[R-390] I probably shouldn't admit this but...

[n4buq at knology.net]

About a year or so ago, I noticed the probe for my HP410B stopped heating 
and I was getting no AC readings.  After checking, I discovered the probe 
has an EA53 (a 6.3VAC filament diode) instead of a 2-01C (a 5VAC filament 
diode) and the heater voltage had been set to 5VAC.  I increased the 
filament voltage to 6.3VAC and the probed started heating again; however, 
it appeared that my AC voltage readings were off considerably.

Figuring the EA53 was failing, I shelved it waiting to find either a 2-01C 
probe or an affordable EA53.  I recently noticed an auction for a probe set 
for the 410B but it appeared to have the AC probe from an ME26D/U.  I 
emailed the owner and asked if he happened to have any EA53s and he did; 
however, he said it sounded like something else was wrong with the meter 
instead of the diode.

I started checking around on it again.  I opened the probe top and fed the 
AC input via a regular silicon diode.  At 60cps, the results were exactly 
the same as the EA53.  With the probe still open and the diode powered, I 
checked whether the EA53 was working and I confirmed with my scope that it 
was functioning as a half-wave rectifier.  It appeared something further 
upstream from the probe might be wrong.

I then happened to think of something.  My test voltages were coming from a 
variac which was being fed with an isolation transformer.  Since I know 
that when measuring line voltages it is not proper to connect the ground 
but only use the probe tip, I was using that method to get sample voltages 
form the variac.  It then occured to me that I was checking the voltages 
across an isolated circuit.  I connected the ground clip and the meter 
again start reading correctly (or nearly so).  After running through the 
calibration steps, the meter now is quite accurate on AC.

Somethimes I feel like the scarecrow from the Wizard of Oz...

Barry - N4BUQ