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Fisher CA-800 Rebuild Thread!

Eastham said:
You say again, what transistor did you replace Q10 with and did you replace Q8 with it's counterpart? I think the KSA1220AYS and KSC2690AYS, would work for replacements if anyone wants to confirm this. You should gain match the pair for each channel and replace one with it's matching counterpart.
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Those 2 are exactly what i used to replace the original drivers. i believe the originals were 2SA1047 and 2SC2441
 
Bassblaster said:
Those 2 are exactly what i used to replace the original drivers. i believe the originals were 2SA1047 and 2SC2441
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Strange they would blow, perhaps they started oscillating and went kapow. What's the voltages looking like on the solder pads of Q10 with Q10 removed?
 
Bassblaster said:
B 44.9
C 0
E 43.5
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Those voltages are low, they should be 55v on the emitter and 54.4v on the base but I wouldn't consider that the reason the driver transistor blew. The collector capacitance is much higher aswell as the hFE on the replacement part. I guess powering the circuit up and full voltage threw things into oscillation and blew the positive side driver transistor check Q8 and make sure it's not blown aswell, I don't think the KSA1220 and KSC2690 are a good pair of this circuit after all. That's the only theory I can come up with. Anyone got any suggestions for a better driver pair?
 
It should actually be closer 65V. im running it on a 40W DBT. Q10 KSC2690 tests good in my tester. Q8 also tests good in the tester aswell
 
Bassblaster said:
R26 good. reads 10.5 Ohm
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That's odd, the only other reason for the low rail voltage I can think of is that it may be due to it being powered on a DBT. Strange both Q10 and Q8 are reading good yet you're getting high voltage on the collector of Q10.
 
well i know the low rail voltage is from the DBT. on the DBT there's only about 75V going into it. i may just end up replacing every semiconductor on this. few transistors, couple diodes. cant be that hard. ill do some more testing in the morning.
 
Bassblaster said:
well i know the low rail voltage is from the DBT. on the DBT there's only about 75V going into it. i may just end up replacing every semiconductor on this. few transistors, couple diodes. cant be that hard. ill do some more testing in the morning.
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I don't think shotgunning parts will get you very far, perhaps Q10 and Q8 are testing good but failing when power is applied. Try replacing them with another matched pair first. Voltage on the collectors could also be floating high with the module removed.
 
Got it working again! issue was what i suspected. the A1240 differential pair was bad. replaced with matched KSA992 and we're back in business!
EDIT: Reminds me of my Pioneer SX-636 i worked on a year ago. it had rail voltage on the output, the 2SA726's in the differential pair were bad
 
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Thats all i have left, and im not sure where to set it at. so i left it on the midpoint.
But i replaced the A1240 on the other channel, so now both channels have the same parts in them

IMG_2599[1].JPG

IMG_2601[1].JPG
 
Bassblaster said:
Thats all i have left, and im not sure where to set it at. so i left it on the midpoint.
But i replaced the A1240 on the other channel, so now both channels have the same parts in them

View attachment 1143447

View attachment 1143449
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I believe rcs16 said, to set it at 15mV accross R33A and R33B so one probe to the side of R33A thats connected to the emitter of Q4 and the other probe to the side of R33B that's connected to the emitter of Q1.
 
Bassblaster said:
So i would poke across pin 8 and 3 right?
View attachment 1144012
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Nope, they're on the output side of the IC, you can also just connect either probes to the emitters of both output transistors, so one probe on the emitter of Q1 and one probe on the emitter of Q4. Also, let the amp warm up to operating temp then make your adjustments then check again after 20 minutes or so.
 
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