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Author Topic: Repairing a Roland JC-50  (Read 1106 times)

inertian

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Repairing a Roland JC-50
« on: June 25, 2018, 12:42:31 PM »
Trying to bring an '80's Roland JC-50 back to life.  Blows both internal 4A fuses on power on, external fuse is ok.

Research indicates the output transistors and possibly the drive transistors are shorted.  Have a schematic, but not 100% sure which are the responsible transistors.  Can anyone point me to the right ones, so I can test and replace?

Schematic uploaded to the Schematics section.
« Last Edit: June 25, 2018, 12:47:05 PM by inertian »

tonyharker

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Re: Repairing a Roland JC-50
« Reply #1 on: June 25, 2018, 01:42:32 PM »
Power transistors, and driver transistors are in the top right of the schematic page 3, Q18, Q22, Q27 & Q28 for later serial numbers, 25xxxx or Q11, Q12, Q13, & Q14 on page 7, for earlier serial numbers 24xxxx. The box around Q13, Q14, Q27 & Q28 means they are on a heatsink or mounted on the chassis

inertian

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Re: Repairing a Roland JC-50
« Reply #2 on: June 25, 2018, 01:54:30 PM »
Thanks very much for the quick and detailed response.

It's a later serial number model  (3xxxxx as I recall) so I will test the transistors ones you called out and find replacements.

On the off chance they all test ok...what would be the next likely culprit?

inertian

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Re: Repairing a Roland JC-50
« Reply #3 on: June 25, 2018, 08:25:45 PM »
Tested the four transistors using the Diode setting on my meter, all have voltage in one direction only.  Think that indicates they are ok.

Time to check for shorted resistors or caps?

Thanks.

phatt

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Re: Repairing a Roland JC-50
« Reply #4 on: June 26, 2018, 07:34:28 AM »
Start at the source,, check the recifier diodes and filter caps FIRST. 8|
Phil.

inertian

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Re: Repairing a Roland JC-50
« Reply #5 on: June 26, 2018, 08:26:35 AM »
Appreciate the suggestion.  Diodes and caps I am familiar with, having spent too much time inside old tube amps...  :tu: