Marshall JCM900 Problem

[roknroll]

I have a JCM900 4100 on the bench. It only puts out 120 cycle hum. When a signal is injected to the return jack of the effects loop I get signal that can be controlled by the Master volumes A or B depending on the push switch. I checked and swapped out all the tubes, checked all pinouts. Checked all major voltages on pream, power, electrolytics in the power string, etc. All voltages were good. Any ideas would be appreciated. Did the chopstick test.
Jack

[Structo]

I suppose you visually checked it all out looking for burnt components and all?

Here are some troubleshooting steps that may help.

http://www.geofex.com/ampdbug/ampdebug.htm

Chopstick around the amp to see if there is a bad solder joint or ground.

There may be a problem that occurs with this amp that some of the techs might be familiar with.
Hopefully somebody that is familiar with this amp will chime in.

[Jana]

I have a JCM900 4100 on the bench.

I'm sorry.


Print off a schematic for it, inject a signal in the input and start probing with a scope and see where it dies. If you can send a signal into the effects return and get a signal that is telling you that it is isolated to the preamp before that part of the circuit.

[Voodoo_Man]

I have a JCM900 4100 on the bench.

I'm sorry.

You're not pokin' fun at the JCM900 line, are you??? Didn't some /or all use op-amp buffers in the effects loop? And why do some people make fun of them. Sure there are better models to buy. But not everyone can afford them. I own a 50 watt JCM900 SL-X and won't ever sell it. JCM900's are the last Marshalls with cap cans and we all know how cool they look stickin' out of the chassis. They are also built better than those new vintage moden thingys IMO. Try working on one of those.

God Bless,
John

[collinsamps]

I'll take a benched 900 over a DSL anything.

You've already isolated the problem to the preamp area between the input and send jack for the effects loop. Just inject a signal(guitar works fine) and find where it's dropping with a scope, meter or listening amp.

[Voodoo_Man]

Have you tried jumpering the effects loop? Just a guitar cable from the send to return.

Good luck & God bless,
John

[roknroll]

Thanks for the repllies. Jumpering the jacks of the effects loop doesn't work, however I did find a dead end to the signal path entering IC5. When I jump out the IC the amp works, so I will order one and see what happens.
Really I have no idea why people like these amps. The distortion is so un-natural and solid state sounding worse than a bad distortion pedal.
Thanks again.

[Wayne]

...Really I have no idea why people like these amps. The distortion is so un-natural and solid state sounding worse than a bad distortion pedal.
Thanks again.

If you bias them hot enough and turn the hot channel up all the way, power amp distortion nearly takes over that annoying preamp buzz. I said nearly.

The clean is great, if you like a Marshall clean. Yes, I know it involves soft diode clipping, but I like it anyway.

Don't get me wrong - I'm not defending it in any way, but in 10+ years of 900 ownership, those seem to be their only redeeming features.

Good luck with your repair, and please let me know how much trouble you have sourcing those switching op amps - I've never tried, but may need to some day.

W

[Voodoo_Man]

however I did find a dead end to the signal path entering IC5.

I recomend getting a dip socket also.

God Bless,
John

[roknroll]

Actually a dip socket isn't a bad idea.
Here is the pdf file link
http://www.st.com/stonline/books/pdf/docs/2172.pdf
and the mouser cataloge
http://www.mouser.com/catalog/637/360.pdf
the one ending in 1N or ln is the one
the one or "l" just represents the temperature range and package style.

[Voodoo_Man]

Actually a dip socket isn't a bad idea.
Here is the pdf file link

I like them because..........................no heat gets applied to the op-amp, and they are secure. They don't cost much either.

God Bless,
John