http://www.nostalgiaair.org/PagesByModel/717/M0004717.pdf
I re-capped and also replaced every resistor in it.
I realigned it and it was playing perfectly. I had not yet cleaned up the chassis rust or anything... just electrically revamped it!
I was letting it play on the bench for a day or so and while listening to it from the next room I heard it make a couple of crackling static sounds then... dead SILENCE!
Good!... I thought... this will make for interesting troubleshooting since EVERYTHING is new...right?
So I see that all the filaments and pilot light are still on... ok.
Then I do a quick audio test with my finger on the CT of the pot ( normally causes hum) ...just dead silence.
So I do another quick test for power supply voltage.
Yep there is 130vDC on the first filter cap and 75vdc on the 2nd filter cap.... hummmm
Well ..yes... OK there is B++ and B+... but funny because I thought I remembered B+ being a bit closer to 90vdc...though not 75vdc.
.. so that part is a little strange?? Humm.
Now I have my nice new o'scope just sitting there so I placed the probe near the local oscillator coil and sure enough the oscillator is working fine... and while watching it I can turn the tuner and watch the frequency change... so all that much seems fine.... hummm
Well ... ok... before I actually have to do any real thinking here, I think to myself... let me take the "quick and dirty" lazy-brain approach and swap a couple tubes from the other working AA5 radio I have on the bench.
I first swap the 50L6... no change.
... then I swap the 12sq7 and the 12sk7.... still no change to anything ...all the voltages remain the same too.
Now I know the 50L6 isn't putting out anything... right?
So time to think a little.
I measure the plate voltage at the tube and only see about -2vdc... WHAT?... how's that?
My first thought was that maybe there is something pulling down the B++ on the plate.... like the 50L6 being turned on real hard or a shorted tone cap from plate to cathode... right?
But gee it seems so odd to be way down at -2vdc... right?... how can that be?... usually if it's low it would still be some low value of +DC not -DC !!
... strange.
Still... no active brain waves or light bulbs are going off in my head.
So I think.... just maybe somehow that nice new .02uf tone cap on the plate might have shorted or something?
So I lift it... same thing... -2vdc on the plate!!!
So I go back and re-measure the other side of the output transformer B++ at the power supply ant it's +130vdc.
Then I lift the opposite end of it off the plate... and sure enough the plate is just floating and so is the output transformer...
It's OPEN!!!
Well.. ok... DUH .. to me.
But I guess I just never expected a passive component like that to actually crap-out right in the middle of playing like that... and then... why the strange low B+ voltage?
Well ok... I replace the output transformer and everything plays nicely again. I remeasure the voltages at the power supply.
B++ is 120vdc
and B+ is 90vdc
Ok.. I understand that when the load is removed fom B++ that 120vdc will rise up to +130vdc... sure fine.
I tested it again afterwards by removing the plate from the output transformer... and sure enough the B++ at the power supply goes right up from 120vdc to 130vdc... and the B+ drops down from 90vdc to 75vdc!!
But why does the B+ drop down like that?
It goes from 90vdc down to 75vdc just because the output transformer is open?
Why is that?
Is it that the screen grid on the 50L6 is starting to draw current?
Any ideas?
It was the 50L6 screen grid drawing current since the plate was no longer energized ...so the tube thinks the screen grid is sort of like the plate.. and it draws current.