[Ragazzo]

The radio works fine, no problem, but in half an hour one of the tweeters becomes very hot.
Namely, item 172, in the bottom part of the schematics. The field coil itself is OK. When I exchange the tweeters, another one becomes hot.
I was suspecting output tubes (6L6G) comsuming too much, may be the push-pul output is not balanced, or something, but without the 6L6 tubes - the same story.
I was not able to find any other suspiciously hot element in the lower or upper chassis. All e-caps are replaced.
Any ideas, why the fild coil becomes so hot?

Leonid

[Chuck Schwark]

You may have an electrolytic wired with incorrect polarity causing excess current draw.
There is at least one filter cap on the audio output grid bias circuit that has POS to ground.
IIRC, there is another that has its NEG side connected to the HV center tap and NOT ground.
(Working from memory here...)

Also, it may be partially shorted internally, making it a much smaller resistance value than it should be causing excess current draw.
Check DC resistance against schematic/service bulletin to compare.

Chuck

[Ragazzo]

You may have an electrolytic wired with incorrect polarity causing excess current draw.
There is at least one filter cap on the audio output grid bias circuit that has POS to ground.
IIRC, there is another that has its NEG side connected to the HV center tap and NOT ground.
(Working from memory here...)

Also, it may be partially shorted internally, making it a much smaller resistance value than it should be causing excess current draw.
Check DC resistance against schematic/service bulletin to compare.

Chuck

Chuck,
thank you for comment.
I checked e-caps and their polarity. All is correct.
The filed coil itself is not a problem, it has correct resistance, and if you change the tweeters, another one gets hot, so the problem must be somewere else.
Leonid

[Chuck Schwark]

Ah, then it sounds like there is heavy partial B+ short in there.

Try pulling one RF/IF/AF tube at a time also - maybe a partial internal tube short? A partially carbonized tube socket maybe? If the sockets are the thin two-layer wafer types, the leakage track can be between the wafers between pins. I've seen it. Also check the upper/lower chassis harnesses plugs/sockets for carbon tracks. B+ flows thru there too.

Everyone gets a 'tough dog' now and then....

Chuck

[Ragazzo]

"Try pulling one RF/IF/AF tube at a time"

Chuck, this is what I was planning to do. Glad we think the same way. It is really time consuming, but so far I can not see any other way to locate the problem...

Leonid

[Ragazzo]

Was checking the lower chassis for a possible short circuit and found a piece of wire stuck under one of the sockets and all the wires - more than an inch long....Looks like it was there since the day the radio was built. Was sure I found the reason... But it would be too simple, nothing has changed.

[Ragazzo]

I decided to calculate, what might be the extra current, that causes the twitter coil to become hot. Let's assume extra 50W are dissipated in the coil. So, 50W=I*I*R, were R=670 Ohm, so there is 0,27A of extra current. The coil is connected to upper chassis via resistor N 69 - 500 Ohm. This is 1/2 W resitor and 0,27A would cause 0,27*0,27*500 = 36W. This power would burn the small resistor, but it is cold even after 1 hour of the radio "on". That means the upper chassis is excluded. Not guilty.

After I checked all sockets, all wires and connectors, all trasnformers and coils - I started to hesitate - may be this twitter coil should be that hot? Remember the 4-section resistor N 163 (and N165), mounted on the chassis? It is always very hot...

So I ask the owners of 38-690 - can you please touch the back of the twitters after e.g. 40 minutes of operation? Are they both cold, or one is hotter, than another one? It is safe to touch the back of the speaker even with the radio "on" - it is grounded. But of course for safety reasons it should be done with the radio "off" for sure.

Well, thank you very much in advance, you can save my brain from exploding!

Leonid

[Ragazzo]

My calculations were wrong. Actual measurement shows 100mA of current through the tweeter field coil (item N172).
The current through big speaker's field coil is 57mA. The current through transformer N152 is 42mA. This corresponds to current through 6F6 audio driver (plate current of 6F6 is 35mA). So, 57+42=appr.100mA... Looks like ALL IS CORRECT, and dissipation in tweeter field is 0,1A*0,1A*670Ohm = 6,7W.
Why on earth then other owners of 38-690 say, that the tweeters should not be hot and have the same temperature???

[Ron Ramirez]

I don't know, Leonid...I wish I could help you with this, but I no longer own a 38-690. I hope to acquire another as soon as I finish college...

[Ragazzo]

Hi Ron,

I see. Let me wish you good luck with college!

Leonid