SA-7500 protection relay clicks after some listening

slowpat

Slowly but surely....
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need help on where to start looking:

I have a SA-7500 in good condition. I have recapped the Phono board with the help of markthefixer awhile back. That came out fine. When powered on, the relay clicks in after 2 sec. Recently, after about 10 min of playing, one channel makes a lot of static noise, the protection relay clicks in and sometimes several times in a roll. When first turned on, DC offset is less than 5mv each side. When the noise starts, the one channel starts climbing to 5-600 mv. I swapped the preamp out , and the high DC remains with the amp. I’m a noob with troubleshooting, so what’s next? Thanks.
 
Something in one of the power amps :) where, well it will take some probing to figure out what is the culprit.
 
Based on my readings, this problem can be caused by the differential pair, but looking through the service manual, I couldn't identify them. Any hints?
 
There are a few transistors in this amp that can cause those problems, namely 2SA725, 2SA726 and the infamous 2SC1451. The input pairs on the power amp are Q1 & 3 for the left channel and Q2 & Q4 for the right channel, all are 2SA726. Replacements are 512-KSA992FBU. Q5 and Q6 on the power amp are the 2SC1451s, replace them with 512-KSC3503DS. Remember to observe pin layout! The 2SA725 and 726 are BCE and the 992 is ECB.

While you're in there, there is a couple others you can replace in the control amp section, it's cheap insurance. Q3 and Q4 on the control amp board are 2SA725s, replace them with the KSA992 as well. You said you redid the phono amp board, I assume you replaced the transistors as well?
 
There are a few transistors in this amp that can cause those problems, namely 2SA725, 2SA726 and the infamous 2SC1451. The input pairs on the power amp are Q1 & 3 for the left channel and Q2 & Q4 for the right channel, all are 2SA726. Replacements are 512-KSA992FBU. Q5 and Q6 on the power amp are the 2SC1451s, replace them with 512-KSC3503DS. Remember to observe pin layout! The 2SA725 and 726 are BCE and the 992 is ECB.

While you're in there, there is a couple others you can replace in the control amp section, it's cheap insurance. Q3 and Q4 on the control amp board are 2SA725s, replace them with the KSA992 as well. You said you redid the phono amp board, I assume you replaced the transistors as well?

Yes...the four or six transistors and a couple of film caps to replace the small electrolytics.

Thanks for the info on the transistor subs. I have a few ksa992 on hand so will do that this weekend.
 
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