Popping Sound JCM900 4102

[norby3]

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.

[MikeH]

1k5 sounds like the grid-stopper...

Is this the correct schematic? http://www.techofsound.com/links/Mar...ll/jmp50c2.gif

[norby3]

1k5 sounds like the grid-stopper...

Is this the correct schematic? http://www.techofsound.com/links/Mar...ll/jmp50c2.gif

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

[Jazz P Bass]

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.

[MikeH]

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.

[soundguruman]

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.

[norby3]

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.

[soundguruman]

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.

[norby3]

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.

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.

[norby3]

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

[MikeH]

Crackle and pop could be a lot of things; you could have some failing solders. Have you tried chopsticking the board?

[soundguruman]

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

It's true that those switches oxidize internally, and should be replaced now and again
the jacks on the board have switches that can also be cleaned

[norby3]

Crackle and pop could be a lot of things; you could have some failing solders. Have you tried chopsticking the board?

Believe it or not I've never heard the term 'chopsticking' in my life. Tap lightly with non-conductive long skinny thing?

[soundguruman]

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?

[norby3]

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.