GK 800RB repair

[Dave M]

Hi All,

I came across a semi-functional 800RB. The 100 watt amp works fine, but not the 300 watt amp. Seems there are some crispy resistors in there now.
I'd like to troubleshoot the cause and repair it back to life. I don't have much in the way of SS experience; a bit with tubes.
The pics hopefully provide some insight. Thanks in advance.

[The Dude]

I'm not sure what the question is.
1) Replace all of the burnt resistors.
2) Check for shorted transistors that caused the resistors to burn.
3) Don't plug in a speaker until there is no DC on the output.
4) Use a light bulb limiter until you get it working.
5) Have a White Russian and play bass.

[Dave M]

Thanks Dude. I guess the question is, what do I need to do to troubleshoot, and correct way to check the transistors (pull them off board?)? Wait, that's more than one question.

I can do #1, and #5 comes pretty easily .
I need to build a bulb limiter, but do have a variac (even tho the bulb limiter is preferred). I'm also curious if the specific damage shown is familiar to repaired 800RBs and if there are common issues that should also be checked.

[The Dude]

I have two of these- one of my favorite bass amps. You can first check transistors in circuit looking for shorts. If you read a short, pull the transistor and check it out of circuit. Here's some info on checking transistors if you are unsure.

Electrical Measurements Part 3 | RSR Electronix Express

Scroll about half way down the page for transistors.

I haven't seen the damage your amp has even though I acquired both of mine broken, so I wouldn't say it's a common problem. Just troubleshoot it like you would anything else. A variac isn't a bad thing to have, but you need to be able to monitor current or the bulb limiter is a must.

[Enzo]

As mentioned above, resistors burn for a reason, usually shorted semiconductors. Q15,16 are almost certainly toast. Q17,21 are highly suspect, as are most of the output transistors. And check all the other resistors in the circuit, just because they didn't burn up doesn;t mean they didn;t go open when all that excess current was flying around.