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Reply With Quote1k5 sounds like the grid-stopper...
Is this the correct schematic? http://www.techofsound.com/links/Mar...ll/jmp50c2.gif
Hi Folks.
I'm getting a pop sound from my amp. The V7 LED lit up, fuse blown. I looked inside the chassis and the 1.5K that goes from pin 4 to 5 of V7 smoked. (what is this resistor called BTW?) I replaced the resistor and still had the pop. I got a new tube, still popping. I replaced V4, still pop.
I had some volume fluctuations before the pop occurred.
I'm an engineer/tech but I'm very unaware of tube amp repair.
Any help? Thanks.
1k5 sounds like the grid-stopper...
Is this the correct schematic? http://www.techofsound.com/links/Mar...ll/jmp50c2.gif
Originally Posted by MikeH
Well the schematic is pretty darn close except there are violet wires instead of white going to r 31 and 32.
The resistor that smoked comes from pin 5 ov V7 and goes to pin 5 of V6.
the board identifier is jmp50c
Pin 5 of an EL34 is the grid.
You may want to mionitor the grid negative supply voltage after replacing the resistor.
You want to see a steady dc voltage (-40 to -35 volts) with very little Volts ac ripple.
The bias cap may be going bad.
Yeah - AFAIK, those resistors generally don't see a lot of juice unless a coupling cap from the previous stage is dumping DC, the bias supply has gone rouge, or a tube has some sort of short and is putting plate voltage on the grid. Either way, it wont bias properly if that resistor is damaged.
Remove the output board (the one with the speaker jacks) and inspect for burning, arcing, check the low / high and impedance select switches..
This amp tends to burn that output board, and it's an expected trouble spot.
Once carbon forms on that fiberglass, it has to be cut or drilled out.
I stuck a newer tube into V6 and it stopped popping. Looking at the old tube I see that it wasn't in too good of shape and was probably arcing. Ordered 2 more JJs to complete the set.
Inspect the board and socket for carbon traces. The carbon traces will make it arc again.
Arcing is often the result of the wrong speaker impedance, bad speaker cable, a defect speaker.
Last edited by soundguruman; 03-17-2012 at 03:25 PM.
I have 4 new tubes in it and I biased it, the tone is great, and it's now doing a little crackle and small pop sounds, which before led up to the big pop sounds. I inspected the hell out of the board and socket and saw nothing. Beats me, man.
Or it could be the dark stuff on the impedance switch. Maybe I shoulda paid more attention to that.
http://i569.photobucket.com/albums/s...tput_board.jpg
password:100w4102
Crackle and pop could be a lot of things; you could have some failing solders. Have you tried chopsticking the board?
It's true that those switches oxidize internally, and should be replaced now and againOr it could be the dark stuff on the impedance switch. Maybe I shoulda paid more attention to that.
http://i569.photobucket.com/albums/s...tput_board.jpg
password:100w4102
the jacks on the board have switches that can also be cleaned
Believe it or not I've never heard the term 'chopsticking' in my life. Tap lightly with non-conductive long skinny thing?
You can tap around on the boards and see if there's a loose solder connection
and a non conductive tool, other than your finger, is recommended, such as wood chop sticks...
you can try playing/ plugging audio into the effects return, try to narrow it down
into preamp or power amp?
Take out V1, V2, is the noise still happening?
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.
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?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.
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.
Last edited by soundguruman; 03-24-2012 at 01:18 PM.
Yes.
I have an analog scope. Not sure what I'm gonna see when dealing with this stuff though.This is trouble when you don't have an O scope.
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?
And what about the little 22pF capacitor mentioned earlier, did we check if that was fitted and try replacing it?
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
Yes take the cap out and see if the popping stops.
Where is this 22 pf cap? C what?
I do not think the JCM900 used that specific cap.
No.Look at the output tube sockets, are any on the connector pins black or burned?
Yes. No effect. There is no evidence of heat in that area or any brown or blackness.Did you resolder the screen grid resistors?
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
Sorry, I think I got confused with another thread about another JCM type amp.I can't find the other thread either.
"Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"
No probs.Sorry, I think I got confused with another thread about another JCM type amp.
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.
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.
"So, for my small experience in the trade I would think that killing customers is, at the very least, bad for business." -Chuck H
Disclaimer: "Take my advice; don't listen to me"-Neil Young. "I'll lie again and again and I'll keep lying; I promise"-Henry Rollins.
correct.
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?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?
So I got a little bit logical about this and decided to follow an approximate path through the circuit. There was/is some black stuff on the terminals of the power transformer on the red and black terminals, though they don't look totally smoked like a resistor might. Differentiating between burns and corrosion on solder is what I needed to do. Anyway a couple of the terminals (B+ I believe) on the tube sockets also look a little carbony but not overly so. What I thought was a little brown discoloration looks more like deposits. I scraped a bit on the power transformer terminals and the stuff came off like you'd find on a burnt chicken leg.
I don't know what the root cause yet. I think I'm correct in my analysis however, but I'm still learning about this stuff.
Now that you've isolated the problem to the power section, I'd start by cleaning up/reflowing the joints in that area. Actually I'd start by chopsticking the area to see if you can isolate the problem. My "Chopstick" is a narrow rubber stopper with a dowel rod fitted into it. With the amp running, tap on the components in the problem area and see if you can recreate the pop. Make sure you tap on any connectors (like the 4 pin, 6pin, etc. connectors that connect the boards together - I've found those to cause crackling due to loose connections or corrosion). If nothing specific shows up, then I'd try cleaning up all of the joints.
As far as the output fuses, power down when removing or installing.
The black stuff you are describing sounds like solder flux. It can get very dark, almost black looking depending on how much heat is applied to the joint.
"So, for my small experience in the trade I would think that killing customers is, at the very least, bad for business." -Chuck H
Disclaimer: "Take my advice; don't listen to me"-Neil Young. "I'll lie again and again and I'll keep lying; I promise"-Henry Rollins.
"A little carbony"
even the slightest trace of carbon can make it arc...in a variety of places...
It would be helpful to post some pictures.
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