[Les Dickson]

So this question may reveal how little I know, but I’m hoping someone can give me some ideas of how to proceed. I’m working on a DeForest W0541 Ch-10DA517. I’ve check the voltages and they are all within limits. When using my signal generator I get results I’ve not seen before. Working from the output tube back to the antenna here’s what I get.

Injecting 400 cycle AF

Tube 35L6
plate = no signal
grid = signal

Tube 12B6
plate = signal
grid = signal

Injecting 455 KC

Tube 12K7
plate = no signal
grid = signal

Tube 12A8
plate = no signal
grid = signal
(oddly, I do get a signal from pin 5 which goes to ground through R4)

Attached is a schematic where the successful signals are in green and the unsuccessful signals are in red.

Attachments:

1. 

   signal-generator-problem-scaled.jpg
   

[John Greenland]

Les,
Assuming that the dc voltage on pin 5 of the 35L6 is neg 5 volts, ( the grid bias voltage ) I would be curious about how much dc voltage is being dropped across the output transformer.
The schematic indicates about 8 volts dc.
This is roughly the same as the difference between pins 3 and 4 of the 35L6.
If this voltage is much less between these 2 pins , then I would think that the 35L6 or its socket is the problem as the tube may have no current flowing thru it.

TTFN
John G.

[Les Dickson]

Thanks for your response, John. The voltage drop across the output transformer is 7.04 volts. I’m reading 93.5 volts at pin 4 and 86.5 volts as pin 3 so that’s a difference of 7 volts which is identical to the voltage drop across the output transformer. So I’m still pondering what’s going on.

[John Greenland]

Les,

I too am wondering.

What are you measuring the audio with ? The gain od the 35L6 is probably 15 to 20 so 1 volt ac on the grid should give 15 to 20 volts ac on the plate.
At the plate of the 35L6 the 80 plus dc volts can play havoc with some voltmeters in ac mode.
The only other suggestion I have is to replace the 35L6, but am also thinking that you may have tried that already.

[Ralph Spracklin]

Evening Les
As a relative newbie in this radio game.I’m not certain if I am even qualified to put in my two cents worth. I think some history on this set is needed. Was it working properly prior to this/these problems arising. Did the proplem(s) occur while the set was being worked on. It seems as though John may be right on the 35L6. Maybe the Cathode wire on this tube is blown off internally.??? Just playing doctor here.

Ralph

[Les Dickson]

Update.
I puzzled over this for a long time. I pored over the schematic to look for irregularities and found nothing, then rechecked the 35Ls tube, swapped in a new tube still nothing – then I discovered that at the locations where I wasn’t hearing anything from the signal generator that there was actually a very, very faint signal. That got me wondering if the radio was really out of alignment so I went through the alignment procedure and it was way, way off. So now I’m receiving stations and all is well.
Just wondering if anyone else has experienced this?
Thank you to everyone who contributed ideas.

[John Greenland]

Hi Les,

Glad to hear that you sorted it out.
I will make a suggestion for future testing — inject a signal that the stage or stages work(s) with.
Had you injected audio at the 12B6 it probably would have sounded fine.
You could then inject your IF at each stage working from right to left on the schematic, knowing at least that your audio section was OK.

I think that you would have probably found the IF stages out of alignment rather quickly.

Anyway — food for thought.

Merry Christmas and a Happy New Year !

[Author]

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