B780 freeze with loud plop

[SquierJV]

I think I have a big problem with my B780. When I switch it on I get a loud bang in the right speaker after a 3 seconds or so. After 10 seconds another one ( and I think after that another and so on, bu I didn't try that ). Meanwhile the machine totally freezes. Not one knob is working only the power...
Can someone help me, have tips what to check first?
Thx in advance Marc

[eckibear]

AW: B780 freeze with loud plop

First off all, you should not kill your speakers by continuing these tests...

The speaker protection system obviouly does not work. Beyond that the amp appears seriously damaged. There's quite a few modules which could be the cause, starting from the power supply, drivers, main amp and possibly even the preamp.

Unless you have at least moderate experience , understanding, tools AND test equipment you should not touch any of those. Save identification and repair of the fault requires a systematic approach (sometimes with additional test fixtures), not the typical "try and error" method of hobbyists. That usually just kills more and more components every time the amp is turned on. Please indicate what your level of experience is and what tools you have at hand.

[emielskey]

AW: B780 freeze with loud plop

Hello, I am repairing the B780 for SquierJV.

I have all the tools and measuring equipment needed at my disposal.

So far all the voltages measure OK (switched to 240 on the back, re-adjusted the +32v line). Also replaced a few components on the power supply (all the elco's and the diode bridges). not measuring strange dips with the scope either during the power up sequence.

so what is happening right now?

-powering up takes 6 seconds, then the relays click. If the speakers A and B are pushed, they switch on as well, if they are both in the "off" position only the Headphone relay clicks, but:

-the headphone relay switches back off immediately, so you hear a double click. The computer doesn't react to any keypress. this repeats every 5-6 seconds.

-when holding the "frequency step down" butting while powering up, the device starts normal. (headphone relay clicks to "ON" and the whole device functions normal)

-when power cycling the device when it has been on for, say, 10 minutes, the fault has disappeared. boots up fine every time. until it cools of again, same Fault reoccurs.

Any thoughts?

[eckibear]

AW: B780 freeze with loud plop

With this new description you could well have an error on the uC board, in particular the power-on reset circuitry. Start by replacing the few elcos on this board. If ICs have socket carefully wiggle them somewhat in an out. Same applies to all the connectors (I'd check any of them inside the B780)
Make sure to observe all ESD precautions !!! The uC is mask-programmed and is quite difficult plus expensive to obtain.

[emielskey]

AW: B780 freeze with loud plop

Thanks EckiBear!

I was searching earlier on on the uC board, replaced the one elco, but (obviously) no luck with that.

But I have some news. I was thinking the double click in of the relay could have someting to do with the DC detection system. And so it seems it does. Just now (cold start) the fault appeard, but when I removed the RIGHT amp output (brown wire) from the speaker-relay-board, the reciever boots up fine.

indeed measuring the output of the power amp, it spikes horribly.
attached a pic of the power-up, with the right-power-amp detatched from the relay and dc detection, and connected to the scope.

https://flic.kr/p/rpeXfZ

notice the huge peaks, first +25v, in the middle -10v when the relay clicks, and then a whopping -50v DC when I shut the revox down.

question is.. is it the power amp or could it be the right-pre-amp?

[emielskey]

Re: B780 freeze with loud plop

it's the power amp. removed the input (left the RPON connected) same error.

advice? rebuild the power amps? both power amps? or just fix the error that is happening right now..

Thanks!

[eckibear]

AW: B780 freeze with loud plop

Good analysis you made! I'd try to run the final amp on the bench with a lab power supply.
Apparently all Elcos have been replaced already(?).

The time to failure is pretty much of thermal nature, which in turn can have many different causes. Somtimes it's a cold solder joint, a bad contact on one of the transistor sockets (including idle current BJT), a bad idle current trimmer, a PCB-track rupture (particularly when board is warped or bent).

Of course any of the BJTs could have similar internal failures as well. You may try to run the amp without final stage on a high-omic load (and feedback closed appropriately) to see whether it's from a final BJT or anything upfront.

However, other than for "learning purposes", this procedure might consume just a lot of time. To be honest, I'd rather rebuilt the whole thing. Even an identified trouble maker could have already lead to weakend spots elsewhere, so after short period of operation you may have to start all over again...
In short, unless you can trace the error and definitely can exclude other subsequent failures by thorough analysis of the schamatics I'd go with that.

[SquierJV]

AW: B780 freeze with loud plop

Thx Eckibear,
We ordered a whole new set to rebuild the power amp: http://www.revox-online-shop.de/epag...oducts/0066-02
Cheers!
Marc