Modulation Monitor Connection

[Steve W8TOW]

I recently acquired a Gen Radio 1931-A mod monitor...After a bit of minor repair, she is up and running, but...
Always a BUT...   
The  manual says it is calibrated with 10 V of RF....When I inject 10 V of RF I only get about 40% of the needed carrier level on the meter...at best...I have evaluated the input circuit, the coil, caps and resistors are fine. All the tubes are now good, except my "Overmodulation" light is always ON...Which I think is yet another issue...(My 2050 fires at 37 V, rather than 27 V)...

Any one with some experience with this?
73 steve
w8tow

[WA3VJB]

Hey Steve,
Congrats on the 1931A. I've had two of them and they're good-looking, functional units.

One thing to make sure you've got right is the tap on that variometer coil.  It's possible to feed 10V RF measured at your signal generator, but lack enough drive for 100 pct reading because of mis-matched coupling.

Do you have a book for it ?


Wondering too which of the panel paint schemes you've got. I had a rearward rack mount 1931B for a while but settled on a 1931A, even though it has the Raytheon Brown glossy front panel intended to be paired with the RA-250, RA-1000 line.

They came in several flavors, including GE metallic blue, glossy Gray, and General Radio textured gray like their other test instruments.

[Steve W8TOW]

Hey thanks, been away for Tday, and now back in the shack!
The 1931A I have is a  metallic grey....I played with the taps a bunch...
Putting in new tubes, I have checked the coil, the dual pot on the front, (R3 I think)
and yes I have the book...
Here is what i was experiencing last week when I was playing with it:
The mod light comes on and stays on after warm up...(with or without modualtion and
regardless where the pot is set...) I tested the 2050 & it fires at 33 v, rather than
the 27 v spec....

With about 20 v inputted into the unit, I am able to peak the carrier level at the
proper point........I experiemented with the freq vs level, and it's uniform, across the freq range....any bright ideas?
73 steve

[Steve W8TOW]

Well, thanks to Bill KD0HG for a solution that worked FB...
Last night on 75m Bill dropped by the Mighty Elmac NET, and heard my
question about mod monitor coupling...
He suggested that I take a UHF "T" connector, remove the center pin from the
male end. Thread a UHF Fmale-Fmale splice in that end running a piece of
coax to the mod monitor from it.
Insert the "THRU" link in series with my rf source....
I did, it did work like a champ...great sampling tap....
thanks Bill
vri 73 steve
8tow
PS, no change in SWR, TX loading, or rf in shack...!!!!

[flintstone mop]

Hey Guys,
Thanks for the tip. That "T" thing worked for my Gates monitor also. But it revealed two  problems.
***The carrier pot smokes. I'll check the circuitry as this was a problem before. I think there is a 5 watt  resistor berfore this pot that's not cooperating. I might have to search for a scheezo for the monitor.
*** When I modulate the carrier using a TS440 driving a Ten-Tec linear, The RF power varies. It has a slight decrease in RF output on the peaks. The excuse for a meter reading peaks on the TS 440 shows no sign of decreases. Could there be too much pos peaks causing this or is it tuning of the linear??
Thanks for any help
Fred

[W2VW]

Hey Guys,
Thanks for the tip. That "T" thing worked for my Gates monitor also. But it revealed two  problems.
***The carrier pot smokes. I'll check the circuitry as this was a problem before. I think there is a 5 watt  resistor berfore this pot that's not cooperating. I might have to search for a scheezo for the monitor.
*** When I modulate the carrier using a TS440 driving a Ten-Tec linear, The RF power varies. It has a slight decrease in RF output on the peaks. The excuse for a meter reading peaks on the TS 440 shows no sign of decreases. Could there be too much pos peaks causing this or is it tuning of the linear??
Thanks for any help
Fred

Try increasing the loading (less capacitance) and see if that improves it.