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SA-8100 bias issues

PaleAleKing

PaleAleKing

New Member
Hey all,

I'm wrapping up service on a SA-8100 and came across a weird biasing problem I haven't seen before.

I just rebuilt the amp board and in part used KSC3503D for the dreaded 2SC1451s. It comes up fine on the dim bulb, except when it's time to adjust the bias. I can set 1 channel to 20mV no problem. When I go to adjust the other channel, the bias in both channels is increased nearly two fold as I approach the 20mV value - starting around 15mV and up. What?! This behavior can flip flop channels too. Resetting the new bourns trimmers to zero and pulling the amp up off the dim bulb results in the bias being too high for both channels without any adjustment. Something is clearly wrong here.

Now, if I put the original 2SC1451s back in, all is normal, on or off the dim bulb. I can adjust each channel's bias independently as expected. 20mV-20mV. The only thing I can think is that the 3503D doesn't have enough gain (HFE ~80 vs 300) and the bias circuitry is unstable. I purchased the 3503's from Digikey, so I don't think they are fakes, etc.

I've heard that the KSC2690Y may be used to replace the 2SC1451. It has a higher gain, but the Cob is higher ~10pF vs ~2pF. Is that enough to worry about? Also, regarding the 1451 Cob, does anyone know what Collector-Base voltage is being applied to determine that spec?

Very strange!
 
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I received a batch of KSC2690AY today and gave those a go on the SA-8100 amp board (AWH-022-B).

Here are some notes:

Amp board rebuilt, but the outputs are original.

The unit is on dim bulb.

MJE15030/31 for 2SD357/2SB527 x2 - I chose these at the recommendation of others here at AK since the compliment to 2690 is EOL.

KSC3503DTSU for 2SC1451
Oscillating? Bias issues as described in the original post.

Put back original 2SC1451, bias adjusts normally.

KSC2690AY for 2SC1451
Bias in the 50s 60s for both channels with trimmers at zero. Damn.

Put back original 357/527 x2
KSC3503 for 2SC1451
Bias adjusts normally!

Boy, this one has some attitude. Very delicately balanced.

I'll leave the 357/527 alone for time being, but I wonder what people are using as subs these days? I heard Toshiba has some offerings folks were messing with?

Anyway, I'm posting this for others to find.

Solved.
 
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