SX-737, no audio on left channel after a while

[zeferinoj]

Hello.

I'm starting to work on an SX-737 that has never been serviced before.

It sounds great for about 5 minutes and then the left channel turns off.

I'm just reading posts and looking at threads such as this one for now, together with the service manual, but I thought I'd ask for some recommendations and suggestions here before diving deeper.

Any help is greatly appreciated.

Thank you.

JZ

 

[fromnowon]

Hi, did you clean the controls yet, including the balance pot and switches?
If you work the controls back and forth does the left channel come back on?

 

[zeferinoj]

Hello. Yes, they were cleaned in advance. After the sound goes out no amount of adjustments will bring the left channel back. I have to turn the set off and back on a few hours later.

 

[AE Bob]

You could likely use freeze spray to cool areas of the circuit, to see if you can bring the audio back. After you find the area, you can localize it to a single component with judicious use of the freeze spray.

 

[zebulon1]

Wet finger amp input check?
Find the pin where the pre amp output signal meats the amp. With the volume advanced slightly, touch that pin and hear if the speaker has a hum.
This will help isolate the issue.

 

[fromnowon]

Could it be related to the protection circuit?
I mean . . . if Q7 isn't strong enough to hold the relay open, I wonder if that could cause just one channel to drop after a while.

My SX-737 was stuck in protection completely, replacing these parts seems to have fixed it:

• Q7 replaced with 512-KSC2690AYS
• Q5 2SA733 => 512-KSA992FTA
• Q6 2SC945 => 512-KSC2383YTA (upgraded part for improved longevity per MTF)
• Q6 shares a pad with C5 so I replaced it also, with 647-UPW1H4R7MDD1TD, 4.7uF 50V

I also replaced the relay and added a flyback diode 1n4004 across pins 9 and 10, protection board. Cathode side to Pin 10.

(then I got distracted and still need to finish the project)

 

[AE Bob]

NO