250rl gallien krueger questions

[Jazz P Bass]

Thanks, I've just discovered that one of the a .33 5watt resistors that connects to the emitter leg of the tip33c is open, which would have to effect the bias?

The .33/ 5 watt resistor is what connects the transistor output lead to the speaker.
It is refered to as a 'ballast resistor'.
When transistors are run in parallel, a ballast resistor helps current sharing between the paralleled devices.

[9 volt]

Thanks for the advice, this is beginning to make sense! There is a small electro cap in the feedback loop which I will replace also!

[9 volt]

Well, I replaced the .33 resistor, rechecked TIP output transistors and am still blowing again. The grounds rail shows continuity with the case.....and the B rail shows a continuity like when testing a capacitor eg slow growing continuity..........starting to lose sleep!

[52 Bill]

Well, I replaced the .33 resistor, rechecked TIP output transistors and am still blowing again.

The ballast resistor could have acted as a fuse and opened up when the related power transistor shorted. Double check the transistor that the replaced resistor is connected to.

When you had the amp up and running did you set the bias on the repaired right channel?

[9 volt]

No, I haven't touched the bias trim pot, though I did have to replace the transistor that goes into the bias trim. I have replaced the both power transistors again I have built a light bulb limiter some will use this next time i test. The diodes that run parallel to the power transistors seem to test ok but I will pull one end tonight to make sure they are ok.

[9 volt]

Well tonight I found the transistor that was popping fuses. It was an mpsa56 which tested ok on diode test but was shorted from c to e. I had it up and running for about two minutes with no distortion then one output transistor baked. I haven't tried to biase the channel which may have been the problem?

[9 volt]

ok, today i finally got it running and biased the transistors, they were sitting on 15mV and are not 5mV. Ran a test (sound through it for a minute) and seems good. Are there any other tests I could do to ensure it will not blow when on for a longer period of time (rather than just seeing what happens)? Would not recommend working on one of these without abit of solid state experience, so compact and complex compared to tubes! Thanks for advice so far hope this will help someone out in the future.