I got another one of theese amps in today to fix. The guy told me that the power wire came unhooked from the amp and touched the heatsink and sat there for awhile. It heated up the power transformers looks like they got red hot it toasted all of the power supply fets,gate resistors, and drivers.
My question is wondering since the power wire was sitting on the heatsink heating the transformers up due to not having an in-line fuse. Wondering if you think the transformers are ok?
I stuck my red probe in the power terminal and black probe on ground terminal and it doesnt read shorted.
My question is wondering since the power wire was sitting on the heatsink heating the transformers up due to not having an in-line fuse. Wondering if you think the transformers are ok?
I stuck my red probe in the power terminal and black probe on ground terminal and it doesnt read shorted.
Having the power wire touch the heatsink wouldn't be likely to do any damage to the power transformers.
The easiest way to test the transformers is to repair the power supply drive circuit (likely only 10 ohm resistors and the PNP driver transistors) and install at least 4 FET (don't have to be soldered to the MEHSA strips for a quick test). If the amp powers up and produces rail voltage without drawing excessive current, the transformers are likely OK. Remember, you have to measure the rail voltage across the red and black twisted wires.
Of course, if there is enamel flaking off of the windings and you can see bare copper (other than at the terminals), the transformer is likely unusable.
The easiest way to test the transformers is to repair the power supply drive circuit (likely only 10 ohm resistors and the PNP driver transistors) and install at least 4 FET (don't have to be soldered to the MEHSA strips for a quick test). If the amp powers up and produces rail voltage without drawing excessive current, the transformers are likely OK. Remember, you have to measure the rail voltage across the red and black twisted wires.
Of course, if there is enamel flaking off of the windings and you can see bare copper (other than at the terminals), the transformer is likely unusable.
shorted transformer
shorted rectifiers
shorted rail caps
defective driver IC
It's also possible to damage the FETs when soldering them to the MEHSA strip.
With only remote and ground connected, you can check the driver circuit to confirm that you have a good drive signal. This will only work if none of the power supply FETs are shorted. If you replaced the FETs and they failed, find the one or two that failed, cut them out of the circuit and check the drive signal with only remote and ground connected.
If you don't get your scope back soon, you should buy another one. Even the most basic scope (less than $100 on ebay) will be significantly better than guessing what's actually happening with the circuit.
shorted rectifiers
shorted rail caps
defective driver IC
It's also possible to damage the FETs when soldering them to the MEHSA strip.
With only remote and ground connected, you can check the driver circuit to confirm that you have a good drive signal. This will only work if none of the power supply FETs are shorted. If you replaced the FETs and they failed, find the one or two that failed, cut them out of the circuit and check the drive signal with only remote and ground connected.
If you don't get your scope back soon, you should buy another one. Even the most basic scope (less than $100 on ebay) will be significantly better than guessing what's actually happening with the circuit.
Measure the DC voltage across the red and black twisted wires. If it's more than a few volts, the caps are not shorted. If the voltage is 0.00v, measure the resistance across the red and black wires.
U21 generally only causes the amp to remain muted. It doesn't cause anything to fail. I was referring to the SG3526. If it has a good drive signal, the 3526 is almost certainly OK.
If you have a good drive signal, install 2 FETs (one in each bank for the transformer) and try powering it up via a current limiter. If it causes the FETs to heat up or drags the voltage down too low, remove them from that power supply and install 2 FETs in the other supply. If only one transformer causes problems it may be shorted.
Did you replace all 8 driver transistors?
U21 generally only causes the amp to remain muted. It doesn't cause anything to fail. I was referring to the SG3526. If it has a good drive signal, the 3526 is almost certainly OK.
If you have a good drive signal, install 2 FETs (one in each bank for the transformer) and try powering it up via a current limiter. If it causes the FETs to heat up or drags the voltage down too low, remove them from that power supply and install 2 FETs in the other supply. If only one transformer causes problems it may be shorted.
Did you replace all 8 driver transistors?
Ok Perry,
With only 4 fets in the amp and new drivers and new gate resistors and new rectifiers.
this is the reading i get
SG3526
1=5.02
2=5.03
3=3.12
4=4.76
5=5.00
6=0
7=0
8=4.42
9=3.69
10=2.06
11=0
12=4.70
13=8.85
14=20.95
15=0
16=8.99
17=20.97
18=5.02
reading across both red and black twisted wire= 202.8
With only 4 fets in the amp and new drivers and new gate resistors and new rectifiers.
this is the reading i get
SG3526
1=5.02
2=5.03
3=3.12
4=4.76
5=5.00
6=0
7=0
8=4.42
9=3.69
10=2.06
11=0
12=4.70
13=8.85
14=20.95
15=0
16=8.99
17=20.97
18=5.02
reading across both red and black twisted wire= 202.8
You need to have one in each bank. The 4 banks of power supply FETs are the 4 rectangles in the attached photo.
Have you checked the drive signal at the gates of the FETs? If not, do it while there are no FETs in the amp or with only remote and ground connected. The DC voltage on all gate pads should be ~5-6v.
If the drive circuit is OK, install one in each bank for the transformer on the left. Then power it up (fused at 10 amps or through a current limiter). The one on the left has priority because that's the transformer that produces the output that's used for voltage regulation.
Have you checked the drive signal at the gates of the FETs? If not, do it while there are no FETs in the amp or with only remote and ground connected. The DC voltage on all gate pads should be ~5-6v.
If the drive circuit is OK, install one in each bank for the transformer on the left. Then power it up (fused at 10 amps or through a current limiter). The one on the left has priority because that's the transformer that produces the output that's used for voltage regulation.
Attachments
Ok I powerd the amp up it idles around 2amps.
Inline of the B+ is a 10 amp fuse and a 1ohm 10 watt current limiting resistor.
The reading i get now are
Sg3526
pin1=5.02
pin2=3.73
pin3=4.23
pin4=4.75
pin5=5.00
pin6=0
pin7=0
pin8=4.43
pin9=3.66
pin10=2.06
pin11=0
pin12=4.94
pin13=.05
pin14=0.75
pin15=0
pin16=0.12
pin17=13.21
pin18=5.02
gates are reading 5.70 volts
voltage across red and black twisted wires both sets = 135.8
All Fets (4 of them) are staying cool
Inline of the B+ is a 10 amp fuse and a 1ohm 10 watt current limiting resistor.
The reading i get now are
Sg3526
pin1=5.02
pin2=3.73
pin3=4.23
pin4=4.75
pin5=5.00
pin6=0
pin7=0
pin8=4.43
pin9=3.66
pin10=2.06
pin11=0
pin12=4.94
pin13=.05
pin14=0.75
pin15=0
pin16=0.12
pin17=13.21
pin18=5.02
gates are reading 5.70 volts
voltage across red and black twisted wires both sets = 135.8
All Fets (4 of them) are staying cool
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