Talk:013-0086-00

When I got to this part of the calibration of my 3A8, I did not have this accessory. Upon reading the calibration procedure, I realized that Tektronix was simply providing a way to watch the Nuvistor grid current charge up a capacitor, and then adjust to null out that current. This procedure required me to adjust and watch for a falling or rising voltage for several seconds and then readjust. This was going to take a LONG time. I realized that all I really needed was a sensitive meter that I could connect to the 3A8 input and watch while I adjusted the pot for null.

I happen to have an HP419A, which is an analog DC meter with a zero center scale that will read down into the pA range. I hooked it up, let the scope and 3A8 warm up thoroughly, and then adjusted it to a null. The HP419A made this trivial. I was able to adjust the grid current to much less than the Tek spec. The limiting factor not the meter, it was how finely I could turn the pot.

This needs to be repeated for each channel.

I let the whole system "burn in" over night and checked again. It had drifted a bit, but was still well under spec. I gave it a very slight tweak and called it done.

In my case, the Nuvistors were the originals in that module, so I think they have already aged as much as they are going to. If you have new Nuvistors, you may need to do this again after a few weeks of use.