Please help me troubleshoot my Roland JC-160!

[zunky]

Hey guys,

This is my first post and it seems that this forum is full of very knowledgable people, so I was hoping someone might be able to help me.

I'm trying to troubleshoot a 70's JC-160. Hasn't been turned on for many years and is making a strange loud noise when I turn it on. I managed to play through it for a few minutes and it sounded okay (however, a little distorted) although it's no longer playable at the moment.

I'm confident with my soldering and have built various pedals and a simple valve amp in the past. Also, I have a multimeter, although don't really know much on the theory side and not really sure where to start, but I would love to learn so thought this would be a great opportunity.

The only thing I've done so far is purchased some DeoxIt and cleaned all of the pots and jacks, which didn't seem to help much. Also, prodded around a bit with a wooden object, although nothing feels to be loose.

Your help is greatly appreciated!

Video of the issue: YouTube - Help With Fault Diagnosis Roland JC-160

[R.G.]

I'm trying to troubleshoot a 70's JC-160. Hasn't been turned on for many years and is making a strange loud noise when I turn it on. I managed to play through it for a few minutes and it sounded okay (however, a little distorted) although it's no longer playable at the moment.

A couple of things come to mind.

First, find a schematic of the amp to work from.
Second, any time something contains the words and phrases "amp" and "many years", especially if it also contains "hasn't been used" or "hasn't been turned on", it's fair to suspect the electrolytic caps. All of them.

While it is possible to find and fix one or two bad ones, the next-most-bad ones will fail next month or next year. So if it were mine, I'd make a list and replace every single electrolytic cap, full stop. Only then can you rely on what you find not to be failing as you go along. It may be a complete fix. While you're at it, it is often a fix for old stuff to remelt every single solder joint with a touch of fresh rosin core solder.

Third, if replacing all the electros and getting the polarities and voltage ratings right doesn't do it, start with the power supply. It's not going to work right until the power supply is operating correctly.

After that, my choice is to measure the DC voltages on the semiconductor pins and compare those against the schematic to see if they make sense. If they don't, fix the sections that don't match. It gets more detailed there. But the cap and solder shotguns usually fix anything that was working when last turned on.

[zunky]

Wow, thanks so much for your detailed reply! After reading your post, I've taken a good look at the electolytic caps and realised that many of them have a hard brown substance underneath them, although I'm not sure if this is a leaky cap or glue?! I've been told if it's solid it's most likely glue? None seem to be leaking from the top, although I have noticed there is a distinct smell near the circuit boards.

I've found a schematic.

Thanks so much for your time!

[R.G.]

A lot of troubleshooting an old amp is finding a stable base to work from. I always start with power supplies, and also the things that I know degrade just sitting there - the electro caps.

The hard brown stuff could be glue. Could be hardened electrolyte stuff too. Hard to say. It's far simpler to remove the question. Get out your digital camera and take some pictures as you go so if you need to, you can refer to how it used to be. This is especially needful where the polarity marking on the PCB for how to insert the polarized caps may be missing or covered up. Notice how they used to be.

Then make notes of the values and voltages of the electro caps, and order them off. Get the same or next-larger capacitance value in microfarads (uF) and the same or next-larger voltage rating, and where space is tight, compare the old cap's measured size to the size listed in the catalog. I recommend Mouser Electronics for good stock and fast service.

Most cap makers only guarantee a shelf life of 5 years on their caps. That gives some idea of how long they think their product is good for when not being used.

Work slowly, be careful not to flex and break off wires when replacing parts. Also, scrape off and clean off the hardened brown goo. It may be conductive. Isopropyl alcohol or acetone will probably work. Acetone may remove lettering from the PCB as a side effect, so notice markings before you swipe it away accidentally. If you have to remove power transistors from heat sinks in this process, take very careful note of how they are mounted and insulated from the heat sinks, so you can put them back that way.

[zunky]

Thanks R.G., you have been extremely helpful and I really appreciate it a lot . I'll try what you have suggested and post back results once complete.

George