Replacement meter has fsd current about three times old damaged meter.

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

American Science and Surplus

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Thank you for the comments. Re. frequency senistivity. No think that enters int it? The unit has two sensor, that look like diodes mounted in cavities in the brass silver plated transmission line of the unit. The ouputs of the diodes are connected to a 6 position switch. Three FWD. and three REV. power positions. The output of the switch is just two unshielde short leads to the meter iteself. There is also a tip/ring headphone style jack on the side of the unit which disconnects the internal meter and extends the circuit so it can be read by a remote or external meter. So at that stage believe everything 'after' the sensor diodes is low current DC. So it would appear necessary to amplify the DC micro-voltage/current by about 3 to operate the replacement meter. BTW tempoarily arranged the existing (damaged) meter (disconnected from the unit) and the replacement in series with a low voltage source and a varaible resitor. With the old meter at (as far as coule be judged due to the damage) at full scale the replacement was about 0ne third. Haven't measured the DC resistance of each yet because that might have an effect on whatever circuit is used to 'step uop' the DC current to operate the meter.

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.

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

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

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!!

On Mon, 28 Jul 2008 20:52:07 -0400, Dave M put finger to keyboard and composed:

I don't mean to nitpick your clever circuit, but you've drawn it as running off *two* 9V batteries. Maybe you should have designated the battery terminals at V+ and V- rather than +/-9V.

Wise words.

- Franc Zabkar

On Mon, 28 Jul 2008 12:53:15 -0700 (PDT), terry put finger to keyboard and composed:

Excuse my ignorance, but wouldn't the 6-position switch be inserting three different series resistances to vary the meter's sensitivity for three different power ranges? If so, then why not just replace these resistors with lower values, assuming the diodes are not affected by the additional current?

FWIW, I found this home brew design that uses a 100uA meter:

- Franc Zabkar

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Great replies: thank for the thoughts and information. Also for the unit specification; had no indication is was 25 mhz. up. But the small size/length of the transmission line unit inside it might have been some indication. Seems to boild down to a) Look at and modify/jigger the values of the resistors in the DC part of the switching circuit to increase meter deflection 'as is'.

Hi,

I opened my Bendix 711-N to have a look-see: The panel meter is by Honeywell. The funny thing is that I have several spare Honeywell meters of the right size, but unfortunately they are all 50uA, and require a three-screw mounting, whereas the Bendix meter uses a four-screw mounting.

Looking at the adjustment trimmers, I see two assemblies, each with two trimmers ganged together. I guess the panel meter itself establishes the 35W full scale sensitivity, and the two trimmer assemblies are for the 75W and 300W scales. The trimmers are ganged to simultaneously reduce fwd and refl sensitivity.

Hi,

Another hint: I see that the shields of the coax carrying the d.c. voltage from the fwd and reflected detectors are returned to the + side of the panel meter. So the + side of the meter is grounded and the detectors put out more a more negative voltage as the rf power increases.

Excellent info and advice. Thanks Ed.