Roland JC120 - Crackling and Noise - Page 1
Forum section: electronics / repair

author: Sigl-Michi
author_rank: Newbie
timestamp: January 18, 2021, 12:14:22 pm
content: hei,please listen:https://youtu.be/qXvF81qqCsQMaybe someone can help, and can give me a hint where to look, when he hears this crackling and noise.The Roland "Jazz Charger", is functional:- when plug in a guitar on booth Amp-Stages i get signal, all Potentiometers are Working - and they do not change any state of the Crackling.The Crackling is only on the right speaker, what could indicate, some problem with the "CHorus-circuit" - since one fact of this circuit is, that one channel let be clean, and the other "chorused" with a rare MC3002 in its heart...When turn of the Chorus circuit, the noise is still here, i believe the crackling is less - but not away.I already changed all Microchips except the MC3002, all germanium diodes with orginal types, and the transistor orginal Types i had (30sk Y fats...), i also changed the Powercaps of the PSU, and all Elkos from the Chorus-Circuit.I checked the cables per View and hit on the cables with a screwdriver, that changes nothing.I have the shematics for it, and i must say its a difficult circuit (that chorus).also there are many different variants of the shematic, but i found the right one:www.siglgut.at/temp/JC.pdf

author: rectifier
author_rank: Newbie
timestamp: February 16, 2021, 11:26:18 pm
content: Hi, I watched your YouTube video and commented.The fault sounds like a leaking capacitor to me. In the signal path. Bursts of electrons crossing a dielectric.But it may not be that. It could be a failing semiconductor or even a dry joint, but from the sound I'd say those are less likely. These are educated guesses.Your random approach, replacing components you happened to have in stock, will rarely fix anything, especially something with this complexity and number of components. I assume the replacements you have made are all exactly correct.I would reattach the loose pcb and ensure everything that should be is tight again. I put gaffer tape over exposed mains connections, fuseholders etc, use an inline-bulb mains dynamic mains fuse, and never put both hands inside a live chassis at once.Identify which schematic version in the service manual matches your amp's serial number, disconnect the right speaker at the PCB end and plug an 8 ohm dummy load into the external speaker socket for that channel (is 'right channel'= 'channel 2'?).Then I would employ a signal tracer (small audio amp with dc-isolated probe and speaker) and the half-split method to locate the faulty component. You will also need the component layout diagram for this. You may find that gently warming the suspect circuitry with a hairdryer will make the fault show up more of the time, which helps when fault-finding intermittent faults. Intermittent faults are the hardest to find, even using logic.When you have found the two or three components where the fault originates, use freezer spray to identify the component that, when cooled, stops the problem noise (or sometimes, makes it worse). If you're lucky this will work; I've done it a few times. The hairdryer can be used to warm the suspect component area to repeat the cooling diagnosis.Alternatively/additionally use d.c. tests and an oscilloscope to zero in on the culprit.Replace it.I hope this all makes sense. I have repaired between 3 and 4,000 amplifiers and other audio electronic circuits over 14 years, and currently have three vintage machines on two workbenches with similar faults.BTW the Service Manual has an error on p1. "Rated output power 22mV RMS/channel" should probably read "22V RMS per channel", although audio amplifier power is really measured in watts RMS. But service manuals often have typos in them. It's a lot of info to get right.

author: fzabkar
author_rank: Super Contributor
timestamp: February 17, 2021, 02:56:53 am
content: I would try this, but be careful.Open the circuit at C23 and C123. That's the decoupling capacitor at the input of the Ch1 and Ch2 power amps. Run a wire from R28 to C123, and R128 to C23.Locate the wire or trace that connects the SP output of Ch1 to the input of the Effect Board. Move this wire from SP/Ch1 to SP/Ch2 .This procedure has the effect of swapping the power output stages between the two preamps, without disturbing any feedback loops. You can then determine whether the crackling moves to the other channel. This should tell you whether the problem is before or after the power stage.I would wait for confirmation from other forum members before trying this approach.ISTM that if SW2a and SW2c are in position #2, then IC1 (MN-3002) is switched out of circuit.