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Power Amp Repair Help - Fuses keeps blowing

spacebiscuit

Dec 28, 2023
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Dec 28, 2023
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I have a Mosfet T-400 Power Amp developed an issue with the volume control so I switched the Potometer. during the change I damaged the tracks a little so soldered in some wires to address this. After reapir I tested on the bench and was ready to reassemble the channel into the main unit.

I then did something really stupid and reversed the polarity, when i switched the amp on there was a brief moment of smoke before I switched the power off but clearly damage has already been done. The amp has two replaceable fuses which have a red and green led. When I power the amp up the green fuse blow immediately and there is then a repeated pulsing sound coming from the speakers.

I've used a multimeter to check the diode (seems there is only one), the transistors and resistors and caps. There is no noticeable sign of burnt out components and the readings I am getting tally with the corresponding compontnet which I am verifying against on the good/working channel.

the smoke looked like it was coming from the bottom right hand corner where the small additional board which is labelled as "cuttoff" sits (detached in photo with brown, orange and green wires). I was convinced this was the issue but switched it over to the good channel and it worked so seemingly this is not the issue.

IMG_20231228_224823.jpg

The small board I swapped out:

IMG_20231228_224855.jpg

This is the rear of the board I repaired after I damaged the tracks:

IMG_20231228_224838.jpg

I'm kind of out of ideas, I have used my bag of spare fuses so my trial and error approach to repair is not working.

I have not removed any components from the board to test with the multi-meter - should this matter?

Thanks in advance.
 

Bluejets

Oct 5, 2014
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I then did something really stupid and reversed the polarity
Not sure but I think you take the cake for the worst ever repair attempt....
For future reference..........
 

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ramussons

Jun 10, 2014
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One standard procedure I follow in troubleshooting electronic equipment is to put a 60 Watt Incandescent bulb in series with the main power supply. It acts as a current limiter to prevent further damage. It also acts as a sort of visual indicator of currents drawn.
You will save on fuses, power devices ... and still be able to track down possible faults.

Whether Incandescent bulbs are available in your market is another story.
 

Delta Prime

Jul 29, 2020
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One standard procedure I follow in troubleshooting electronic equipment is to put a 60 Watt Incandescent bulb in series with the main power supply. It acts as a current limiter to prevent further damage. It also acts as a sort of visual indicator of currents drawn.
You will save on fuses, power devices ... and still be able to track down possible faults.

Whether Incandescent bulbs are available in your market is another story.
Talk is cheap. so is that method.
But effective.
DimbulbSketch_1611737928883_1684430580847.png
Oh look at that! there's a switch mode power supply there on the input wow!
Let that be a lesson to you. Just put your amplifier there....same difference. It says 150watt but 60Watt will work.1703901726996.png
 
Last edited:

H2814D

Nov 4, 2017
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Messages
215
Any progress on this OP?

Based on your first posting, you said there appeared to be smoke, after you powered up, from the bottom right side of the PCB where the smaller PCB is attached in your first picture above. Just above and to the right of the Mosfet numbered TR4 is a green resistor (it may have a different purpose than just being a resistor, because it is green, although that is what it is marked as on the board) numbered R17. Looking at the top of R17, it appears different than all of the rest of them, as if the lead has separated from the body, but I can't really tell because of the orange wire in the way. That would make sense as far as where the smoke came from and it appears the green LED is also in that part of the circuit. If that resistor failed quick enough, it may have saved the Mosfet, but because you are saying replacement fuses are blowing right away, probably not. They usually fail to ground, in my experience, and thus the blowing fuses on startup. Am I right about the top of that resistor?
 
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