35v DC at speaker terminal!

[onejay09]

Re: 35v DC at speaker terminal!

ive got driver transistors being delivered as a complimentary pair so ill change those with the outputs, might start replacing the resistors that are like the jumper, corroded...

[onejay09]

Re: 35v DC at speaker terminal!

the jumper went from + from rectifier to r512 and r503

[budm]

Re: 35v DC at speaker terminal!

Glad to hear that you found the problem, the black glue, was it soft? it looks like they use regular silicone RTV which is corrosive instead of using electronics grade silicone RTV.

[budm]

Re: 35v DC at speaker terminal!

the jumper went from + from rectifier to r512 and r503

That explained why you did not get enough bias to the positive drive section.
Now one thing is that you had -28VDC on the speaker terminals, that would have badly damaged speaker.

[onejay09]

Re: 35v DC at speaker terminal!

its an 8inch very heavy sub speaker! it seems fine.
im worried that something is a miss, if not the fact that the mica insulater slipped from behind the tip36c, why wouldnt this cause both to blow?
i have took them out and theyre both dead short

[onejay09]

Re: 35v DC at speaker terminal!

it ran great for about 1 minute.
well the black stuff where not charred to a crisp was more like tar and where it was hot it was more like burnt plastic.
after i blew the outputs i decided id pass the time by cleaning the board where i could, replaced all the jumpers that were brown and crispy, also pulled some bigger parts off to get the last of the RTV off. went over all the solders, made some traces bigger.
noted the resistors id like to replace..
just waiting for these parts

[SteveNielsen]

Re: 35v DC at speaker terminal!

I don't like the looks of that cap below Q603. The R906(?) resistor, and that black near the top of the white block on the left above it looks like it might be from a flame-flash. Does that Resitor check out ok?

[onejay09]

Re: 35v DC at speaker terminal!

i have recapped the board all except 1 or 2. the black stuff is glue, but yea one of the poly caps blew near there.

[budm]

Re: 35v DC at speaker terminal!

I do not think any speaker can handle 28VDC on the Voice coil., @ 8 ohm, that will be about 3.5 DCA, almost 100W of steady state of power.

[onejay09]

Re: 35v DC at speaker terminal!

ill check its impedance when i get home but it sounded ok, im wondering if the new outputs arent matched then could i increase the value of r524 and r525, if i understand right these are emmiter resistors? would it give some lee way if the power transistors are not matched up well, what would the side effects be if i increased the values?

[onejay09]

Re: 35v DC at speaker terminal!

just got the outputs today, what should i be checking before i put these in?

0.4 v dv at r519/r520.

[ben7]

Re: 35v DC at speaker terminal!

just got the outputs today, what should i be checking before i put these in?

0.4 v dv at r519/r520.

You should check all the other transistors in the amplifier circuit, and the resistors too. Unfortunately, amplifier circuits can suffer cascade failures, where one component blows and causes more to blow with it!

[onejay09]

Re: 35v DC at speaker terminal!

its fine it turns on and runs, just waiting for new vbe and driver trans delivery

[onejay09]

Re: 35v DC at speaker terminal!

put the new output transistors in and a new (q507) vbe in, turns on but no sound nothing!

[onejay09]

Re: 35v DC at speaker terminal!

nevermind its working again, it was the q507

[rievax_60]

Re: 35v DC at speaker terminal!

The only way for Q507 to cause no sound without resulting in permanent damage is if its Collector or Emitter pin was in contact with the grounded heatsink.
If any of Q507's pins become disconnected from its circuit, it will cause both output transistors to turn on hard, drawing excessive current from the rails and then likely to both go short.
R523 going open will have the same result.

[Adder]

Re: 35v DC at speaker terminal!

Hi. I have a dead VW-8 which seems to have failed components in the AC power supply.
The circuit diagrams which have been posted don't seem to match mine.
The main AC input seems to go to a control relay which isn't pulling in (no AC getting to the mains transformer). It also goes to a large resistor (R1 4.7ohms I think) which seems to be open circuit. This resistor seems to feed into a bridge rectifier which has a failed inductor on it's output side (casing cracked) and a diode also looks like its overheated.
None of these components seem to show on the schematics here!
Does anyone have any different schematics for this amp?
Thanks

[Agent24]

Re: 35v DC at speaker terminal!

It would be greatly helpful if you can post photos of your unit, the blown components, etc.

The soft-on relay circuit may be simple enough that you won't need a schematic anyway.

[Adder]

Re: 35v DC at speaker terminal!

Here should be some photos of the board which seems to have the fault:
https://drive.google.com/open?id=0B5...ndMQ2NkWmpRQVk

Resistor R1 seems to be open circuit (8Mohm across it)
Inductor L8 has a cracked case but seems to have continuity still.
I thought diode D2 looked overheated but it still measures 0.5v drop across it ok.
There seems to be around 24 ohm across the output of bridge BD1 (seems a low value)
The mains switching relay doesn't pull in (hence no AC to transformer).

I didn't see any of these parts on the schematic which had been posted for the VW8.
Connector at end of board is marked as CN3A.

Any ideas?
Thanks

[Agent24]

Re: 35v DC at speaker terminal!

I've attached your photos here so that 1) they're easier for everyone to view and because 2) external links have a habit of disappearing after some time


Anyway - replace R1, obviously.

D2 may test OK but it's definitely been stressed and should be replaced.
Maybe a good idea to replace the bridge rectifier for the same reason.

L8 may still have continuity but may have lost inductance (shorted turns) so replace that too.


Probably with that low resistance reading you will find the LNK364PN controller IC has shorted. This may have been caused by D2 going bad in some way - it appears to be part of a snubber network which protects the IC. Or maybe the IC went bad and burned D2.

Check C2 is not bad, check any SMD parts (R2 underneath?) connected to D2\C2 as well. Check C1 does not have reduced capacitance. Check\Replace D1.

You may find that the U3 optoisolator has been blown up as well, if the LNK364 has died spectacularly enough.


After doing all that, power up with a series lightbulb to the mains supply and see what happens.