Roland JC77 Dead Short Only When Both Speakers are Connected

[Gahoo]

Hi all,

Recently got a Roland JC-77 on which the reverb stopped working. One of the wires to the input transducer broke right at the bobbin so I'll have to buy a whole new pan .

However, before turning the unit on I plugged it into my light bulb limiter. When powered up through the limiter the bulb came on full brightness. Quickly shut it off, pulled the chassis, and set it up on the bench. Checked for shorted output transistors and all was well. Checked for shorted bridge rectifier and it was fine. Plugged into the limiter (no speaker load) and the bulb lit only dimly. Checked both 8 Ohm speakers and both are fine. Plugged in only one speaker and powered up the amp. Bulb lights up dimly and it passes signal. Connect a load to the second channel only and get the same result. With loads attached to both channels the bulb lights brightly and there's about 5VDC on the output of either one.

I guess my question is what element in the circuit could be giving out when the amp is fully loaded? And why would both channels work fine when they're isolated like that?

Schematic here:Roland jc-77-schematic.pdf

[drewl]

Check the speakers and jacks are wired correctly

See where the speakers are grounded

[Gahoo]

Speakers are fine, the brightness when two loads are connected occurs with other speakers as well.

Outputs aren't shorted and are grounded correctly. Again; either channel works fine when the other channel doesn't have a load connected.

Also may be worth mentioning that the 5VDC offset on the output only occurs when the bulb goes bright (two loads connected)

[g1]

Does it happen with no test gear connected (aside from the limiter lamp) and with speakers that do not have frames grounded?

[Gahoo]

Yep, it'll happen without any other test gear connected and with non-grounded speakers.

I'm fairly sure that the schematic I posted is the correct one, were the changes between variations dramatic? Everything I've seen in the amp and referenced to the schematic has been spot-on...

[g1]

It's probably correct one then. May have to do with loading down the supply and one side collapsing.
Monitor the supply rails when the bulb is lit.

[drewl]

Measure the resistance on the output jacks without speakers connected and with speakers connected just see if there's something funny going on with the jacks.

Otherwise the extra current draw from both is causing something to break down

[g1]

Another possibility is it doesn't like starting up on the lamp when loaded.
Try connecting the loads while it is powered up on the lamp, but do it with the connections at the speakers, not by plugging in and out of any jacks.
As Mr.Fahey would say, it may be "turning on dumb" with the lamp when loaded.

[Gahoo]

Just had it set up on the bench with two dummy loads plugged into both outputs, no bright flash... Very odd.

I'll try your suggestion of hooking up the loads after it's powered up and see what effect that has. Good tip!

[Gahoo]

Well hooking up the second load after the amp is powered on doesn't set the light bulb off. Pretty stunned here, I would've been losing my mind trying to pin this down! What causes the amp to sense the load of the light bulb limiter and start putting DC on the output?! Can someone explain what could be going on?

With this information do you think it's safe to plug directly into the wall and try out?

Thanks for your guidance here!

[52 Bill]

The light bulb will reduce the voltage going into the amp and therefore the dc power supply voltages as well. The amount of voltage reduction will be dependent upon the current flow through the bulb.

When the power amp starts up, it rarely comes up evenly, meaning the two halves of the circuit (push and pull) will each ramp up at different rates. Under normal power up conditions, this uneveness happens so quickly that it just passes and creates a thump in the speaker.

With the bulb reducing the voltages, it might cause the circuit to latch-up at the most uneven point of the start up, causing the bulb to glow brighter and reduce the voltages even more.

I would remove the limiter and power up with no speakers. Then add the speakers and then if all is good, power up with the speakers.

[Gahoo]

Did just that and all is well. Guess I was making a problem where there wasn't one...

Glad to have learned this, thanks everyone for the help. Any advice on how to spot an amp that might throw this kind of problem in the future? or do you just have to have a general awareness about what's going on with those power supply rails?

[g1]

It seems to be kind of random. The DC that you had at the output only when on the bulb was a tip-off. This is due to the latching up to a supply rail that 52Bill mentioned.

[The Dude]

Sometimes (certainly not always) this is a sign of a filter cap starting to dry out and taking longer to charge. I might check C88 & C89 if you have a good cap tester.

Edit: A symptom of this would be that the offset occurs when the amp has been powered off for some time and then powered on. The offset does not occur when the amp has been powered on for a while and then power is cycled.