Another Dual VCA question

[Captain Biscuits]

I recently built a dual VCA. The B channel kind of worked, A didn't work. Having checked over the board I replaced both of the chips this evening and now both channels work but I'm not convinced that they're working as they should.

I've looked on Ray's page and he shows some very clean and clear oscilloscope shots of triangle wave modulation which are symetrical around 0v.

When I try to duplicate Ray's traces I get very different results. Everything is above the zero line.

I've adjusted the trim pots as per Ray's instructions. I can't entirely get rid of the CV bleed through but can get it pretty low.

The VCAs do work but seem a bit odd. Am I doing something wrong? I've only bought the oscilloscope recently so am not very familiar with it, but everything else I've used it for has made sense. This is baffling me.

Can anyone out there tell what might be happening from these pics?

Many thanks

Ian

---

IMG_2508.JPG
Description:	Linear
Filesize:	1.75 MB
Viewed:	244 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

---

IMG_2509.JPG
Description:	Log
Filesize:	1.76 MB
Viewed:	243 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

---

[Captain Biscuits]

ahhh - having followed the adjustment instructions to the letter I've just connected the oscilloscope and wiggled trim pots randomly until the trace looks right. It is heading in the right direction!

I'm too tired to play with it now so I'll have another look tomorrow.

Still very grateful for any advice

Cheers

Ian

[Captain Biscuits]

Putting aside Ray's method and just using the oscilloscope I've managed to get my VCAs looking and sounding lovely.

I've no idea why Ray's method wasn't working for me but watching the output on the oscilloscope and using the bias pot to centre the wave either side of 0v and the trim pot to get the amplitude sensible all is working as expected.

Cheers

Ian