Older JC120 - Buzz/speaker driven fw

[fdesalvo]

Hi, all.

Out of my element with these solid state amps, but have a great customer and want to do him a favor. After replacing a shorted bridge rectifier, the amp now powers on, but one of the speakers is being driven fw and that power amp is producing a buzz. Previously, this speaker was blown. I'm seeing 30vdc on the orange/brown pair. Is this a common issue with these amps? Can anyone set me in the right direction?

TIA!

Frank

[Enzo]

Common? You have a blown power amp. I know of no make or model that never had an output fail. If you leave that 30vDC on a speaker for mor than a moment, it will burn up the speaker.

Start by looking for shorted output transistors.

[fdesalvo]

Ty, Enzo. I ran the diode test on all 4 and the driver. They all have identical measurements. Of course, I could’ve failed to execute the test properly. .5 on two pairs and 1.5 the other on all. Can’t recall bce pins off hand.

[g1]

There are quite a few schematic variations for this model. Can you post the serial # and whether it has 'bright' switch?

[glebert]

Don't burn up the speaker if it is original, those are hard to find.

[fdesalvo]

Thanks for the replies. I did a little homework prior to posting and made sure to only power on the amp briefly and on my limiter. I do not have the rear panel, so no access to serial number. It does have bright switches however.

[g1]

no access to serial number. It does have bright switches however.

Attached is the only JC120 schem. I have with the bright switch.
Run the amp with the bad side unloaded and post your DC voltages for the output transistors on that bad side.
You can compare voltages to the good side which makes troubleshooting a bit easier.

[Chuck H]

FWIW... If this is a speaker that has blown before (as reported) there may be an ongoing problem that was though to be remedied at some point. I might be looking for shorts, ground faults or anything else that could cause an operational failure in the PA. You're not necessarily looking for a blown transistor, though that seems most likely. Just mentioning that there may be another cause for DC at the output.

[fdesalvo]

Outstanding gents! Ty will report back.

[fdesalvo]

Hi, all.

Apologies for not providing closure lol. My left index finger was treated to a delightful full thickness central and lateral extensor tendon laceration. I'm typing this as I emerge from the post surgical fog of pain meds - agh!

After replacing the quad of transistors with 2SC2921s, the issue is still present on the brown/orange pair/associated power amp. Are there any other places I can look for solutions? The red/black pair has no such issue. I'm seeing about 30vdc at the speaker.

TIA!

[g1]

You replaced 4 output transistors, I guess you also replaced the ones on the good amp?
Need to figure out which amp side we are talking about so we can refer to correct part designations. Does orange connect to board pad #20 or #28?
DC voltages on appropriate drivers would be good to check.

[fdesalvo]

You replaced 4 output transistors, I guess you also replaced the ones on the good amp?
Need to figure out which amp side we are talking about so we can refer to correct part designations. Does orange connect to board pad #20 or #28?
DC voltages on appropriate drivers would be good to check.

Yes all 4. Ok I’ll get some measurements. Slow going with this dang finger. Appreciate the help.

[fdesalvo]

Apologies for the morose update, but I’ve decided to return the app to my friend. I’m out of my depth here and with family conditions in my hand don’t have the cycles necessary to learn on this job. I appreciate the help however.

[g1]

Hope you have a full and speedy recovery Frank. Unless you are a lefty I guess that is your fretting hand so that sucks. I guess that is the back of hand tendons?
A friend did that to the other side (flexors) on his fretting index a couple years back. It took quite some time but he is finally gigging again. Trying times ahead but be patient and do your physio!

[fdesalvo]

Thank you and it was the extensor side thank god! I get the pins out in about a month so I can’t wait to get back on the fretboard. Lol I thought my hand would heal faster than my ego after that Roland stole my lunch money and called me a nerd in front of my girlfriend, but a customer just brought in a Valveking with no sound and I found the culprit quickly- a shorted cap on the low voltage supply. Gotta roll with the punches!