Belar Modulation Monitor (FMM-2) crystal replacement

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Posted by w0ep on October 28, 2018 at 4:31 pm

[Yesterday’s run: 8.x miles in Starkville, a lot of walking]

A few months ago I picked up a surplus Belar Modulation Monitor, model FMM-2.  I’ve been wanting to get it working again at WMFH-LP.  I have a portable modulation monitor that I use to check our modulation.  But I would also like to get this one going in the equipment rack.

I was able to download the operating manual which includes schematics and parts information.

The unit is crystal controlled.  It was configured for 91.3 MHz and I needed to move it to 95.5 MHz.

The manual talks about how to put in a new crystal.  First there is a calculation:  The crystal is the desired operating frrequency + 0.650 MHz.  The crystal in the unit as received is marked 91.950  which does follow from the equation in the manual:

OperatingFreq + 0.650 = MarkedCrystalFreq

91.3 + 0.650 = 91.95

I was able to pick up a strong carrier at 91.95 using a pick-up coil in proximity to the oscillator board.

I don’t think there are many crystals (if any) that are made on a fundamental frequency of 90+ MHz.  Such things are done by taking a lower frequency crystal and operating it on a multiple or over-tone.  I was able to do some measurements of the as-delivered crystal and came up with a figure of 18.390 MHz which is 1/5th of the marked frequency.

CrystalFundamentalFreq = MarkedFreq / 5

18.39 = 91.95 / 5

I used an AD9850-based frequency source and replaced the crystal with a direct injection of 18.39 MHz and I then attempted to detect the 91.95, which was successful.  Below is a picture of my wire-and-hot-melt-glue crystal replacement plug.

I then calculated the needed injection frequency for use of the unit on my operating frequency of 95.5 MHz

95.5 + 0.650 = 96.15

96.15 / 5 = 19.23

I then changed my frequency injection to 19.23, adjusted the oscillator coil as instructed in the manual for crystal changing, and detected a strong carrier at 96.15 MHz.

I have to leave the project for a little while.  I think my next step is to get the off-air signal into the RF port of the modulation monitor.  I would rather get the signal off of the air than use a tap on the transmission line.  I probably will need to use a pre-amplifier to get the off-air signal up to the right voltage level.

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