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Messages - ilyaa

#301
Amplifier Discussion / Re: roland jc120 weird woes
February 19, 2014, 09:16:43 PM
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??

#302
Amplifier Discussion / Re: roland jc120 weird woes
February 19, 2014, 12:06:01 PM
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....)
#303
Amplifier Discussion / Re: roland jc120 weird woes
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....
#304
Amplifier Discussion / Re: roland jc120 weird woes
February 13, 2014, 01:39:06 PM
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.....
#305
Amplifier Discussion / roland jc120 weird woes
February 13, 2014, 03:21:05 AM
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......)
#306
moving forward with the amp working, should i be considered that the negative voltage out of the main power supply is a couple volts off? by the time it hits the preamp the + and - Vs are leveled out, but i can still see a bigger ripple and larger negative voltage on the main rails. not huge, just a volt or two -

should i just use the amp and see if things stay normal or try and see if the power supply needs some fine tuning?
#307
Alright!!

Another victory (let's hope)!!

With the preamp shorted out via the mains in, I could still hear the amp making kind of a weird sound with no signal or anything going into it - perhaps a quieter version of the THUMP??

i scoped the output and could see tiny little hiccups when the sound happened. i scoped the preamp out and saw HUGE spikes - 10+ volts - that coincided exactly with the sound. briefly unplugged the shorting lead and sure enough it was my THUMP.

so the problem was (yes, i said was!) definitely in the preamp.

i had been suspecting the weird too high negative voltage for some time, so i thought id start with the power rails going into the preamp. sure enough, right away i saw something funny on the negative rails. if i checked one side of the 330 ohm, 10 w power resistor (the first thing the power coming from the supply sees), id get one voltage, then id check the other side and get something way lower, but not consistent and not around -16 V (what i should be getting). checking across the resistor id get zero volts....i figured thered have to be at least SOME drop across the resistor...i checked and it didnt seem shorted....what the hay????

THEN all of a sudden i realized, while prodding it, that it was loose!!! and as i prodded it, i heard the amp making a little mini-thump. i disconnected the shorting lead from the f/x loop and found i could reproduce the thump exactly by wiggling that little chalky mofo.

i remembered how roly had mentioned that the wrong voltage seemed to suggest there was a loading issue somewhere and here it was! the loose resistor was cutting out V-. it wasnt so loose as to be predictably disconnected, but once i wiggled it a bit it became clear.

just a dash of solder later and i think the problem is fixed!!!

im now getting +38-39 V and -39-41 V on the main rails, which seems much better, right?

the power resistor feeding the preamp DO seem to get kind of hot kind of quick, though...think that's a bad sign? they are big 10 watters but still, they are hot to the touch....could this issue have fried the zobel resistor? should i just roll with the fix for now and see what happens??

either way, im stoked!! thanks for your help, everyone!
#308
alright:

if i short out the loop contacts altogether with a known good lead - still thumps
if i short out the preamp - no thumping

the thumping is unpredictable, though, so its hard to say 100%, but i did not get a single thump with the preamp shorted out. which points to that part of the amp as the culprit, probably, correct?

whats the next step - start checking voltages on the preamp?

because of the weird negative ripple coming off the power supply, and the two high negative voltage, would it make sense to start with checking any V- hookups on the preamp? and the preamp power supply, knocks the 39v down to somewhere near 16?

there are a lot of ICs around - are they likely suspects?
#309
oh wait you mean the 'effects loop'?

sorry got confused by the terminology. if i plug something to the main in it will short out the preamp? cool ill do that.

and i did clean those contacts on the outside, but not inside - ill do that, too.
#310
wait, what's the 'main in'?
#311
there is a molex-style connector between the pre- and main amp. its got 5 pins, a power ground, a V+ and V+, and a signal and signal ground (i think). should i remove the connector altogether and just fashion a cable that will allow me to send signal into the main amp via the two signal pins left on the male end of the connector?

the amp THUMPs when its just sitting there everything turned all the way down nothing being played. could i just as well disconnect the preamp and just turn the amp on and see what happens? saving me the trouble of trying to inject a signal in between pre and main amp...if it THUMPs then we've achieved the same half-splitting diagnosis, right?
#312
okay some progress, but we might need to reassess our theories.

i DONT think its an oscillation issue. looking at a larger amplitude waveform from my guitar, there is NO discernible fuzz or distortion other than what the amp naturally provides. it looks pretty clean, in other words.

the amp has been behaving strangely. the previous time i plugged it in, it played PERFECTLY without thumping once for like 15 whole minutes. cranked, too! after successfully fixing my acoustic 370 via a deep clean, i thought id do the same to this one. AFTER the clean - the boards looked alright, but there was some stuff here and there, notably on some of the interboard connectors, the power supply filter caps, and on the mainline fuse - i turned it on and right away THUMP THUMP THUMP. in fact, it was doing something weird at first where it wasnt working at all as far as passing signal. it wouldnt switch channels and i could not get it to make a sound....

after i turned it on and off that went away and it was working fine, but THUMPing.

when a THUMP occurs:
1) signal goes away but comes right back afterwards
2) the LED channel indicators flicker and switch between themselves
3) DC voltage on unused speaker out - the amp has two parallel outs - goes from between 150 and 300 mV (when not THUMping) either down to near 0 or up to as high as a volt or down to like -700 mV (during a THUMP). in other words, kind of wacky. is that an okay way to measure DC volts at that point, with the amp plugged in to a speaker and the measuring tools on the other speaker out?
4) o-scope shows what you'd expect, the waveform flickers and goes nuts for a split second.

the THUMPs seem to be totally random. sometimes itll be just one in a long while, sometimes they will happen many one after another. so far, symptoms seem ambiguous

BUT

earlier i said that i measured the voltage on the rails and got something strange. where i shouldve seen +/- 39V, i get +39 and -46 to -52V. so somethings weird on the negative end of the power supply. i followed my nose and scoped the rails and saw something strange.

on the positive rails, the scope shows a small but very steady ripple - about 20 mV. on the negative rails, the scope shows a ripple that is twice as large, about 50 mV, but this ripple is NOT steady. every so often it totally wigs out, turning into some crazy zig-zaggy angular thing, sometimes for a few seconds at a time - it gets distorted and spikes and does all kinds of stuff. it returns to the normal ripple waveform, but this process is repeated and predictable.

so something is up with the negative power rails. now, unfortunately, if i have the speaker plugged in and watch the scope on the ripple, the ripple distortion/spikes do not necessarily correspond to the THUMPS. only sometimes. they happen much more consistently and frequently than the THUMPs do. what are we looking at here? bad filter caps? is this a likely cause for the amp making the THUMPs? what do people think?

a side note: at one point i was trying to measure somewhere else along the power rails and touched my ground o-scope probe to a point i thought was ground (turns out it was V+) - it arced/sparked and the amp blew a fuse!! wtf. luckily, i had a replacement handy. the fuse that blew was 250V/5A. i put in a 250V/3A slow blow. are slow blows okay for solid state mains? what the f happened, anyway, to make it blow?

ALSO, sometimes when i turn the amp off, 2 volts remain on the power rails and the Red ON indicator LED will stay lit!

similarly, sometimes after a THUMP the amp will stay on channel 2 (the channel switch wont work) and it will not pass any signal. then randomly itll start to work again.

#313
yeah i think we did it! the real test will be playing it again and just keeping my ears open, but things definitely seem to be better.

thanks, guys!
#314
alright!

so i gave it a realllyyy good scrub today. got ALL of that gross, dark orange, kind of amber/sap looking stuff out of the board(s).

preliminary test before going to bed indicates that maybe the noise is gone. i dont want to jinx it, of course, but i was bashing that board with a drumstick and it was silent. and the amp continues to work fine, otherwise.

so maybe it was just a gross, oxidized/corroded board, whatever that orange stuff means! ill do a thorough check tomorrow and report back, but it may have been a strictly mechanical issue all along. i really cleaned the modular connectors, too, as i think they may have been part of the problem. but, we'll see - its an intermittent issue so who knows, might come back with a vengeance tomorrow.
#315
it wont be easy to tap the signal feed - the modular connectors are soldered onto the boards. every precaution has been taken to keep any part of this amp from being removed from any other.

BUT it does have two line outputs coming straight from the preamp. would a CRO on showing the symptom point to the preamp as the culprit? guess i may as well check!