terry wrote:
Repairing a used VSWR meter; which has a badly damaged meter. Meter
front broken off, most of the meter needle missing etc. If had another
similar meter would have a go at fixing the meter and making a scale
for it but nothing of similar manufacture!

Unit is Bendix Microwave Devices: Micro Match Model 70N? (Nameplate
slightly scarred so maybe it is model 7 ON?) Has 3 ranges Fwd and Rev.
30, 75, 300 Watts. 'N' type Load and Transmitter connectors.

Have replacement meter (said to be from a Heathkit) approx same
dimensions which mechanically fits perfectly; but has an fsd of about
100 microamps (I think) whereas the old damaged one appears to be
about 30 microamps fsd.

Idea is to fit a small DC amplifier mounted on a piece of vero-board
etc. inside to amplify the DC signal that comes through the range and
FWD/REV switch from the probes.
snip

Hi,

I have one of those RF Wattmeters. It is a Bendix model 711-N
(presumably the "N" indicates type N connectors).
It is the same as the M.C. Jones Micromatch model 711N,
Bendix bought out M.C. Jones. You can get a manual
for the M.C. Jones 711N from A.G. Tannenbaum,
but the manual lacks the schematic and circuit description.


Data is as follows:
Freq Range 25 to 1000 MHz
Impedance 51.5 Ohms
Accuracy +/- 5% of full scale on each range.
RF Power Range 30W full scale, 75W full scale,
and 300W full scale, fwd or reflected.
There is a jack on the side
for a remote indicator, but I have no data for that.
The meter movement is 30uA d.c. full scale.
My panel meter is marked DEL MS24,
there is no panel meter maker's name.

Sorry, I have no practical experience with op-amps,
but the d.c. current amplifier should be straightforward.
You'll probably need a load across the op amp input
with a resistance about the same as the original panel meter.
I figure the original 30uA meter had a 2K resistance or so
(if GE, Simpson or Triplett), or about 1.5K if Weston.
Caution: Don't just put an Ohmmeter or DMM across your new panel meter,
to measure its resistance, you'll probably destroy it.

73,
Ed Knobloch