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Dynaco-Doctor Forum Welcome! Need support, you got it. Or share your ideas and experiences. |
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Dynaco-Doctor Forum Welcome! Need support, you got it. Or share your ideas and experiences. |
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In Reply to: Dynaco ST-70 Power issues posted by mjstriker5@yahoo.com on February 24, 2014 at 17:11:37:
The testing of the power tranny can get quite extensive. Another source of the power issue could just be the on/off switch had its day, unless it was already replaced. Also, if it had the power supply board mod, why did you change the quad cap? That is usually disconnected.
You have to see if the primary wires to the power tranny is getting AC voltage. I would also change the fuse even though you might think was good. It uses a slo-blo. make sure you put in the proper amperage value. Don't go higher. It is sized for the circuit.
As for the bias supply. You can put in a larger value resistor(3-5K larger) in there to let less negative voltage to pin 5. Could be just those tubes need a larger range across the pot. 1.4VAC is good for both the tubes. With the higher VAC voltages coming out of the wall outlet the tubes will draw more than they should if kept at the stock 1.56VDC. Also, the bias resistor from the tube pin 8 to ground might be out of tolerance. That should be changed to a 10 ohm and then bias both tubes at .80VDC(40 MA per tube if they are matched).
You should also measure pin 5 of both the output tubes to see if you have the proper negative voltage. Should be about -36VDC or so(I forget totally.
The big thing right now seems to be the power tranny not turning on. I really don't thing you did anything to blow it up. That's what the fuse protects, among other things.
Hope this helps!
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Follow Ups
- RE: Dynaco ST-70 Power issues - xaudiomanx 15:27:59 02/27/14 (2)
- RE: Dynaco ST-70 Power issues - mjstriker5@yahoo.com 15:48:44 02/27/14 (1)
- RE: Dynaco ST-70 Power issues - xaudiomanx 22:48:06 02/27/14 (0)
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