Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 29, 2024, 03:54:50 AM

Login with username, password and session length

Recent Posts

 

roland jc120 weird woes

Started by ilyaa, February 13, 2014, 03:21:05 AM

Previous topic - Next topic

ilyaa

okay ive had this little combo for a bit - bought it cheap and knew it had problems - figured id get around to them sometime and here i am!

but a frustrating evening, after all.....something seems to be wrong with my scope - the screen keeps flickering and spiking even with nothing plugged in....i dont have the patience to troubleshoot my troubleshooting equipment!!! argh

anyway, heres whats up with the amp:

works great EXCEPT

1) if bright switch is NOT engaged, there is a significant loss of volume at louder levels
2) distortion circuit does not work at louder volumes either. there is signal and it's distorted, but i cant turn it up without it getting all quiet and congested

now i was ready to start figuring these things out on my own when i was foiled by

1) my scope crapping out and
2) i just cant seem to find the right schematic for this amp!!

there are a few floating around the internet - one for a 1982 jc120, one for a 1979 jc120, and one for a 1984. i think mine is like 1981 - its made in japan with a s/n something like 141xxx (i dont remember the last three now).....the 1982 schematic seems close but the part of the amp i started focusing in on (around the bright switch) does not match up......

in my amp, the bright switch either connects or disconnects the treble pot wiper to the volume pot wiper via a 22k resistor and a smallish cap. something like that.

at this stage, because i havent done any real testing, i wont ask for any help other than advice on the whole schematic issue.......are there other ones floating around?? i looked and looked but came up empty-handed. im not really confident enough with this stuff to get into it without a schematic to help me out....

(side note: taking some measurements (not carefully) i fried a non-component part of the board - a green trace was touching the chassis and got really hot and just blew up!! so i had to jump the blown out part with a fat wire. is there a good way to securely fix a broken trace issue like that??? im a moron......)

teemuk

#1
Quoteat this stage, because i havent done any real testing, i wont ask for any help other than advice on the whole schematic issue.......are there other ones floating around??

Yes.

If you search persistently enough you will probably find about eight to ten different schematics for different revisions of the JC-120. After circa 1982 Roland made less and less changes to the design and because of that JC-120 amps made post 1982 have a lot in common with each other. Conversely, JC-120 amps made before ca. 1982 can be almost entirely different from one another depending on their circuit revision.

Do note that at least one of the JC-120 schematics most commonly found is a schematic for the JC-120H, which is the head only version. For the combo amps this circuit diagram is entirely useless as no other JC-120 bears even slightest similarity to it. Fortunately the diagram is easy to distinct, not only from the year (1984) but also from overall features it shows the amp having as a combo JC-120 commonly lacks most of them. (e.g. the head is a single-channel circuit and has a "hi-treble" control).

So yes, do make sure you get the right service data. Wrong ones in this case are often totally useless and even misleading.

Roly

Quote from: ilyaai dont have the patience to troubleshoot my troubleshooting equipment!!! argh

First you fix your test gear.  Would you try and fix an amp using a DMM with a flat battery?  Or with a bucket over your head?

What's the make and model of your CRO?

This might help.
http://www.ozvalveamps.org/testgear.htm#cros
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

its a phillips pm3265

ive looked and looked and i cant find the right schematic! anyone here have it by chance? post-79 pre-82 jc120??? i even called roland but they gave me the cold shoulder because im not an authorized service center.....

teemuk


ilyaa

you sure that file still exists?

i tried to download it and my computer read it as 0 bytes....

Roly

Quote from: ilyaa on February 13, 2014, 04:10:33 PM
you sure that file still exists?

i tried to download it and my computer read it as 0 bytes....

Same result; site reports that I have "exceeded my d/l limit" before I've had a byte.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

#7
found the file here:

http://www.epanorama.net/schematicsforfree/Audio/Musical/Products/Roland/index.php

!!

gotta figure out if one of these schems applies but looks like it will!

okay its the one labeled SM_JC-120_11016! woo!

in the meantime (between the last activity on this thread and now) i opened it up, resoldered some of the pots that were wriggling a bit and did a bit of cleaning, and the bright switch problem is gone.

now the thing id like to address is the distortion issue. i played someone else's jc-120 (very similar edition to mine) the other day and seemed like it maybe had the same problem....basically the distortion circuit seems wonky. if i play without the amp's distortion circuit, i can get a very clean, loud tone and even with a big muff or something crazy plugged in the amp volume and tone and density, etc stays good. BUT if i use the distortion circuit in the amp, the distortion kicks in and brings with it a kind of congestion and sort of warbliness and weakness to the signal. it doesnt stay as loud and definitely does the kind of thing where the louder i try to play the more the signal seems to get squelched.

should i do a sound clip? maybe i will when i get a chance.

any experience with that kind of thing? ill get into the board soon, too (still gotta figure out my o-scope issue....)

ilyaa

heres a sound clip (attached)

its a little hard to convey the real issue - the distortion IS distorted, but accompanied by a significant loss of volume and low end. the congestion factor is hard to distinguish in this sound clip, but some words to describe it are boxiness, fartiness, over-saturation, warbliness, you get the idea.

any idea where i should start troubleshooting it? what parts of the distortion circuit are likely to blame? anyone heard this kind of thing before? like i said, i played a similar era jc-120 the other day and it sounded like it had the same issue, more or less....does this amp just have a crappy distortion circuit??


J M Fahey

Yes, simple as that.
Use an external distortion pedal.
Even a simple MXR Distortion+  is miles ahead.

DrGonz78

Yeah I too agree that the distortion on those chorus amps is less than desirable. As you turn up the distortion it gets really noisy and the signal does not push through the white noise sound. I am pretty sure people love those amps for the rich clean chorus channel sound and not the distortion.
"A person who never made a mistake never tried anything new." -Albert Einstein

ilyaa

well i guess thats a good and bad solution!

good in that ive got no fixing to do and bad in that i was hoping id be able to get something out of this 'feature'

oh, well. thanks guys!! at least i found the right schematic for future use if i need it.

phatt

Quote from: ilyaa on February 20, 2014, 01:32:36 AM
well i guess thats a good and bad solution!

good in that ive got no fixing to do and bad in that i was hoping id be able to get something out of this 'feature'

oh, well. thanks guys!! at least i found the right schematic for future use if i need it.

Yes sadly that would be the case, wise to not use that part. :-X
Frankly the worst distortion of amps from that era that I ever used. :loco
Though it might work for a few old songs like "Spirit in the sky". :lmao:

The stereo chorus was hip at the time but really only of novelty value nowadays.
Phil