general-el: Audio Dies Out

I recently started a full restoration of this classic radio.  Before the restoration, the radio was working, but I never let it run very long for fear of damaging components before recapping.

I swapped out all the usual suspects, including the electrolytic caps.  There's only 1 cap that hasn't been replaced (C20) a 1000v, but I don't think that this is the problem. (Just a guess)

What happens is that the radio works great as it warms up. I receive a strong signal and can tune the radio, but after 10 seconds, the sound starts to weaken and quickly is gone.  

I'm just looking for some guidance on what to be looking for (i.e. a tube problem or a resistor problem).  Any help is appreciated!

thanks.

craig.

 

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Craig, Look at your voltages, especially your B+.  If the voltage comes up and then quickly goes down at the same time your audio dies, you may have a circuit that is loading down the power supply.  Also look carefully at your grid voltages.  You notice in the voltage chart that some of your grid voltages are slightly negative.  If they are even slightly positive, excessive current flow through the tube could place an excessive load on your power supply.

Question: when you say that the audio dies, have you verified that in fact it is the audio stages that have failed (using a signal tracer), or does the entire radio including oscillator and mixer stage fail to function?

All the best,
Howard

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Hi Howard,

I checked the rectifier tube and it was bad.  The filament lit up like expected, but then after a minute it went dark and my tube checker indicated that the tube was bad.  The new tube arrives tomorrow so I'll let you know if that doesn't do the trick.

cheers,

craig.

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Hi Howard,

After much searching and freting, it turned out that I had the electrolytic capacitor installed the wrong way.  What a silly mistake!  Nonetheless, I reversed the capacitor and it immediately started working correctly...

Now I moved on to voltage checks and saw some things that were still off by 20% here and there and tested the resistors and found 3 or 4 that were more than 30% off, and quite a bit off in some cases.  

Now I wait for the resistors to arrive, but am going in the right direction.

cheers,

craig.

 

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