What does it mean when you have a dead short on your driver board

scootchu

scootchu

Dear Sir or Madman
I have continuity on everything on the left channel of my Citation 22. Everything seems to be tied to ground. The output transistors have continuity to ground on each leg. WOW. Small wonder I was blowing fuses. Where can I look to see what is causing this? Diodes? Resistors?
Well going cycling for a bit and this will give me something to think about while I endure the pain.
 
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Since the quarters inside this amp are cramped I desoldered the left side output drivers and low and behold a pair is shorted. Every combo of the MM leads results in a short reading on the meter. The others seem to be okay in that they match each other with current going one way and not the other.

I won't rule them out but I think I found the two culprits.

My question now is what caused this and it still doesn't explain why I get nothing out of the right channel. This does make me feel good though in that I actually found two bad transistors. I will pull them and get the part numbers. Hopefully they are still around.
 
Parts are Toshiba A1516 and Toshiba C3907. I see a guy on Ebay has both for $1.95 and $3.95 respectively. Would you trust them? My head hurts from trying to find them on Digikey or Mouser.
 
Thanks. The seller is just a guy in Orlando with a 97% pos feedback.
Should I replace or at least buy 8 of each just in case or are these a common trans?

I will cross each bridge as I get to it, but now what do I check? Diodes and resistors?
 
With outputs out of circuit.Verify that the power supply is no longer shorted to ground.The shorted outputs you have , could very possibly be all thats wrong.
 
Seems I've read that Toshiba is one brand that is fairly commonly counterfeited. So, just be sure (somehow??) that you're gettin' the real deal.
 
whoaru99 said:
Seems I've read that Toshiba is one brand that is fairly commonly counterfeited. So, just be sure (somehow??) that you're gettin' the real deal.
Click to expand...
This is what I've heard also..I think I'd go with B&D..:yes:
 
avionic said:
With outputs out of circuit.Verify that the power supply is no longer shorted to ground.The shorted outputs you have , could very possibly be all thats wrong.
Click to expand...

Since I removed the two shorted outputs and desoldered the rest everything seemed to clear up on the left side channel. Now I have to go and double check when I get home, but I am pretty sure there were no more shorts.

What do you think is causing no sound or even a level indication from the right channel? It seems to be okay in that it doesn't blow the fuse. It has 14.9mv at the speaker terminals, but when I tried to run a source through it... nothing. The protection light isn't on.

I appreciate the help ( aka hand holding) I am learning a lot.
 
Parts come tomorrow!!

I am very eager to see if this solves the issue. I ordered new mica washers and I am assuming that RS heatsink compound is okay to use.

Believe it or not I think that some of this is sinking in... that's until I see a new challenge that makes my head explode.
 
scootchu said;
" I am assuming that RS heatsink compound is okay to use."

NO, NO, don't use that stuff!!! It is the worst stuff I have ever used. Stick with Wakefield or Thermalloy compound.

Even MG Chemicals is good stuff. The Radioshaft stuff is like runny snot, smells like it too,heh,heh.....
 
scootchu,

If you can swing the expense, I'd recommend replacing ALL the outputs on the left channel! When one set shorts, it sends a pretty big current spike through the others and can stress them, especially if you had slo-blo fuses in place.

It may work just fine for awhile with some of the old outputs in place, and you never can be sure how long, but a customer of mine recently had to have an amp rebuilt for the second time, because the first tech to work on it didn't replace all the outputs. The tech also used a hodgepodge of output transistor brands, and forgot to put heat sink grease on most of the outputs, which didn't help matters either.

Just a suggestion though . . .
 
BrassTeacher said:
scootchu,

If you can swing the expense, I'd recommend replacing ALL the outputs on the left channel! When one set shorts, it sends a pretty big current spike through the others and can stress them, especially if you had slo-blo fuses in place.

It may work just fine for awhile with some of the old outputs in place, and you never can be sure how long, but a customer of mine recently had to have an amp rebuilt for the second time, because the first tech to work on it didn't replace all the outputs. The tech also used a hodgepodge of output transistor brands, and forgot to put heat sink grease on most of the outputs, which didn't help matters either.

Just a suggestion though . . .
Click to expand...
Good advice...:thmbsp:
 
Okay I plan on installing the new drivers in the Citation tonight when I go home and I have some questions.

First I don't have a variac, nor do I know if I need one, but what do I check after I am done soldering the transistors?

I had read that you should be using a heatsink on the trans legs as to avoid damage while soldering. I also read where an alligator clip would work. I am a decent soldering guy. I can get in and out fairly quick. (oh God how life draws its parallels) Is a heat sink necessary?

After I get answer to number one I will then ask should I close my eyes when I hit the power switch.
 
Okay I have made a dim bulb tester and so far so good with a 60 watt. I started with a 40 watt. It goes bright on start and then immediately dims. I have a 120 watt bulb. Can I move to that? My speaker terminals have about 129 mv running with the 60 watt bulb.
 
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