"Chuck Harris" wrote in message
...
Norm Dresner wrote:
I have an HP200CD that needs some loving care. The manual says to
measure
the difference in the voltage between two different points in the
circuit --
it shouldn't be all that much but the voltages between these points and
ground is in the hundreds of volts range. The manual also says that I
should be using a meter with 220M ohm or greater input impedance to make
the
voltage measurements. Any suggestions where such a beast could be
bought/rented/stolen?

TIA
Norm


The only real reason that HP required you to use a 220M ohm input
impedance
meter is their HP410C was a 220M ohm input impedance meter. As was their
earlier HP410B, and earlier HP412A.


Gotcha. It's pretty much a nonstandard value these days -- and I think it
was back then too.

Related questions: The HP200CD service manual says, in part, "The DC
voltage between the cathodes (pin 3) of the 6AU5 tubes should be 1 volt or
less. If this voltage is excessive, it indicates a bad tube in the
oscillator (V1-V4) which must be replaced to insure low distortion in the
output waveform."

1. What is "excessive"? 1.5V? 1.1V? anything over 1.000V? ...

2. The cathode voltage for each tube is given as 3.5V. Clearly if one of
them is way off, it's pretty much the one that should be replaced. What if
they're both close but the difference is a little over 1.0V? Should I just
replace both?

3. Should I just replace all 4 tubes in the circuit? There's also a
rectifier tube whose output is filtered by a couple of 10 uF capacitors
[actually three sections of a single capacitor) and a 6H choke. How much
60/120 Hz ripple on the DC output of the power supply would suggest that the
tube and/or the capacitors should be replaced too?

4. The really hard question: Am I still trying to polish a turd and should
just replace all of the electronics with a good quality IC-based Wein Bridge
using the dual variable capacitors as the basis for this?

Norm