and resistance measurements. Here's the breakdown:

V1:
1: 24.5K | -9.3 VDC
2: 2.1 Ohms | 0 VAC
3: 14 Ohms | 13 VAC
4: 26 Ohms | 25 VAC
5: 16.9K | 107 VDC
6: 16.9K | 107 VDC
7: 3.7M | 0 VAC

V2:
1: 3.1M | 0 VAC
2: 1 Ohm | 0 VAC
3: 26 Ohms | 25.7 VAC
4: 37.5 Ohms| 39 VAC
5: 16.9K | 107 VDC
6: 16.9K | 107 VDC
7: 182 Ohms | 252 VDC

V3:
1: 13.7M | -.6 VDC
2: 2 Ohms | 0VAC
3: 14.5 Ohms | 13 VAC
4: 1.7 Ohms | 0 VAC
5: .657M | -2.3 VDC
6: .657M | -2.3 VDC
7: 242K | 69 VDC

V4:
1: 300 Ohms | 9.5 VDC
2: 447K | .02 VAC
3: 85 Ohms | 90 VAC
4: 37 Ohms | 40 VAC
5: 447K | 0 VAC
6: 16K | 115 VDC
7: 15K | 123 VDC

V5:
1: 16.24K | 115 VDC
2: 112.8 Ohms | 121 VAC
3: 110.3 Ohms | 117 VAC
4: NC | 0 VAC
5: 110.3 Ohms | 117 VAC
6: 15.2K | 127 VDC
7: 84.8 Ohms | 90 VAC
8: 15.2K | 127 VDC


I'm noticing a few wonky resistance measurements around the 50C5, and pins 5 and 6 on both V1 and V2. Any idea where I should be looking next?

So it would seem that the speaker and audio stages are at least alive... Have you tried a fairly long antenna wire, perhaps 20 feet or so, to perhaps get some signal through the converter and if stages?

jr

When I first powered it on after my recap, it was pulling in a station (I have an in-home transmitter) that gradually faded away within the first few minutes of it being turned on. Whatever burned out happened within that time frame. I'm looking to get it back to that point.

Couple of observations:

There seems to be a note missing in the Sams... the ohm table shows a tag for V1 and V2 pins 5&6, V3 pin 7, V4 pins 6&7. I suspect that these resistances should be measured to the 35Z5 cathode (pin8) and not to ground. With this assumption, your “wonky” restance readings look reasonable.

252 volts on V2 pin 7 can’t be correct or the 100 ohm cathode resistor would be smoking. I would double check the voltage and resistance values at that pin, and around that tube. Perhaps the problem is related to the area around V2. :scratch2:

jr

Okay, I rechecked Pin 7 on V2. I wrote it down wrong.

Today it was hovering around 230 mVDC. I forgot to write down the M when I was taking notes. The resistance is measuring the same, though.

In looking at the schematic, that one may be a little too low if anything. Could that difference cause my audio to disappear?

While I suspect it will not be the big red X that we are trying to find, I would go ahead and replace the 100 ohm resistor in the cathode connection of V2... 182 ohms is likely too far off for good performance.

jr

Got it. I'll put one on order and see if it helps.

Okay. I've swapped out the questionable resistor. Not hearing any change.

Any thoughts on what I should check next?

Have you tested/substituted all tubes?

jr

I have not. I ordered some spares a while back and they've been delayed. That being said, since this is an AA5 for all intents and purposes, I might could borrow some known good ones from my AM radio and see if that has any impact.

I think I have a signal generator set up somewhere. Would there be any value in checking my audio section with one? I know my speaker output is good since I'm getting some audio hum from it, so it has to be a breakdown somewhere in the signal line, right?

I thought that we verified that the audio stages were working...do you have a RF generator? Swapping tubes from a working set seems like a good idea since your shipment of new tubes has been delayed.

jr

The audio stage is humming, but I'm not getting any signal through. I can get a video of the noise if that will help.

And yeah, I'll pull some tubes from working sets to see if that makes any difference.

Went ahead and created a video: https://www.youtube.com/watch?v=Nyrr8iUFRzE

I also tried new tubes for everything except the 35Z5. I don't have any spares of that for now, but it also doesn't seem like the problem is in the power supply.

I don't believe I have an RF generator, but I do have a digital signal generator I could set up. I haven't used it before but it looks like it might work for running some tests.

Again, I think the audio section is ok... I hear slight hum which changes with volume control rotation... some volume control scratch ... hum increase with body capacitance... noise from band switching. Do you get noise or change in noise as you connrct a wire to the antenna terminal?

What is the frequency range of the signal generator? can it generate a square wave? (rich in harmonics).

jr

Looks like it can, yeah. I'll have to do a little research into the specific functionality (It's a sort of combo oscilloscope and signal generator) but the interface definitely allows for square waves.

(It's a sort of combo oscilloscope and signal generator)

Never seen such a unit, what model is it? Does the oscilloscope have sufficient bandwidth/sensitivity to display RF/IF signals from the radio?

jr

https://store.digilentinc.com/analog-discovery-100msps-usb-oscilloscope-logic-analyzer-limited-time/

One of these things, but I've got an adapter to give it BNC connectors for the Scope functions. It plugs into the computer for the display.

Interesting! thanks for posting. I’m not sure that I would be comfortable exposing such a device to the fairly high voltages present in the SW-54, even if you used an isolation transformer.

jr

So I'm thinking my best plan of attack is to use my multimeter to start testing individual resistors and capacitors. It'll take some time, but hopefully that'll give me a better picture of what failed.

I know my main electrolytic is fine (I actually swapped in a spare to be safe) so it's a matter of checking the standard caps and resistors.

A quick question, since I haven't gone this in depth before: Do I need to desolder one lead of each cap and resistor before I check it, or can I just turn the power off and start running tests?

Many parts can be accurately tested without unsoldering a lead... look at the schematic to see if it is connected to anything that could affect the accuracy of the measurement.

It might be a good learning exercise to go ahead and measure all the parts connected in circuit. Then if a part measures out of spec, try to determine why... is the part bad or merely connected to another part that affects the measurement. Verify by disconnecting one end of the part in question.

jr

Good plan. I'll get on that. I'm sure if I work my way down the line the problem will jump out.

Okay! So I've got a few parts marked as suspect. There's some out of tolerance resistors, some caps that are damaged, and one in particular that has no reading at all across it! So I'm about ready to place an order.

A question, though. There's a component across the tuning capacitor. The original was a small white cylinder, and the SAMS lists that as a capacitor. However, I have a parts chassis I'm pulling a new headphone jack from, and I noticed that that component looks like a resistor there!

My set has a few other differences from the SAMS, namely the presence of a small network in place of a few discrete components on the SAMS, and some paper caps going to a few of the antenna jacks that aren't on the SAMS. The parts chassis I have also has that network, but it has a few other differences.

I'm including some pictures of both sets, and I'm going to see if I can dig the white cylinder I pulled out of my trash bin. It's somewhere in there, I know.

I don't think that's the source of my dead audio, because my audio faded out and the set started smoking before I replaced that part. I did notice a bulge on the side of it when I replaced it, though, and my multimeter is just kind of hanging there (not giving OL, but not displaying any measurement at all) when I try to measure my replacement.

Any thoughts here? I can also scan the schematic that came with mine if that would be helpful. I'm not entirely sure how to read that part of it.

I have seen small value caps that look like resistors before likely that is the 3uuf cap that is shown between the two sections of the tuning cap.. C9.

jr

Gotcha. Perfect. In that case I'll place my order today and get started.

My parts set also has the same network. If replacing the group of components I've identified doesn't fix things, I'll swap that in as well to see if there's any internal damage.

What's the purpose of the filament shunt resistor?

Mine keeps blowing. It was blown when I first got it. I swapped it originally, had no issues, had no issues still when I finished replacing a whole batch of components today, and then I swapped in a bunch of tubes from a non working parts set I picked up and suddenly my R15, which is listed as the "filament shunt" started blowing. It's also blowing out my dial light. Thankfully I have quite a few spares for both.

I'm taking a break before swapping it back again. My desoldering station needs a part and I had to order it, but my next test is going to be swapping those tubes back and replacing the resistor again. Could a dead tube cause that resistor and the dial light to blow on power up? I could very well have an open filament on one of the tubes.

What's the purpose of the filament shunt resistor?

Mine keeps blowing. It was blown when I first got it. I swapped it originally, had no issues, had no issues still when I finished replacing a whole batch of components today, and then I swapped in a bunch of tubes from a non working parts set I picked up and suddenly my R15, which is listed as the "filament shunt" started blowing. It's also blowing out my dial light. Thankfully I have quite a few spares for both.

I'm taking a break before swapping it back again. My desoldering station needs a part and I had to order it, but my next test is going to be swapping those tubes back and replacing the resistor again. Could a dead tube cause that resistor and the dial light to blow on power up? I could very well have an open filament on one of the tubes.

The rectifier tube should a heater tap for the dial lamp that part of the rect heater that is in parallel with the dial lamp goes open the dial lamp would have too much power flow burn out then the resistor in parallel will cook. If any other tube goes open the whole heater string should go dark but otherwise be fine. For a different tube to cause the dial lamp to fail it's heater would have to short... though in some cases the dial lamp/tap/resistor also acts as a B+ fuse and a B+ short could also cause dial lamp failure.

I would ohm check the 35Z5 heater... guessing it is open between pins 2 and 3.

jr

I would ohm check the 35Z5 heater... guessing it is open between pins 2 and 3.

jr
Jumper pins 2&3 for now, to finish trouble shooting. That's a trick, I learned over 60 years ago, by an old time radio tech.

Quick check confirmed it, pins 2 and 3 were open on that tube. I swapped a new resistor in and swapped in another 35Z5. I'm back to testing, though I'm still waiting on the parts for my station before moving forward much more.

Are there any solutions for injecting signals without a signal generator? I have a signal generator, a Heathkit SG-8, but I haven't restored it yet. I'm trying to decide whether I should step back and get that work so I can start injecting signals, or if there's something else I could use.

I have one of those AM radio transmitter kits, tested and working. Do you think I could clip in a lead from the antenna output via a coupling capacitor and start injecting along the grids of each tube?

I need a crash course in signal tracing or something.

I rebuilt (or think I did) my Heathkit SG-8 signal generator. New electrolytics, replaced the paper caps, and I swapped in a 1N4004 for the selenium.

I tested that by placing it along the antenna of an AM radio and was able to pick up some warbling along the B band when I moved the dial around. It moved with the dial, and went away when I switched off the generator. So I think I have that working. This thing has RF out and AF in and out.

From what I understand, I need to connect the ground clip to the chassis through a capacitor, then connect the other end about 450 KHZ on the B band to the input of the grids on the tubes. So I started at Pin 5 of the 50C5, no dice. Just that humming.

Help? Any suggestions? I'm really at a loss on this at this point. I've tested components multiple times and can't seem to find any more that are out of spec. I'm reaching a point where I'm tempted to just replace everything and hope that gets me something.

Use the audio output for testing the audo stages, 455 khz for testing the IF circuits and 550 khz and up to test whatever frequency the radio is tuned to.

jr

Mine has an AF and RF output. Which should I be using?

Should I be hearing the signal come through the speaker on the radio?

AF is audio frequency ... use for audio stages.
RF is radio frequency ... use for 455 khz IF and frequency that the radio is tuned to.

yes

jr

Okay.

So additional question, how do I know if my signal generator is outputting anything on its AF output? I haven't found anyone that has a good test for deciding if that part works yet.

Say I connected the leads directly to a known good speaker. Should I hear something coming out on that?

Okay.

So additional question, how do I know if my signal generator is outputting anything on its AF output? I haven't found anyone that has a good test for deciding if that part works yet.

Say I connected the leads directly to a known good speaker. Should I hear something coming out on that?

If the audio output is strong enough it should drive the speaker to where you can hear with your ear next to it...if not do you have a set of computer speakers or an audio amp with an audio input? You can connect a weak audio output from the generator to those and hear the audio tone.

Generator opperation note: if your generator is similar to my Heathkit IG-102 the compound audio input output jack will only output audio if the internal/external modulation switch is set to internal. And the modulation/AF out pot will set the audio out level.
You can probably use this generator as an AM transmitter if you want...if you set modulation to external and connect an music source to the audio input jack it. Should transmit it...Some of these tube generators (like my Heathkit) need a powerful audio input (output of an amp) to modulate noticably/sufficiently.

Too weak to drive a 8 ohm speaker... I can faintly hear mine when connected to earbuds or my Sony 7506 headphones. Switch set to internal, output pot set at max. Scope (high impedence) sez it is about 10 volt peak to peak at 535.7 hertz.

jr

Looks like my signal generator isn't working. I hooked it into a USB scope. It shows an identical waveform when the unit is turned both on and off.

Guess I'll need to debug this before I can get back to the radio.

What frequency?
What amplute?
How is the scope connected to the signal generator?

jr

Looks like my signal generator isn't working. I hooked it into a USB scope. It shows an identical waveform when the unit is turned both on and off.

Guess I'll need to debug this before I can get back to the radio.

Remember that on a tube generator the lytics store enough power to run .5-10seconds and the tubes typically don't loose emission immediately after loosing heater voltage either.

I did a little more research. Looks like if I pick up a frequency counter I can get a better idea on whether I have any output.

Any suggestions on models to pick up?

I have to admit, I feel like I'm burying myself in a hole here. It sounds like I need a known good signal generator before I can fix my radio, but I'm having a bear of a time figuring out whether my generator is any good. Is there really no way to take, say, an AM radio transmitter (which I have one of that I know works because I use it daily) and just directly run the antenna output to the grids of the tubes?

I did a little more research. Looks like if I pick up a frequency counter I can get a better idea on whether I have any output.

Any suggestions on models to pick up?

I have to admit, I feel like I'm burying myself in a hole here. It sounds like I need a known good signal generator before I can fix my radio, but I'm having a bear of a time figuring out whether my generator is any good. Is there really no way to take, say, an AM radio transmitter (which I have one of that I know works because I use it daily) and just directly run the antenna output to the grids of the tubes?
That won't work unless the you only inject between the antenna and the converter/mixer stage, your transmitter is capable of being tuned to 455KHz, or the radio is a TRF set.
One other thing you can do is if you have a working analog tuned AM radio you can run it at the same time as the set you are working on and dial around the AM band tuning on both...at some point the oscillators will be at the same freq and beat against each other creating a whistle...if you have two working AM sets put them next to each other and you can observe this behavior.

I use a vintage Tripplet 7000 counter....not because it good so much as it works and was the first cheap used counter I could find.

At RF/IF tube service generators often are non-sinusoidal and produce a double humped wine wave some generators can't accurately read. My best solution to dial my tube Heathkit generator in on frequencies under 30MHz is to dial in the digital tuner of my Sony ICF-SW7600GR world band radio to the desired freq then play with the generator till the Sony gets peak reception then hook the Heathkit up to the vintage radio I plan to align.

Looks like my signal generator isn't working. I hooked it into a USB scope. It shows an identical waveform when the unit is turned both on and off.

Guess I'll need to debug this before I can get back to the radio.

I think it would be better for you if you stayed with one thing until the investigation is exhausted before jumping to something else.
The audio signal frequency should be somewhere between 300Hz and 500Hz, well within the range to use a USB scope. What scope is that anyway? You need to give us more details so that we can check the scope specs. A USB scope is a digital scope, what it displays is actually in digital memory. It may work like what would be a "storage scope". It sounds to me that with the generator off the scope isn't triggered so it displays what the last thing it saw before the triggering went away. Does the scope have an "AUTO" trigger mode? That would allow it to sample the signal without enough of a signal amplitude to trigger it.
Once you have the USB scope working you can find the frequency, amplitude and wave shape of the audio signal. A frequency counter would only cloud the issue at this stage since it also needs to be triggered in order to display anything.

Sorry, you're right, I'm jumping around. Too many things going on at once.

Let me get that info for you on the scope.

In the meantime, I'm investigating another route on the radio. I realize I've been reading the schematic and chassis wrong. C10 may be my issue.

C10 looks like a resistor, but it's a cap on the schematic. It's listed as 100 mmfd (which I understand is pf). I'm measuring it as .5nf, which doesn't appear to match up.

I've ordered some 100pf capacitors and will see if replacing that brings my signal back. It's moving from the tuning cap to pin1 of my 12BE6.

While I'm waiting on that, let me get more info on my signal generator. I'm using one of these little kit oscilloscopes: https://reference.digilentinc.com/reference/instrumentation/analog-discovery/start

It's basically a 0 feature, simple scope. I technically have a slightly more advanced heathkit scope, but it's unrestored and untested so I'd need to tackle that before I could rely on its readings.

OK. You are still kind of getting ahead of yourself. If you tested C10 while it is still connected then the reading will be wrong. The reading would be effected by other components. You should ask about things here first, then if necessary order parts. At this point you don't know enough to make some decisions before forging ahead.
Wow, so that Digilent Analog Discovery can do a lot of things, it isn't just a oscilloscope. In a way that makes it harder to figure it out. You can change things around a lot in the software. Since I don't know the software I don't think I can help with setting it up right.
One thing that I did notice is that the maximum input voltage is +-25V.
That voltage is fine for most modern solid state circuits but tube circuits can have hundreds of volts DC on them. Unless you use a DC blocking capacitor on the inputs you could easily blow the scope input circuits. I would use a 0.1uf, 400 to 600V film cap.
I notice that there are trigger inputs on that device. They may or may not be what you should use with the scope inputs, the trigger inputs might be digital inputs used when the unit is used as a logic analyzer.
Probably few people on this forum know how to use that unit, you might get better understanding on it somewhere else.
The frequency and input resistance are fine. It may not be very sensitive to small signal levels however, but tube circuits do have a high signal level in general.

I've got a board that plugs into it to give it BNC scope probes, though the input voltage may still be an issue. I'm using it to read the probes, though, not anything internal to the signal generator, so I'm confident that I'm not seeing any dangerously high voltage there.

That being said, though, do I have any options to test my SG-8 without a known good probe or frequency generator? Assuming that device is out of the question (which it sounds like it just isn't a good option for this) where do I start? It's starting to feel like this whole thing just goes around in circles unless I get a known good piece of equipment that I can use to test everything else.

I've got a board that plugs into it to give it BNC scope probes, though the input voltage may still be an issue. I'm using it to read the probes, though, not anything internal to the signal generator, so I'm confident that I'm not seeing any dangerously high voltage there.

That being said, though, do I have any options to test my SG-8 without a known good probe or frequency generator? Assuming that device is out of the question (which it sounds like it just isn't a good option for this) where do I start? It's starting to feel like this whole thing just goes around in circles unless I get a known good piece of equipment that I can use to test everything else.
Do you have amplified computer speakers or some audio amplifier with an audio input?...that's all you need to test the audio generator half of the generator. Any signal in the 100-5000Hz range will be fine for signal injection testing of audio.

I kinda make it a point not to buy scopes or any piece of test gear currently made broken...If there is a good used one I can afford then a broken one is not worth wasting my life fixing.

Let me check. I know I have a little 9V battery powered amplifier. I'll have to see if I have a cable I can use to connect to it.

The audio output of my SG 8 is a nice sine wave around 500 Hz, should make a pleasing tone on your amplifier... I could hear it with headphones connected directly to the SG 8 without needing to use an amplifier.

https://live.staticflickr.com/65535/50141558693_1c24da649e_w_d.jpg

jr

I decided it was easier to show what's going on with the AF and RF output using video. That way you all can see what I'm talking about.

AF test: https://youtu.be/kNM5KqScNIY

RF test: https://www.youtube.com/watch?v=0GfKehD-BKw

I also checked the AF output with a pair of headphones. Didn't hear anything.

Can you connect the amplified speaker directly to the AF output of rhe SG 8 without going through the transmitter? Directly to the output connector, without using the old SG 8 cables? use clip leads or just hold the connector of the amplified speaker against the output connector of the SG 8.
Set SG 8 to “internal” to turn on the audio generator.
RF appears to be working well enough to drown out 60 Hz interference buzz from power supplies, lamp dimmers, chargers LED house lights and other switch mode power supply pests.
Do you have a manual for the SG 8?
http://tubularelectronics.com/Heath_Manual_Collection/Heath_Manuals_S/SG-8/SG-8.pdf
jr

It looks to me a problem with the connection. It would be better to diagnose if the wiring was as direct as possible. Making various adapter cables is an important project. I would not use the detour to the transmitter even if it isn't plugged in, it could act as an antenna for hum. just clip the wires end to end. None of the RF controls should effect the AF output. I've replaced the AF connectors with RCA (phono) connectors on my SG-8. I'm very busy today or I would check some things myself.
The probe adapter for that USB scope may have a 10X attenuator that will increase the max voltage you can look at.

I kinda make it a point not to buy scopes or any piece of test gear currently made broken...If there is a good used one I can afford then a broken one is not worth wasting my life fixing.

Of course some of us consider getting old test equipment working just as interesting and useful as fixing up a radio or TV. But you do have to start with some working equipment you can believe in.

Of course some of us consider getting old test equipment working just as interesting and useful as fixing up a radio or TV. But you do have to start with some working equipment you can believe in.

Therein lies my issue with this whole thing. But I do agree, I got this specifically because I wanted to learn how to get it working. I just didn't realize how complicated confirming it as working would be.

I decided last night to pick up another patch cable that I can use instead of jumping through the transmitter connection. It's the same cable I'm using there, just not connected to anything. I'll plan to open the cabinet, bypass the connector and cable with some alligator clips into the unit, then repeat the AF test. Maybe I'll have more luck.

While I'm in there, I'm going to compare the diagrams on the schematic with what's actually wired there. I know I didn't change anything, but I have no knowledge on the history of this unit. It's possible it was assembled incorrectly.

You could also check the cables and connectors for continuity with a meter.

I dug out my SG-8. I had modified it somewhat. I remembered that I had changed the audio connectors to RCAs and changed the RF output to a BNC.
I also remembered tweaking the audio oscillator to 400 Hz, which is a standard test frequency, and adding a tube to improve the modulation. What I didn't remember was how I changed the modulation switch hookup. I think you need to have the modulation switch set to "INT." for the internal audio oscillator to work.
At any rate, you should be able to get up to 20Vpp ( that means 20V peak-to-peak) out of the audio output.
Now a little education, in case you don't understand the subject, 20V peak-to-peak is what you would see with a oscilloscope, 20V between the positive peak to the negative peak. Since a sinewave with no DC on it is symmetrical around zero volts, the peak voltage (Vp) is half that or 10V. What most meters would display is RMS (Root-Mean-Squared) AC volts, the equivalent DC value. That means it produces the same power (and current) in a resistor as that DC value. For a sinewave only, the RMS voltage value is equal to 70.7% of the peak voltage. So for a sinewave with a peak of 10V the RMS value is about 7V.
That is a big signal, way bigger than what the battery powered amplifier you tried is intended to deal with. What you probably are hearing is hum due to a bad connection, maybe the ground. The audio oscillator (6C4) may not be working at all.

Trying to make sure I'm understanding.

Am I correct in saying that I can measure the AF output with a DC voltmeter, and should see about 10VDC coming out of that if I do? Or are you just saying that, were I to plug this into an oscilloscope, I should see it moving from 20V above the 0 line to 20V below the 0 line?

Trying to make sure I'm understanding.

Am I correct in saying that I can measure the AF output with a DC voltmeter, and should see about 10VDC coming out of that if I do? Or are you just saying that, were I to plug this into an oscilloscope, I should see it moving from 20V above the 0 line to 20V below the 0 line?

You can't measure audio with a DC meter. He was explaining to you the difference between AC Vpp, Vpk and Vrms (and how rms delivers equivalent power to the same DC voltage will when driving a resistive load).

You can't measure audio with a DC meter. He was explaining to you the difference between AC Vpp, Vpk and Vrms (and how rms delivers equivalent power to the same DC voltage will when driving a resistive load).

If your DMM has an AC range, likely it has a wide enough frequency response to reasonably measure a 400 Hz output. Using an inexpensive DMM, I measure about 3 Volts rms from the audio out of my SG 8. See picture on my post # 71 for peak to peak (about 9 Volts), rms Volts and frequency of my un-modified SG.

jr

Electronic M and jr_tech took the words right out of my mouth.
The measurements I did were with a 10Megohm scope probe and a 10Megohm input DMM, so they did not load the output very much at all. Part of the mod I did was to take the plate signal on the 6C4 through a 0.1 uf capacitor directly to the output connector. It does not connect to the 1 Megohm pot so it isn't variable. It is an example of the maximum signal you might see. The original circuit allowed it to be decreased with the pot.
Also I confirmed that you need to set the control for internal modulation.
When you select external modulation the oscillator circuit is rearranged to form an amplifier/driver that takes the input audio signal and drives the modulation. In your video I think you had it set to external.

Finally had a chance to get back to this. I've gone ahead and taken a video.

https://www.youtube.com/watch?v=gLhlQPAO7_4

I tested the AF output again, this time with a direct connection to the same speaker. Still no tone. So I opened it up and ran some continuity and value tests on the AF section. Everything in the AF section (including the wires to the AF out socket) shows continuity and appears to be within tolerance.

I traced the filament pins of the 6C4 tube to where pin3 on the 6C4 connects to pin 4 and 5 on the 12AU7. I discovered that pins 4 and 5 on the 12AU7 appear to be soldered together, which seems odd to me, but it also looks like it works on the schematic.

One question I do have: is Pin 3 of the 6C4 meant to connect there? The schematic I'm looking at is a little blurry, and Pin 3 there leads off to either an M or an N or an X or something. Pin 5 on the 12AU7 has another hard to decipher mark on it. I think those two link up but maybe someone with a better schematic can confirm. If my 6C4 is miswired of dead, that looks like it could kill my AF out.

I also need to check my pot values, but I'm not sure how to test when I have the three pins coming out of it. Any notes on how to best check the resistance reading on one of those?

Finally back with a major update.

Regarding the SG-8: It's finally working. It turns out that the generator fine the whole time, but my probe had an intermittent break in the cable. A new probe sorted it out, and I've been able to get steady and strong RF and AF output.

So now, back on the SW-54. I started at the input pin of the 50C5 (pin 5)and worked my way backwards. I got a steady, strong tone from Pin 5 of the 50C5, pin 7 of my 12AV6 (which is output but it means my network is good) and pin 5/6 of the 12AV6.

I ran into trouble at my 2nd IF transformer. Injecting the AF signal to pin 6 of the transformer block gave me a loud but heavily distorted signal, and injecting the AF signal into pin 1 of the block gave me a loud click but no signal.

So I'm thinking maybe my issue is inside my 2nd IF transformer, but I'm still seeking input before I do a swap. I have a spare if necessary.