HELP!: HH1 Rev D - IC8/R23 differential gain calibration issue

[barwildi]

I am building a HH1 and I am following Carl's cal. procedure, adjusting R23 for null on pins 2 & 3 of IC8. However, the issue is that R23 has NO effect on the pins' voltage level. Currently, voltage ACROSS pins 2 & 3 is 0.00 V and voltage for both pins when referenced to ground is +1.95 V. Voltage on both pins stay constant at +1.95 V (ref ground) regardless of R23's wiper position. I know I am trying to achieve 0 V, but this does not seem like normal operation. Correct?

Here is what I have done so far for this part of the process:

- set R18 for max on TP4
- checked power pin voltages
- shorted SW2
- set R34 to max (also tried min)
- checked PCB for bad joints and shorts
- made sure Q8, Q9, and IC8 are removed
- shorted IC8 pin 1 to ground via IC9 pin 4

Not sure if it matters, but R24 & R25 are 1%, while R26 & R27 are 5%. This was not on purpose, it's just what I had on-hand. Also, the board is a Rev. D from Silverdog's site.

Other than start swapping parts with spares, I'm not sure what to do next. I have been able to resolve a few minor issues on my own so far. But now I'm stuck. I do have a scope if needed, but I'm working in limited space right now and just trying to push forward to complete the PCB with basic measurements, if possible.

I would sincerely appreciate any assistance.

Thank you,

Barry

[6666]

Is r23 mechanically faulty , that is, does the wiper move ?

[barwildi]

Is r23 mechanically faulty , that is, does the wiper move ?

I had ruled that out since I can feel the "clicks" when I adjust it to both extremes (multi-turn). I suppose there could still be a problem with the wiper. I'll remove it and check if it is functional.

[barwildi]

R23 checks good out of circuit. The wiper varied resistance from both ends over the entire resistance range. Also, I replaced IC7 with a spare and verified that R22 is shorted. Unfortunately, no change in operation. I still have the issue.

[barwildi]

May be no problem

Maybe my issue is illegit. I have searched the forum to exhaustion trying to find an explanation as to why, contrary to the original HH build instructions, R23 may not affect
voltage on IC8 pins 2 & 3. I found 1 or 2 threads where other folks had no reply.

I have decided to take Reg's advice from a few years back on this post: http://www.geotech1.com/forums/showt...8477#post68477

I am only trying to understand what - if anything - could be incorrect in the circuit and correct it before I finish building it, since (I assume) that was the purpose of the stepped calibration and testing.

Hopefully this thread and Reg's advice above will help others in the future.

- Barry

[greylourie]

Looking at the schematic, if you are set for max out of TP 4.... it would be reasonable to see changes on pin 1 IC7a and pin 4 of IC7b - when adjusting R23.
Checked out the post you linked to above. Good info there. Based on his Regs input, if I were building the HH, might be tempted to do away with R23 altogether...

Real mystery why that R23 is not showing changes in output when adjusted. What if you try with IC7 removed ?

[barwildi]

...What if you try with IC7 removed ?

I did try with IC7 removed. Results were the same, even checking pins 1 and 4 of IC7.

With the voltage at max on TP4, it almost seems that R23 needed to be a *larger* value. So I tried reducing R18 for about +0.3V on TP4. Still no change.

I still have to make a test coil and put everything together. Just have not had enough time yet. Hopefully this issue is not going to be a problem once everything is done. But I have my doubts.

I do thank you for your suggestion and if you can think of anything else I may have missed, please do reply.

- Barry

[greylourie]

Cant think of anything. Go with the info from that post you linked, set the pot so reading from wiper to either output of the pot gives an equal resistance is all you can do....... At least that way, one uncertainty is crossed off your build/calibrate list.