"James Sweet" wrote in message
news:_Xt3d.4555$je.2531@trnddc04...

"Norm Dresner" wrote in message
...
"Jim Adney" wrote in message
...
On Fri, 17 Sep 2004 15:58:23 GMT "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?

There are such devices, but any modern DVM with a 10 MOhm input
impedance will work just fine to measure the voltages in your 200CD.
That warning is there because when the 200CD was made the common VOMs
of the day had much lower input impedances, so HP wanted to keep you
from using those.

I've worked on 200CDs with 10 MOhm meters with complete satisfaction.


Jim

Thanks for the encouragement. I've almost decided not to try to repair
it
but to build a modern, solid state oscillator using the mechanical
components as the basis. The dual variable capacitors are an almost
impossible item to find and there's more than enough space in the
chassis
for anything with transistors or IC's. And, as a bonus, the bottom of
the
interior foundation should serve to isolate the incoming AC from the
sensitive circuitry in the upper compartment.

Norm


If you do decide to retrofit it with modern internals, a guy I know has a
book, I think it's called Analog Circuit Design, anyway he showed me a
schematic in there of a solid state oscillator using an incandescent lamp
to
stabilize it just as the HP uses, he built it and said it performs very
well.


Yup. The book is "Analog Circuit Design" . It's published by
Butterworth-Heinemann in their EDN Series for Design Engineers. The editor
is Jim Williams and the article you're referencing is #7, "Max Wein, Mr.
Hewlett, and a Rainy Sunday Afternoon" by Jim Williams, p 43ff. The book is
on a shelf in my library already.

Norm