Quested VS2108 active monitor shematic

[csantos]

Hi,
i need the schematic of the amplifier quested VS2108 active monitor.
One of my is blowing the power MOS-FETs and the fuse of channel A (woofer) as i turn on the power.

Anyone has it? I tried to find it everywhere but i can´t find it.


Thanks.

[csantos]

So, nobody can help me? I really need this schematic please....

Thanks

[singa]

Hi, Please understand not all the world's products or rather manufacturers
of electronic products do not freely distribute their product's schematics
as they are regarded as IP ( interlectual property) but in fact you can't
copyright a circuit( that is the basic building blocks) because if it is allowed then humanity would be kicking itself in the butt because that would limit it's use and no further development is possible for practicality sake in general.

No matter as there are books that teach you how to troubleshoot without
schematics just by understanding an electronic device such as transistor or
mosfet etc.So you just have to learn and practice.

As for your Quested VS2108 they are studio monitors and you are anxious to repair them because you have limited budget perhaps it should comfort you to know that when a fuse blows it means a major failure ,a dead short or
open circuit so it is easy to find the bad component but also check all
relevant components before turning power on or else it's money wasted
blowing another set of transistors. Good luck. Singa

[csantos]

Hi, thanks for you reply.
I understand that not all manufacters freely distribute their schematics, but as i have seen so many schemes in this forum, i thought that anyone could have it.

I have spend some hours with the pcb, trying to figure out why the power mosfets blows again but i just can´t understand why.
It seems that theres no short anywhere but keeps blowing.
Thats why i asked for help.

Thanks

[singa]

Originally Posted by csantos

Hi, thanks for you reply.
I understand that not all manufacters freely distribute their schematics, but as i have seen so many schemes in this forum, i thought that anyone could have it.

I have spend some hours with the pcb, trying to figure out why the power mosfets blows again but i just can´t understand why.
It seems that theres no short anywhere but keeps blowing.
Thats why i asked for help.

Thanks

Hi csantos,
That's an expensive way to troubleshoot.
1.The first thing you should do is a visual check ie. look for burnt parts/pcb or bulging or deformed capacitors.Discharge the power filter caps with a 5-10W
resistor about 1Kohm or more to ground.

2. If the fuse blows then there's definitely a short somewhere ,if it's not the
power mosfets then check other components.Check the power transformer
output secondary (pull connector from main pcb) for ac voltage.

3. I hope this is not the case but check the resistance of the speaker,if it is
rated 8 ohm it should measure less at about 6 ohm ,if it is zero 0 ohm
then it's a shorted voicecoil (pull connector from pcb).

Do the above 3 points, that should get you going. Singa

[csantos]

Hi,

thanks again for your help.
I´ve checked the woofer and its not shorted. And visually theres nothing wrong in the pcb, i have checked other components like small transistors, resistors and capacitors and i could´t find any short.
The way the fuse blows it´s not like short circuit (the fuse inside doesn´t goes black, it stays transparent) i hope you understand me, i´m portuguese lol.

I tried powering up the circuit with a voltage regulator to see if anything warms before the fuse and the mosfets blows, and at one point as i increase the tension, the woofer starts to make a noise "hum" and seconds later the fuse blows and the woofer starts making the same noise but higher.

I leave here a link with some fotos of the pcb if you want to take a look to better undesrtand the circuit.
http://dl.dropbox.com/u/9574849/Foto...20Downsize.rar

Thanks

[csantos]

Hi again,

I made some progress, i put new mosfets and without the fuse that blows in the circuit i can have the rest of the amp working.
Then i reconect the fuse and start increasing the tension with my voltage regulator carefuly, touching the mosfets to see if it warms a lot.
At some point i realized that the amp is working (touching with my finger in the input) so the amp section is working but one or two mosfets are very very hot!
Of course i can only do this with the overal tension at maybe 70% of what it should be. If i put the tension at 100% the fuse and mosfets blows again.
So, could this hapen because of a wrong bias setting?
Give me your opinions please.

Thanks

[Bone]

It seems to be that the bias IS set too high and the FETS are passing too much current, or they are oscillating (unlikely). Measure the voltages across R2, 4, 5, 7, 9 & 11 they should all be similar. Compare them with the voltages across the corresponding resistors on the other channel, again they should be similar. If the bias current is high then the voltages will be higher and the bias should be adjusted until they are the same. This assumes that you havent touched the other channel.

[csantos]

Hello again,

I´m getting crazzy about this amp.
I´ve compared every tension value in every resistor of the circuit between the good and the damaged amp, and there´s no diference between them.
Then i removed the fuse that blows and measured the current with my multimeter on the socket.
Inicialy the current is normal as on the good monitor, but seconds later starts to increase, the mosfet gets very hot and i have to disconect the amp or it damage the transistor again....

I don´t see any more clues of what causes this.
What do you think?

Thanks.

[singa]

Hi csantos,
This pcb seems difficult with no burnt or exploded parts.

Now this is important,I noticed your repair of the output mosfets are not in good contact with the heatsink,ie. there are gaps or spaces between mosfet
and heatsink.This is a sure disaster and the new mosfets will fail quickly.
Better yet to use heatsink grease/compound for better heat conduction.

From your description only the mosfets on channel A is the bad side.
And because of the improper mounting is causing the problem? Tip: clamp
the mosfets first and then solder ,that will ensure good contact with heatsink
and also better to use heatsink compound with them.

As the post header says there are differences of both channel A and B of
the circuit components of U1 and U6 OP275: See uploaded image TOP_A
I have put red arrows to show the differences on both circuits.

ChannelA Channel B Good channel

R19 is empty = R64 = empty

R25,26 (9.1Kohm?) = R71 (9.1Kohm) , R72 (empty)?

C3 = 10nF 100V = C33A = empty?

I would think that both channels A,B in this part of the circuit (driver?)
should be the same? Please check all other components for short or open
circuit. Singa

oops second picture is a mistake, I meant to post the bottom side of pcb so others
can trace the connections.