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Old 3rd December 2012, 12:47 PM   #21
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i'd be checking the caps on board three for shorts or high ESR.
if the problem is showing up on other boards it may a main supply fault as in bad bridge rect.
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Old 3rd December 2012, 12:49 PM   #22
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argh now no sound from 991 right at all. Bias fine on both
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Old 3rd December 2012, 12:49 PM   #23
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Keep it to just the one unit and problem Things get really really confusing otherwise.

So it looks like the bias was sky high. Did you use the bulb tester ? because that should have lit brightly. Also in post #11 I mentioned,

1. Disable the vbe multiplier to reduce output stage current to zero on both channels.

Thats a good safeguard for initial testing because it forces the bias to zero (assuming no other issues are present) and allows confirmation that all is OK.
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Old 4th December 2012, 07:29 AM   #24
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Hi,

I must admit I do not own a bulb tester or know how to disable the vbe multiplier, so I might have skipped those steps =-(
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Old 4th December 2012, 09:29 AM   #25
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While I am here what are your guys thoughts on these photos, is this glue or a bled caps?

Evidence for glue : On same caps on both boards, on the larger caps

Evidence for bleed: It is reacting with the copper rails. The glue for screws is green and the glue for the caps looks different?

Any ideas?
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Old 4th December 2012, 09:29 AM   #26
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Click the image to open in full size.


Click the image to open in full size.

Click the image to open in full size.

Last edited by inteificio; 4th December 2012 at 09:33 AM. Reason: Incompetence.
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Old 4th December 2012, 09:51 AM   #27
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Glue
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Old 4th December 2012, 10:58 AM   #28
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I checked with Rotel.
It is glue that becomes conductive with ages (what a crap glue).

So current had been flowing through the glue precipitating crystal growth.

Time to clean off some glue!!!
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Old 4th December 2012, 11:45 AM   #29
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Glue is somthing of an industry wide problem...

The Vbe multiplier can be simply shorted out as shown with a bit of wire. This removes the bias from the outout stage. A good amp will still work normally, all you might hear is a little crossover distortion at very low volume giving a "rough" sound).

The bulb tester is a 100 watt mains filament bulb in series with the live mains feed to the amp (used like a fuse)
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Old 4th December 2012, 12:24 PM   #30
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Wish I had asked for clarification earlier, that is quite simple.

If only they still made GLS lamps. I'm sure I can find something though.
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