Belar FMS-2 Meters

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jthorusen
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Belar FMS-2 Meters

Post by jthorusen » Tue Aug 13, 2019 11:45 pm

I recently acquired a Belar FMS-2 stereo modulation monitor. Looks like a nice clean unit, although it is an early version (slide switch to select AC line voltage).

My question is this: Both meters are pinned off-scale low with no power applied to the unit. I know that sometimes meters are deliberately set up this way to allow for ballistics or circuit peculiarities but the manual says nothing about this. Before I stick my Xcelite greenie in there and try to adjust the meters for mechanical zero, I thought I'd better ask if this is normal or not. Anyone out there with a Belar FMS-2 happen to know what the meters do with no power applied?

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James K. (Jim) Thorusen
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Dale H. Cook
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Re: Belar FMS-2 Meters

Post by Dale H. Cook » Wed Aug 14, 2019 7:35 am

It has been years since I have worked with an FMS-2, but they should not be pinned with power off.
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Bill DeFelice
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Re: Belar FMS-2 Meters

Post by Bill DeFelice » Wed Aug 14, 2019 11:35 am

@Jim: My FMS-2 and FMM-2 are set to rest on the lowest part of the scale. I replaced the meters on my FMS-2 years ago due to faded scales and the screw adjustment was all I needed to get the movement in the proper position at rest.
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Re: Belar FMS-2 Meters

Post by radio_guru » Thu Aug 15, 2019 7:45 am

Before going nuts replacing costly meters, simply see if the zero adjustment works. It's entirely possible the spring tension changed and a simple adjustment is needed. And have you consulted Belar's tech support?

Another thing to keep in mind is the monitor system has a zero (0%) adjustment and the system might actually reference a null voltage which counters the full left spring. Much the same as the FMS-1 pilot freq. meter which is a centered meter.

FWIW, I have a FMM-2 for sale....

RG

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Re: Belar FMS-2 Meters

Post by jthorusen » Thu Aug 15, 2019 5:45 pm

Not to worry.... I don't have the bucks to replace the meters. Based on earlier comments by Bill and Dale I think the meters are supposed to sit on zero with no power applied. So, I removed the front panel and the meters, adjusted the second hair spring position and got the meters to zero nicely. I did a calibration verification of both meters afterwards. Some tricky work there, since the meter scale is linear for most of its travel but not at the very top end. Turns out that although the meter reads 133% full scale, the actual full scale reading in equal divisions is 127. Once I figured this out (and realized that I had the numerator and denominator reversed in my calculation) I was able to make up a table of error for each meter. Neither meter is very linear; the worst one has a 7% full scale error right around 120%, but I think they will work well enough to do the job.

Belar, as I understand it, has pretty good electrical engineering, but the guy who did the mechanical engineering on this thing ought to have been drowned at birth. Getting the front panel off is an exercise in frustration. The panel is secured by 11/32 nuts that are too close to the edge of the basket chassis walls to get a normal socket on. I had an extra socket which I was able to grind thinner. What is really required is an 11/32" thin-wall flex socket. Not having one of these, I replaced the original nuts with 1.5 inch long hex threaded stand-offs. At least, you can get at these things to turn them.

Anyway, after a monumental struggle, it is back together with the meters mechanically zeroed. Now I just need to run through the calibration. I won't be able to do it all as I don't have a stereo generator (second project - build one) but hopefully, I will at least be able to tell if its working.

Thanks to all who offered advice.

Regards,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
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NW Oregon Consulting Bdcst Eng.

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Re: Belar FMS-2 Meters

Post by kcbooboo » Fri Aug 16, 2019 7:29 am

I took a look at the manual. I didn't see any zero-adjust pots inside, which led me to believe the front panel meters are simply AC voltmeters and are supposed to sit at zero with power turned off. Some of the Belar products do use center-zero meters, such as those that also indicate FM pilot frequency or AM carrier shift.

I'm surprised the meter linearity is so far off. Can you determine whether it's the mechanical meter movement or the electronics that's causing that? For example you could feed DC to the meter movement in series with a DMM and see how much current it needs for full-scale, then check the meter at various other points. Also, the electronics may be trying to simulate the ballistics of a VU meter and that could be the cause of some non-linearity. Sometimes a mod monitor will have a fast response to peaks but then a much slower release, so it's easier to see the peak modulation value.

There may be a few things you can check without a real stereo generator, by using external modulation on a decent signal generator. Also, there are some inexpensive stereo transmitters (milliwatt) and even ICs that can generate the required signals such that you can at least verify the pilot and separation levels.

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Re: Belar FMS-2 Meters

Post by Deep Thought » Fri Aug 16, 2019 11:27 am

Those are custom scale meters so I would guess that Belar used that to compensate for something in the electronics or mechanics of the meter itself.
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Re: Belar FMS-2 Meters

Post by jthorusen » Fri Aug 16, 2019 6:01 pm

For Bob M.:

That's how I determined the meter linearity. Weston lab standard 0-1 mA mirrored scale milliammeter in series with a 22K resistor and my low voltage bench power supply. 400 uA full scale (spec for the Belar meters according to what is printed on their scale cards) is roughly 10 volts from the supply.

Unfortunately, I don't have a lab standard meter in the 500 uA range, so the low end of the scale readings were pretty far down on the 0-1 mA meter. I go from 0-1 mA down to 20-0-20 uA in my lab standard meter stock; I suppose I should do something about that gap.

Interestingly, the first meter I took apart got a bit abused in the process, as I didn't get the front zero adjust mechanism quite lined up when I first put the front cover back on. In the process, I discovered that in order to adjust the rear hair spring tension, it is only necessary to remove the brass nuts and a little plastic cover over the back of the meter; the front cover need not be removed. The second meter, therefore, never had the front removed; only the back cover. The interesting thing is that the more abused meter is the more linear and accurate of the two. Go figure.

I would attach the spread sheet, but I don't how.

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James K. (Jim) Thorusen
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Re: Belar FMS-2 Meters

Post by kcbooboo » Sat Aug 17, 2019 7:15 am

On an AM mod monitor the linearity past 100% could be questionable depending on the demodulation scheme, but I'd hope that the demod is linear on FM out to 100 kHz. You could also attach an external meter of almost any kind to the FMS-2 and compare its actual uA readings with the front panel meters.

IF you have the appropriate test equipment, you can use a spectrum analyzer and FM sig gen to create a very accurate FM modulation level using Bessel nulls. I found my sig gen was a bit low in its indicated modulation level, something like 73 kHz when set to 75 kHz, so a slight compensation can be applied to at least check an FM mod mon.

I checked and calibrated one of the earlier Belar FM mod monitors (FMM-1) with the separate stereo unit (FMS-1) using a lab-quality frequency counter, RF sig gen, and spectrum analyzer. With that unit I was able to feed the stereo unit the signals it needed directly, rather than go through the RF input and demodulator.

Sounds like you're gaining on it though.

Bob M.

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Re: Belar FMS-2 Meters

Post by jthorusen » Sat Aug 17, 2019 9:09 pm

Re Bessel nulls: This is something I wrote a paper on when I still worked for CBS. I am quite familiar with the technique; I used it to calibrate the TFT-723 Main / Mono modulation monitor now in service at the station.

Dale was kind enough to send me the NAB instructions for an FM stereo proof, and their technique requires a stereo modulation monitor. The FMS-2 that I am now going through will take composite out from the TFT (after I massage the TFT output stage for more gain) and enable me to see things like left and right modulation instead of just total (as displayed by the TFT-723), cross-talk, separation, pilot level and it will provide a pilot output that I can connect back into the TFT so that its frequency counter can measure the pilot frequency... something it cannot do by itself.

The calibration instructions from Belar call for both single audio generators for some steps and a stereo composite generator for other steps. No RF involved; just composite generated locally and input direct to the Belar. If I had an old Optimod lying around, that would be perfect.... but those things go for gold. I am looking into home-brewing a stereo generator around the NJM2035 chip. The only catch is that one step in the Belar procedure calls for a stereo generator with > 60dB of separation.... and the NJM2035 chip can only manage 40. There is another DIY generator that is speced at >50 dB separation but it is more complicated to build... it is essentially a software defined encoder.

However, a simple audio generator will suffice for setting up the metering board and a simple stereo generator will suffice to provide the composite signal needed for most of the calibration.... so I will wing it when I come to setting the separation null.

Thanks to all for your input.

Regards,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
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NW Oregon Consulting Bdcst Eng.

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Re: Belar FMS-2 Meters

Post by ChuckG » Sun Aug 18, 2019 10:29 pm

How would a software encoder work for this? you'd need a decent sound card.

https://www.diffusionsoftware.com/airomate.php
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Re: Belar FMS-2 Meters

Post by jthorusen » Mon Aug 19, 2019 12:49 am

Hi Chuck:

To quote Artie Johnson, "Verrry Interesting!" :D

I will look into this.

Thanks,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
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NW Oregon Consulting Bdcst Eng.

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Re: Belar FMS-2 Meters

Post by radio_guru » Wed Aug 21, 2019 3:58 pm

This isn't an analog generator. It's only AES/192Kbps output....er digital.

I like the concept however.

RG

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Re: Belar FMS-2 Meters

Post by jthorusen » Wed Aug 21, 2019 8:33 pm

According to what I read, it uses two sound cards. One is used to convert local analog audio to a digital format. This can be eliminated by using an internal audio source such as a media player as the signal source. This is converted to a 192 KHz digital data stream which is software converted by the Airomate software into a digital data stream which drives the output sound card... and its output is an analog composite multiplex signal which can be applied to an exciter or in my case, to a modulation monitor composite input for test purposes.

Regards,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
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NW Oregon Consulting Bdcst Eng.

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