35v DC at speaker terminal!

[onejay09]

Re: 35v DC at speaker terminal!

so i know its unbalanced and i assume thats where its failing to operate correctly, would biasing help, eg: replacing a resistor with a pot and re biasing?

[onejay09]

Re: 35v DC at speaker terminal!

I forgot to mention that it started intermittently, once hot it would do this buzzing, then i left it off overnight and it had gone, it started again when it got hot and got worse

ive ordered some parts, ill check all values of all parts inside the red box, thanks budm

[Agent24]

Re: 35v DC at speaker terminal!

Got any freezer spray? If the problem is intermittent with heat you might be able to track it down by freezing various parts

[rievax_60]

Re: 35v DC at speaker terminal!

What is the voltage drop across R512 and also R515?

[rievax_60]

Re: 35v DC at speaker terminal!

The circuitry in the bottom left corner is difficult to make complete sense of. I dont think its
for DC protection. Its threshold seems to be tens of volts rather than a few volts and I cant see a link to the speaker relay control.

[onejay09]

Re: 35v DC at speaker terminal!

the bottom left corner on the schematic? theres an op-amp and where it says iso thats an led and photo resistor,

or photos from the prev repair?

the schematic on the right the relay. i think its a soft start protection? perhaps

[rievax_60]

Re: 35v DC at speaker terminal!

I am certain now that its for automatic level limiting. The OPTO LDR must tie into the signal path somewhere.
The important point is that there is no obvious DC protection circuit.
Only a delay for the relay and also current overload detection which is able to latch the relay off.

[rievax_60]

Re: 35v DC at speaker terminal!

I see now. The LDR is in the amplifier's negative feedback path, clever!

[onejay09]

Re: 35v DC at speaker terminal!

cool, ive just found something

[onejay09]

Re: 35v DC at speaker terminal!



i was trying to clean the board and pushed a corroded jumper, its snapped at the base. if this does fix it im going to replace them all with new,
blame a shoddy repair the first time around as i got exited and left it in a poor state, this is what the black glue does to stuff...

[onejay09]

Re: 35v DC at speaker terminal!

so if this was making poor contact?

[rievax_60]

Re: 35v DC at speaker terminal!

I would be curious to see where that link is situated in the schematic to work out what exact effect it would have, being open.
You have yellow/brown glue there. It starts off yellow and harmless. It turns brown when exposed to elevated temperatures and moisture. It also becomes conductive and corrosive.
It needs to be removed. Components that are now partly corroded, mainly small glass diodes and resistors are likely to fail later.
It totally baffles me as to how even well know manufactures are still oblivious to the destruction that this glue causes. I have been aware of it for 30 years.
I have lately seen it in a Marantz and a Sony product.

[Khron]

Re: 35v DC at speaker terminal!

Can you trace out what components (and legs) was that jumper connecting?

[Agent24]

Re: 35v DC at speaker terminal!

It totally baffles me as to how even well know manufactures are still oblivious to the destruction that this glue causes.

Maybe they do know and they use it to ensure product failure. Sony certainly knows, IIRC Budm linked to an article that came from Sony which explains how the failure works.

Black corrosive glue is a new one on me though. Maybe people have stopped buying the white stuff so the dodgy manufacturers changed the colour!

[onejay09]

Re: 35v DC at speaker terminal!

well it worked, tested the sub and it sounds fine, but the mica film for the outputs slipped out and i blew the outputs.good job i ordered new ones yesterday, so ill get back on here when ive fitted the outputs, maybe tomorrow. ill let you know where the jumper went when i unscrew it all again.

[onejay09]

Re: 35v DC at speaker terminal!

yea it was a complete mess anywhere theres heat+black stuff, theres corrosion, i think it took me 2 days just to clean it off with a pin

[rievax_60]

Re: 35v DC at speaker terminal!

I would not expect a missing insulating washer to damage the output transistors. It should only cause a main rail to short to 0V or ground through the heatsink.
Be certain that the bias transistor is in thermal contact with the heatsink.

[onejay09]

Re: 35v DC at speaker terminal!

is that q507?

[rievax_60]

Re: 35v DC at speaker terminal!

is that q507?

Yes, it needs to sense the heatsink temperature so that it can keep the idle current bias for the output transistors correct.

[onejay09]

Re: 35v DC at speaker terminal!

yes that is bolted down along with the outputs, its an mpsa13