Roland Jazz Chorus JC77 blowing main fuse

[copycat]

Hi gang, first post here. Extreme amateur looking for guidance in solid state amplifier repair.

i have an 80s Japan Roland Jazz Chorus 77. I have owned this combo amp for a few years with little use. One day it wouldn’t turn on... blown main fuse (3a 125v)
Replacing the fuse yields the same result.

I suspect something happened to it that my studio mates won’t disclose.

I have taken the Amplifer part out of the cabinet and began testing with a multimeter to find the shorts.

Here are my findings so far:

disconnecting the three transformer secondary wires (30v/0v/30v) from the main amp circuit board, I am able to power up the transformer and the fuse doesn’t blow.

The secondary windings are outputting around 25vac

With the transformer powered off and drained the resistance between the black 0 wire and orange 30v wires on the secondary side of the TF is very low, about 0.2ohms.

I also measured the resistance between the transistors and case ground.

with the transformer disconnected from the main board I get about 1.5MOhms

with the transformer connected Q11 and Q28 read around 3Mohms while Q9 and Q25 read shorted 0.5ohms.

thanks for taking the time to read, I am looking forward to any help possible.

best regards.

[copycat]

Here is the schematic that I am using.

[Enzo]

Your transformer powers up on its own without blowing fuses. It is OK then, move on.

Each transistor has three terminals, When you found Q9 and Q25 shorted, which leg were you measuring? What matters is if the emitter is shorted to the collector (mainly) Also, disconnect the speakers. If you try to measuer the transistor case to ground on those negative side ones, you will be measuring the speaker.

[copycat]

Thanks for the reply Enzo,

i was measuring the transistors from from the body to the nearby case. Which I believe is the same as the center leg. Just easier to get a good contact on.

The speakers were were disconnected and wires isolated from case/each other.

[copycat]

Am I crazy to think that the resistance between the x-former output wires (0.2ohms) directly translates to Q9 and Q25 being shorted out (0.2ohms) when I connect the xformer to the board.

[copycat]

I am at a loss, asking for help to diagnose further. Anyone have a direction they can point me in to troubleshoot?

[Enzo]

It blows fuses? And if you disconnect the secondaries from pins 38 and 40 and power up, fuses do not blow? Meaning the transformer is not shorted. SO leave the transformer disconnected. Use your ohm meter to test all combinations of legs on the main rectifier bridge D30. Any sides shorted? Is either DC terminal shorted to ground? In other words does C88 or C89 appear shorted?

Leave power removed. Your four output transistors are all TO3P or similar. Measure between middle and right leg of each - collector to emitter - are any shorted that way? Disconnect speakers for this.

[copycat]

Thank you for helping me Enzo, here are today’s findings.

Rectifier bridge was shorted between ac and dc +
Replaced with a good new one.

Amp now powers up for a second before blowing fuse...

C88 and C89 desoldered and tested good, reinstalled.

no shorts on any combination of transistor legs when Xformer is disconnected.

Speakers disconnected for all tests.

Where should I look next?

[g1]

C88 and C89 desoldered and tested good, reinstalled.

I think he wanted you to measure resistance across each while soldered in circuit. It's a shortcut method of looking for shorts on the power rails.

[copycat]

Just checked, the cap terminals don’t appear to be shorted.

[copycat]

Bump