Dynaco SCA-80 No Bias (fixed bias amp)

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Raccoon1400

Raccoon1400

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Joined 2013
#1
I'm working on a Dynaco SCA-80 and there is no bias in one channel. The other channel has about 40-50mV across the emitter resistor.

I've tried a different set of outputs, and swapped the drivers from the other channel but no change. Last time I looked at it I checked most of the resistors and they seemed good.

I'm not sure if something is wrong or if something is just a bit off spec. If the later, I could modify this amp to have an adjustable bias? What is my best option/course of troubleshooting?

I should also mention it shows 10-15mV bias when I power it on then it quickly goes to 0.
 

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#3
Raccoon1400 said:
I'm working on a Dynaco SCA-80 and there is no bias in one channel. If the later, I could modify this amp to have an adjustable bias? .
Click to expand...
Agree with John Snell that C3 & C4 could be problematic at this date. 5.1 v zener can be measured although zeners this old are also often a problem.
This bias arrangement was notorious for not providing enough bias cold and soft. Loud & warm was okay.
d joffe designed a closed loop bias control circuit at 20 ma for the ST120, looks like it could be used here too: Dynaco Stereo 120...can be beautiful
I've had it blow up the sense transistor and go to 200 ma bias, so don't use 2n3904 for the sense. I've had better luck with ztx653 but more common 2n5551 might work.
My pcb got rather damaged on one channel in the ST120 and replaced it with a point to point AX6, which also sounds rather nice. It has a conventional bias control pot, which I paralleled with a diode as a clamp if the wiper oxidizes and loses contact. Reused the output transistor & capacitor. Retro Amp 50W Single Supply Warning this AX6 is not compatible with PAS3 preamp, input resistance is too low. My mod on page 22 might have worked but haven't tested it with the PAS2 yet, which developed paper capacitor problems that weren't resolved with mylar capacitors.
 
#4
indianajo said:
Agree with John Snell that C3 & C4 could be problematic at this date. 5.1 v zener can be measured although zeners this old are also often a problem.
This bias arrangement was notorious for not providing enough bias cold and soft. Loud & warm was okay.
d joffe designed a closed loop bias control circuit at 20 ma for the ST120, looks like it could be used here too: Dynaco Stereo 120...can be beautiful
I've had it blow up the sense transistor and go to 200 ma bias, so don't use 2n3904 for the sense. I've had better luck with ztx653 but more common 2n5551 might work.
My pcb got rather damaged on one channel in the ST120 and replaced it with a point to point AX6, which also sounds rather nice. It has a conventional bias control pot, which I paralleled with a diode as a clamp if the wiper oxidizes and loses contact. Reused the output transistor & capacitor. Retro Amp 50W Single Supply Warning this AX6 is not compatible with PAS3 preamp, input resistance is too low. My mod on page 22 might have worked but haven't tested it with the PAS2 yet, which developed paper capacitor problems that weren't resolved with mylar capacitors.
Click to expand...

I'll check out his mods.
I should have mentioned all electrolytics are new.
 
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