National SW-54 Thoughts

[Dubis7]

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.

[Dubis7]

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?

[jr_tech]

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

jr

[Dubis7]

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.

[jr_tech]

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

[Dubis7]

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?

[jr_tech]

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

[Dubis7]

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

[Dubis7]

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.

[jr_tech]

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

[Dubis7]

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.

[Dubis7]

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.

[Electronic M]

Originally Posted by Dubis7

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.

[jr_tech]

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

jr

[dieseljeep]

Originally Posted by jr_tech

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.