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Old 20th January 2013, 02:56 PM   #61
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Here they are.
First Bias set cold
ColdBias_svt | Flickr - Photo Sharing!

Second, bias pot half way
HotBias_svt | Flickr - Photo Sharing!

EDIT:
I will get a scope tomorrow.
What kind of probes would you suggest to use here?

Last edited by served; 20th January 2013 at 03:00 PM.
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Old 20th January 2013, 03:08 PM   #62
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Quote:
Originally Posted by served View Post

EDIT:
I will get a scope tomorrow.
What kind of probes would you suggest to use here?
any standard 10x probe will be fine!
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Old 20th January 2013, 03:36 PM   #63
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But still. At the moment there is still something out of balance. Scope will not help much here.
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Old 20th January 2013, 03:51 PM   #64
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Quote:
Originally Posted by served View Post
But still. At the moment there is still something out of balance. Scope will not help much here.
If the negative side is breaking into oscillation it will draw higher current then the positive side. a scope will help you visualize the problem!
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Old 20th January 2013, 04:05 PM   #65
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Have you replaced the mosfets on the negative side? if those gate resistors were open, those mosfets are likely bad!
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Old 20th January 2013, 04:44 PM   #66
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Just for conclusion I will post a picture showing parts that were replaced.

ReplacedSVT | Flickr - Photo Sharing!

Oh and I also replaced Bias Trim pot.
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Old 21st January 2013, 07:24 AM   #67
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If I look at the schematic with your markings of the changed parts; I would suggest the following:
1. Take a clean schematic.
2. Mark al the parts with a fluo pen (eg red) you replaced and are KNOWN to be good.
3. Then start to measure (or remove and measure if you are insure) from the output towards the input of the power amp and mark them green if they are OK or Red if they are bad and need replacement.
Do not try to write it down! Mark them in color!
4. Continue until you encounter a node (where the components touch each other) where all components are green.
There you stop end continue with another part of the schematic.
5. When all redmarked parts are surrounded by green marked parts; the chance that the amp, component wise, is OK is very great.
6. Come back with an annotated schematic with the marked parts in green and red.
Wait for conclusions by severall members before swithing your amp back on.
7. Patience is the message right now...
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Old 26th January 2013, 03:00 PM   #68
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Hi.

So I did it.

Found that D6 or D7 was bad, replaced both of them.

There is only Q7 and Q11 in the schematic others are moved to minimize possible MOSFET failure.

After replacing D6 and D7 I got the rail Zero and it alsa was biasable, so if I turned the bias pot I got no voltage on output.
Also checked all the other diodes and caps and resistors to make sure. This was my only find.

Can previously described imbalance be caused by D7 or D6?
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Old 26th January 2013, 04:50 PM   #69
Tarzan is offline Tarzan  Belgium
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Glad there is some progress.

BUT, BEFORE applying power...

Check the components as asked.
Then come back with the markings on the schematic in red and green .

Other members will comment and if it's safe according to them, apply power with a light bulb in series with the ac and check if there is no short or exsesive current taken.

Only then apply the full ac to the amp.

Off course no load applied...

Patience is half of the repair in your case.

You were lucky that nothing burned up.

So it's not to late to show us that annotated schematic in color.

Cheers
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Old 26th January 2013, 10:03 PM   #70
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Replace the cheap and nasty bias pot for a good quality cermet type and that will cure the problem. The old pot changes value!
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