Ampeg PF350 - IRS20957s no oscillation

[Chris56000]

An update on this for the OP and others :–

I am now looking at the unit at home for the OP foc, and it looks like the fault is related to the switch–mode power supply coming on at too high a voltage that is causing the IRS20957s to trip out into a protective mode, so the ±73 V the OP said he measured  at switch–on is not correct!

I am continuing to check the Power Supply and it's regulation and feedback components and I will report back in due course!

Chris Williams

[jboy32]

Good morning Chris,

Thanks for the update, from my side, I did check the power supply part with a focus on the KA3525A that is the IC regulating high +/- voltages, and didn't didn't find any issue their, could it be the KA3525A himself ... I was thinking to play with the PWM on KA3525A to adjust the high voltages in case that is the root cause. I tested the amp by powering with a VARIAC to move from 230Vac to 190Vac and get +/- 55 Vdc. all other voltages still good as they are regulated. Even with that no change on IRS20957s.

In my case by changing C61, IRS20957s seems not to be in fault now as I have +5,576 Vdc taken to GND. I'm still trying to understand why I have the same voltage on VS on IRS20957s than on HO. It seems that something is wrong on the output.

When I plug a 8R resistor on the output,

VS Pin 13 moves from 2,71Vdc to 0Vdc
VB Pin 15 moves from +9,75Vdc to 7,7Vdc
CHS Pin 16 moves from 3,7Vdc to 1Vdc

I followed with an oscilloscope the input signal and it goes to Pin 3 of IRS20957s.

It is a REV H of the AMP with some differences with the schematic that we have, so maybe on the REV H +/- 75Vdc is the voltage expected ...
In the differences :
1. no 12V regulator, U7 is missing
I did test by adding one but no changes.

2. on the output, there are 4 additional components that are not on the schematic.
R94 = 10R
R98 = 10R
C84 = ?
C97 = ?

3. D34 is an ES1D diode and not an ER1D diode

they make a bridge (R98+C97) from D to S on Q13
they make a bridge (R94+C84) from D to S on Q10

I did replaced IRS20957s but maybe I have a fake IRS20957s. I don't know if I have a DC voltage on VS pin13 because I removed Q10 and 13 ... I have tried with the 8R resistor on the output that moves VS to 0Vdc and replaced Q10 and Q13 but again they died after few seconds ...

In your case with Q10 and Q13 in place are they safe?

I have ordered ES1B diodes to replace them also, for D32 and D33, I will get them next week.

I add a link to an interesting video in fixing a PF350

[jboy32]

Good afternoon,

Some update on my investigations, I did not yet received the ES1B Diodes.

I put a 440Hz signal on the input of the amp to follow it. it comes to Pin 3 of IRS20957s with a square so seems to be good, I was thinking that ADTL082JRZ IC could be the culprit.

IRS20957s might be a fake IC that I ordered, so I'm going to buy some from some websites that I hope certified they are true.

I found lot of information there that I'm still trying to understand, situation is very similar. https://www.diyaudio.com/community/threads/behringer-1400w-class-d-for-b1800d-repair-help.247903/