Model 240 Interesting Problem

hirscwi

AK Subscriber
Subscriber
I'm working on a Model 240 that has in interesting problem. After it warms up (an hour) the left channel cuts out occasionally, with static. It sounds similar to poor FM reception.

I have the service bulletins and I know that I need to have beta matched transistors in several spots. But before I go too far on a wild goose chase I thought I'd see if anyone has a suggestion.
 
If smacking it doesn't indicate some kind of loose connection, then perhaps Q501 and Q502 (differential pair)?

Tom
 
Thanks Tom. I've already tried tapping things with a wooden stick but that didn't seem to have any impact. I have replaced the differential pair with 2N5961. I read in one of the many posts that someone had a problem with the 2N5961 and went back to the original SPS1928 but I don't have those anymore. Perhaps I didn't get teh Hfe matched close enough so I'll try again.
 
Hi.
I had a similar problem. Cleaning of contacts of the relay of protection has helped. Model 240 has no protection cover on the relay, cleaning will be not difficult. Very much I hope that it will help. All benefits.
 
Not a real fan of freeze spray.
Have used it in the past to find a noisy transistor like described in the past.


Barney
 
Here's an update. I lowered the bias across R531 & 532 to 8mV - as per a post I saw in the many I've reviewed. Also readjusted the DC offset and Max Power Output (R541 & 542) as per service manual. It seems to be working now but I'm going to test the heck out of it before I conclude success. Thanks for the suggestions.
 
This is my 240 hirscwi was working on and, I would like to publicly thank him for his fine restoration and diagnosis work on what has been a deep rabbit hole for us both. Flawless and sounding great since I got it back.... playing to the back porch as I type.
 
I would make sure those trim pots are as clean as possible. My 240 had issues that were related to those pots being filthy
 
Don’t tell me you were involved in our little quagmire! I know hirscwi did a lot of research on this so, extended thanks to all involved. Although I know hirscwi turned the wrenches!

Yes, Bob was the brains behind the operation.
 
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