Stetsom 5k

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Hi everyone

I'm new to the forum and hope someone can help.
Bought a stetsom 5k, one of the older gen ones, bought it as a faulty unit with the hope it would be an easy fix.
Everything on the board measures fine, no blown ps fets, no blown outputs, amp has never been touched.
Amp powers on, blue light, builds good squarewaves on the transformer primaries, as soon as it reaches full duty cycle the amp shuts down. I then get solid red led for about 5 seconds followed by 3 flashes of red led.
I see nothing wrong with any components, been through just about everything on the board.
I'm using a 30A regulated bench supply at around 13v so don't think it's a power or low voltage issue.
Current draw at startup is around 5-6A
 
It builds rail voltage, the amp turns on but as soon as it gets to full duty cycle on the power supply and max rail voltage it shuts down. According to the online manual the fault code is low voltage. I've powered it up from a 30A supply and even powered it from my battery bank, it does the same thing, supply voltage or current are definitely not an issue.
 
This is the board
 

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I think so too Perry, I have repaired a few of these stetsom amps but have never seen anything like this before, I see the 4427 drivers on the power supply have been replaced or maybe just resoldered, the ps fets have never been replaced so I don't know why the 4427's were replaced. I've seen these amps use an ir4427 or the ucc27524 this amp has the tc4427 I don't suppose that may be an issue?
 
Your undervolt circuit it located in the circled area.

I have previously post good voltages for that circuit.

You can bypass the protection for undervolt at the PIC16 IC. I have posted about about how to do that as well.

I haven’t seen one of these amps build full rail and then shut down. But when that happens, does the rail voltage coming out of the power supply sag significantly or is there a spike in the current draw from your power supply just before the shutdown. This could indicate something is shorted or damaged in the output section of the amp.

If you don’t spot any anomalies then the problem is most likely the undervolt circuit. I would bypass the protection first to see if the amp will boot up and stay operational. Then you’ll know if the problem is in the undervolt circuit without a doubt.
 

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The PIC is the heart of the amplifier. It does the power up self check, it sends out the pulses to the power supply driver ICs it handles all fault codes.

Grounding pin 8 should have eliminated that fault code. The amp will do one of two things. It will boot up and run or it will give you a different fault code which will most likely be the underlying fault. I’ve had to do this a few times to discover what the real fault was.

Double check your work to make sure that pin 8 is grounded. If no mistake was found then you can be fairly certain that your problem is not an undervolt at this point, but rather something in the output section could be triggering the fault.

Have you checked the rail voltages to see if they sag quickly? Amp draw spike?
Can you get a glimpse of the drive signal in the output section? Is the drive going all the way back to ground? Do you have equal voltage on you speaker terminals as referenced to ground? Each one should have half the rail voltage on them and should be even within a volt or two.

In these Amps like this that I’ve dealt with that give a false undervolt is a sometimes a leaky transistor, mosfet, or the current sensing ICs located close the output mosfets. They look like tiny 5 legged ICs but only 2 sometimes 3 of the legs are utilized. Since yours is faulting after rail voltage has built I would look at the output transistors and the current sensing ICs closely. Make sure that the insulators are in good shape. Pull the board from the heat sink and make sure that you don’t have some foreign conductive object floating around. Sometimes just a little piece of speaker wire in the wrong place can do this. You may have to pull the output transistors from the board and test them individually.
 
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