M100...another jewel that was fiddled with

[wesleyb420]

So on the heels of getting the M50 working, i opened up the M100 to see what the issues were with this guy.

1. Input Caps...replaced them and one was bad. Replaced the bad one.
2. Resistor mod...done very badly by the PO. I tried to repair but the resistors tested good. I will mark that as good, just horrible looking.
3. Dim green LED...It is dim but it is on. Not really sure what this means but my guess is that something is wrong.

Any suggestions on where to start? the M50 was straightforward enough and had not been touched...this one has been messed with and I dont know where to start troubleshooting. From my testing of the IRFz44 transistors, they seem open.

Thanks again for the help.

[Silverhorse]

pictures of the board?
maybe the amp pulls to many current so you have a voltage drop.
Can you measure how many amps it uses? What is your power source?

[wesleyb420]

pictures of the board? - I will post some tonight...be warned...it's bad.
maybe the amp pulls to many current so you have a voltage drop. - Possible, see below
Can you measure how many amps it uses? What is your power source? I am using a computer power supply which "should" supply 12v 18A. I know it is on the low side, but the amp still should start, right? I didnt even think to measure how many amps it is pulling, ill check that tonight as well.

[Silverhorse]

12v 18A power supply is ok for testing.
if the amp uses more than 3 amps idle than something is wrong.
is the red LED also lighting up?

[wesleyb420]

No red light, just an extremely dim green light.

[wesleyb420]

Draw at idle is 4.7a. Red light turns on then goes off normally during startup.

[wesleyb420]

Here are some photos. Let me know if they are too crappy.

[ttocs]

have you checked the voltage on the led yet?

[wesleyb420]

Yes, it is 1.75V

[ttocs]

I am not a pro but that seems low. what voltage are the other leds reading?

[Silverhorse]

seems like a bad connection somewhere. Maybe a bad via.
this is a difficult one...

[wesleyb420]

This may sound silly but would the vias for the resistor "mod" that was done, cause this? I can take them out along with the caps to check everything.

Should i/How can i rule out the transistors? I have seen ways to test them but I have no experience with it. The PO said that 4/6 power side transistors were bad but he also said that this board had not been messed with.

[Silverhorse]

I'm not an expert, the experts are stil on holiday I think
What I would do:
- take out all the components that are not original ( the 4 white ressitors to start...)
- take out the 4 IRFZ 44 and test them individual when they are out
- take out the 6 PSU caps
- take out other components that are looking not good
- when they are removed, remove all the solder with desolder braid or desolder pump
- check that you have connection between both sides of the PCB (when the via is pulled out you have no connection)
- if you have a bad via restore them (you can find info on this forum how to do this)

- put the 4 resistors back in
- put back the other components that you have removed, when they are discoloured or burnt replace them with new
- put the 4 IRFZ 44 back in (if you have new FET's in stock, use them)
- put back 1 capacitor
- put back the heatsink on the board (never power up without heatsink)

- power up the amp back up, use a 10 amp fuse
- when it powers up fine you can place the other 5 PSU caps

[wesleyb420]

Where can I find the irfz transistors?

Whats a solid procedure to test the transistors?

I had to do a few via holes on the m50 so I’m familiar with that process...good call on testing them.

[ttocs]

https://www.youtube.com/watch?v=b0jCUsenNsY

[wesleyb420]

The guy in that video says his method will not work with fets...I have not found a swell video yet.

[ttocs]

https://www.youtube.com/watch?v=RkWy1EirEu8

[Silverhorse]

you can buy them here:
https://www.digikey.com/products/en?keywords=IRFZ44ZPBF

[wesleyb420]

Replaced the 4 irfz44 and she is back in action!!! Thanks guys!!!!!