'49 Admiral audio problem

[wa2ise]

Originally Posted by kbmuri

Thanks again, wa2ise, I've looked at that schematic a hundred times, and have seen pin 2 as a cathode and pin 5 as plate every time (I guess I've been looking at too many 12AX7s). So I've been imagining the top diode backwards. When you said the diodes were in series, I said, "huh?". Then I looked a little closer. Ok, so I see why the secondary coil has a DC potential from chassis ground. And, like you said, .024V is about half of the .054 on pin 5 (also at the cap +). So I guess that circuit is correct.

Sounds like the detector is probably working, and the problem may be a dead sound IF stage

Like I said earlier, C37 was not present originally (solid wire intead). No sweat to try an .02 mFd.

That's not likely to be the problem, then. Just leave the wired connection.

I'm willing to replace all the resistors in this circuit, just in case one is failing under a load more extreme than my Ohmmeter. Although the whole kit-n-kaboodle is operating in the 0.1-Volt neighborhood, so it's hard to imagine they're particularly stressed out.

If they read reasonably correct (20%) with the ohmmeter, then they should be okay and not need replacing.

If I'm understanding well, the DC potential at the + side of cap C5 could be said to vary in proportion to the amplitude of the AC signal as seen at the secondary of the IF transformer. Demonstrated by the fact I could get pin 5 to read nearly 3 Volts with the signal generator applied. If so, then I suppose a low DC reading at pin 5 might still be a symptom of weak signal from further up the line. If so, then I'm still just as "in the dark" as I ever was. The Sams isn't clear whether the 0.1V expected at pins 1 & 2 are with or without a signal applied.

The capacitor symbol connecting across the IF transformer secondary has no part number and doesn't appear to exist on the TV chassis. I've been assuming it's physically inside the IF transformer's shell (or a measurable property of the transformer itself, perhaps?). Could this have gone bad? I'm reluctant to pull the transformer and start digging inside it on a "maybe". Just throwing it out for debate.

That cap is inside the IF can, and is not likely to be bad. Though if it did go open circuit, it would screw up the resonant circuit frequency. Not likely.

You may have noticed I can only work on the set on the weekends. Other pressing tasks. So any suggestions and comments during the week are more than welcome, even if it takes a little time to get to them.

thats okay :-). I think you said that you did check the circuits in the sound IF strip (do you get voltages on tybe plates, and do you see a volt or so of voltage across cathode resistors of those tubes?). A broken IF stage could ruin the sensitivity of the sound detector. Maybe an IF transformer went open circuit.