On Sun, 27 Jul 2008 08:24:12 -0700 (PDT), 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.

The DC amp. Will require small battery (probably a 9 volt attached
with one of those snap on pigtail connectors) that can perhaps be
activated by either a push switch or maybe a two position toggle
switch (Momentary/On). To avoid running down the battery might add an
LED "ON" warning as well? It will also be necessary to adjust the DC
gain to 'calibrate' the meter deflection. There is sufficient space
inside IMO to easily accommodate all this.

Where, as an old 'tube' man, could I find a suitable circuit or buy a
suitable little module to fit module?

Help/advice most appreciated. terry


==============

A transimpedance amplifier is what you need, as Sam suggested. Here's a circuit
to start with. You might have to fiddle with exact resistor values, depending
on the Op-amp you use, which isn't critical, except for input offset voltage.
Select a dual Opamp with low offset voltage, such as the Texas Instruments
OPA2227.
Since it only sees DC currents and voltages, there are no issues with frequency
response. The circuit runs off a 9V battery. Current drain is reasonably low,
but if you want to leave it on for extended periods, you'll want to fashion a
line-operated DC power supply (a wall-wart is good for this).

The bottom half of the opamp is used to split the battery voltage in half to
provide a virtual ground. This keeps the opamp well within the linear region,
away from the battery's rails.

View in a fixed-width font such as Courier.




|+9V
|
|
|
| +---------+
+---------|--+ Rf +-+
| | +---------+ |
| |\ | 33.2K 1% |
| | \| |
+------+------+- \ |
Input Current | \U1A |
0 - 30uA | /------------+ Vout=0 - 1V
| / | (0 - 100 uA)
+----++ / |
| | /| +-+-+
| |/ | | |
| | | |Rscale
| | | |10K 1%
+++ | | |
+ | +-+-+
| |
-9V |
| 100uA
*** Meter
* *
* *
* *
+9V ***
| |
+-+-+ |
| | +++
| | +-------------+ +
10K | | | |
1% | | | +9V |
+-+-+ | | |
| | |\ | |
| | | \| |
| +--+- \ |
| | \U1B |
| | ------+
| | / |
+-----------++ / |
| | /| |
| |/ | |
+-+-+ | +++
| | -9V +
| |
10K | |
1% | |
+-+-+
|
-9V


Cheers!!!
Dave M

Never take a laxative and a sleeping pill at the same time!!