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Motorboating -38-12 - Printable Version

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Motorboating -38-12 - Eric Adams - 08-21-2013

Every time I think I know what I'm doing, I get hit with something like this. (Although, once I solve the problem I sure feel great)
OK, 38-12. The motorboating is a putt-putt sound, about 2 per second. In other words not real fast. Doesn't change with volume or station position. It was there before I did anything.
What I've done so far: All tubes tested and replaced 78 and 6A7. All caps replaced. All micas tested, within specs. All resistors tested and almost all replaced. All coils and transformers tested ok. I can put a signal to the center terminal on the volume control and it comes in loud and clear, along with the putt-putt.
All voltages check ok except one. The screen (S) voltage on the 78 tube should be 90v and I have 27v.
So now I'm stuck. Any ideas?
http://www.nostalgiaair.org/PagesByModel/236/M0013236.pdf
Thanks.

RE: Motorboating -38-12 - codefox1 - 08-21-2013

9 out of 10 times it is a flaw in the voltage divider, and then usually a filter capacitor, crispy critter resistor, or a missing tube shield. Of course that is if you did not change orientation of components when replacing ancient capacitors. Start there.

RE: Motorboating -38-12 - Eric Adams - 08-21-2013

Thanks, just what is the voltage divider? Filter caps are new, resistors were checked and replaced as necessary. It doesn't have tube shields. Since it made the sound before I touched it, I doubt that the orientation of components is the issue.

RE: Motorboating -38-12 - codefox1 - 08-21-2013

Other (tubular caps ) replaced ? Voltage divider is chain of resistors and capacitors used to isolate stages and eliminate interference with each other. Plenty of stuff on line about that and motor boating as well. Obscure causes too!. Best!

RE: Motorboating -38-12 - Jim Dutridge - 08-21-2013

There should be a shield around the 6A7.

RE: Motorboating -38-12 - Eric Adams - 08-21-2013

As I stated, all of the caps have been replaced. Can you give me a place to look? Like what is the voltage divider on the 38-12? I looked on line but I still don't know what to look for on this specific radio.

http://www.nostalgiaair.org/PagesByModel/236/M0013236.pdf

RE: Motorboating -38-12 - Eric Adams - 08-21-2013

actually, the shield is on the 75, but that isn't the problem. The problem was my screw up. It is working now. Icon_crazy
I think I need a vacation!
Thanks for your input.

RE: Motorboating -38-12 - Ron Ramirez - 08-21-2013

So, what happened, and what did you do to correct the issue?

RE: Motorboating -38-12 - jerryhawthorne - 08-21-2013

Yes Eric, let us know what you did. This is how we learn.
Jerry

RE: Motorboating -38-12 - Eric Adams - 08-21-2013

Ok guys. I'd rather not say. but I will tell you that a 75 tube will fit in an 84 socket and a 38-12 only uses 1 75 tube!Icon_lol

RE: Motorboating -38-12 - morzh - 08-21-2013

OK, I guess this is where "the more the better" does not apply Icon_lol

RE: Motorboating -38-12 - jerryhawthorne - 08-21-2013

Thanks Eric, I think most of us have done something like that and a good reminder to check!
Jerry

RE: Motorboating -38-12 - Eric Adams - 08-21-2013

Yup! At least the good news is that after all the testing/replacements the radio really sounds good!