S&H Only sampling negative voltages

[numbernone]

Revisiting my non functional S&H module, I have it back on its feet. However, it is only sampling negative voltages.
With a bipolar input it is going negative and staying at its baseline for the positive excursions. For positive only voltages, no effect at all.

Any leads appreciated.

[modulus in moduli]

I don't have this module, nor have I studied it in detail, but if nobody else gives you advise, Ray's documentation is actually pretty good, so I'd try to check these:

U1D, pin 14 should be at v- (-12volt if that's what you use) except for small pulses, when it goes up to v+. Does that work?

Do you get the pulses on the gate of Q6? I'm not sure if they would go all the way to v+, probably not but I think they need to be high enough to open the Q6 for all input voltages. If they only go to -5v, that might be your problem.

You can also check pin 1 of U3, that should be an attenuated version of the input signal.

I hope you have an oscilloscope.

In any case I think your problem is might be around Q5 and Q6. Try to verify all the resistor values around there (R39, R40), see if the nfets are the correct type and if they have signs of melted plastic (meaning they are probably toast). If you have your opamps in a socket you could try replacing those. The nfets are probably the easiest to blow up. But desoldering them is a pain and when you do that too often you can damage the pcb. So if an nfet is blown, make sure the new one won't blow up right away too.

Hope this helps.