[Patrick]

Hi all! Back again!

This time, I've got a 48-475. Replaced all the caps (left most of the mica), checked wiring, replaced resistors if out of tolerance. Got a hum that varies with volume, but not too concerned with that as it will probably go away once I figure out this problem.

I inject an audio signal on pin 3 of the 7C6 ("D" of section 2) and get a good, loud output from the speaker. But when I inject the same signal on pin 6 of the output tube ("B" of section 2), I get no audio. All it does is silence the hum. Tube checks out O.K., but someone did replace it (should be a 7B5) with a 7C5.

Looks like the 7C5 should work in  place of the 7B5.

Voltages everywhere check out O.K., albeit about 20V low.

I've spent waaaaaaaay too much time on this radio already, since it had extensive repair in the past. But I did trace down all the wiring, and everything looks O.K. The only componet that was NOT in the radio was R312 (6.8Meg), but that goes to WS-2® which should only affect FM.

This is only half the schematic. I'll upload the other half if needed.

[Image: http://i1367.photobucket.com/albums/r796...ccckac.jpg]

Ideas anyone?

Thanks,

Pat

[Patrick]

Guess I caught everyone on vacation! Or napping like I usually do. Anyone? Thanks.

[Radioroslyn]

Don't think it matters much as most of the audio gain is developed at the 7C6. Wouldn't loose any sleep over it, cause you've got good output from the 7C6-7B5. The 7B5 will be a little easier on the transformers.

Terry

[Patrick]

Don't think it matters much as most of the audio gain is developed at the 7C6. Wouldn't loose any sleep over it, cause you've got good output from the 7C6-7B5. The 7B5 will be a little easier on the transformers.

Terry

Yes, but why don't I get any audio out when I inject a signal at test point "B"?

Pat

[Raleigh]

Are you injecting your signal through a .1 uF cap? If not capacitively isolated, any DC on your Sig Gen lead could change the grid bias.

But don't ask me why the same thing doesn't happen at point "D". It's a mystery to me.

Everything I've found on line says that the 7C5 is an acceptable substitute for the 7B5 & the specs look similar enough.

[Patrick]

Are you injecting your signal through a .1 uF cap?  If not capacitively isolated, any DC on your Sig Gen lead could change the grid bias.

But don't ask me why the same thing doesn't happen at point "D".  It's a mystery to me.

Everything I've found on line says that the 7C5 is an acceptable substitute for the 7B5 & the specs look similar enough.

Yes, I'm using a .1 uf cap.

Here's the troubleshooting sheet I'm using. All voltages are good. C201 and C203 are both new. R202 is within tolerance. There is also no audio output when injecting a signal at test point "A" either. 

Even though the voltages checked out O.K. on pin 2 and pin 3 of the 7C5, could there be a problem with it? I should have just stuck in another tube. 

[Image: http://i1367.photobucket.com/albums/r796...fdnunr.jpg]

[Raleigh]

OK, here is a silly question. Troubleshooting for section 2 says to use an audio signal. Just to be sure, are you using an audio signal or an audio-modulated RF signal?

If you don't get any audio out of test point A, try walking your probe past C207, the volume control, and C210. At some point you'll get a signal, because you already know you get strong output from the 7C6 grid at point D. Then you'll have a good idea what was wrong between points A and D.

Ditto if you walk the audio signal from point A up to the IF transformer. If you are supposed to hear audio injected at point A, then you should hear it if it's injected anywhere along that path.

Is there a particular problem you are trying to solve? If you hear audio injected at point D, then you know the AF path works from the speaker back to that point regardless of your indication at point B, so I wouldn't sweat that for now. What do you hear when you turn the radio on and try to get a station, or if you put modulated RF on the grid of the IF tubes?

[Patrick]

OK, here is a silly question.  Troubleshooting for section 2 says to use an audio signal.  Just to be sure, are you using an audio signal or an audio-modulated RF signal?

If you don't get any audio out of test point A, try walking your probe past C207, the volume control, and C210.  At some point you'll get a signal, because you already know you get strong output from the 7C6 grid at point D.  Then you'll have a good idea what was wrong between points A and D.

Ditto if you walk the audio signal from point A up to the IF transformer.  If you are supposed to hear audio injected at point A, then you should hear it if it's injected anywhere along that path.

Is there a particular problem you are trying to solve?  If you hear audio injected at point D, then you know the AF path works from the speaker back to that point regardless of your indication at point B, so I wouldn't sweat that for now.  What do you hear when you turn the radio on and try to get a station, or if you put modulated RF on the grid of the IF tubes?

The original problem was no signal from the IF. So I just thought I'd try the troubleshooting steps, instead of just going to the IF coils. I did find that the volume control pot is flaky. I was able to finally get a good audio signal on the left of R205 (100K), but very weak on the  test point A side. Is a 100K resistor going to reduce the audio that much? Then I discovered that I got no signal from terminal 1 of TB400, so none of the trimmers did anything. I disassembled the 3rd IF can, and found a bad mica cap. So I'm just going to check all three IF cans out, but I had to order more mica caps and fish paper, as the original insulation was stuck like glue to the insides of the can.

So it will be a week or 2 before I attack it again. But I've got a Deforest Crosley 1621, a Midwest 181, a Philco 41-608, a Philco 37-660, and a Philco 38-3-3XX  staring at me waiting for restoration. I'll hit the Deforest Crosley first.

[Patrick]

Now I'm baffled. Looking at the schematic, I get a strong audio signal on pin 3 of the 7C6. I also get a strong signal on the volume control, as well as the left side if R205. I get a very weak signal on the right side of R205.

I get no signal from the plate of the 7C6, and therefore to the input grid of the 7B5.

So how is this strong signal getting to the speaker? What path through what components could it be taking?

[Patrick]

OK, here is a silly question.  Troubleshooting for section 2 says to use an audio signal.  Just to be sure, are you using an audio signal or an audio-modulated RF signal?

If you don't get any audio out of test point A, try walking your probe past C207, the volume control, and C210.  At some point you'll get a signal, because you already know you get strong output from the 7C6 grid at point D.  Then you'll have a good idea what was wrong between points A and D.

Ditto if you walk the audio signal from point A up to the IF transformer.  If you are supposed to hear audio injected at point A, then you should hear it if it's injected anywhere along that path.

Is there a particular problem you are trying to solve?  If you hear audio injected at point D, then you know the AF path works from the speaker back to that point regardless of your indication at point B, so I wouldn't sweat that for now.  What do you hear when you turn the radio on and try to get a station, or if you put modulated RF on the grid of the IF tubes?

O.K. John. I didn't read your post thoroughly. I was using an audio-mod RF signal. When I went to an audio sig, things worked. So I've been looking in the wrong place. The problem appears to be around the 1st IF amp (6BA6). I should be able to find it now.

Pat