jcm900 high gain dual reverb very low volume

[cooldude666]

Hi there forum,
I'm working on finding a fix JCM900 that has so far eluded me.
A couple of days ago the tubes red plated in my friend's amp. I put in a working set of 6L6GCs and rebiased it to a nice 38mA (plate voltages are around 475V). Thing sounded great for about 2 days.

2 Days later- volume drops while my friend is playing it. I come down and find that V1's socket is a little bit loose, wiggling the tube brings back full volume so I sit down to retension and clean the sockets, and haven't been able to get the volume back up since. Despite this precursor to it sitting on my bench, the problem really doesn't seem like it's in the preamp. The amp sounds like a JCM900, just really quiet. EXCEPT for right when I turn it on. If I start playing right when I flip the amp on I get the full 100watts for about a second, and then it drops back down. Running the effects send into another power amp alleviates the volume problem, and the distortion characteristics are those of a JCM900. What should I be looking at?

I swapped tubes (power,preamp, and phase inverter), no dice.
I replaced the filter caps, no dice.
Plate voltage is nice and high. No resistors or caps are blown open and all of the resistors I tested were reading their proper values.
I cleaned all of the tube sockets and jacks, and I jumpered the effects loop send/receive

Any suggestions? All of the geofex ideas involve the preamp (aside from replacing the power tubes).
Thank you for any help leant.

JCM900-PREAMP-JMP52C-1990.pdf

[mozwell]

it could be dirty contacts on the send & receive sockets, plug a lead into the send & recieve sockets, do you get full volume back

[cooldude666]

i wish

it could be dirty contacts on the send & receive sockets, plug a lead into the send & recieve sockets, do you get full volume back

no, i wish it were that easy. i tried that. should have said I did in my post.

[pontiacpete]

could it be that the power tube sockets need to be replaced?

[g1]

First off, what is the exact model number? You posted the preamp schematic, but need to post the power amp.
So it is weak if you plug your signal into the return jack? You need to check for proper DC voltages at V2, V3, and the power tubes.
Earlier you were able to make the problem come and go by wiggling V1. However, you determined it is not a preamp problem, so it was probably due to flexing of the circuit board while you were wiggling V1. This would imply there is a bad connection somewhere, and it would seem you have isolated it to after the return jack. There are also IC's in the "post return" circuit along with V2,V3, and the power tubes. I'm using the term "post return" as there is a bit more stuff after the return jack than just the power amp.

[cooldude666]

First off, what is the exact model number? You posted the preamp schematic, but need to post the power amp.
So it is weak if you plug your signal into the return jack? You need to check for proper DC voltages at V2, V3, and the power tubes.
Earlier you were able to make the problem come and go by wiggling V1. However, you determined it is not a preamp problem, so it was probably due to flexing of the circuit board while you were wiggling V1. This would imply there is a bad connection somewhere, and it would seem you have isolated it to after the return jack. There are also IC's in the "post return" circuit along with V2,V3, and the power tubes. I'm using the term "post return" as there is a bit more stuff after the return jack than just the power amp.

I posted both schematics. I checked voltages on the ICs after the return and they seemed about right (although I don't have them handy this moment). Broken trace is a distinct possibility- would that make sense with the loudness at power on and then drop off?

[g1]

Check DC voltages at pins 1,3,6, and 8 for V2 and V3, post results.

[cooldude666]

Check DC voltages at pins 1,3,6, and 8 for V2 and V3, post results.

V1
1 193.6
2 1.3 mv
3 1.5
6 347.9
7 194
8 194

V2
1 244.1
2 4.7mV
3 2.26
6 211.5
7 1.6mv
8 1.6

V3
1 303.7
2 27.85
3 45.4
6 298.8
7 29.38
8 45.3

[cooldude666]

194 volts on the cathode of V1A seems odd to me. Is this right??

[g1]

Yes, those voltages are ok for V1 as it is a cathode follower. Also, you said the preamp out (send) works fine so V1 must be ok.
Your V2 and V3 voltages also seem ok. Are you still seeing the same idle current at the power tubes?
Aside from that, you will probably have to signal trace to find out where the signal is getting weak. I usually look at the 5201 IC's as I have seen many failures, in this case IC8 would be suspect.

[cooldude666]

ahh ok. isn't that a little close to the max heater-cathode voltage for a 12ax7? do i need a scope to signal trace?

[Lee_ranaldo]

check this:

If you took the loop/output board out to be able to adjust the bias (for any reason). make sure the wires underneath didn't break loose and got disconnected. Make sure you drain the caps before this happens.

This amp holds some voltage (even on stand by mode) on the filter caps so turning it on without the standby-on can hold a few hundred volts. this avoids wearing the rectifier and filter caps out due to the inrush current .

Re.check every wire (yellow,blue, green and others) that come from the board to the sockets. They get cold joints many times and might create problems that are usually fixed by "wiggling" the amp a bit.

Check that pins1-8 from the bases that hold the power tubes are all tied to ground.

Make sure you have bias voltage on every tube (pin 5). if any of those voltages seems odd or "too low" just resolder the resistor, if not, just replace it. The JCM900 has only 2 grid stopper resistors.

Make sure you get around 420 volts (voltages may vary) on PIN 3 of each power tube socket
around the same voltage for the grids (PIN4).

If you ruled out that is not a power amp issue (feed the jcm900 from another preamp that you know it works normally).

Then its a IC issue. Its common when IC's "DIE" that they just short themselves and you need to "KICK" them with a HIGH input level
to "wake them up" somehow. The amp will work "normall" after that point. Of course is not healthy for the amp either

If the pre-amp works fine when its connected to another power amp there might be the LM1458 IC that drives the signal coming from the "RETURN"
of the amp . In some newer JCM900's this IC is a TL072. Any of them will work.

Recheck that every op-amp is receiving a simetric voltage since the zener based +/-15 volts regulator tends go drift and deliver uneven voltages. Replace those diodes
with sturdier ones and also the caps. Your amp will be happy , specially those Matsushita Ic's (M5201) that are discontinued and quite expensive on ebay.

[cooldude666]

So I think I've traced this problem to the phase inverter. Everything sounds fine up until the PI. Signal is strong coming into the grid of the non-inverting side from the preamp, but is pretty weak on the grid of the inverting side (via a signal tracer run to another amp). Off of the plates the signal is weak for both of them. Is next step to test resistances of the PI's resistors and make sure it's functioning? A tube swap for good measure would probably be a good idea although I've already tried that once. Should I try and read signal off of the cathode?

[cooldude666]

AH. Actually here's what's almost definitely the problem. heaters are sitting at 3.5vac.

[cooldude666]

No, heaters are fine. Measuring to pin 9 on v1 gives me 6.8vac. Back to suspecting the PI?