Gemini XPM-900 - Burnt thing on my amp, what is it? *pics*

[MikeHunt79]

One of the channels on my Gemini XPM-900 has stopped working recently, so i whipped the top off to find this:


It's just behind the red cable in the middle at the top:


Now, to me it looks like a toasted resistor, but I haven't seem that many burnt things, so maybe someone here would be able to tell?

Also, has anyone got any idea's what caused this and how to fix it? Also, whats the best way of cleaning the black crap off?

[Jan Dupont]

Hello Mike,

That doesn't look good
The resistor just below the opamp seem to be OK (just got a little burned) but the opamp looks to have cracked...
Try to wipe some of the burned away from the PCB. Maybe it's possible to see/guess what component went up in smoke...

[jaycee]

It looks like the op-amp that handles the balanced inputs has gone up, and at a guess, those resistors are part of the regulator circuit that power the opamp.

Here is a service manual:
http://www.geminidj.com/disc._prod_m...0-900-1200.pdf

Unfortunately - the PCB and schematic diagrams are useless as they are woefully under-size bitmaps

To determine if youve got further damage I would remove the blown parts and then try to power on the amp using a series lightbulb. The unbalanced input may still work just fine.

[Mooly]

Can you make out the type no of the I/C . Use cotton buds and WD40 initially to clean P.C.B, then use buds and Iso. Can't really make out circuit, but the Op-Amps on left of diagram look like fed from simple series res and zener, no current capability there, the bottom I/C looks to be fed direct of it's own unreg supply ? .Is it this one ? Had another squint, is it the resistor to pin 8 +supply ? If it is would advise change all components on that line e.g rectifier, smoothing, caps filter caps and the zeners. Were you working on it at the time, it's very weird.
Regards Karl

[MikeHunt79]

Originally posted by ACD
Hello Mike,

That doesn't look good
The resistor just below the opamp seem to be OK (just got a little burned) but the opamp looks to have cracked...
Try to wipe some of the burned away from the PCB. Maybe it's possible to see/guess what component went up in smoke...

Ok, I wiped some of the black stuff off, and here's another pic. The opamp is ok, it just has some black goo on it which I have now wiped off...


Definately looks like a resistor that went bang IMO.

Originally posted by jaycee
It looks like the op-amp that handles the balanced inputs has gone up, and at a guess, those resistors are part of the regulator circuit that power the opamp.

Here is a service manual:
http://www.geminidj.com/disc._prod_m...0-900-1200.pdf

Unfortunately - the PCB and schematic diagrams are useless as they are woefully under-size bitmaps

To determine if youve got further damage I would remove the blown parts and then try to power on the amp using a series lightbulb. The unbalanced input may still work just fine.

Thanks for the manual - you are right about the resolution tho, it's too low. I've e-mailed them asking for a higher resolution version...

Do you have info on how to test with a lightbulb? I can try unbalanced, as it's got TRS and XLR in's. Do you just wire a 100W lightbulb to the speaker outs?

Originally posted by Mooly


Can you make out the type no of the I/C . Use cotton buds and WD40 initially to clean P.C.B, then use buds and Iso. Can't really make out circuit, but the Op-Amps on left of diagram look like fed from simple series res and zener, no current capability there, the bottom I/C looks to be fed direct of it's own unreg supply ? .Is it this one ? Had another squint, is it the resistor to pin 8 +supply ? If it is would advise change all components on that line e.g rectifier, smoothing, caps filter caps and the zeners. Were you working on it at the time, it's very weird.
Regards Karl

TL072CP. I've pulled the burnt resistor, and got another pic:


Unfortunately I cannot make out what is going to what pins on the opamp, as it seems all the op amp pins seem to be on traces on the other side of the board... I will to take the PCB out tho, as there could be burnt bits of resistor under there...

[Mooly]

Hi Mike, think it was Jaycee that mentioned the bulb trick, anyway you use the bulb to limit the current the amp can draw. Try a 60watt bulb (filament not energy saver ) and connect it in series with the mains supply to the amp. Do not connect any speakers.

[Mooly]

What test equipment do have ? If you have meter measure on low ohms (amp off) for continuity from pin 8 and then pin 4 of that I/C to one end of the where the burnt part was. Is this where it goes to ?

[MikeHunt79]

Originally posted by Mooly
What test equipment do have ? If you have meter measure on low ohms (amp off) for continuity from pin 8 and then pin 4 of that I/C to one end of the where the burnt part was. Is this where it goes to ?

I have a fluke meter with a scope, and loads of normal meters... I also sometimes use the PC as a sig gen.

I'll give the light bulb method a go...

I'll try and test the resistance to pins 4 and 8 of the op amp right now, gimmie a min.

[Jan Dupont]

I think it seems more like a blown Zener

[MikeHunt79]

Ok, I can't actually get the probe to connect, as there are just holes as I pulled the burnt resistor... I'm gonna have to pull the PCB to do this...

I'm gonna try the bulb then first, then pull the PCB.

I did manage to find another burnt part while looking around tho.



Looks like I may be needing a new tank cap!