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.

[fstarocka]

I tried the light bulb test, measured voltage but nothing shows, not even a glow?? maybe i missed sth. Anyway - replaced the rectifier bridge and some transistors I found faulty - (desolder / diode test / resolder) -

Can anyone suggest what parts to possibly focus on in the pic below? I havent checked the other caps, only the 2 big ones were replaced - many forums keep insisting output transistors, those would be the bigger guys - ill retest them, not too hard to desolder - what about the little black chips with 6 legs, could they be shorting? arent they also basically a bunch of diodes? then i can test those as well -

[fstarocka]

Also whats the MOST likely culprit? Caps, resistors or transistors? Everyone keeps telling me transistors..?? im not qualified i know, but im sure i can keep trying with amultimeter and hope the issue is something i can fix?

[52 Bill]

Also whats the MOST likely culprit? Caps, resistors or transistors? Everyone keeps telling me transistors..?? im not qualified i know, but im sure i can keep trying with amultimeter and hope the issue is something i can fix?

Welcome to the place. What kind of amp are you trying to fix? What are the symptoms?

If there is no voltage then have you tested the fuses with a meter?

[R.G.]

I tried the light bulb test, measured voltage but nothing shows, not even a glow?? maybe i missed sth. Anyway - replaced the rectifier bridge and some transistors I found faulty - (desolder / diode test / resolder) -

Light bulbs are simple: if current flows through them, they glow, unless it's far below their normal current. A 100W light bulb needs 100w/120V = 0.83A to glow at full brightness. Brightness falls off as the fourth power of the applied power (volts * amps) so they dim fast as current drops, but there's a little glow way down. If you're not seeing any glow it's possible that (1) there is too little current to make a big bulb glow; screw in a lower-wattage replacement or (2) there's really no current flowing.

Can anyone suggest what parts to possibly focus on in the pic below?

Probably not, at least not usefully. Looking at parts in pictures is only helpful if there are parts that are visibly burned and blackened or otherwise damaged. Otherwise, pictures are not much help at this stage in debugging.

many forums keep insisting output transistors,

In general, the more forums you submit your problem to, the more confusing the results. The internet is incredibly wide, but it's not very deep.

those would be the bigger guys - ill retest them, not too hard to desolder - what about the little black chips with 6 legs, could they be shorting? arent they also basically a bunch of diodes? then i can test those as well -

There are many little black chips with six legs. Need a schematic and perhaps a parts list to tell what they are. Six legged chips may be diodes - or they may be optoisolators and not testable with a simple meter at all.

Also whats the MOST likely culprit? Caps, resistors or transistors?

That's not a useful question. Simply guessing at what MIGHT be the problem will take a very, very... very long time to get to a fix, even if the errors introduced in testing don't add other problems. No proficient tech starts with only what the most likely kind of parts are.

Everyone keeps telling me transistors..??

Again, unless you're giving them a lot more information than you've given us, they will have no particular clue what the most likely thing is, and it costs them nothing to send you off on a wild goose chase. The more internet forums you read, the more likely you are to get confused unless you're already an expert and know things independently to cross check what you get told.

im not qualified i know, but im sure i can keep trying with amultimeter and hope the issue is something i can fix?

Be aware that the longer you mess around in there, the likelier you are to accidentally touch a hot AC power wire. There is some real risk to you to keep messing on a hot amp with a multimeter.

The very best advice I can give you is to:

1. Find an accurate schematic for this amp, and post a link to it here.
2. Post here which resistors were burned, and that parts you have tested and/or changed.
3. Be prepared to do further tests as suggested here. You have the attention of Enzo, who is one of the most knowledgeable and helpful techs you're going to find.
4. Quit going off to many forums for advice. Pick one you trust (for whatever reason) and work with them.