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« Last post by W1DAN on January 30, 2021, 05:40:26 PM »
Per Anders:
More tests on IC5:
Add -15v to IC5 socket (no IC inserted) pin 5, TP1 moves from -180mv to -190mv
Add +15v to IC5 socket (no IC inserted) pin 5, TP1 no change T -238mv.
Shorting IC5 pin 5 to ground, TP1 = -237mv.
New tests on IC1:
IC1 (OP07C inserted), input shorted to ground, IC5 socket empty:
Pin 1 = +14.975
Pin 2 = -1mv
Pin 3 = -149mv
Pin 4 = -15.011v
Pin 5 = +0.2mv
Pin 6 = -14.321v
Pin 7 = +15.033v
Pin 8 = +14.919v
With IC1 removed, TP1 is at -300mv. Adding -20v to IC socket pin 6 (no IC inserted), TP1 reduces to -170mv.
Added 22K across R29, added 100k across R27. Bench supply of -9.5v at pin 6 now creates 0v at TP1!! With IC1 (OP07) installed, TP1 is now -7.5mv. Good! Will this increase noise from OP07 to preamp?
Inserting a 1KHz sine wave to the input causes IC1 servo to oscillate at about 40hz. Adding 10uf at the output of R29 eliminates this. Using an oscilloscope, I now see crossover distortion on the test sine wave at the card output, possibly from the RIAA stage-will look at this later.
If this servo change is approved by you, we now need hints on eliminating the 284MHz RF oscillation (this is not from a Radio Sweden International transmitter, but generated in the preamp). With the input shorted, the RF signal is 70mv p-p at TP1.
Thank you!
Dan