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Most common reasons for solid state power amp failure?

Roccorobb

Member
Messages
1,168
This past weekend I was cleaning out the garage and I stumbled upon a friendly relic - my old Randall 1x12 combo, my first 'real' gigging amp. Its as heavy and as loud as a Fender Twin, and (if I remember correctly) it had a really great sound for a solid state amp. Then one day it died. It's been sitting in the garage for almost 20 years now.

It powers up and the preamp still works. I can run the fx send to another amp and everything functions correctly, so whatever got fried only affected the power section. I have a 10 year old now who is all into electronics and I thought it may be a fun project for us to try and figure out what's wrong with it, but I don't know where to start.

If it were a tube amp, I'd roll the rectifier and power tubes, and then maybe look at the OT. But I don't know anything about how SS amps work. Where should I start?

I know someone will chime in with the standard psa about how amps can kill even if they're unplugged. I am aware of the risks involved and certainly won't venture beyond my known limitations, especially with my son at my side. But if there are simple things to check I think i may give it a go.
 

Hacksaw

Time Warped
Platinum Supporting Member
Messages
10,213
There is usually a couple final transistors mounted to a heat sync, do a visual inspection and I bet you will see a hole on one of them where the smoke leaked out. There are usually a pair of them so you can get the part number off the one that didn't leak smoke.

You know there are high voltages in the wall so don't hold the amp anywhere near the holes in the wall. :D
 

fusionbear

exquirentibus veritatem
Platinum Supporting Member
Messages
10,990
There is usually a couple final transistors mounted to a heat sync, do a visual inspection and I bet you will see a hole on one of them where the smoke leaked out. There are usually a pair of them so you can get the part number off the one that didn't leak smoke.

You know there are high voltages in the wall so don't hold the amp anywhere near the holes in the wall. :D
This is what I see most. Some amps use a NPN and a PNP output transistor/s so look at all of them and write down the numbers...
 

teemuk

Member
Messages
3,225
If you're about to embark on some kind of a DIY repair project remember to plug that amp to some kind of current limiter while you test it. That is, if you wish to avoid potentially ruining hours worth of time and $$$ parts in an eyeblink.

Job of amp's fuses is to prevent that thing from catching into fire. The delicate parts inside need much better protection than mere fuses to survive fault conditions intact. Hence you need a current limiter.
 

Paully1

Senior Member
Messages
279
If you turn on a SS amp and dont turn it right off but wait till you can play thru it THEN turn it off.
All hi voltage will disapate in 15-20 minutes and
It will be safe to work on.
THIS DOES NOT APPLY TO TUBE AMPS
STAY THE HELL OUTTA TUBE AMPS
 

J M Fahey

Member
Messages
2,643
Only 1 useful answer so far (Teemuk's) and no hopes for better ones, this being a Forum inhabited by Guitar players (not Techs) and 99.99% tubeheads.
Fine with me, but useless in this case.
I suggest you ask this same Tech question at Music Electronics Forum.
 
Messages
536
It is probably the power transistors, transistor output stages do not like shorts on the output. You can see if they are getting warm but be warned they can have quite high voltages on the cases. If you have a scope then trouble shooting is relatively straight forward.
 




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