I have an Atwater Kent Model 42 in about a dozen pieces, ready to reassemble and rewire. While I'm fitting the power transformer into the power unit case, I realize that I have no ground lead as shown in the diagram.

https://lh5.googleusercontent.com/-72kXZcOaFsA/VG05WKv_uyI/AAAAAAAANF4/Niz-pucn0TY/s520/AK%252042%2520200.jpg

I have the other 12 leads properly identified and voltage-tested, but I think there should a separate lead to ground--I can't see any other way that the transformer would be grounded. I've been staring at the transformer for several weeks, but the solution has not appeared.
So, perhaps I broke off a copper-wire lead and can't find it?

https://lh4.googleusercontent.com/-l7JKiwlGR0A/VG05XHfunHI/AAAAAAAANGY/2llRWREKGGs/s520/AK%252042%2520204.jpg

Another question: What for is that reddish fabric strip (upper left)? I saw a photo somewhere that explained why it was there, but I forgot the answer and can't find the photo again.

Check the resistance across the leads of the rectifier plate winding, then check to see if you get about half that value between either end and the iron core...If you do then the core is your ground.

Good luck.

Thanks. I did that already--300 ohms across the HV secondary, looked for 150 ohms from that winding to every possible contact--but no continuity to the core or any other place. I'll recheck though.

Winky are ya sure that IS a fabric strip , and not a strip of woven fine copper wire that could be your missing HV winding center tap ?

Good thought--I had to double check, but it's cloth.

Did you check it with an ohmmeter just to be sure?

Absolutely. And I scraped a clean spot on the core to be sure of contact.

Could there be a fairly fine wire broken off inside of the cloth braid?

jr

I just shredded some of the cloth braid. No wire.

I got a good replacement transformer. What I lost on the original wasn't the ground connection; it was one end of the primary. The missing connection was on the inside of the primary coil, about two inches up where the stick is.

https://lh3.googleusercontent.com/-aOTaTU7cM8k/VKOO_gGZSlI/AAAAAAAANR4/7RjUeo-jnT8/s340/AK%252042%2520208.jpg

Now that I have power, I got all the components cobbled together on the bench; chassis, power supply, speaker, variac, and isolation transformer. All components meet original specs except the three filter condensers which were replaced with .5-, 10-, and 10-uF caps as recommended in an article posted on atwaterkent.info by Ken Owens.

https://lh6.googleusercontent.com/-RmikRBQMrKQ/VKM9EgnYo1I/AAAAAAAANRc/ke-F4Yj_rGM/s640/AK%252042%2520221.jpg

With 110V input, I got good reception and very good audio with NO HUM. BUT...the medium-strength volume was practically unaffected by the volume rheostat and I couldn't receive the signal through my indoor antenna--I had to connect the antenna post to an earth-ground. After quadruple checks on my wiring and disassembling the rheostat I ran the voltage tests, and everything was good except the power tube +B voltage was only157V versus the specified 180V.

So, I reassemble everything in the cabinet, turn it on, and get no sound at all. Quick check shows (Aha!) no filament voltage at the power tube socket (but the voltage is present at the power supply, so that's an easy one, even for me). BUT...now the +B voltage to the power tube is 200V!

So, I disassemble the power supply until I find (Aha!) a broken contact on the speaker choke (also referred to as the plate choke).

So, I repair that and reassemble the power supply and think the problem is solved. BUT...now I have 240V!! +B to the 171A. Further investigation discloses (Aha!) that I have a broken connection to the RF plate circuit resistor.

So, I pull the chassis, repair the broken connections to the plate resistor and the power tube filament voltage and try again. BUT...now the power tube plate voltage is 300V!!! (and the RF plate voltages are high as well). Only the detector seems normal.

I retested all the tubes to make sure that I hadn't killed any of them. I need one more "Aha!" to proceed with the project. Would I be risking damage to the power transformer if ran the set without tubes in? Does anyone know where I might find another "Aha!"?

On the bright side, the refurbished vernier dial works perfectly!

https://lh6.googleusercontent.com/-IsQLLDuhH5w/VKMq5YVJx8I/AAAAAAAANQo/dW41ggkFs9I/s460/AK%252042%2520405.jpg

It's probably okay to run the set without (any) tubes, but it might be a good idea to put a dummy load on the B+ winding just in case its voltage is able to increase beyond the insulation rating of the transformer (which is a bad thing). Can calculate that load based on the current drawn by the B+ and the voltage using ohm's law.

Thanks. I'll give it a try.

As far as volume control goes, if the 42 is like the 40 and 44, don't
expect much control until it is in its in its case with the top on.
You're probably picking up local stations on the input to the 2md
RF.

And any sets that use a volume control in the antenna circuit need a good external ground connection in order to properly adjust the volume.

I understand the issues with ectopic signal reception in these sets. I have that problem with a Model 37 (but only if the lid isn't on the set), and I'm pretty sure that the situation with my Model 42 is different.

I'm not sure how I accomplished this, but I got all of the voltages OK at the power supply and at the tube sockets. After I was sure that problem was fixed, I spent a lot of hours checking continuity, resistances, and capacitances throughout the set. But I still have the same issues--reception only when the antenna is connected to my earth ground and barely perceptible volume control. Good quality audio, though.

(My "earth ground" is 10 feet of bare braided copper wire running down the side of the house, buried 2 feet deep [where the soil is always wet] with the strands splayed out over an area of about 2 square feet, which I hope is enough to ground a radio signal.)

Also, I get the strongest reception when I hold the ground wire (attached to the antenna post) and put a finger directly over the 2nd RF tube. And putting the metal lid on the cabinet changes nothing. Except, of course if I forget to take my hand away from the 2nd RF tube--the lid is pretty heavy, so I get bruising on my wrist.

So, the signal is either bypassing the volume control, coupling transformer, and 1st RF tube (?); or the volume control is somehow screwed up (?). I'm posting this now to see if someone has a suggestion other than to pull the chassis again to disassemble the volume control.

I don't understand how this is supposed to work. Here are two diagrams of the same thing except the right diagram doesn't show the shielding on the antenna lead.
With the volume control set up like this, I get resistance between the antenna post and the 1st RF grid from 16 ohms to 230 ohms and then back to sixteen.

https://lh3.googleusercontent.com/-cC5Xnzg6ceE/VKeEc_2M87I/AAAAAAAANSI/ScR6lnd0hFY/s460/AK%252042%2520223.jpg

I have a Model 37, which is set up like this. This works pretty well with no external ground.

https://lh3.googleusercontent.com/-e52tLktOM4Q/VKed4WqaTvI/AAAAAAAANSY/rao6MQlXrjo/s360/AK%252042%2520224.jpg

cancel, bad info. Ignore attachment.
Sorry, coffee hadn't kicked in yet.

Actually I tried a couple of other configurations which didn't work, but they were based on my lack of understanding of the antenna coupling transformer. So I'll disassemble the set tonight to see if I can find faults.

Now, looking closely at the coupling transformer, which I viewed as a single coil, like a choke, tapped in the middle for some mysterious reason. I see that at the antenna connection (red), there are two wires--one for an inner winding which ends at the yellow terminal and the second for an outer winding which ends at the black terminal. So there are effectively two coils which mysteriously act upon each other when the signal current is present. Does this work like a step-up transformer?

https://lh6.googleusercontent.com/-Ug8W-r7nZas/VKh_UPFltaI/AAAAAAAANSo/G5TnJWkMAzw/s400/AK%252042%2520225.JPG

The antenna transformer is what is known as an auto-transformer.
http://en.wikipedia.org/wiki/Autotransformer

If you know how a variac works then it makes the description simpler. Your antenna transformer is used as a voltage step-up transformer.

Of course I know how a variac works. You turn that knob on the top and watch the needle go up....
But seriously, I tried to research this earlier but I needed that word, "AUTOtransformer."
Thanks much.

I'm at the point where I'm repeating tests without changing anything and expecting different results. The only change I've made is to increase the plate voltages which were all lower than normal. This made a very slight change in the radio's performance.

These socket voltages look good to me:

https://lh4.googleusercontent.com/-hOOe0XFQvXw/VK9kDbmT_4I/AAAAAAAANTA/iOOFkYQfpRs/s460/AK%252042%2520228.jpg

Still, I can only pick up a broadcast signal if it's very strong and the antenna post is connected to my "earth ground." With the radio's lid on the output is medium-low audio volume at the rheostat's lowest, increasing as expected when the volume is turned up, like half the signal is going through the rheostat. But if I pull the coupling tube, there no appreciable change in the audio output.

On the other hand, if I use a signal generator (induction through two loop antennae) to the antenna post, then pulling the coupling tube stops the audio output.

I've also tried aligning the variable condensers but haven't changed anything. Notably, rotating the antenna coupling condenser 360 degrees has no effect on the output.

I'm hoping that these results will resonate with someone. Otherwise this set goes back in the box and I move on to the next project ("Going from failure to failure with undiminished enthusiasm").

https://lh5.googleusercontent.com/-CUuIQlzPI0M/VK9wHdPgp5I/AAAAAAAANTc/vy3IiFkMEWM/s400/AK%252042%2520901.JPG

I left that link at the end accidentally.

Could the fabric strip be a dipping thread? Electricians in the motor rewind shop used to dip the windings in Varnish, and used shoestrings or paracord - strong, light, and with just a slight flex they could break all of the dried varnish off it for reuse.

Do you have zero resistance between your antenna ground and your HV center tap (chassis ground)?

Possibly it's a dipping thread. The second transformer I looked at has one on top and one on the bottom, and they're long enough for that purpose. I don't think Atwater Kent would have needed to varnish the outside since they were entombed in sealer, but I don't know if Atwater Kent made the transformers or if they came from a different manufacturer.

The thick tar likely would not penetrate the windings as deeply as thin varnish (which aided in the dielectric strength of the windings.).

Kevin: Yes, zero ohms from the center tap to any other chassis ground. And (this is something I don't understand) zero ohms from antenna post to ground when the rheostat is at the lowest setting. When the rheostat is turned (440 ohm element), the antenna-ground resistance goes up to about 100 ohms, then back to zero at the highest volume setting.
Tom: Makes sense. I also think it might have facilitated placing the transformer into its chamber. It's a tight fit with very little room for fingers. I have to use a pair of pliers on the core to pull it out.

According to the schematic - with the volume control all the way up(volume control center wiper at bottom end of antenna coil), you should basically be reading 6 ohms across your antenna terminal to ground(DC resistance of the lower half of the antenna transformer in parallel with 400 ohm volume control). With the volume all the way down, the volume controls middle wiper should be shorting the antenna post directly to ground. So if you're measuring 100 ohms at some point, that would seem suspect. Have you taken resistance measurements of the two windings of the antenna transformer? It would be completely reasonable for that ant coil and/or volume control to have been zapped by lightning at some point, although that would normally leave something open circuit, it could also leave a carbon track. You can try hooking your antenna directly to the top half of the antenna coil(1st RF grid). At that point the volume should get much louder.

https://lh3.googleusercontent.com/-cC5Xnzg6ceE/VKeEc_2M87I/AAAAAAAANSI/ScR6lnd0hFY/s460/AK%252042%2520223.jpg

Thanks. I've covered this ground before, but I rechecked both issues. The antenna autotransformer shows 6.4 ohms and 18.8 ohms. I have a spare transformer that shows the same resistances within 1 ohm. The rheostat has been disassembled twice and totally rewired (but saving the shielded lead), carefully inspected and tested. I just tried bypassing the coil with the red lead (directly to grid) but it made no difference in any way. Putting the lid on the set reduces the volume by 80 percent.
The rheostat has no effect on the volume in this configuration.

I think I've tried every other configuration from rheostat to grid. I have a Model 37 that works well. It's nearly identical to the Model 42, but instead of the antenna transformer it simply has a choke from grid to ground. I was thinking of disconnecting the antenna transformer and using the choke from the older set just to see what happens. Your thoughts?

I think I've tried every other configuration from rheostat to grid. I have a Model 37 that works well. It's nearly identical to the Model 42, but instead of the antenna transformer it simply has a choke from grid to ground. I was thinking of disconnecting the antenna transformer and using the choke from the older set just to see what happens. Your thoughts?



Hmm. I think something else is wrong. When you hooked the antenna directly to the 1st RF grid, it effectively took the volume control out of the equation. I need to think about this for a while. :scratch2:

What happens if you connect your antenna wire directly to the grid of the 2nd RF tube?

This is a dumb question, but have you visually verified that the filament of the 1st RF tube is lighting?
And can you verify that the tube pins for the grid and plate are contacting the socket lugs?

Just got back from dinner and connected antenna wire from rheostat directly to 2nd RF grid. Amazing. The three strong stations come in, each over a broad range, volume control works (but some signal is still present at lowest volume setting), and the rest of the stations I usually receive are present as well.

I can't relate the 0-100 scale to frequencies, but at the low end of the band a single station is present from 0 to 20. At the upper end the reception ranges become normal.

What do you think? I'm going to recheck continuities on the 1st RF transformer, then look at the 1st variable condenser more closely.

It's normal that you're loosing a bit of selectivity when only running two stages of tuned RF amplification. Now if you were to run your antenna wire to the grid of the third RF amp, it will tune even broader, and with less gain. But that's great news that you've narrowed things down to the first RF stage.

In case you don't already have it, here's more modern schematic that's much easier to navigate.

http://www.atwaterkent.info/TechData/Drawings/Images/AK09800sch.pdf

Whoops! I didn't notice that I had the coupling tube pulled. All of the above is without the coupling tube.

When I inserted the coupling tube it reverted to original--only three strong stations with no control from the volume rheostat.

I do have the modern schematic, but I learned to read schematics from Riders Vol 1-3, so the old ones are easier for me. They're more pictorial.

Not sure what you mean by the coupling tube?

Is it possible you could have the polarity of one of the 1st RF transformer windings reversed?

Sorry, I'm rushing too much.
The coupling tube is the 1st RF.
I just repeated trials with antenna direct to 2nd RF grid, both with and without the 1st RF tube. The result for both was the same--what followed "Amazing," lots of reception with good volume control of all but the strongest stations.

The 1st RF transformer connections have tested correctly according to the (old) schematic, and the connections have not been changed from original, and are the same as on my junk Model 42. But I'll recheck them.

The 1st RF transformer connection to the 1st RF plate lead was "iffy." I had to do some pushing on the test probe to make the connection. So I resoldered that one.

And somehow I missed Old Coot's response yesterday. The filament lights and I get good Gm and emissions with my testers. Plus, I tried swapping tubes anyway. I also cleaned all contact surfaces of pins and socket and verified that they were connecting.

I just put the coupling transformer back into the circuit--wired normally except the black lead goes to the 2nd RF grid. The result is the same: Three uncontrollable loud stations and the rest of the area stations come in normally.

Kevin: Yes, zero ohms from the center tap to any other chassis ground. And (this is something I don't understand) zero ohms from antenna post to ground when the rheostat is at the lowest setting. When the rheostat is turned (440 ohm element), the antenna-ground resistance goes up to about 100 ohms, then back to zero at the highest volume setting.


Backing up a bit here. I measured the resistance between antenna and ground on my 42, and I get the same - close to 100 ohms at about mid volume. Looking at drawing again, it makes perfect sense - at mid setting the 400 ohms is basically split into two 200 ohm resistors in parallel, but the lower half is in series with the 6 ohm winding of the antenna transformer. So at mid setting you have 200 ohms in parallel with 206 between ground and antenna. A little hard for me to visualize looking at the drawing.

https://lh3.googleusercontent.com/-cC5Xnzg6ceE/VKeEc_2M87I/AAAAAAAANSI/ScR6lnd0hFY/s460/AK%252042%2520223.jpg

I just put the coupling transformer back into the circuit--wired normally except the black lead goes to the 2nd RF grid. The result is the same: Three uncontrollable loud stations and the rest of the area stations come in normally.

I'm not clear on what you did, or did differently?

Previous configuration was red (center tap) to 2nd RF grid.
I put red back to the center tap. Then disconnected black from 1st RF grid and connected it to 2nd RF grid.

Then I remembered to try it with the lid on the set. The lid blocks nearly all of the non-antenna signal. In that configuration the set receives normally, all stations volume-controlled--except for a negligible amount of the strongest signals getting through the cabinet.

So, it works well bypassing the 1st RF tube, 1st RF transformer, and 1st variable. condenser. I'm assuming there's a problem with one of those components or with the connections. Right now I'm removing the 1st RF transformer so I can inspect it thoroughly and check the continuities again.

The other part of the puzzle is that I cannot detect any signal if the set is connected to my usual 40-feet-around-the-ceiling antenna. I don't get any detectable signal unless I connect the antenna post to "earth ground." (I put that in quotes because it's not an REAL earth ground with iron rods 8 feet into the whatever)

And I would guess that the inability the get signal through my normal antenna is related to whatever keeps the signal from getting through the first RF stage.

I'll take a close look at the RF transformer, then try to simplify the results with only the essential variables.

Ok, another goofy theory from jr... How about your indoor antenna is picking up the amplified signal from the TRF stages and the radio is actually oscillating at some frequency, obliterating the weaker signal from radio stations? Do you have an oscilloscope?... it might be worth checking the output of the last TRF stage for a strong CW carrier.

jr

jr: Not having an oscilloscope, I approached this theory differently. I took down the antenna. No change in symptoms.

Here's a recap of what the radio and I are doing:

https://lh5.googleusercontent.com/-Qe48NNBegd4/VLGfQnyhurI/AAAAAAAANTs/IXt5NM73FvM/s460/AK%252042%2520226.jpg
Normal configuration on left. Configuration on right bypasses 1st RF stage.

Normal configuration, lid on, ANT post to indoor antenna:
No audible signal at max volume.

Normal configuration, lid on, ANT post to earth ground:
Receive 3 stations, some audio at zero volume, otherwise normal volume control.

Bypass 1st AF stage, lid on, ANT post to indoor antenna:
No audible signal at max volume.

Bypass configuration, lid on, ANT post to earth ground:
Receive 7 good stations and 3 weak stations with normal volume control. Only the strongest station gives some audio at zero volume.

(I pulled out the 1st RF transformer and it appears to be OK, with 2.2 to 2.4 ohms through each winding.)

A couple of questions;
1. Is there *another* path to ground perhaps?... such as a new 3 wire power cord grounded to the chassis for "safety" reasons, or perhaps "leakage" between the power transformer primary and the chassis?
2. Does the volume control pot work better if you connect the yellow and green wires together at the back of the pot?

This is a real headscratcher,
jr

I keep missing Kevin's posts. This is from yesterday. I will try antenna directly to grid RF2 and see what happens.

jr - No, I didn't ground the chassis to the house ground. I actually did that on the first-ever piece of electronics I worked on a few years ago. Couldn't figure out why the circuit breaker tripped every time I turned on the power. Then I read the sticker on the back panel, "DO NOT GROUND THIS CHASSIS!"

Potentiometer? I guess this is technically a pot, but I don't think they had started using that word in 1928. The wires exit the side of the Bakelite shell, and they're not accessible without pulling the chassis. The volume control apparently works properly when the signal goes through it, but the next time I pull the chassis I'll look at that.

Power transformer primary leaking to ground? Probably not, but easy enough to check.

So, this evening I'll (1) Try antenna wire to 2nd RF grid, (2) Check resistance between power transformer primary and ground, and maybe (3) Set the Model 42 up like the Model 37 using the antenna choke from the older model.

Oh, I almost forgot that I have a Heathkit signal tracer that I "restored" last year, but it's never been used. If a signal tracer would be helpful, I'll see if I can figure out how to work it.

Here's a recap of what the radio and I are doing:

Normal configuration on left. Configuration on right bypasses 1st RF stage.

Normal configuration, lid on, ANT post to indoor antenna:
No audible signal at max volume.

Normal configuration, lid on, ANT post to earth ground:
Receive 3 stations, some audio at zero volume, otherwise normal volume control.

Bypass 1st AF stage, lid on, ANT post to indoor antenna:
No audible signal at max volume.

Bypass configuration, lid on, ANT post to earth ground:
Receive 7 good stations and 3 weak stations with normal volume control. Only the strongest station gives some audio at zero volume.

The symptoms you've delineated here could occur if the grid of the 1st RF tube were shorted to (chassis) ground.
With the tube removed, and the coupling xfmr disconnected from the grid lug of the socket, there should be infinite resistance (open circuit) from the lug to ground. Is that what you're getting?

I keep missing Kevin's posts. This is from yesterday. I will try antenna directly to grid RF2 and see what happens.


You did that, only you included the volume control and antenna transformer, which is fine. I'm fairly confident there's nothing wrong with your volume control, because it seems to control the volume going into the 2nd RF stage just fine.

Old Coot: There is infinite resistance between 1stRF grid lug and chassis ground (With no connection of grid lug to anything).

I replaced the old (magnet wire) indoor antenna with 50 feet of 14 gauge, 7-strand copper "antenna wire." It works better, and I am no longer connecting the antenna to earth ground for reception. So this is a bit of a restart which will simplify things for me.

1. In "normal" configuration, I get very weak audio at maximum volume on two or three strong stations.

2. With antenna to 1st RF grid, as below, I get slightly better audio output from the strongest stations.

https://lh5.googleusercontent.com/-TzqPIul2aLc/VLMAbJN57XI/AAAAAAAANUI/ZTA9C-ibLOI/s400/AK%252042%2520231.jpg

3. With antenna to 2nd RF grid, as below, I get loud audio from many stations.

https://lh4.googleusercontent.com/-xraa50EBiDQ/VLMAbz8tsbI/AAAAAAAANUM/IqhbFiz9a7w/s400/AK%252042%2520232.jpg

I've also pulled the grid resistor and the RF transformer to examine them out of the circuit, and I can't find anything wrong with them. I played with the signal tracer, but all I could do was pick up audio.

Question: With the diagram below, I still read "short" between stator and rotor. I see no physical connection of the stator with chassis ground. Is this short to be expected--is enough current conducted between rotor & stator plates to show resistance of only 4 ohms?

https://lh3.googleusercontent.com/-j5U8JrVUWNg/VLMAarGBMUI/AAAAAAAANT8/n3X4EaKZrcE/s300/AK%252042%2520238.jpg

No tuning capacitor should ever show DC resistance (the only kind a DMM will measure)!

If that capacitor is shorted then it would completely explain the symptoms you've been discussing. Find that short and fix it.

Also if the grid lug of the first RF tube shows resistance to ground with the tube removed and nothing connected to it, then the socket is bad...Which means find and remove the conductive path or replace the socket.

Both of those problems will kill a whole lot of signal if left be.

No tuning capacitor should ever show DC resistance (the only kind a DMM will measure)!

If that capacitor is shorted then it would completely explain the symptoms you've been discussing. Find that short and fix it.

:yes:

The little signal that is getting through is probably feedback through the common filament bias network.

Confused. The set is not in operation. 1st RF tube grid lug has been disconnected from the coupling transformer. Tube is pulled. Those are the only electrical connections to the lug. It's mounted on Bakelite and has no electrical connection. The DMM show infinite resistance between the lug and ground. There is no conductive path. What am I missing?
The statement about tuning capacitor is also confusing to me. It shouldn't show DC resistance--but if it's shorted it's bad? I'm missing something here as well.
Help?

Missed Kevin's last post. What little signal? I only understand 85 percent of the thousands of meanings of "signal."

Confused. The set is not in operation. 1st RF tube grid lug has been disconnected from the coupling transformer. Tube is pulled. Those are the only electrical connections to the lug. It's mounted on Bakelite and has no electrical connection. The DMM show infinite resistance between the lug and ground. There is no conductive path. What am I missing?

NOTHING! That is how it should be. One of YOUR earlier posts indicated that the test you just described gave you a resistance reading that was less than infinity...So I was telling you readings less than infinity in that situation are bad.


The statement about tuning capacitor is also confusing to me. It shouldn't show DC resistance--but if it's shorted it's bad? I'm missing something here as well.
Help?
By should not show resistance, I mean there should be INFINITE resistance (so high you can't measure it) between the side of that capacitor that is connected to the RF transformer and ground in the circuit shown below.
TO BE DOUBLE CLEAR IF THERE IS A SHORT OR A RESISTANCE LOWER THAN INFINITY THEN SOMETHING IS WRONG.
https://lh3.googleusercontent.com/-j5U8JrVUWNg/VLMAarGBMUI/AAAAAAAANT8/n3X4EaKZrcE/s300/AK%252042%2520238.jpg

I usually use the term 'no resistance' to mean an open circuit....I guess I should use 'no measurable resistance' next time to make it more clear. If you don't know yet that a capacitor is supposed to have infinite resistance between it's terminals (unless something is wired in parallel with the capacitor), then you REALLY need to brush up on the basics.

Hi Tom, There's a lot of folks enjoying the radio restoration hobby that don't necessarily have a formal background in electronics and basic electrical principles. And everyone learns at a different pace, so we need to cut them some slack. Now I'm curious to find out what is causing that tuning capacitor to be so fully shorted. :scratch2:

Missed Kevin's last post. What little signal? I only understand 85 percent of the thousands of meanings of "signal."

I mean any little signal that's getting past an apparently defective(shorted plates) tuning capacitor.

Kevin: I've been looking for a leak to ground, and that's why I was asking about my finding of a short through the first tuning capacitor. The stator is grounded through the RF transformer secondary. The rotor is grounded directly. I recognized that the rotor and stator would short through their ground connections, so disconnected the RF transformer ground before testing stator-to-rotor connectivity.

I can't see or feel any contact between stator and rotor plates yet I still found a stator-rotor short, measuring 4 ohms. The RF transformer coil also measured 4 ohms, which seemed like a peculiar coincidence.

So, I was trying to find out if there might be a another explanation for a stator-rotor short other than plate contact before I proceeded.

Thanks.

Well you can turn the rotor so it's completely out of mesh from the stator - that eliminates any possibility of plates rubbing. You can disconnect the wire wound(grid stop) resistor and the RF transformer wire from the stator by removing the top and bottom nuts on the rear of the stator - that eliminates any possible leakage through those two points. Other than that you have the stator supported to chassis by the top and bottom phenolic insulators. I suppose there's a remote chance that something metallic is bridging across the phenolic pieces to chassis ground. I can't see any other possibly path to ground.

Yeah. I figured out the un-meshing thing. The plates were in a position that couldn't be unmeshed without first pulling the chassis, loosening the pulley, and repositioning the plates. Still had 4 ohms. Having done that, I pulled the capacitor from the chassis and still had 4 ohms. Then I saw the problem. I'll give you a hint, "NEVER USE STEEL WOOL AROUND ELECTRONIC EQUIPMENT."

BINGO! Good find!

jr

Thanks to all who had the patience to help me track down the culprit. I'm posting this photo so others with limited experience might come across it and benefit from it. The lesson is, "Don't use steel wool near electronic equipment." This problem resulted from my use of steel wool two feet away from the set.

https://lh4.googleusercontent.com/--Omlweo__z4/VLRGgXri13I/AAAAAAAANUg/LrrI1aFkzNM/s400/AK%252042%2520985.jpg

I think this set's going to work beautifully when I reassemble it. I'll do one last post when it's complete.

Absolutely on steel wool being banished from the electronics work area. Ditto keep it away from speakers: their magnets want to "wool gather."

https://lh4.googleusercontent.com/--Omlweo__z4/VLRGgXri13I/AAAAAAAANUg/LrrI1aFkzNM/s400/AK%252042%2520985.jpg


I'd like to know what you used to get those capacitor components so sparkling clean? Have you by chance documented this complete AK 42 restoration process some place?

If those plates are aluminum like on the model 40 sets, then they tend to stay fairly shiny and clean on their own...I've worked on/with 3 model 40's, and opened the lids of a few more..

Interesting. The ones I find seem to have a dull gray patina on them, but I admit I haven't done a thing to this one besides vacuuming the dust off. Was just thinking...in another 10 years a lot of these late 20's sets will be 100 years old, true antiques. :)

http://i452.photobucket.com/albums/qq245/Kuehn/Atwater%20Kent%2042/AtwaterKent42chassis002_zps0a086a89.jpg?t=14211701 64

That looks like it spent its life in a coal-fired factory. Here's a photo showing the Model 42 tuner before it was cleaned.

https://lh4.googleusercontent.com/-6VKeDUPqkBo/VLsGQFbAjdI/AAAAAAAANkg/Uw7eohIUam0/s600/AK42%252041%2520Interior.jpg

I have an untouched AK Model 30 chassis sitting behind me, and the variable condensers look just as shiny as these.

I just posted an album of photos of the Model 42 restoration. You don't want to miss the definitive guide to restoration of Model 42 ball feet!

https://picasaweb.google.com/coldrb/AtwaterKentModel42Restoration?authuser=0&feat=directlink

Excellent slide show of the restoration! Thanks for the info.

Winky, You have some amazing restoration skills there! :thmbsp: