Popping Sound JCM900 4102

[norby3]

I'm gonna observe it here for a couple days, it's so slight...before I open the thing up again. May try the valves.

I ran out of the preamp(bad amp) into another amp's effects receive...no noises in other amp but I heard the pops/crackle coming from the speaker.(bad amp). So it looks like it's in the power amp.

I investigated the impedance switch(with the black stuff on the leg) so I measured across the legs, got zero ohms no matter switch position...that was weird. I decided to remove it and measure...switch is not shorted. What am I supposed to see across the output transformer resistance wise?

What are the carbon traces supposed to look like, BTW? I've seen burnt traces in pictures and other crap I've worked on but I'm not exactly sure what I should look for in this case.

[soundguruman]

I ran out of the preamp(bad amp) into another amp's effects receive...no noises in other amp but I heard the pops/crackle coming from the speaker.(bad amp). So it looks like it's in the power amp.

I investigated the impedance switch(with the black stuff on the leg) so I measured across the legs, got zero ohms no matter switch position...that was weird. I decided to remove it and measure...switch is not shorted. What am I supposed to see across the output transformer resistance wise?

What are the carbon traces supposed to look like, BTW? I've seen burnt traces in pictures and other crap I've worked on but I'm not exactly sure what I should look for in this case.

You still hear the crackle with the phase inverter pulled out? Try narrow it down more if possible...Is it the phase inverter section, or the output tubes?
This is trouble when you don't have an O scope. I hate to start replacing things randomly...so try to avoid that

You will see a tiny (or bigger) black spot in the fiberglass, between two copper circuit traces, you will sometimes need to use a lighted magnifier, or shine a flashlight through the fiberglass, to see the burned spot inside the fiberglass. Sometimes these things are hard to spot, and you take the parts off the board and measure between the circuit traces, testing for leakage, with an ohm meter, set to the highest scale. It's not unusual for a carbon track between two traces to measure 2.2 megaohms, or even higher...when the high voltage turns on, the carbon acts as a resistor (an extra resistor in the circuit).
You can use a q tip, dipped in denatured alcohol, to swab underneath the power tube sockets. swab around the solder lugs,
Are the Q tips coming out with black carbon, like charcoal colored? There can be a burned spot, under a tube socket, or on the socket, that you might never see until you take the socket off the board.
Resistors get crackles too. you can try replacing the grid resistors, the bias splitter resistors, and the bias circuit, clean or replace the bias control, replace the screen grid resistors, etc...but try to narrow it down first more...
Did the old output tube socket pins look burned? Which socket did the burned pins come from?
Did you try reheating all the connections in the power amp, take the tubes out first before resoldering the sockets...

But I have seen these amps slowly burn a hole through that board, little by little...I have seen these amps where techs have cut out the carbon, leaving a hole where the fiberglass was burned. (click on the pic for a better view)





Seems to be happening between the screen grid resistor area and the switch, it's that area right there that melts down.
Maybe you have bad solder joints on your screen grid resistors, right there, in the hot area on that board. Try resolder the screen grid resistors.

[norby3]

You still hear the crackle with the phase inverter pulled out?

Yes.

This is trouble when you don't have an O scope.

I have an analog scope. Not sure what I'm gonna see when dealing with this stuff though.

[soundguruman]

Yes.


I have an analog scope. Not sure what I'm gonna see when dealing with this stuff though.

If the crackle is still there with the phase inverter pulled out, you know that it must be in the output section.
A bad socket, a bad resistor, a bad connection in that section...
Look at the output tube sockets, are any on the connector pins black or burned?
Did you resolder the screen grid resistors?

[Steve Conner]

And what about the little 22pF capacitor mentioned earlier, did we check if that was fitted and try replacing it?

[soundguruman]

And what about the little 22pF capacitor mentioned earlier, did we check if that was fitted and try replacing it?

Yes take the cap out and see if the popping stops.

[norby3]

Yes take the cap out and see if the popping stops.

Where is this 22 pf cap? C what?

[Jazz P Bass]

Where is this 22 pf cap? C what?

I do not think the JCM900 used that specific cap.

[norby3]

Look at the output tube sockets, are any on the connector pins black or burned?

No.

Did you resolder the screen grid resistors?

Yes. No effect. There is no evidence of heat in that area or any brown or blackness.

I'm not finding anything in terms of clues for this problem. All I know for certain is that the 1.5k between V7 and V6 smoked and a fuse blew at some point. And that I replaced a tube and the bad popping got way better.


Also: what is the deal in this photo? The flux gets old and turns brown? This isn't a carbon thing is it?
http://i569.photobucket.com/albums/s...brownstuff.jpg

[Steve Conner]

Sorry, I think I got confused with another thread about another JCM type amp. I can't find the other thread either.

[norby3]

Sorry, I think I got confused with another thread about another JCM type amp. I can't find the other thread either.

No probs.

[DrGonz78]

Yeah that brown stuff looks to be old flux and I have always wondered when it looks like that if it is time to clean up those joints...? It is tough to see in the picture but those jacks look like they have some cold solder joints or at least from what I can tell from the picture. Not sure really.

[norby3]

Yeah that brown stuff looks to be old flux and I have always wondered when it looks like that if it is time to clean up those joints...? It is tough to see in the picture but those jacks look like they have some cold solder joints or at least from what I can tell from the picture. Not sure really.

The flash does weird stuff...the joints are firm and shiny, but not beautiful. In fact, the soldering job the manufacturer did on this is far below my standards.

[g1]

Sounds like at this time you have replaced all the output tubes and still have the popping sound, and it is still there if the phase splitter (V3) is removed. Is this correct?
Try removing one of the output fuses to see if the popping quits. Then put it back in and remove the other one. Can you get the pop to quit by removing either of the OP valve fuses?
The grid stopper resistors are a bit of an odd arrangement in that one pair of tubes gets 1K5 worth, but the second pair gets them in series for 3K worth. As V5 & V6 get the series arrangements, make sure the series "mate" of the resistor you replaced is good. (you said "The resistor that smoked comes from pin 5 of V7 and goes to pin 5 of V6" so R25 in the linked drawing is in series and may be suspect). At this point I would suggest you just replace it.
Aside from that, I suppose it is possible for screen resistors (2K2 4W) to cause popping sounds.

[norby3]

Sounds like at this time you have replaced all the output tubes and still have the popping sound, and it is still there if the phase splitter (V3) is removed. Is this correct?

correct.

Try removing one of the output fuses to see if the popping quits. Then put it back in and remove the other one. Can you get the pop to quit by removing either of the OP valve fuses?

can I remove them while the amp is running or should I remove one then power up, then power down, replace it, remove the other, power up?