Fender SPL 6000 power amp blowing fuses on one side...

[Neal]

I feel like I'm brain-dead. I'm used to working on tube amps, but I'm clueless on solid state stuff.
I'm trying to help out a bandmate with a Fender SPL 6000 power amp with one side not working. Inside I found 2 blown 8 amp fuses (4 internal fuses total, the other two are fine).
What would be the obvious things to check, and how do I check transistors with a multi-meter?
I need more coffee...

[52 Bill]

Get a copy of the schematic from Fender, at least that way you will have something to follow. The most common problem is output transistors. Depending upon the type of transistor, identify the three leads Base, Emitter and Collector.

Basically, you test the transistors by looking for shorts between pins. The outputs are often in parallel, so if one is shorted, they will all test as shorted in circuit. The final test will be out of circuit.

[Enzo]

Troubleshooting is a universal skill, it doesn't matter what technology you apply it to. If a tube amp blew fuses, what would you do? I'd look for shorted output devices, or shorted power supply rectifiers, or even shorted filter caps. Well your solid state amp has those things too. They just look different. Beyond that, the key is isolate the problem.

[Mick Bailey]

I would start by building a bulb limiter if you don't already have one.

Power transistors sometimes take with them their driver transistors, so don't assume if you find a shorted output device that's all there is to it. You also need to check any emitter resistors as these can fail at the same time. An open emitter resistor means that its associated transistor can't work, so additional load is placed on the others. The only other issue with failed output transistors is you can remove them to find one or two shorted out, but you don't know if the rest on that channel have been stressed, which can give rise to future failure. The safest bet is to replace the set and this also avoids current-hogging due to mismatched devices.

Always run a rebuilt amp on the bulb-limiter (without a load) to make sure there's no significant DC on the speaker outputs. I also check the voltage drop across each emitter resistor - they should be the same.

[Neal]

Thanks, guys! Just what I needed to kick-start my feeble brain!
I really appreciate it!

[Neal]

Thread resurrection.... once again I'm throwing myself on the mercy of the Forum Brain Trust.
This amp has at least one blown output transistor. I haven't had time to pull them to isolate which one(s) it is.
I figured I'd just replace all of them on that side, including the driver transistor. So, here's my two-part question...
First, the parts are obsolete. Can anyone recommend a drop-in replacement available from Mouser?
Transistors in question are all Toshibas, numbers:
2SA1302
2SC3281
Driver transistors:
A1306
C3298

And secondly, should I replace the (good) transistors on the working side of the amp to match the replacement transistors on the bad side?
Once again, thanks for the help, everybody!

[Jazz P Bass]

On Semi makes a replacement for those output transistors.
MJL1302AG/ MJL3281AG.
FindChips: Electronic Components, Distributor Inventories, Datasheets

Or you can use the standard 2SA1943/ 2SC5200.

The 2SA1306 can be replaced with the 2SA1837.
The 2SC3298 can be replaced with the 2SC4793.

Digikey has all of them available.

I see no reason to change the other working side.

[Neal]

Once again, exactly the info I needed, almost instantly. Thanks very much, Mr. Bass!
And thanks to all others who responded.
As a side note, Enzo has helped me out so many times over the years that I owe him a steak dinner and a couple of beers. Next time I get to Michigan....

[Enzo]

And after MSU beat OSU? of course OSU repaid the favor to U-M.

[Neal]

And after MSU beat OSU? of course OSU repaid the favor to U-M.

I was politely not going to mention that.

[Neal]

Fixed! Thanks for the help, everybody!