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

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.

[Stevem]

Is the amps +and - 5 volt supply working?
You have 2 zener diodes and 2 transistors in that section and many times in amps you will find a shorted zener !

If need be use a hobby knife to cut open traces in a boards power supply to see if un-powering a section will stop the fuse from blowing, BUT NOTE THIS, if the circuit is powered by a plus and minus power supply rail you must open both rails as many times having only one side of the rail powered will in and of itself take out semiconductors!!!

[gui_tarzan]

Yeah, all the power circuits out of the PT are working. I'll check those parts tonight and see what they do.

[Stevem]

You say all the ac outputs are working from the pT, but are all the dc supply's working, as one of them must be getting pulled down to blow the fuse!

[gui_tarzan]

That's just it, the fuses are all good and it didn't blow any when I plugged it in.

[Stevem]

unsolder both of the output transistors and prove them out with a ohm meter check, next check both R34 and R46 the 5 watt .33 ohm emitter resistors and make sure they are not open.