I completed my 3rd modification of a Sony 685 CD/DVD/SACD player by placing a 12AU7 ground cathode gain stage to take the analogue signal just before the OP amps (front right and left only).
A few problem developed that I have not had with my other exact mods to this player.
BUZZZZZ. Lots of audible buzz, too much to be listenable.
Loss of gain, a .5 volt square wave input nets a .3 volt output.
See screen shot, this is with the intput signal before the 1.5 K resistor, the lower trace is the output.
This is the trace with the signal inputed after the 1.5 K resistor.
The 3.3uF caps seem to be guilty as handling them increases the buzz.
Additionally the stage only biases in the .4 volt range on one side and .2 volts on the other. Changing tubes does not make a difference.
TubCad calculates that the bias should be -3.42 volts, a gain of 22.7 db, my B+ is 270.
POWER SUPPLY FILTERS
TUBE STAGE
Any suggestions for trouble shooting would be appreciated.
dr_sleep
An externally hosted image should be here but it was not working when we last tested it.
A few problem developed that I have not had with my other exact mods to this player.
BUZZZZZ. Lots of audible buzz, too much to be listenable.
Loss of gain, a .5 volt square wave input nets a .3 volt output.
See screen shot, this is with the intput signal before the 1.5 K resistor, the lower trace is the output.
An externally hosted image should be here but it was not working when we last tested it.
This is the trace with the signal inputed after the 1.5 K resistor.
An externally hosted image should be here but it was not working when we last tested it.
The 3.3uF caps seem to be guilty as handling them increases the buzz.
Additionally the stage only biases in the .4 volt range on one side and .2 volts on the other. Changing tubes does not make a difference.
TubCad calculates that the bias should be -3.42 volts, a gain of 22.7 db, my B+ is 270.
POWER SUPPLY FILTERS
An externally hosted image should be here but it was not working when we last tested it.
TUBE STAGE
An externally hosted image should be here but it was not working when we last tested it.
Any suggestions for trouble shooting would be appreciated.
dr_sleep
Well, I am concerned about the grounding scheme as the outputs and the gain section are grounded to the chassis while an additional separate ground is formed with the power supply, a ground taken from the cathode resistors, grid leak resistors and output ground. Perhaps using insulated outputs, not grounded to the chassis would cure the problem.
I still am confused about the bias which seems low for the operating conditions. All the resistor values check out and it is not tube related.
My plan will be to insulate the output from the chassis by changing the RCA's and not grounding the cathode resistors to the chassis also but just the main star ground.
Any other bright ideas?
Thanks,
a "Casual" dr._sleep
I still am confused about the bias which seems low for the operating conditions. All the resistor values check out and it is not tube related.
My plan will be to insulate the output from the chassis by changing the RCA's and not grounding the cathode resistors to the chassis also but just the main star ground.
Any other bright ideas?
Thanks,
a "Casual" dr._sleep
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