Help repairing Infinity powered sub plate amp

Thread Starter

Jon Hoover

Joined Oct 10, 2019
11
Hi, I have been getting my butt kicked by this Infinity SW12 powered sub plate amp for a few months and am reaching out for some help. When it was brought to me, it had no sound. I had good signal going into the power amp but nothing coming out. I replaced the output MOSFETS Q11, 13, 15 & 17. I also replaced the PWM driver chips U7 & U8 and the op amps on the power amp board U9 & U10. Now I have sound but there is a very high frequency that is on the output signal that is there most of the time but occasionally it goes off and I get clean signal. I poked and prodded around every component with my wood chopstick looking for a trouble spot but didn't find any. I resoldered a ton of connections and the issue is still there. I checked all caps for bulges, leakage and unsoldered a few and checked for value. All ok there. I moved to the power supply and replaced BR1, Q6, Q8 R76 and R78 due to hot spots noticed on the PC board. Still have the noise. I have posted a screen shot of the output signal with the noise. I have also included PDF attachments of the schematic. First picture is of the amp on but no signal applied. Nice and clean trace. Second picture is of the signal with the high frequency component. Third picture is the output after the signal has been removed. The high frequency is still there. Any suggestions would be super helpful. BTW, this amp has numerous surface mounted components so it's a pain in the neck to work on.
1 Amp On No Signal.png2 Amp On Signal On.png3 Amp On Signal is Removed.png
 

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Thread Starter

Jon Hoover

Joined Oct 10, 2019
11
Reaching out again to see if anyone can give me a little direction here. I have contacted Harman tech support but really got nowhere. I think they farm their stuff out to a 3rd party. They seemed able to recite the owner's manual to me but that seemed to be the extent of their knowledge :( Was hoping that someone on here might have a little more experience with class D amps and may be able to help.
 

peterdeco

Joined Oct 8, 2019
27
When I was doing repairs I got an occasional "dog" myself. With your amp making the high frequency sound, you have to scope out every stage from input to output to see where it begins. I remember a similar problem on a transistor amp that would go into oscillation (1MHZ RF - you couldn't hear it) and heat up the output transistors. Turns out it was an intermittent disc cap across a base and collector - to prevent oscillation - that would intermittently open up and the amp would oscillate. So when you said you checked a few and all are ok there, you really don't know all are ok. Of course you can't remove every cap, you need to trace the stage where it begins. I printed out the schematic for your amp. Boy! You have a job ahead of you. Good Luck.
 

Thread Starter

Jon Hoover

Joined Oct 10, 2019
11
When I was doing repairs I got an occasional "dog" myself. With your amp making the high frequency sound, you have to scope out every stage from input to output to see where it begins. I remember a similar problem on a transistor amp that would go into oscillation (1MHZ RF - you couldn't hear it) and heat up the output transistors. Turns out it was an intermittent disc cap across a base and collector - to prevent oscillation - that would intermittently open up and the amp would oscillate. So when you said you checked a few and all are ok there, you really don't know all are ok. Of course you can't remove every cap, you need to trace the stage where it begins. I printed out the schematic for your amp. Boy! You have a job ahead of you. Good Luck.
I know! I checked all the leaded electrolytics but not the surface mount caps....... Who wants to pull them out and check them and then try to tweezer them back in if they are ok. I'm more of an "old school" electronics guy used to working with point to point or single sided circuit boards. I will check some of the bypass caps as suggested. I already informed my customer that this amp may not be repairable. He is actually ok with that. But, if I can get it fixed, I can be a hero!
 
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