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Dynaco PAS 4 phono stage is barely audible

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Apr 28, 2021
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Hope this is in an okay place considering the blurb for this section was about showing off DIY built stuff. I have a 90s model PAS 4 and the phono section is extremely low/quiet. I swapped the tubes for the phono sockets into an SCA 35 and the record player produced the regular level volume you'd expect. The other inputs for cd etc produce regular volume in the PAS 4. If the tubes work in an alternate integrated with phono stage fine enough, and the other inputs work on the PAS 4... any ideas what the problem could be? Its an early 80s Denon deck with a 2M bronze Ortofon cart. Also, I plugged my phone into the phono stage using TRS headphone to stereo RCA and the volume from it is also extremely low. On a Dynaco specific forum I asked for help and the single reply that I got was that this particular preamp has a weird biasing and that I should verify that C207 negative reads -3.3 volts. I got it to a work space and it read -2.93 volts, but then the user quit responding. Any ideas would on how to troubleshoot/fix would be greatly appreciated.
 

DVDdoug

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May 27, 2021
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Also, I plugged my phone into the phono stage using TRS headphone to stereo RCA and the volume from it is also extremely low.
If it was working that should blast the thing into bad clipping/distortion. A phono preamp has a gain of about 40dB (100X) at mid-frequency.

I found the schematic if you don't already have it. There are multiple 300V power supplies and you might have a bad MOSFET or something else in the power supply.
 
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