The chassis had been playing beautifully for several days on the bench.
....But then, UN-fortunately ... or maybe fortunately, as soon as I got the chassis back in the cabinet... the radio started exhibiting LOUD static crashing noises, not unlike silver-mica-disease.
...( however since this set uses conventional IF cans I didn't think it could possibly be that!) Right?
However it's driving me screwy.
Arrrgh.
Here's the print:
http://www.nostalgiaair.org/Resources/062/M0025062.htm
On top of the static problem, ... while I was troubleshooting it, one of the 14C7 tubes' filament opened up!
I didn't have any "14" tubes around and put some on order.
In the mean time Norm Leal helped me out by suggesting that I can substitute a 7A7 or a 7B7 for the 14C7... Just the 6 volt version.
So I put in a 7B7 and just cut back the AC input by 6-vAC on my Variac.
So that was a great help so I could get back chasing down this mysterious static crashing.
The problem is that it would come and go. But mostly when it was active... I couldn't seem to isolate it to any section.
At first I thought that maybe there was shorting inside a tube somewhere.
I began to suspect the first RF tube (14C7) so I put in another 7B7 substitute ... but the problem continued.
It seemed to get worse when there was a signal. And seemed to be less so if I reduced the signal.
But nothing was consistent. Anything I touched appeared to make it happen.. but that was a false indication because it kept up EVEN when I didn't touch anything too!
I've been fighting this thing for three all-night sessions.
I thought it was the mixer (14Q7) but since I don't have one.. I made a socket adapter and subbed in a 12SA7.... But ..no help, SAME loud static crashes.
It was all over the place and I tried by-passing the 1st RF tube and putting the antenna right to the grid of the mixer... but no help.
The whole audio section appeared clean when I inserted a 455khz modulated signal to the 1st IF amp.
So I was suspicious of the RF section... but it was too random to localize.
So I tried a little shot-gun-style troubleshooting by swapping things around.
Checking stage by stage for a sign of help...
I swapped the 1st IF can with a spare IF.... but no help
Same static.
I then disconnected the entire RF stage from where it was feeding the 1st IF amp tube.
Then I tried again to feed in 455khz there... and it seemed fine.
But when I left the signal generator connected to the grid of that 1st IF amp tube.. but turned OFF the modulation... the static started again.
So now I could see it was over there, somewhere AFTER the RF section.
As I fiddled around here and there... eventually the static was there with or without modulation...
BUT it went QUIET if I did not inject a signal.
What?
Signal dependent?
... Yes... somehow... only if I inject a signal.. or with the radio's own RF feeding it, the static was there now.. LOUD crashing!
Remove the RF and it was quiet.
I took a long hard look at the 2nd IF can on the print... and the only unusual thing is that weird split trimmer on the secondary side.
So anyway.. at this point I was so frustrated... I decided to try swapping in a spare regular-4-lead IF.
Well that seemed to instantly stop the static crashes!
What the heck??
So I decided to tear out that whole IF can and open it up for inspection.
It looked pretty clean and normal inside.
I loosened up the 2nd trimmer cap screw a lot and I could see several layers of interleaved plates. I tried blowing a little air in between the plates even a little contact cleaner.
But when I tack-soldered it back in place... the STATIC was back... as loud as ever!
So I can see that the split trimmer has a lead that goes to ground.
I wondered if that area was the source of trouble.
So I disconnected it from ground... it's that 5th wire coming from the center of the split trimmer to ground.
SURE enough.. that's it!
As soon as I disconnected it, the static stopped.
Put it back ....and the static starts again!
So.. it seems there's some leakage path in the ceramic housing of that trimmer.
I'm not sure exactly what the purpose is for that split trimmer.
Perhaps it has an effect on the tone because the radio seems to work well without that lead connected.
... but I think that the audio tone is a bit sharper now and has less bass.
Do any of you ( maybe Norm?) know what it's for?
This has been so frustrating... and it's been the absolutely toughest AM radio problem I've had to chase down.
However.... way back in my memory ...I sort-of seem to think I may have had this same problem years ago... ...like way back around 8 or 9 years ago... because I seem to have vague flashes about this IF can causing problems like this.
Anyway... I fought the good fight!
I slayed that lousy static-breathing-dragon! .... lol
This is part of a filter to remove IF frequency from audio. It operates along with the 47K resistor and a capacitor on each side. Operates like power supply filter caps only at IF frequency.
You can replace caps on each side of the 47K to ground with fixed 100pf.
Norm
:The other day, right after I finished the cabinet restoration on this radio... I put the chassis back in the cabinet to listen it AND to sit back and admire all my handi-work.
:...lol
:
:The chassis had been playing beautifully for several days on the bench.
:
:....But then, UN-fortunately ... or maybe fortunately, as soon as I got the chassis back in the cabinet... the radio started exhibiting LOUD static crashing noises, not unlike silver-mica-disease.
:...( however since this set uses conventional IF cans I didn't think it could possibly be that!) Right?
:
:However it's driving me screwy.
:Arrrgh.
Thanks Norm:
I can see that now.
But how exactly does that IF filtering function relate to the purpose of C13 (500pf) on the plate? If this already filters off the IF-RF ... then why is C-13 needed too?
Since the grid already get's its "filtered" input via the vol pot why is the C-13 IF filter needed too?
Anyway...
I hung my scope on the high side of the vol pot and the RF hash was very apparent there.
Since I had already cut off that 5th lead up inside the IF can, I didn't want to have to take off the IF can again and tear it all open, to get to the other side of that 47k resistor....
So instead I just hung a 130pf cap on the high side of the pot and sure enough that RF hash disappeared immediately.
I knew I was hearing it as a bit of annoying distortion in the audio... now that's all cleared up and it sounds fine!
Thanks again Norm.
Norm's Reply:
Peter
The added cap, C13, filters out remaining IF signal just like added filter caps removes hum. It also removes some high frequency response in the audio. Stops hissing noise. In some cases, by eliminating high frequency response, it will stop a radio from going into oscillation. You could remove this cap and in most cases not notice much difference.
Norm