Jcm900 Issue

[loapathy]

This is my first post, so of course it's about a jcm900. Because these amps were made basically to be problematic, as far as I can tell.

Anyway, thanks in advance if anyone can help me with this.

Problem: jcm900 4100 head--when on the clean channel, A, there is no treble/presence response whatsoever, and it's woofing. However, on the high gain channel, B, everything sounds exactly how it should sound.

Plate Voltage is 474. Tubes (el43) are running at about %60, highest at 31mA.

Tried playing around with preamp tubes and no change. So the preamp tubes and the sockets are all working.

Didn't see any bulging around the caps, and can't find any burn damage anywhere visible.

I've gone at the jacks and pots with Contact Cleaner and already tried jumping the effects loop--no change.

Again, this is only channel A. Oh, and the gain knob on channel A has no effect unless you bring it all the way up, and when you do that all sound completely dies out of the channel.

I hope I don't have to take the PCB boards out because they look fragile as all hell and I've never done it before. However, this amp when running right is worth maybe $400 and the cheapest tech I've talked to quoted me $300--so if I have to take the boards out, I will. And while doing so I'll wish all amps were hand wired with turrets, ha.

Anyone have an ideas? or do I have to pull the boards out?

[Rick Erickson]

[the cheapest tech I've talked to quoted me $300--so if I have to take the boards out, I will.]

And then you will find out why the "cheapest tech" quoted you $300.00...

RE

[loapathy]

I'm not knocking techs, I understand why they get paid what they get paid. My point was that this amp is hardly worth the money to fix it.

If this amp were worthwhile it'd already be at a tech with experience with PCB stuff. But it's not.

I figure if I have to take the boards out to fix it, I'll either ruin it or learn how to fix it. That to me is worth more than $400. And if I ruin it, I'll just take the boards out, build a ptp board and put it in, and never buy another PCB based amp. Ha.

But I still really hope it's not something under the boards. I've heard the copper on the boards is incredibly fragile and the board will warp with heat the copper will flake off.

[Tubenit]

I gutted and rebuilt my friend's JCM900 into a modified Plexi 50w reverb. Sounds incredibly good now. If you'd like more details and a layout with schematic ...... leave me a PM.

http://s28.photobucket.com/albums/c2...escription.jpg

After trying different mods ........ he wanted a paralleled 5751 V1 and cathode biasing with pentode/triode switch.

With respect, Tubenit