I took another whack at it tonight and it seems to be working
Here's the AC Zero circuit. What I did was short out that 15K resistor and rotate the control all the way over. Effectively, grounding the end of the 56M resistor.
Actually, I did temporarily directly ground the resistor and it had the same effect, but I want to preserve the control action in case I did need it at some point.
That brought the needle down to 0 on every AZ range.
What I found confusing was why did the AC range need a special Zeroing control at all ?
I figure maybe the original EA53 diode tube introduced some offset and this control nullified it ?
I see that the
tubedepot list price for a EA53 is $99.95 each
At any rate, I rigged up a crude probe with a 1N4007 diode, fed in some AC and compared it to my digital scopes RMS reading. I can get them to agree fairly well.
That leaves picking up some better HF diodes and making up a real probe. A trip to the hardware store turned up these parts.
I'll cut the tube down a bit shorter and stuff the four components inside. I'll solder a short ground wire and clip right to the tube.
I figure I can drill a hole in the end cap for the coax and a hole in the other end for a probe needle.
I'll put some teflon or PVC tubing around the probed need so it doesn't short to the copper tube.
Perhaps I'll put some heatshrink tubing over the probe body or clear coat it so it's not shorting out everything it makes contact with.
