Electronics > Repair
Ampeg PF350 - IRS20957s no oscillation
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jboy32:
Hi @Mojoampguy

Did you have a look to this discussion ? https://www.eevblog.com/forum/repair/irs20957s-based-amplifier-sound-loss/ ?

In my case on the revision that I have REV H01, U7 do not exist, it seems that on your picture you have it may be a good point to investigate there, to get exactly 12V?

The floating voltage that I have is +14,97Vdc so >12Vdc expected ... So I don't know if on the new revision U7 that is a 7812 regulator is an add on or if it has been removed.
Mojoampguy:
Yes I had seen that post. 

Mine is the newer Rev H version like yours with no U7 12 v regulator. The voltages I measured are shown in an earlier post from me above. I too measured ~ 15v on that pin. We are chasing a similar fault, I suspect?

. I ran out of ideas after several months working on this. Kindly Chris said he’d take a look, so I’ve posted the amp down to him. Fingers crossed.
jboy32:
Good afternoon Dave & Chris,

I did some progress during the week end  :)

I have been focusing on the CSD pin2 oscillation.
Mute should be 0V dc
Unmute should be around 5Vdc and stable taken to the GND.

I checked all the parts of the mute circuit and found that C61, 10uf was not very stable while measuring outside the board. So I replaced by a 10uf SMD capacitor and now I have a stable voltage on Pin 2 of IRS20957s = 5,537Vdc taken to the GND.

All other voltages on IRS20957s are the same, just getting now +1Vdc on LO Pin 10 taken to COM. Q10 and 13 are still off the board. I'm thinking to put new one in place but was is weird are the voltages on pin 12,13 and 14. I'm thinking that I should get 0Vdc on VS pin 13. So without fixing this zone it may burn again Q10 and Q13. Have have still 8 in spare. Maybe some other 10uf are in trouble in this area I'll check.

Julien
Mojoampguy:
In my case CSD is always sat at 3.2v. To unMute it must be greater than 5v.

I replaced the mute circuit 10uF cap twice, no change in my case. But that was with irs20957 and Mosfets in place. I believe iCSD is essentially going into protect. Don’t know why? Possibly because rails are in fact over voltage?

Best of luck.

jboy32:
Good morning  :)

I'm now more focused on the rail voltage, and looking to KA3525A and oscillation,
I understand that this IC is supposed to drive the output voltage on the rails so trying to understand how it works and checking al parts around. Maybe replacing it, very cheap.
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