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Author Topic: New here - JC-120 #492020 - chasing down info  (Read 769 times)

JB074

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New here - JC-120 #492020 - chasing down info
« on: December 05, 2017, 12:44:30 AM »

Hi,
New here.
I googled JC-120 and ended up on an epic thread, had to join.

It was a bit of a two-pronged attack - I was chasing a JC-120 and saw the thread, which made me feel a little better about troubleshooting.

I subsequently found a JC-120 and picked it up today, for  $150. It has a few issues, which I'm looking to sort out.

Namely, the effects aren't working, but the amp is OK.  Yet to strip it down and look inside, but I'm guessing new caps and possibly a coule of diodes/transistors. - maybe an FET/BJT or something - I will have to find the correct schematic first, but let's see how we go!

Wish me luck. :tu:
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Jazz P Bass

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Re: New here - JC-120 #492020 - chasing down info
« Reply #1 on: December 05, 2017, 12:56:52 AM »

Good Luck!
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DrGonz78

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Re: New here - JC-120 #492020 - chasing down info
« Reply #2 on: December 05, 2017, 02:40:09 PM »

Welcome to the site!! What is your experience level troubleshooting faults in amplifiers? Simply we don't want to diagnose the amp with ideas in our head while trying to replace parts on a whim to prove such ideas. Instead we want to actually locate the fault and fix it by using troubleshooting techniques that will prove to be useful if you decide to play along. First tell us about your abilities with working on amps and what tools you have to get the job done.

Edit: Removed dead link...
« Last Edit: December 08, 2017, 09:31:19 PM by DrGonz78 »
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“A person who never made a mistake never tried anything new.” -Albert Einstein

JB074

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Re: New here - JC-120 #492020 - chasing down info
« Reply #3 on: December 07, 2017, 05:02:32 PM »

I have a few years under my belt building and repairing hifi/tv so I should be OK. :dbtu:

I got the amp on the bench for only a few minutes yesterday. both channels ok. some minor hiss - more than I'd like, but not too bad. no 50Hz ac hum though.

Got the factory service manual from Roland emailed to me, parts guy informed me of xmas closures and next shipment from japan not arriving till mid Jan, so I have plenty of time to let this sit on the bench over xmas.

No distortion on 2nd channel, we have vibrato, but no chorus.  I haven't put a scope on it yet. It needs a good clean. footpedal jacks are in need of a good clean, pots are all in very good nick.

So I have a few places to start looking.  Effects board is early ET-7 052-190 rev0 (s)


Someone has been inside it before though - the pcb screws show signs of over-enthusiastic 'mechanic' work.
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g1

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Re: New here - JC-120 #492020 - chasing down info
« Reply #4 on: December 08, 2017, 12:25:09 PM »


Schematic linked here...
https://elektrotanya.com/roland_jc-120_jc-160.pdf/download.html
Doc that link does not find anything.  I read about other Roland/Boss manuals being removed from elektrotanya so there must be something going on over there.
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DrGonz78

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Re: New here - JC-120 #492020 - chasing down info
« Reply #5 on: December 08, 2017, 09:32:48 PM »

Here try this instead... I know OP got a copy but here is one that everyone can follow along with.
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“A person who never made a mistake never tried anything new.” -Albert Einstein

g1

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Re: New here - JC-120 #492020 - chasing down info
« Reply #6 on: December 09, 2017, 02:06:47 PM »

First thing would be to verify the dist. and chorus switches with a meter.
If you've been through "the ultimate JC-120 thread" here, you probably already know that if the vibrato works, most of the chorus circuit is also working.
So that part is good news.
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DrGonz78

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Re: New here - JC-120 #492020 - chasing down info
« Reply #7 on: December 10, 2017, 02:12:27 PM »

And obviously you read that the depth and speed (rate) have no effect on the chorus, which is in a fixed state. The vibrato uses the depth and rate as it is adjustable.
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“A person who never made a mistake never tried anything new.” -Albert Einstein

JB074

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Re: New here - JC-120 #492020 - chasing down info
« Reply #8 on: December 17, 2017, 02:19:28 AM »

Just a prelim update.
pulled the box down yesterday, took tray out, pulled drivers, cleaned the entire unit, there's 35 years of dust in there. was pretty abysmal.
Luckily the cones were in good condition, the centre caps have taken a knock at some stage, but I was able to pull it back with vacuum. polished them up and re-lacquered them. Applied same to cabinet insides, and the edge trim/studs. Unit looks almost new except for a tiny nick in the grille cloth.
then general dust removal, chassis cleaning, etc etc.

Put the tray on the bench, started visual inspection. Then we had a massive thunderstorm, complete with lightshow, so the bench was vacated until the house ceased to become a lightning conductor. Lots of fun.

Other half comes home from shopping, my 'free' time was subsequently schottky'd to her xmas planning requirements.  :grr

Will let you know how it goes DrGonz... When I get my 'free' time back...  :lmao:
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