Problem with my jcm 900 (almost no volume)

[generic_user]

its a 2500 series head and i have had it for about 2 years and it has always worked on and off. but now when i try to play it the volume is really weak. if i turn the volume all the way up still it feels as though it is set on 1. the high low switch on the back does nothing either. so i was hoping someone may have a guess as to what is wrong. i am capable of doing the repairs myself but im not sure what is wrong.
thanks in advance

[Amp Kat]

I'm sure you have replaced the tubes right ? I would isolate the problem from preamp to power amp. Use the send jack and route that to another power amp in or the front of another amp or return. If it works using the preamp gain control to vary volume then it is in the power section from the return to the output. Also try patching a cable from the send to the return jack and see if it comes back. A scope would help hugely but you may be able to get by with an AC meter.

[generic_user]

when i patch the send to the return i get the same amount of volume. i did however patch the send to the in of another PA power amp i have and the gain control varied the volume level but it sounded absolutely horrible. is that normal. if what i did was right the problem should be in the power section right?

[StratTone]

I had this problem one time that had me going in circles for a MONTH!! lol Ended up being the resistor across the V1 preamp tube. I can't remember the designator but give that a check and see if it is open.

[Amp Kat]

What are you saying as horrible ? It should be clear and loud if playing thru the clean channel. If you have it set up for clean and it's distorted it's possible you have the gain on the input PA to high. Some PA's have a preamp some don't. If your PA doesn't have a preamp and it still sounds distorted it's possible your overdriving the input to much with the 900. If it still sounds distorted running clean to the PA in you most likely have a problem in the preamp and as stated above could very well be a preamp tube. Most low volume problems however,are usually power amp related but not etched in stone as I've seen both. If the send out gets loud with the PA it is most likely a power amp problem and we'll need to start checking some voltages and do some resistance checks if your up to it.

[generic_user]

yea i would be up to checking this out. but yea i was in a rush because i had to go to work and i had the distortion on that probly why it sounded horrible.

[nico.verduin]

A familiar problem with JCM900's is that the Effectloop jacks get dirty. If you plug in a guitar cable from the send to return and the problem is gone than there is your problem. A little contactspray in the jacks, putting the plugs in and out a couple of times should do the problem.
Nico

[generic_user]

sorry but i have already tried that the problem seems to lie somewhere else

[nico.verduin]

Do you have a scope? You could trace the signal from tube to tube. Are all the powertubes operating (not just glowing?).

[StratTone]

Did you check that resistor by any chance?

[generic_user]

a new discovery i found when looking under the board, it seems that the bridge rectifier between V3 and V4 looked pretty scarred. it looks like something i should deffinitely replace. would this be the cause of my low volume. if i do replace this i wonder what caused it and will it happen again.
thanks

[catstrat]

Jcm 900

Hi everyone. I'm a new guy to this site so please be gentle with me. I own an amp repair business here in Pensacola Florida and i have a JCM 900 higain duel reverb that's doing about the same thing.Earlier it came to me for no signal at all and i found r35 was so badly burned that it damaged the pc board tracings. I replaced it with a 100 ohm 10 watt resistor that i remotely mounted to the chassis. That cured that problem. But it's back again for another problem. Low distorted output. My scope shows a good signal into the pi tube and a good signal into the output tube grids. The tubes are about 6 months old. I substituted test tubes at the output with no change.My dc voltages are good. What kind of signal voltage should i get into the o/p tube grids? I'm always unclear about that. The other thing at the o/p tube plates i have some strange sine wave i can't identify. At the base line it has the typical upward slope but at the positive peak as it starts its downward slope it has a squiggle and as it reaches the base line it only goes a small amount past the baseline and as it moves back up it has the squiggle again. That tells me the negative part of the signal is disproportionate to the positive half of the signal.I admit, my wave identification skills are not all that good. But i've never seen this waveform before. Since it seems i always get problems no other techs ever get i need some advice. Thanks

[Mars Amp Repair]

Hello catstrat & welcome!
I'm sorry that have no time right now to reply, but FYI you'll get better exposure & response if start a new thread as opposed to replying to an exisiting one.

glen

[Enzo]

Yeah, hi and welcome, but start a new thread on this insteaqd of at the end of a thread that has been dead for over a year. And we'll be wanting to know the model number of the JCM900.