9710 VCA Hum
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Author:  Kyhotay [ Thu Aug 11, 2011 2:18 pm ]
Post subject:  9710 VCA Hum

Heya Scott,

I wanted to see if you have a solution to my 9700 VCA problem. On the "A" output I have a low frequency component to the sound. More of a harsh growl than a hum. The other VCA outs are all clean. I can send you a sample if you need to hear it.

Got any thoughts?

Brian Folkes

Author:  PAiA-Scott [ Fri Aug 12, 2011 9:22 am ]
Post subject:  Re: 9710 VCA Hum

It might just be the modulator on a fast cycle, bleeding through. Does adjusting the AR controls affect the frequency of the tone? Check page 22 of the manual for details about the adjustment of the Carrier Null trim (set the AR to an audible cycle rate and adjust to minimize the tone).

Author:  Kyhotay [ Fri Aug 12, 2011 10:51 am ]
Post subject:  Re: 9710 VCA Hum

Thanks, Scott, I'll check it out and let you know if that solves the problem or not.


Author:  Kyhotay [ Sat Aug 13, 2011 3:29 pm ]
Post subject:  Re: 9710 VCA Hum

It really did need some adjustments on both the MOD and CARRIER pots but, sadly, I still have the hum. It isn't a variable pitch. It does sound like it is probably 60 HZ. Again, the L, R and L+R VCAs are clean.

Author:  PAiA-Scott [ Mon Aug 15, 2011 5:41 pm ]
Post subject:  Re: 9710 VCA Hum

Maybe one of the voltage regulators has failed. This would let ripple though on the power supply voltage, and result in more, or less, voltage and be a reason the trims weren't as expected.

To check the voltage regulators, just measure for DC volts on the
324 op-amp power supply pins. Pin 11 should have the negative DC
and pin 4 the positive DC. Look for -12 and +12 readings at each of
these points with one probe on ground. If the tester is analog,
reverse the polarity so the needle doesn't try to swing the wrong
direction. Note the regulators are two different parts even though
they look similar. IC2 is a 7812 and IC1 is a 7912.

The V+ circuit runs up to the front panel to R109-1. From there is
also runs over to R111-3. Look for any possible contact with nearby
ground points in this area. Note the proximity of R111-3 with the
mounting rails on the FR7 on a 9710 in a P9700S.

Feel the ICs to see if any feel particularly warm or hot. Double-check to be sure the transistors and ICs are in the proper
places with the polarity observed.

IC7 is associated with the modulator and it's LED which should light
with the cycling of the modulator. Output pins on sections of IC7
should show dc voltages that vary in a range from about 0-10v dc at
the rate the LED should be varying (according to the modulator A D S
and R controls).

Here are some other readings in the VCA A section going sort of left
to right from IC6c to IC6D according to the schematic. Set ADSR
controls to 9:00 and modulator to Cycle.

IC6pin9 0V
IC6pin10 0V
IC6pin8 4.9

R34 adjusting disk (appx. calibrated setting) 4.4V

IC4pin10 5v
IC4pin8 5v
IC4pin2 -0.73
IC4pin3 -0.074 to -0.091 (-0.74 to -0.91) edit 8/23
IC4pin1 -0.01 to -0.02
IC4pin4 -0.02 to -0.20
IC4pin14 -12
IC4pin5 -10.47
IC4pin6 6.33 to 6.35
IC4pin12 6.35 to 6.37

IC6pin13 4.75 to 4.90
IC6pin12 5.35 to 5.37
IC6pin14 -0.04 to 0.23

Author:  Kyhotay [ Mon Aug 15, 2011 7:36 pm ]
Post subject:  Re: 9710 VCA Hum

A thorough reply, as always, Scott! I'll take a look at the measurements and let you know what happens.



Author:  Kyhotay [ Sat Aug 20, 2011 5:05 pm ]
Post subject:  Re: 9710 VCA Hum

Happy Saturday, Scott!

I Followed your instructions. Everything seems to be within the range of your specs. I'm not quote sure about IC7 I checked the outputs with the ADSR at 9 and on Output 1 it was .22 - 8.0, pin 8 was -11 to +10 and 14 held at 10.82.

R34 measured 5.0

On IC 4 did you mean for pin 3 to have a swing from -0.074 to -0.091? I'm reading -0.73 to -0.98. Pin 4 is -0.01 to -0.30. Pins 6 & 12 are steady at 6.2.

On IC6 pin 13 holds at 5.44, 12 at 5.41 and pin 14 swings from -0.155 to -0.170.

The space between the bottom row of pots is tight so I bent them up a bit for better clearance.

It seems to sound better but I can still hear the hum. It totally goes away using the next gen dual VCAs. Do the readings above help narrow things down?


Author:  PAiA-Scott [ Wed Aug 24, 2011 3:42 pm ]
Post subject:  Re: 9710 VCA Hum

These all look in order Brian. I checked on the IC4pin3 readings and sure enough, I'd noted those an order of magnitude too small (I fixed this in the earlier post).

I listened to the one here and could hear some hum too. After making a few checks, I found the simplest fix was too increase the filtering on the dc supplies from the MIDI2CV8 out to the module. This can be a patch on the MIDI2CV8 board, or, wired into the 4ckt power connection tap. I'll mail you the 'insert' I made for checking it out. Unplug the VCO from the MIDI2CV8, and plug this in its place. Then plug the VCO into it.

I'll get back with a schematic and photo of the board patch and insert assembly as I can get these things together.

Author:  PAiA-Scott [ Thu Aug 25, 2011 1:06 pm ]
Post subject:  Re: 9710 VCA Hum

Below is a link to a pdf with details for adding resistor-capacitor filtering to minimize ripple on the DC supplies of a P9700S. Adding this should make any hum through the 9710 A VCA much less noticeable, if at all.

DCfilter82511.pdf [723.61 KiB]
Downloaded 1190 times

Author:  Kyhotay [ Wed Sep 07, 2011 11:31 am ]
Post subject:  Re: 9710 VCA Hum

Scott, I did get the ripple reduction module the other day. I have't had a chance to install & test it since getting my brand new knee replacement 2 weeks ago! Would there be an advantage to this type off added filtering to the VCO or VCF?

Finally, would there be an advantage to using the rack wing power supply in place of using the MIDI>CV for power? Is the wing supply any


Brian Folkes

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