[cwc]

New person here.  I had a radio repair business in the 50's when I was in HS and worked my way through college as an electronics tech.  All of this indicates that I should be able to solve this myself, but then I'm always ready to learn from others especially if it saves me time.

I bought this 38-12 at a garage sale and it worked initially then one day I turned it on and there were no signals.  I verified that the LO was working by listening for it with another radio.  Then  poking around with a scope I found the oscillation at the plate of the IF stage.  This oscillation is at 470 KHz and is tuneable with the first IF transformer.  Touching the grid cap of the IF amplifier stops the oscillation.

I verified the voltages at the tube sockets and they all were ok.  I wiggled the tubes in case there was corrosion on the pins.  All of the coils measure ok resistance wise and weren't shorted to somewhere they shouldn't be.   I bridged all of the bypass caps with a cap of the same value and that didn't help.  I intend to replace the caps but won't be able to get enough of the right values until next week.  

I googled this a bit and didn't come up with anything.

Is this a common problem with a known fix?

Any suggestions will be appreciated.

[klondike98]

Welcome to the Phorum!
I'm sure some of the folks that are better at troubleshooting than me will chime in soon.

[Radioroslyn]

Hi and Welcome,
Methinks you'll be just spinning your wheels till you get some new bypass caps in it. Also having a look at the resistors isn't a bad idea either. They drift in value. When that's done if the problem persists try detuning the IF transformer. After taking a quick look at the diagram couldn't hurt to add a .05 or .1 form red wire on the IF can to gnd.


http://www.nostalgiaair.org/pagesbymodel...013236.pdf

Terry

[David]

Welcome to the phorum, there is a great 8 part youtube series on rebuilding  the Philco 38-12. On the phorum look up the file for Philco changes find your set. I have the big brother 38-15 which only added a short wave band. Found some differences in parts and circuits which matched the changes listed for my set.  I Replaced all caps and most of the resistors. David

[klondike98]

Changes.zip file can be found here
http://philcoradio.com/phorum/showthread.php?tid=215

[cwc]

Hi and Welcome,
 After taking a quick look at the diagram couldn't hurt to add a .05 or .1 form red wire on the IF can to gnd.

Terry

Yes, that does look like a good idea.  I'll try that when I get the other parts.

[cwc]

Changes.zip file can be found here
http://philcoradio.com/phorum/showthread.php?tid=215

Thanks for the link.  I check to see if my radio has/needs the changes.

[cwc]

There is some progress.  I've replaced all the caps except the two on the primary side of the power xfmr, replaced a couple of resistors (not the 39K in the photo below) and found an intermittant short in one of the diodes in the 2nd detector tube.  I'm running with just one anode of the 75 tube connected right now.

The IF is still a little unstable so I think I should do one of the fixes shown in the "changes" file mentioned above.  

It involves re-routing the wire in the photo that is marked with the arrow.  The Philco description is a little vague.  I don't want to re-invent the wheel here so if anyone has done this fix I'd appreciate any suggestions as to the best way to route the wire.  

[Image: http://i179.photobucket.com/albums/w290/...f0l4wz.jpg]

[cwc]

After some new caps, a new tube and an alignment my 38-12 seemed to be working fine so I put it in the living room  to see if it would run all day without a problem.

Well, it didn't.  After 2-3 hours the local oscillator changed frequency.  It didn't "drift", it was an abrupt shift down in frequency by about 35 Kc.  I repeated the test a few times and it repeated the shift consistently.

I assume this is a heat problem and know how to test the various components to track it down, but I thought I'd ask if this is a known problem with a known solution to save a bunch of tedious testing. 

Any suggestions will be appreciated.

[Radioroslyn]

Hi Charlie,
I'd keep an eye on the voltage on the second grid of he 6A8 (osc plate) to make sure it's not changing much. A volt or is fine 20 or 30 not so good.
Also if you have a 110mmf mica (6) I'd swap it with the one that is in there.
I see there are a few ground connections if they are done with rivets they are suspect.

Merry Christmas!
Terry

[cwc]

Hi Charlie,
I'd keep an eye on the voltage on the second grid of he 6A8 (osc plate) to make sure it's not changing much. A volt or is fine 20 or 30 not so good.
Also if you have a 110mmf mica (6) I'd swap it with the one that is in there.
I see there are a few ground connections if they are done with rivets they are suspect.

Merry Christmas!
Terry

Thanks.  I'll look at those things.  

Unless I'm missing something (always a possibility) maybe I can just monitor the B+ supply and look for a change there since there is just 3.5 ohms between the supply and the 2nd grid??