my wonky voltage quantizer

kindredlost · Joined: Mar 02, 2011 Posts: 22 Location: usa

My voltage quantizer is not working at all. It is outputting zero volts in WHOLE and FOURTHS mode and 90mv in HALF mode. No response from either the Initial Voltage Pot or external voltage inputs.

I'm running on a 15 volt supply but I've changed R13 to 51K as specified.

My scope read 52.17KHZ on pin 2 of U1 clock. I changed R1 to 220K so it is slower at 47KHZ but there is still no improvement.

I have some over-rated caps. C18 (.002uF) is 500 volt rated and C1 through C4 (100pF) are 200 volt rated. I have some 50 volt caps coming in.

Any ideas of where to check?

forshee · Joined: Dec 16, 2010 Posts: 46 Location: Hopkins, MN

I'd check your off-board wiring first that's always easy to screw up. But if the switches are wired right I would guess its in the ADC section. But poke around with a scope and see what info you can get.

Don't worry about a high voltage rating on the caps. As long as its 35v or better and fits on the board it shouldn't be an issue.

kindredlost · Joined: Mar 02, 2011 Posts: 22 Location: usa

Thanks forshee.

I'm leaning toward the DAC as well. I need to get a good picture in my mind about the counter behavior so I'll be doing a down 'n dirty on that aspect next.

The thing that has me really baffled is that I've built four of these to have as a quad module. All of them output the same thing exactly.

It's maddening because I tend to troubleshoot back to the supply voltage and start again everytime. The clock works great and the signals all look good on the inputs and thru the clocking portion of the circuit so the supply seems right. Such as positive leading edge on the CLOCK waveform and negative leading edge on the NOT-clock etc..

I have noticed the DAC does not step up to 1V+ and trip the TL071 to change state. There is a static high voltage coming from U5-A pin 1. It seems as if the entire system is stuck in reset so there is no comparator or counter operation. Is there a chance the U4 flip flop is not getting a clock? (thinking out loud now, or better yet - grasping at straws).

It seems I still have a lot of details to check, but that pesky global problem shared across four modules haunts me.

Thanks again for your insight forshee.

forshee · Joined: Dec 16, 2010 Posts: 46 Location: Hopkins, MN

Rarely you get a bad chip but it is something to check. And lots of places use confusing house numbers on CMOS stuff. Just something to think about since I think I just got a bad set of OTAs.
But if your building them assembly line style its easy to make the same error four times.