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Old 18th May 2009, 09:02 PM   #11
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It is quite rare to encounter such a level of mismatch between parallel
bipolars as the Vbe is quite consistent without matching. Matching simply
means selecting for high beta (current gain) for low distortion. It may
be that the resistor in question had been toasted to a higher value or that
the transistor was experiencing some sort of failure.

If you can obtain the same part numbers, I would simply replace the broken
parts and try again. Get a Variac - you will need it.

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Old 19th May 2009, 07:53 AM   #12
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Well, the amps are already 25 years in service so a failure may be expected after such a time.
When I got the amps they both had a biasvalue measured over several emittors of 6 mV. That was way to low the amp only became 30 degrees Celcius warm. After turning up the bias problems maybe emerged after the amp got stressed a bit more.
Luckily my Acoustats were also protected with a 5 amp fuse that did blow also.
In Europe we expect that all American 19 inch highend equipment, that looks to be built to last forever, you don't expect a failure of this magnitude.
I just have to find the time to correct this problem.
Thanks for the support though.
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Old 20th May 2009, 07:55 PM   #13
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Found this Threshold spec on this forum about biasing values fort all the Threshold amps kindly supplied by member R-K Rønningstad. Plus the comlete procedure how to bias the amp wit graphs and sequential instructions:
Threshold 400a Parts Substitution & Mods.
Wish I had it before...
It specifies for the non opticalbias versionof the SA/1 and the S/1000 to by the way only 42 degrees Centigrade final value after a one hour warmup.
The temperature rod should be in the last hole from the back of the amp.
I learn in the proces....
Can't open the amp where I needed because do not have the right American 1/8 Inch hexagonal wrench that's long enough to get one section of the heatsink of so I can't find out what kind of NPN/PNP powertransistors this amp uses.

Probably paired ML15024/25 but I'm not sure.
Anybody knows for sure?

'member djk' suggested to replace them all (40) but I'm reluctant for obvious reasons to perform such an action.
I'm inclined to let a skilled technician who also is the Krell technician for the Netherlands to do this job but I'm willing to give it a try to get more hands on experience with this kind of work and it's rewarding when it succeeds. So I ordered a Variac and I give it a go.
Any last hints?
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File Type: pdf threshold bias procedure attachment4.pdf (45.6 KB, 91 views)
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