Replacement meter has fsd current about three times old damagedmeter.

[terry]

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

 

[Samuel M. Goldwasser]

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

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

Google will find one.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Gnack Nol]

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.

more deleted:

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/

 

[terry]

more deleted:

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 Saleshttp://www.fairradio.com/

American Science and Surplushttp://www.sciplus.com/

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.

 

[Edward Knobloch]

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

 

[Franc Zabkar]

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 ...

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.

+9V
|
+-+-+
| |
| | +-------------+
10K | | | |
1% | | | +9V |
+-+-+ | | |
| | |\ | |
| | | \| |
| +--+- \ |
| | \U1B |
| | ------+
| | / |
+-----------++ / |
| | /| |
| |/ | |
+-+-+ | +++
| | -9V +
| |
10K | |
1% | |
+-+-+
|
-9V


Cheers!!!
Dave M

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

Wise words.

- Franc Zabkar

 

[Franc Zabkar]

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.

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:
http://www.qsl.net/vu2msy/homebrew/SWR_Bridge_from_Calcutta_VHF_newsletter.pdf

- Franc Zabkar

 

[terry]

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

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'.

 

[Edward Knobloch]

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.

73,
Ed Knobloch

 

[Edward Knobloch]

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.

73,
Ed Knobloch

 

[terry]

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.

73,
Ed Knobloch

Excellent info and advice. Thanks Ed.