Electronics Forum / Repair / July 2008

You need a transimpedance amplifier, which is a trivial op-amp circuit.

Google will find one. :)

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Terry you should check to see if the meter circuit contains a shunt and if
so try adjusting it for the coser to the proper range. If you can obtain a
movement that is identical except for the scale the original meter face an
probablt be placed on the working mechanism to render a more original look.

Using an amplifier in a sensitive circuit like a swr bridge is problematic
since the amplifier itself will respond with frequency sensitive gains.

You might want to go to some of the surplus sites that still do exist like:

Fair Radio Sales
http://www.fairradio.com/

American Science and Surplus
http://www.sciplus.com/

<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

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

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!!