Roland Jazz Chorus JC-77 Hiss

[blvd]

Hi there,
Got a good deal on ebay for an old JC-77. Its got a few problems...mostly, it would seem, from sitting around accumulating cobwebs and getting pretty dusty.

There is also a substantial amount of hiss, even with the volume set all the way on zero. I've read that the Jazz Chorus line has a lot of hiss but this seems excessive. A friend brought his JC-90 to my house a couple of months ago and it was nowhere near this noisy.

Is it likely that the hiss could just be caused by dirty pots? And in that case would it make sense to just replace them? Or is there something I can spray in there to clean the contacts but also get any dirt out? It seems like they will have to be disconnected from the circuit board because the opening is on the side facing the board.

Is there anything else that could be causing hiss?

Is there a place to order replacement parts for these amps? The pots look to have a smaller than 3/8" bushing. Also the Chorus switch seems like it should be replaced since it is cutting out when set on "manual" mode

Thanks for your help!

[Jazz P Bass]

Caig DeOxit is the best cleaner for the pots.
Roland is your best source for parts.
Rather than "throwing" parts at it, why not sit down & troubleshoot the amp.
Narrow down the problem.
An often stated procedure is to use the send & return jacks.
Send the signal to another amp.
If the hiss is there, your problem is in the preamp.
Input a signal into the reurn jack.
Still hisses, the problem is in the power amp section.
After you have isolated the issue to one half of the amplifier, you need to further narrow it down.
This takes disciplined troubleshooting technics, with preferably the schematic in hand, and a number of pieces of test equipment.

[blvd]

Sitting down and patiently troubleshooting things is not my forte!

Well this particular model doesn't have Efx send/return so I disconnected the speakers and took the line out right into my soundcard. It seemed much less hissy to me and I was able to turn the volume pretty much all the way up without really any noise. So I guess that would suggest the noise is coming from the power amp section which rules dirty pots out of the equation.

Where would I start here? I know a little bit about tube amps but solid state is a mystery to me. I don't have any testing equipment other than a multimeter so maybe its best to bring to a tech? thanks for your help!

[Jazz P Bass]

That is interesting that the line out is quiet.
It comes right off the speaker connection through a resistor divider.
I do not think this "proves" anything.
I have been reading that the JC 77 , 120 ^ 160 amps "are" hissy.
Although the 55 is not.
Link: Roland Jazz Chorus Noise Floor Hiss - The Gear Page

[km6xz]

All JC's had a high noise floor and all sounded very much alike despite going through several complete redesigns of the Chorus circuit, going from discrete to monolithic devices. Yes, nothing sounded as good in chorus effect for jazz. It is a lousy rock or blues amp but great for jazz and related genres

As Jazz rightfully suggested, patient logical trouble shooting is the key to effective repairs and having some test instruments that can give quantitative values is essential to arrive at an appropriate diagnosis......or you can keep throwing parts at it until it, by luck, starts working, or is beyond hope from the introduction of lots of new problems.
If you value the amp, take it to a tech who has the test equipment and knowledge to focus precisely onto the problem. It will be much faster and cheaper than the alternatives. Ask if you can watch and to explain while he is working would be a good learning experience.
Noise problems in solid state gear is not like finding a noisy tube and subbing it.

[blvd]

Actually lots of great rock guitarists played these amps, too, specifically Adrian Belew and Jeff Buckley. Just comes down to how you define different genres I guess...

Well I think with some patience I might be able to get somewhere but I'm thinking taking it to a tech is probably the best idea. There is a very good repair shop nearby and they charge a flat fee of $175 plus parts. Since I picked the amp up for about $125 I think its still worth it if it means they can get it into top shape.

Jazz, I'm not sure the line out comes off the speaker. At least on an 84 schematic of a related model I found online it comes earlier in the circuit. On these amps the right and left speakers are driven by separate channels... The right speaker seems to have a louder hiss than the left.

Anyway, thanks for your help.

Tyler

[Jazz P Bass]

Yeah, I see that now, that the line out is preamp.
If the line out is indeed hiss free and the right speaker hisses, then it appears the right power amp may be at fault.

[mikeskory]

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