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Old 27th March 2020, 03:10 AM   #1
barber02 is offline barber02  United States
diyAudio Member
 
Join Date: Jun 2018
Location: Georgia
Default Stetsom 10.5k power supply wonít boot up

I have a Stetsom 10.5k amplifier that came into my shop with some shorted
output mosfets. I replaced the mosfets on both sides and the gate driver ICs
on both sides and the logic gate ICs on both sides. I also replaced the gate
resistors that were out of spec and few of the diodes around the output
mosfets were replaced due to shorting.

When I feed the amplifier 14.5 volts the amp gives me an undervolt error. I
replaced the diodes in the undervolt circuit and replaced Q27 with a BC817.
I also checked the resistors and capacitors in the circuit and they all
checked good.

I removed R127 and jumpered pin 8 to pin 14 and started the amp up and now I
get an overvolt error. I was wondering if anyone could offer suggestions as to what may be the issue.

The power supply never tries to boot up. The amp goes straight into
protection. No rail voltage is being produced.

The amp has a 14 pin PIC16 chip. These are the pin voltages before I installed the jumper.

1. 4.97
2. Crystal
3. Crystal
4. 4.11
5. .005
6. 4.70
7. .002
8. .005
9. .002
10. .003
11. 4.64
12. .010
13. 5.60
14. .003

I am including a photo of the area in question.

Thanks,
David
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Old 27th March 2020, 03:31 AM   #2
barber02 is offline barber02  United States
diyAudio Member
 
Join Date: Jun 2018
Location: Georgia
I’m feeding the amp 14.5 volts and no current limiter. The amp is drawing very minimal current. The only thing running are the fans. I get B+ voltage on the the areas circled in the power supply. They feed B+ to the center legs of the power supply mosfets. I also get battery voltage through the small circuit in the picture up to the black x marked on the board but nothing to the large and small voltage regulators which figures since the power supply is not oscillating.

The other picture is of the PIC16 and the undervolt circuit.

I get B+ voltage at the cathode of D36 and D33.
I get 6v on the anode side of D36, 5.82v on the anode side of D33, 6v on the cathode side of D37 and 0v on the anode side of D37.

D31 I get 13.5v on cathode side and 9.4 on anode side of D31. D29 I get 3.8v on cathode side and 0.0v on anode side of D29. D30 I get 4.12v on cathode side and 0.0v on the anode side of D30.

On the linear regulator in front of the PIC16 I get 13.5v in and 4.95v out.
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File Type: jpeg 00163B4F-2ABD-4A36-9D26-BF5F151879B0.jpeg (475.6 KB, 28 views)
File Type: jpeg 90529D8E-A88F-4EFB-B47D-2B1BA043E398.jpeg (405.2 KB, 27 views)

Last edited by barber02; 27th March 2020 at 03:33 AM.
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Old 27th March 2020, 08:43 PM   #3
barber02 is offline barber02  United States
diyAudio Member
 
Join Date: Jun 2018
Location: Georgia
I got lucky and got a Stetsom 8k into the shop that I just repaired and it’s working fine. So I took some voltage readings in the undervolt/overvolt circuit.
Feeding the amp 14.5 volts

D36 K 13.5v A 0.65v
D33 K 13.5v A 5.23v
D37 K .065v A .064v
D31 K 14.09v A 9.95v
D29 K 3.87v A .064v
D30 K .064v A .064v

Linear regulator 14.1 volts in and 5.05 volts out

Q27 B .067v E .771v C .064v
The emitter is connected to ground.

Test points next to the linear regulator

VPP .078v
PGD .075v
PGC 4.9v
VDD 5.0v
GND 0.0v

So I’ve got some good numbers to go on now. Maybe I can figure this thing out.
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Old 27th March 2020, 10:38 PM   #4
Perry Babin is offline Perry Babin  United States
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Join Date: Nov 2003
Location: Louisiana
What happens if you power the amp up with 12.5v?
__________________
Anyone who purchased the repair tutorial, email me (babin_perry@yahoo.com) if you want to download the latest version.
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Old 27th March 2020, 10:55 PM   #5
barber02 is offline barber02  United States
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Join Date: Jun 2018
Location: Georgia
Same error. I tried that yesterday.
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