JCM900 4100 Problem

[dan2002]

I have a JCM900 4100 head that has been giving me trouble lately. I took the amp in awhile back to have a problem with an IC fixed, and the tech didn't have the MC1458 IC5 required so he substituted it with an RC4558 in the meantime, until I was able to get an exact replacement. It worked for awhile and I finally ordered the correct chip and I went and soldered in a DIP socket in case anything happens again. Since then the amp has been giving me nothing but crackling out of the cab. I took it apart and noticed firstly, that IC8, R33, and R34(pre amp) were running hot to the touch, to the extent that I smelled something burning. I tested the voltages going into R33 and 34 and they read 26 and -26 respectively. I replaced the 15V Zeners D5 and 6 I believe and the voltages to the IC's all give me +15 and -15 but there still is a loud cracking and the R33 and R34s are still running really hot. Any ideas, I dont really know how to approach this from here.

[52 Bill]

...noticed firstly, that IC8, R33, and R34(pre amp) were running hot to the touch...

I don't have a copy of the schematic for your amp, so I can't refer to your part numbers.

Any time a chip get hot, that's usually a sign of a problem. If R33 and R34 are part of the power supply, getting hot may be normal.

If IC8 is an opamp, read the voltages on the output pins. If this is in the signal path you should find nearly no voltage there.

If all of this started since you replaced the reverb chip, double check your work and see if there are any solder bridges on the pc board, bad connections, etc.

[Jazz P Bass]

JCM900 4100

The amp "worked for a while" & then you added a socket.
Make sure the socket is soldered properly.
No bridges. No opens.
Is the IC inserted correctly (ie: not backwards)?

[bluesfreak]

I used this schematic as reference: Dr Tube 4100 PreAmp and apparently something draws too much current from the +/- 15V Bus. Have you checked if all the ICs are ok and correct voltage is supplied on Pin 1/8?
The 900 is known to have an underrated bridge rectifier BR3 used in the PreAmp Tube Heating which can cause the burned smell that you already noticed. I usually replace this rectifier with a 10A chassis mounted version and connect via wires. Using the chassis as a heat sind you can extend the lifetime of the rectifier drastically.

[dan2002]

Thanks for the replies guys. The voltages going into the ICs all check out, but I have ordered all new ICs for the amp, they're relatively cheap and it'll be one more thing I can cross off the list. IC8 no longer gets hot, but R33 and R34 both still get really hot, when I touch them it literally burns me. I also tested pin 5 on the preamp tubes, and its reading -40V. I've gone through and haven't found any bad connections or double solders, but I know these PCB's can be tricky. Oh and I believe its only the SLX's that have the BR3 in there, I couldn't find it on any of my schematics and don't remember seeing it on my board.

[g1]

Those resistors will run hot. Are they discoloring or burning up? If not it may be normal. There are also fuses for the low voltage supply which I think would blow if there was a problem.
You should have 6.3VAC at the heaters, from pin 9 to pin 4, and pin 9 to pin 5.
Do you have signal with crackling or no signal just crackling?
If you put a signal into the FX return jack do you get sound? Crackling?

[dan2002]

Pins 9 to 5 and 4 of the power tubes? And I plugged my iPod into the return jack and its sketchy, low volume static, but its there, and on both channels. The EQ and volume controls do nothing. Also only the bottom 2 of the 4 clipping LEDs light up, and when they light up they light up really bright.


-Dan

[g1]

Sorry, you said earlier -40 on pin 5 of the preamp tubes. Using the fx return bypasses most of the preamp so the eq and channel volumes should have no effect on anything.
Can you run the fx send into some other power amp? If we can isolate the problem to before or after the fx loop it will make it easier.

[dan2002]

Sorry, you said earlier -40 on pin 5 of the preamp tubes. Using the fx return bypasses most of the preamp so the eq and channel volumes should have no effect on anything.
Can you run the fx send into some other power amp? If we can isolate the problem to before or after the fx loop it will make it easier.

I'll be trying this tomorrow, but are there any specific areas of the power amp I should be looking at?

[g1]

Maybe the 5201 switching IC for the master volumes. You could try swapping it with the one for the reverb. (assuming it doesn't feel hot when running, if it's hotter than the other 5201's just replace it)

[dan2002]

Little update, I plugged my ipod into the input jack and the signal came out of the FX send jack just fine. I plugged the ipod into my floor pod amp sim, and plugged that into the FX return and I got no signal out of the cabinet.

[bluesfreak]

That would put the focus on the lower part of the schematic, speaking V2, Reverb Circuit (any difference if Reverb is turned on/off?) and especially IC8....

[52 Bill]

Yes and IC5 is still part of the equation.

If you shake the reverb tank can you hear the springs rattle in the speaker?
If yes, double check V2, IC5 and wiring, if no then V2 and IC8.

[dan2002]

I replaced IC5 and IC8 with some spares that I had, but I'm not sure they are good ICs. Most of the crackling and popping stopped after that, but there is still no signal coming out of the amplifier to the cabinet.

[dan2002]

I dont know if this is useful information or not, but I have noticed when I flip the standby switch and power switches off the last couple of seconds before the sound fades out the FX line in gets louder and clearer.