I finally have the gear I need to test and calibrate my Pharmasonic 704 VCA's. I've tried following the directions provided by Pharmasonic to calibrate it and am not getting the desired results as far as I can tell.
I tried to follow the Linear Adjustment step in the calibration instructions:
Here is what I did: I've got a Mutable Instruments Shades set up to provide a +10V signal into the 1st of the 2 modulation inputs on the module. All pots are turned fully 'off,' and the switch on the VCA is set to 'linear'. I have the VCA out headed into a voltage meter so I can monitor the output voltage.The 1st step is the adjustment of the DC balance.
Set the switch to LIN, all pots to 0 and feed one of the modulation input with 10V.
Probe the VCA output with an oscilloscope (use high gain) and adjust the DC BAL
trimmer so changing the CV amount produces minimum change.
If I understand the instructions correctly, I should be watching the output voltage of the VCA while adjusting the voltage of the signal being fed into the modulation input, and then adjusting the DC BAL trim until I don't see the VCA output signal changing as I adjust the input CV level.
What I'm seeing is that the VCA output is at a constant +6.5V regardless of what is fed into the modulation input, and adjusting the DC BAL trim doesn't seem to change this result in any way.
Next calibration step:
When I send a 10Vp-p sine wave into one of the inputs (knob turned up to 10) and a 10V CV input into one of the modulation inputs (also turned up), I am seeing a square wave output that is clearly heavily distorted. Adjusting the VCA gain trim does not seem to change this behavior in any way.Once done, feed one audio input with a 10Vp-p SIN wave and set the level to 10.
Feed one of the modulation input with 10V and set the amount to 10.
Probe the VCA output with an oscilloscope and adjust the VCA GAIN trimmer so you get a 10Vp-p SIN wave.
So my takeaway is that something is probably wrong, and the 1st thing I suspect is my ill-gotten 3080 ICs from China, as the LM301's that I sourced in a similar fashion are bad and folks here are telling me not to order cheap components from China again, and also that there are a lot of bad 3080 chips out there. So I'm hoping someone will be able to help me to troubleshoot one of my VCA's so I can confirm that the 3080's are the root of the problem and I can some replacements ordered. If they were cheaper, I would just order them straight away, but at $5 a pop, I figured it would smart to check this before hitting 'buy', as I'm going to need a lot of them. The good news is that both VCA's are displaying this same behavior, so at least my problems are consistent. I've also done some preliminary tests with my LFO's and have had bad results from those as well, and they also sport the 3080 chips in them, making them even more suspect.
Would anyone be willing to help me determine that things around this chip are good so I can confirm that these 3080's are crap? Many thanks for any assistance.