Fixing non-functional chorus in Roland JC-120

[tmh]

I have a Roland JC-120 Jazz. It's not a Jazz Chorus anymore, since the chorus circuit is totally non-functional. All other aspects of the amp work fine.

Looking at the schematic, it's a nightmare for me to make sense of. For whatever I know about tube amps, my knowledge of solid-state amps is probably less than 1% of that, so...

Any ideas where to start tracking down the problem? I have an Eico 667, Hickok 6000A, an oscilloscope, and a couple DMM's at my disposal... Just haven't a clue as to how to try and trace the source.

My gut tells me it's a failure in one of the IC's, which is my worst fear. I have no clue how to test them or if replacements are readily available.

Thanks for any help or suggestions!

Scott

 

[unclejed613]

you want to check the square wave pulses coming out of IC7 first, and make sure they are there. if they are, you need to feed in a sine wave into the preamp, and make sure you have the sine wave at the input of IC6(IC6 pin 3). if you do then check the base of Q27 and make sure you have your sine wave there. if it's not there, chances are IC6 is bad. if you don't have the square wave pulses on the outputs of IC7(IC7, pin 2 and 8), then IC7 is probably bad. if you don't get a sine wave at the input to IC6, then IC5 is probably bad. now the bad news, IC6 is a bucket brigade delay chip, and may not be available anymore, and i'm also not sure about IC7. the reason i didn't tell you to look for the sine wave at pin 6 of IC6 is that the waveform there hasn't yet been filtered, and if you don't have a lot of experience with a scope, the display you see would be a bit confusing to you, since the sine wave there would be mixed up with high frequency square waves.

..... just did a web search for MN3007 (IC6), and ay least one company is claiming to have over 50,000 of them in stock

 

[tmh]

Wow, thank you so much! I'll report back after I get in there and check it all out. Thanks again!

Scott