Roland KC-350

[gui_tarzan]

This amp came to me with a "blowing fuses" problem.

So far I've traced the power all the way to the power transistors being what Roland says it should be according to the schematic. My issue is I don't know what the voltages are supposed to be out of the transistors themselves, that's not listed on the schematic.

I removed the power transistors Q7 (2SC5242) & Q17 (2SA1962-O) and get the following readings:

Resistance test:

Q17

Black on pin 1, red on pin 2 - Base to Collector - 11.1 Mohm
Black on pin 1, red on pin 3 - Base to Emitter - 12.7 Mohm
Black on pin 2, red on pin 3 - Collector to Emitter - open

Red on pin 1, black on pin 2 - Base to Collector - open
Red on pin 1, black on pin 3 - Base to Emitter - open
Red on pin 2, black on pin 3 - Collector to Emitter - open

Q7

Black on pin 1, red on pin 2 - Base to Collector - open
Black on pin 1, red on pin 3 - Base to Emitter - open
Black on pin 2, red on pin 3 - Collector to Emitter - open

Red on pin 1, black on pin 2 - Base to Collector - 12.65 Mohm
Red on pin 1, black on pin 3 - Base to Emitter - 13.2 Mohm
Red on pin 2, black on pin 3 - Collector to Emitter - open

Diode test:

Q17

Black on pin 1, red on pin 2 - Base to Collector - .519
Black on pin 1, red on pin 3 - Base to Emitter - .522
Black on pin 2, red on pin 3 - Collector to Emitter - open

Red on pin 1, black on pin 2 - Base to Collector - open
Red on pin 1, black on pin 3 - Base to Emitter - open
Red on pin 2, black on pin 3 - Collector to Emitter - 0 - edit - reads open

Q7

Black on pin 1, red on pin 2 - Base to Collector - open
Black on pin 1, red on pin 3 - Base to Emitter - open
Black on pin 2, red on pin 3 - Collector to Emitter - open

Red on pin 1, black on pin 2 - Base to Collector - .520
Red on pin 1, black on pin 3 - Base to Emitter - .522
Red on pin 2, black on pin 3 - Collector to Emitter - open

I get the expected +55v and -55v on the collectors when they're in the circuit so I know power is getting all the way through the circuit and the fuses aren't blowing at this point with the power board and power amp board hooked up.

This is the first solid state amp I've worked on in many, many years so I'm very rusty on my theory, but the zero reading on Q17 collector to emitter is suspicious to me. I checked the numbers twice to make sure I had good contact on the leads.

[Jazz P Bass]

Why the resistance check of Q17 shows Open & the diode check shows Zero has me baffled.

[g1]

Why the resistance check of Q17 shows Open & the diode check shows Zero has me baffled.

And only with the probes in one direction, with the probes reversed it's open.

Please recheck Q17 C to E readings.

[gui_tarzan]

I just took measurements again and the zero actually reads open now. I must have done something wrong the first time but I have no idea what. The meter went from something like .5 to .1 to zero. Static maybe?

[g1]

I'm confused about when it blows the fuse, you said with the outputs installed you got proper voltage rails and no fuse blowing?

Edit: maybe you are on a lamp limiter?

[gui_tarzan]

The owner said it won't power up, the shop owner that sent him to me said it was blowing fuses. So far it hasn't and I've seen correct voltage from the power cord to the output transistors so I'm confused as well.