guitar amplifier going bad ..... why???

[dirtyb15]

Okay, one of the two diodes that i saw disoloration is shorted. I think it is a 1n535. I have no problem replacing these, but what would cause it to short like that? I made sure there were no places the board was grounding out or anything like that.

 

[Nigel Goodwin]

Okay, one of the two diodes that i saw disoloration is shorted. I think it is a 1n535. I have no problem replacing these, but what would cause it to short like that? I made sure there were no places the board was grounding out or anything like that.

What's the circuit reference on the PCB?, so I can see what they do!.

 

[dirtyb15]

cr56 and cr57 on the pcb. I dont have the schematic you sent me with me so i assume these labels are on the shemcatic also.

 

[Nigel Goodwin]

cr56 and cr57 on the pcb. I dont have the schematic you sent me with me so i assume these labels are on the shemcatic also.

Yes, that's the reason for the labeling.

They are 16V 5W zener diodes, used to provide the +/-16V rails from the main +/-40V supply.

 

[dirtyb15]

I had my multimeter on the wrong setting, neither is open or shorted. I have 16 and -16 at the correct test points. So what should i look for now? All i get is a loud hum. Does not change with volume, guitar inputs or anything.
I apologize for turning this into such a big post, I probably should have just taken it to the repair shop

 

[Nigel Goodwin]

I had my multimeter on the wrong setting, neither is open or shorted. I have 16 and -16 at the correct test points. So what should i look for now? All i get is a loud hum. Does not change with volume, guitar inputs or anything.
I apologize for turning this into such a big post, I probably should have just taken it to the repair shop

If it's a seriously loud hum, it could be the ouput stage blown, measure the DC voltage on the output - it should be zero (or VERY close to it).

 

[dirtyb15]

Well its definitely not 0. It is at 40V.

 

[Nigel Goodwin]

Well its definitely not 0. It is at 40V.

It rather looks like you have blown the output stage then :cry:

If you've got 40V DC sat across your speaker it won't have done that any good either!.

 

[dirtyb15]

Well i did not leave the amp on for very long so i hope my cabinet is okay.
Upon closer look at the board, i noticed that cr27 diode was not near as low to the board as the other 3 on the heatsink bar. I pulled on it a bit and sure enough it was not even soldered in, or had broken at the joint. Could this have caused it to blow? Is it worth trying to repair the output stage? Thanks again, sorry for all the questions.

 

[Nigel Goodwin]

Well i did not leave the amp on for very long so i hope my cabinet is okay.
Upon closer look at the board, i noticed that cr27 diode was not near as low to the board as the other 3 on the heatsink bar. I pulled on it a bit and sure enough it was not even soldered in, or had broken at the joint. Could this have caused it to blow? Is it worth trying to repair the output stage? Thanks again, sorry for all the questions.

Yes, if that was OC it would blow the output stages, it's one of four series diodes that set the output quiescent current - if the diode goes OC then the transistors all turn hard on, with disasterous results!.

I would suggest checking all four output transistors, and see if any read SC between collector and emitter. They are commonly available darlingtons, and wouldn't be too expensive to replace.

But make sure you solder CR27 first (and check the other three), if you're REALLY lucky (how many times did you win the lottery last week?), soldering the diode 'might' be all that's required?.

 

[dirtyb15]

Soldered the diode back, but no lottery wins ever for me

I have shorts on both q13, q14, and i suspect cr38 is also damaged ( will will pull it to be sure.)

 

[Nigel Goodwin]

Soldered the diode back, but no lottery wins ever for me

I have shorts on both q13, q14, and i suspect cr38 is also damaged ( will will pull it to be sure.)

I wouldn't expect CR38 to have failed, but it's across Q13 and Q14, so you can't test it while the transistors are in circuit.

However, these are the negative side transistors, if they are short then you should have -40V on the output, I though you said 40V, not -40V?.

 

[dirtyb15]

I was quick when i did my measurment and must have had my probes backward. I just verified it is -40V. I also removed cr38 and it is fine.

 

[Nigel Goodwin]

I was quick when i did my measurment and must have had my probes backward. I just verified it is -40V.

You 'might' be able to just change those two then?, have you disconnected them? - as they are in parallel only one might be faulty?, if so remove the faulty one and try the amp with the other one. The amp will work fine with just one pair of transistors, but will only feed 8 ohms (60W) and not 4 ohms (100W) - but it will be fine to try it like that. If you feed 4 ohms at high volume the transistors will overheat and die though!.

 

[dirtyb15]

Thanks, ill try that out right now. I dont have them out yet, but working on it. I need more hands and irons.

edit:: sorry about these double posts, i keep pressing back on explorer and it reposts them.

Thank you very much Nigel. I got the first transistor out and it was still shorted. The other was fine, so i left it in and turned it on. 0V at the output now. Dont have my cabinet or guitar here to try it out, but i might as well order the new transistor while i have it all open. Thanks again,
Dave

 

[dirtyb15]

Thanks, ill try that out right now. I dont have them out yet, but working on it. I need more hands and irons.

 

[dirtyb15]

well i picked up a new TIP147 at the local electronics shop. Soldered it in, checked everything over once and powered it on. Measured output and had 0V. Started smelling something so i checked again and i was at -40V.
Powered it off and took the new one out again. The other tranisistor is now shorted.

 

[Nigel Goodwin]

well i picked up a new TIP147 at the local electronics shop. Soldered it in, checked everything over once and powered it on. Measured output and had 0V. Started smelling something so i checked again and i was at -40V.
Powered it off and took the new one out again. The other tranisistor is now shorted.

Are the four Vbe diodes all OK?.

 

[dirtyb15]

cr25, cr26, cr27, and cr28 all checked out okay.
Any other obvious components to test?

 

[Nigel Goodwin]

cr25, cr26, cr27, and cr28 all checked out okay.
Any other obvious components to test?

Everything around the output stage really, but when you test it, don't bother fitting all four transistors - just one top and one bottom is enough for testing.