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Old 27th September 2006, 09:01 PM   #1
glen65 is offline glen65  United States
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Default Transistor failure, opinions please.

I replaced the output transistors in an amp several weeks ago
and they are out again. I noticed a circular rough surface on the
back of one of them when I removed it (see pic). I’m thinking that
there wasn’t adequate contact between the device and the sink.
What do you think? The device is an MJL21194.

Thanks
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Old 27th September 2006, 09:07 PM   #2
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Did you use silicon paste and mica washer?
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Old 27th September 2006, 09:54 PM   #3
anatech is offline anatech  Canada
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Hi glen65,
Looks like too much grease, but not a transistor killer normally. Did you use new mica insulators?

The other questions would target the driver and bias circuits, also the speaker load. I have seen speaker leads that run into walls that develop intermittent shorts.

When you did this repair, did you check the bias current? Was the current stable with time? Is there enough air flow where it was mounted. What was happening just before the failure?

Make and model number?

Many questons.

-Chris
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Old 27th September 2006, 09:56 PM   #4
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Quote:
mica insulators
yeah, the words I was searching my mind for.
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Old 27th September 2006, 10:12 PM   #5
anatech is offline anatech  Canada
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Hi darkfenriz,
You can see the heatsink grease squeezed up through the central mounting hole as well. A sign of too much grease.

-Chris
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Old 27th September 2006, 11:50 PM   #6
glen65 is offline glen65  United States
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What’s the make and model?”

It’s an Onkyo ASV610PRO integrated amp.

“Did you use mica insulators?”

Yes I used mica insulators,
I just pulled the insulator off of the heat sink and it looked
like there was very little if any grease on that side. I know I
greased it, What was there looked pretty runny almost like water
instead of white. I just purchased a new tube of GC brand heat sink compound before I worked on this unit. It seems to be quite a bit more on the runny side than the last stuff I bought which was the same brand.

“Did you check the bias current?”

I made all adjustments according to the service manual.
The adjustments were pretty touchy but once a had them
set they seemed to be stable. Their using a cheap single turn
pot. I think that this time I will replace them with a multi turn pot
to make the adjustments a bit more manageable.

“Is there enough air flow where it was mounted?”

Yes,
There was nothing sitting on top of the amp to restrict air
flow.


Here are a couple of pic that show the damage.
The driver IC is a UPC1342V which I am having quite a time
trying to locate.
Click the image to open in full size.

Click the image to open in full size.
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Old 28th September 2006, 01:35 AM   #7
anatech is offline anatech  Canada
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Hi glen65,
It sounds like your heatsink grease separated. It needs to be mixed. Somewhat difficult in a tube.

There is nothing wrong with the control. It may have too much range in the design. Multiturn controls do not do well in that application. There is DC current running through the control.

It also looks like the heat radiator has very high thermal resistance. Unless there are some healthy fins somewhere I wouldn't expect it to be able to get rid of a lot of heat.

If you didn't change the drivers and bias transistor, something may have failed later (related to the original failure).

Any of these things may have over stressed the output stage.

-Chris
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Old 28th September 2006, 01:41 AM   #8
PB2 is offline PB2  United States
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R53 looks cooked, don't you think?
Or is it just the picture?

Pete B.
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Old 28th September 2006, 01:46 AM   #9
anatech is offline anatech  Canada
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Hey Pete,
The one on the left looks worse!
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Old 28th September 2006, 01:51 AM   #10
PB2 is offline PB2  United States
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Hi Chris, Opp, yeah ... I thought that one was supposed to be black!
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