Need help trouble shooting blown fuses in solid state amps!

[srvprodigy]

I need help trouble shooting a vintage Kustom TRB400 Bass head. It had two blown resistors which I have replaced, but it keeps blowing the fuse as soon as I power up the amp. I cannot see anything indicating something is wrong. I need advice on where to start looking first. Any advice would be appreciated. Thanks.

[Enzo]

Resistors don't burn up because they are bad, they are bad because something burnt them up. They can't burn on their own, something had to cause excess current to flow through them. If the fuse pops at power up, then something is shorted. Probably an output transistor or two. Possibly a main rectifier. Not likely, but even possible shorted main filter cap. Whatever those resistors connected to would be a great first suspect.

[srvprodigy]

Thank you very much! That is a HUGE help in figuring this out. Much appreciated. :)

[srvprodigy]

I tested the diode rectifiers and found that two, side by side, were reading as closed (shorted) so I replaced them. AFter I replaced them I retested the new ones and they read as closed also. I then tested the old ones after removing them and they read as open. What would be the most likely cause of this? And should I test the output transistors the same as the rectifiers or on a different setting? Thanks so much!

[trevorus]

diodes should test closed one way, open the other. Start there, and you might have to take this to a tech, though, if a transistor is bad. They can be tricky to troubleshoot.

[srvprodigy]

I tested the diodes both ways and the read "shorted" each time. Thats what has me a bit puzzled? I'm wondering if something else being shorted would cause them to read as shorted also? I can't imagine so, but solid state amps aren't my thing. :-)

[52 Bill]

Testing power supply diodes in-circuit can sometimes be tricky, as the power transformer windings and the filter caps can sometimes cause unusual meter readings.

When testing diodes with a meter, first set the meter to the diode test position or to a low ohm meter setting. Place the red lead to the marked end (stripe or plus sign), and place the black lead on the other end. Your meter should read infinity or open. In circuit, your meter may start out with a low reading, but if the diode is ok, the reading will slowly rise as the voltage from your meter charges the power supply filter cap.

Now reverse the meter leads and you should get a low, but not zero reading. In circuit you may get a zero reading, but again if the diode is ok, the reading will slowly rise to a non-zero reading.

If your meter has a diode test function, this reading will be somewhere around .6-.7 volts for a silicon diode.

Hope this helps.

[srvprodigy]

A few more questions..... I am checking the transistors mounted to the heat sink. SHould I check for continuity or simply test same as the diodes? When I test the same for diodes the transistors 79m15,78m15,C4793,A1837 read OPEN. The other transistors C5200, A1943, D15XB20H all read SHORTED. When I test 79m15 & 78m15 from outside pin to outside pin they read SHORTED. Also, are all of the transistors mounted to the heat sink considered Output Transistors? What am I missing here? Where should start next? All your help is Very appreciated. Enzo.... Please help!!! :-)

[Enzo]

I think this project might be a bit over your head. Do you know anyone versed in electronics in your area who might help in exchange for dinner or a beer?

If your meter is a digital one, there should be a "diode" test function on it. That is also used to test transistors. Read the meter owners manual on how to use the function.

Output transistor means about the same thing as output tube. It refers to the part's function in the circuit. Yes it is on the heat sink, but no, not everything there is an output.

The C5200 and A1943 are your output devices. WHen ordering new ones, there is an understood "2S" in front of those numbers. The parts are really 2SC5200 and 2SA1943. Do thaty for all the C and A transistors.

78M15 and 79M15 - and you will often see them without the M - are not transistors. They are voltage regulators. They make the positive and negative 15v power supplies for the preamp respectively.

To me, "vintage" Kustom comes from the 1970s. This amp doesn't sound very old to me with these particular parts in it.

Any transistors measuring shorted should be removed and rechecked out of the amp. If they are still shorted, then replace them.

[srvprodigy]

Thanks so much Enzo. I'll admit, it is different than what I am used to working on. I do believe with the info you have graciously shared that I can handle it. You actually confirmed a few things I already knew, just wasn't certain of myself, you know what I mean? Unfortunately, there isn't a qulitfied amp repair man with 100 miles of me. Anyone looking to relocate??? Immediate opening for repair guy! Thanks Enzo! You da Man!!!!

[olddawg]

After you replace the shorted active devices and the resistors you should run the amp up slowly on a variac while checking the current draw. It will keep you from just thunking out power transistors and drivers. If you don't have access to a variac an old trick is to put a 100 watt light bulb in series with the power cord. If it starts to glow bright, unplug the amp quickly. You should also have an isolation transformer. Seriously, if you are miles from any service techs and want to do this stuff yourself you should get some good books and understand what you are doing. You could hurt yourself.