National SW-54 Thoughts

[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.

[Dubis7]

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?

[Dubis7]

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.

[jr_tech]

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

[Dubis7]

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?

[jr_tech]

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

[Dubis7]

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?

[Electronic M]

Originally Posted by Dubis7

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.

[jr_tech]

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

[Dubis7]

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.