80's Randall RRM 2-120 Power amp

[Memphis]

I have the above power amp here. When I turn it on the breaker trips almost immedietly. So where do I begin troubleshooting. I have never worked on Solid state equipment. Tube stuff I can do all day. Any and all help is appreciated. Schematic attached.

Bruce

[Jazz P Bass]

Looks like a nice amp to get your feet wet.
There are a lot of voltage markings on the schematic.
That is a plus.
A blown fuse at power up can happen for a number of reasons.
Wrong fuse value/ type is the first thing to check.
Starting at the power cord, you can check the primary winding with a meter, for a short.
The secondary into the bridge rectifier can be checked for shorts.
If you could disconnect the B+ & B- (marked A & B)wires that would isolate the power supply from the rest of the circuit.
If the power supply checks out, then you have to go hunting for a short in the output section.
Most instances are the output transistors.
And certainly build yourself a lamp limiter (be quick, they are outlawing our favorite test lamp)

[olddawg]

Check Q10 and Q11 for short. Don't keep turning the amp on.

[Memphis]

OK, I will start with P Bass's recommendations first. I have a 6amp breaker I am going to swap first. Then the PT windings. After that I'll check back.

Bruce

[Memphis]

OK, Checked the PT Both primaries and secondaries appear to be ok. No short to ground. OK next question. Checking Q10 and Q11 These I assume are output transistors. How do you test these ?

Bruce

[Enzo]

Troubleshooting is troubleshooting. DOn't care tube or solid state. If a tube amp pops fuses instantly, where do you look first? I look for shorted output devices first, then shorted rectifiers, then shorted caps. SS no different, look for shorted output devices, shorted rectifiers, shorted caps.

Output transistors will be the largest ones, mounted on heatsinks of some sort. Shorted is shorted. Q8,9,10,11 are all outputs. Look for shorts from the transistor case to either of the two pins underneath. If in doubt, pull the part and check it while free. Checl all those power resistors for opens too.

Look up "light bulb limiter" and make one and use it.

[Memphis]

Enzo,
Thanks for the reply. I went through all the output transistors and found one bad one. Replaced and working. The limiter is something I realy need to build. In fact tomorrows project for my intern.

Bruce

[Enzo]

Well, a socket and a light bulb... hope he's up to the challenge.

Glad you fixed it.


Still, always check the power resistors for open. An open one can leave it out of circuit putting the whole burden of operation on its buddy.