Calling all Amp techs!!!

[Be Loveless]

My Roland Jc77 is.......distorting. If you turn it up past halfway it startsto grumble. The housing/cabinet is tight and the pots seem to be clean except maybe the volume pot. Wild guesses? TRouble shooting? Suggestions appreciated. I must stay CLEAN.

[dragonworks]

Originally Posted by Be Loveless

My Roland Jc77 is.......distorting. If you turn it up past halfway it startsto grumble. The housing/cabinet is tight and the pots seem to be clean except maybe the volume pot. Wild guesses? TRouble shooting? Suggestions appreciated. I must stay CLEAN.

Tube or transistor?
Anyway, make sure you start with what is in front of the amp.
How many channels, does it do it on all channels?
If only one channel it suggests something in that preamp section, tube, trans, resistor, cap, etc.
If both channels it suggests something past the preamp sections or
Possibly power supply related i.e. not delivering proper specs to circuits
Possible bias mismatch.
There are many possiblilities at this time.

[Be Loveless]

SOLID STATE, 1 channel, high and low input. Did a little more puttzing around. The distortion only seems to occur when using the "high gain" input. LOw input is as clean as a whistle ( makes me want to carve some soap) but with a lot less volume , or just LOW er gain. Also tried different guitars and different cables...problem still occurs.

[dragonworks]

unless you have putzzed around inside amplifiers before, it is time to have her looked at.

[Be Loveless]

Uggh..... Dont know of any reputable techs in the area (and a little strapped for cash too). What do you suspect? I've opened up the amp before ( pain in the ass to get back into the cabinet) to clean pots and such and see if anything was obviously wrong (Hey that dere wires broken, HEy that there thingies on fire ). That was when I was getting this static rumble thing happening, I cleaned the pots and that went away. I would like a few more opinions before I spend a couple hundred on an amp guy who I don't even know 50 miles away from me.

[Dani Pace]

Wgen you say "turn up past half way," do you mean the volume or the gain? On all amps turning up the gain will increase distortion, I've got an old Fender that sounds like i connected a fuzz box if i turn the gain more than half way up. Try setting the gain low and the master volume up. Back off on the EQ or tone controls too, boosting anything more than about two thirds of maximum will increase distortion. Another thing, does the amp do this with all guitars or just one? If it's just one you might turn down the volume on the guitar or possibly need to lower the pickups, if it's a problem with any guitar you need to get the amp checked.

[Be Loveless]

1volume pot no gainand I rarely boost the eq past halfway (5 out of 10) I keep the eq set pretty flat. Distorts with different guitars and different cables. Let me reiterate ROland JAzz chorus-77. Its the older version of the Jc90 only without an effects loop. Dual 45 watt amps 90 total two ten inch speakers. The JC 120 is a dual 60 watt amp 120 total ,two twelve inch speakers. This amp is supposed to be nothing but clean. The CLEANEST in fact.
Now I have never had to turn this amp past halfway but I got curious. After hearing the distortion at high volume I assumed that it probably is distorting at lower volumes also, I just can't hear it, thus affecting the overall sound quality.
Lately I've been on a quest for a huge Thundering Clean tone and trying to find every possible tonal variation that the amp is capable of without effects. Well I found one that should'nt be there.

[Be Loveless]

Okay.... I think I have found the problem, There is a "distortion" effect on the amp. I almost always have it turned off, all the way down then click..OFF!!! However I just plugged in the footwitch and..... the distortion effect does'nt seem to completely bypass/disengage unless done with the foot switch!!! Unoticeable at lower volumes but just a little buzzy when cranked. Now, could that be a problem with the "distortion" effects potentiometer? I wish I could remove this aspect of the amp entirely I have no use for it.

[dragonworks]

One thing, as you turn up volume you turn up distortion. There is no amp made that can escape this. As you approach a certain percentage of the amps power it will start to distort. One spec for this THD(Total Haromic Distortion). The lower the THD the better. You could bypass that circuit but then again it would probably take a tech. Since you seem to have found your answer it would be best to just make sure the circuit is turned off.

[Be Loveless]

Still only a temporary fix. I can get the circuit to turn off but not the way it should, the "Distortion" pot sitched in the OFF position. Yes your right it will take a tech to make OFF mean OFF. thanks for the responses folks.