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Old 18th April 2013, 10:25 PM   #1
srinath is offline srinath  United States
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Default How did I blow this fisher BA6000

I have this fisher BA6000 with the stk0100ii power amp chip.

That amp came to me with a dead stk chip, I pulled the good chip and put it in he other ba6000 I had and sold that.

So now I needed 2 stk0100ii's which I got and installed in this amp.
It worked for about 2 hours at about around 1/3rd-1/2 volume. Then I saw 1 meter was not working, so I tapped like you knock on a door on the case just on top of that meter. I heard a pop in the speakers and the amp is dead.

The stk's have been removed and the safety relay clicks. There is no obviously burnt spots in the board. I can replace the stk's but is that all there is to it ?

This is the schematic. I have a feeling one or more of you guys would knock this out of the park like you did with my nad meter pegging issue.

Thanks guys.
Srinath.
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Old 18th April 2013, 11:05 PM   #2
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If tapping it blew it up then there must be a bad or loose connection somewhere.
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Old 18th April 2013, 11:51 PM   #3
srinath is offline srinath  United States
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Oooo worth checking, output panel especially or to the bottom case.

These generic chips also have a reputation for having no overload tolerance at all ... and I know they have legs that break off very easy, OK I will look over it.

Cool.
Srinath.
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Old 21st April 2013, 12:48 AM   #4
srinath is offline srinath  United States
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OK I have dirty and weaker output in the channel that blew the power amp chip.
Bad caps in signal path ? anything else ? Power supply ? that's common to both channels isn't it.
Cool.
Srinath.
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Old 21st April 2013, 05:07 AM   #5
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You don't say who you dealt with to replace those "hybrids". If they were pulls from scrapped amplifiers, it's likely they were marginal or already blown in one transistor, as you describe above. There is no real guarantee that pulls are fully functional and many that have been bought over the last 10 years or so, to nurse old amplifiers back to life, have to be returned for credit - not really a viable way to do business.

If the power supply rail voltages are correct and remain within acceptable limits under load, only thermal runaway or any DC above the correct marked bias level at R52,62/R51,61 would cause failure of these Darlington output stages (which is all the Hybrid modules are) unless they are driving a significantly heavy load or there is a short at the output. The protection circuits themselves are simple current limiters (Q25,26/23,24) and DC speaker protection (Q27-31), for both channels) so it is just a matter of time for this amplifier to fail due to unusual speaker loads, faults, intermittent shorts etc.

Check the voltages marked on the schematic of the upper channel. If there are significant differences in DC readings at the output (R95,96), for instance, it will be a failed module as you say. Then go on to compare DC measurements between relevant areas of the channels as they are shown - I can't read them on this attachment, perhaps others can. If everything up to the hybrids is OK within a few % and they were correctly fastened to the heatsinks, you only have the hybrids themselves or the output wiring and load to blame. There are other possibilities, but without other indications, this is more likely, I think.
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Last edited by Ian Finch; 21st April 2013 at 05:32 AM.
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Old 21st April 2013, 01:33 PM   #6
srinath is offline srinath  United States
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These stk0100ii's were not used pulls, these are new replicas. I have had enough BS with these, 1 of 3 came in blown, then I noticed soldering and desoldering maybe 1-2 times gets their legs to break off ... This amp needs to be killed. In nay case I will look for what you suggested, though a bad stk means I will not be buying any more - I may change the guts to one of em stk4046 kits off ebay, or something else. The problem is that its 57vdc. I cant really use it for an lm3886 or other amp.
Thanks so much, I knew I'd get my answer ... just may be not what I wanted.
Cool.
Srinath.
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Old 21st April 2013, 06:38 PM   #7
llwhtt is offline llwhtt  United States
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B & D Enterprises has real Sanyo STK0100IIs for $45. This is the place to get real Japanese numbered parts.
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Old 22nd April 2013, 12:19 PM   #8
srinath is offline srinath  United States
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I think I am going to go onto something else, I would probably try a stk4046 kit off ebay in this chassis. Cos the solder traces in both channels are begining to go, the safety relay has a melt spot in it, and several things are not on par.
Cool.
Srinath.
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Old 22nd April 2013, 02:38 PM   #9
jaycee is offline jaycee  United Kingdom
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The chassis looks nice - I'd reuse it and build a nicer amp in there.
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Old 23rd April 2013, 02:00 AM   #10
srinath is offline srinath  United States
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Yea I know right. I am hoping to even have the meters working.
First however, I would rather try my hand @ this el cheapo Onkyo TX27 I got ... so I can get some momentum going. The nice trick would be to just eliminate the power amp section, but leave the tuner, the switching and everything else intact ... I am even thinking of how I can leave the protect mechanism in there. That amp basically fried a few resistors in the output in 1 channel, without frying the chip or anything else.
The relay clicks, the amp makes sound in 1 channel good but scratchy and crappy in the other.
Tomorow I will try swapping the 2 at the amp board to see what happens - if it switches sides.

Cool.
Srinath.
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