35v DC at speaker terminal!

[onejay09]

hi guys im trying to repair my active subwoofer its an expensive piece of kit. so id rather fix it than bin it. its a monitor audio vw8 active sub its a push pull ab amplifier board, ive got the scematics here ill post below, ive already repaired this once before:

https://www.badcaps.net/forum/showthread.php?t=32471

this time as the title sudgests ive got 35 volts at the speaker terminal!,
im not smart enough to say whether this has an output thats capacitor coupled..

the speaker hums like 60hz or 120hz, theres no input and the volume doesnt change the sound atall. i unplugged the pre amp section and the hum doesnt stop.

transformer is centre tapped ground but it seems ok
outputs are tip35c/tip36c, they seem ok too
driver transistors seem ok
filter caps were replaced theyre ok
bridge rectifier is new also

shouldnt the protection relay prevent this?
What am i Missing? any help would greatly bee appreciated, thanks for reading guys

[onejay09]

Re: 35v DC at speaker terminal!

power amp stage

[onejay09]

Re: 35v DC at speaker terminal!

power supply

[stj]

Re: 35v DC at speaker terminal!

check Q512 & Q513 for shorts.

the hum would be c901 & c902

[stj]

Re: 35v DC at speaker terminal!

oh - i see you did that stuff!

[onejay09]

Re: 35v DC at speaker terminal!

yes i checked the outputs off the board for shorts, none.
q508 q509 q510 q511 seemed to be ok, but when onboard q511 reads both ways from leg 1 to 3 which is a 2sb647. -neg to pin1 +pos to pin3 .874 other way around is .709

[onejay09]

Re: 35v DC at speaker terminal!

its almost like the outputs are allways on

[budm]

Re: 35v DC at speaker terminal!

" but when onboard q511 reads both ways from leg 1 to 3 which is a 2sb647" That is because you are reading the PN junction of Q513. What DC Voltage do you get at E-B of Q513? Right now more likely that the bottom half (the negative) of the output is not on.
Since you did the repair work on it before, did you verify the soldering works that was done last time?

[onejay09]

Re: 35v DC at speaker terminal!

Hi budm! Glad your here, you helped greatly last time,
Yes the soldering was fine I even increased the copper traces with solder to make good.

I'll check in about an hour when I'm home
Thanks

[onejay09]

Re: 35v DC at speaker terminal!

from pin 1 to 3 q512 read .345 volts, and q513 it read .485 vdc

both outputs are 1base 2collector 3emitter.

[onejay09]

Re: 35v DC at speaker terminal!

ive heard of outputs being capacitor coupled? would that put this much dc on the output?

[budm]

Re: 35v DC at speaker terminal!

This direct couple output amplifier and there is speaker protection relay to disconnect the output of the amplifier from the speaker so when you have DC VOLTAGE (>2V) it will not damage the speaker. You should not see any DC voltage at the speaker terminal right now since the relay should be OFF at this point.
Q512 (the top positive drive) has less bias than the bottom negative drive Q513, which should have made the output have more negative DC than the positive DC at the output.
Is it 35V plus or minus voltage at the output? Polarity indication needed for that voltage reading

[onejay09]

Re: 35v DC at speaker terminal!

theres definatley dc at the speaker terminal. when i turn it off the relay clicks off and the voltage drops to 0

so black dmm lead to negative speaker terminal red to positive = -28.5v dc

[onejay09]

Re: 35v DC at speaker terminal!

is r518 the bias resistor or r519/r520?

[budm]

Re: 35v DC at speaker terminal!

1) One thing bad right now is that the protection circuit is not working because you do have voltage at the speaker terminals, the protection relay should stops that voltage from getting to the speaker terminals.

2) I know you have DC voltage at the out but I do not know if it is positive voltage or negative voltage since this amplifier uses bi-polar power supply (it has + and - voltage with ref to the circuit ground). So now I know that the output is negative 28.5V which correspond to the Bias voltage reading on the two output transistors Q512 (POSITVE DRIVE) which has less bias than Q513 (NEGATIVE DRIVE). In simple terms, the the output is not balanced, you have more negative drive than the positive drive, if both are equal, the output will then be 0V.
So we need to find out next why the positive drive Q512 has low bias than normal.
Q507 is an active bias circuit for the output section.
You should check everything in those section at least.

[onejay09]

Re: 35v DC at speaker terminal!

the negative speaker terminal is on the same trace for center tapped ground from the transformer.. if that helps

[onejay09]

Re: 35v DC at speaker terminal!

so its like one of the outputs are stuck on?

[budm]

Re: 35v DC at speaker terminal!

One is not turned on (bias on) enough to balance out the output to 0V. see updated post 15.
"the negative speaker terminal is on the same trace for center tapped ground from the transformer" That is correct connection and that is how it should be.

[onejay09]

Re: 35v DC at speaker terminal!

q508 q509 q510 q511 were replaced the last time i repaired it, could a resistor fall out of value?

had another look at post 15
how do we find the issue, ive had q508 q509 q510 q511 off the board and tested, no shorts but im unsure if they are leaky or failing, they read at most .78 nothing higher

[onejay09]

Re: 35v DC at speaker terminal!

q507 "mpsa13" seemed a little high at .86 on some readings