Belar FMS-2 Docs Needed

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jthorusen
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Belar FMS-2 Docs Needed

Post by jthorusen »

I am attempting to resurrect a Belar FMS-2 stereo modulation monitor. I have downloaded the FMS-2 manual from the Belar web site. Unfortunately, the A1 board in their manual (given as Revision A) is completely different from the A1 board in my monitor. The op-amps are different and the pot values are different, and the board layout is considerably different. The op-amps are LM318's instead of the more modern TLO series shown in the manual copy I have, so I suspect that I have an early version of this unit. The lack of component layout information makes it very difficult to do the required audio setup for the auto-ranging attenuators. It will be extremely difficult to trouble-shoot the board without the correct schematic and I think this might be necessary.

So... does anyone have an OLD Belar FMS-2 manual lying around that might have data on an early version of the unit? I would pay for a scan or good readable photocopy if you have one. I will drop a line to Belar tomorrow, but I'd like to cover my bets in case they tell me they no longer have the data.

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

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Bill DeFelice
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Re: Belar FMS-2 Docs Needed

Post by Bill DeFelice »

Any luck, Jim? I have a manual somewhere that I'd be glad to scan but I'd need a way to spot the older version from the current version.
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jthorusen
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Re: Belar FMS-2 Docs Needed

Post by jthorusen »

Thanks, Bill... but I was able to speak to a very kind gentleman at Belar named Carl, who informed me as to the differences between the two versions of the FMS-2 and he E-mailed me the pdf version of the old manual.

The principal differences are in the A1 board which is the metering board as I mentioned in my post. The older board is an averaging board, while the new board is an RMS board. When this change was made, the board layout and schematic were completely different, so having the correct manual is virtually essential... especially as I am having some difficulties in following the setup procedure.

The first step in setting up the metering board is to apply .55 volts RMS at 1 KHz to the input pin on the board and set the input gain for 100% with the meter in "Peak" mode. One then switches to "Auto" mode (which is the averaging mode) and sets another pot for the same level to equalize the peak and average readings for a sine wave input, which, of course, they should be.

Unfortunately, in my case, I can only do this for the right channel.... the left channel averaging pot only gets to about 92% when wide open. The right channel can be adjusted +/- 8% of 100% so there is a problem in the left channel. I think I have traced this down to a bad diode in a pair of matched diodes in the averaging amplifier. In the right channel, the output waveforms of both diodes are identical in amplitude and shape. In the left channel, this is not the case. Close, but no cigar.

So, I am now looking for a matched pair of AA119 diodes.... or trying to figure out how to do my own matching. I have already yelled for help over on the ARF on how to do this: https://antiqueradios.com/forums/viewto ... 9&t=371637 .... so rather than repeat everything I said there I will just post the link.

Any suggestions welcome.

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

COMMENG
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Re: Belar FMS-2 Docs Needed

Post by COMMENG »

jthorusen wrote:
Sat Feb 01, 2020 2:03 am
...So, I am now looking for a matched pair of AA119 diodes.... or trying to figure out how to do my own matching.
...
Any suggestions welcome.

Thanks,
Might I suggest theses suppliers and order say 10 of them and check them on your DMM for the best 2 matches.

https://www.rf-microwave.com/en/telefun ... ode/aa119/


COMMENG

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jthorusen
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Re: Belar FMS-2 Docs Needed

Post by jthorusen »

Thank You for the suggestion. Based on information from the ARF thread mentioned previously, I have decided to go with NTE-110MP which is a factory matched pair of NTE-109's which are the replacement for the AA119 diodes.

Unfortunately, the outfit I placed the order with neglected to inform me that they were back-ordered. They hope to have some in tomorrow (2-13-20) and will ship then. I will check with them again on Thursday or Friday and if they are still unable to ship, I will go elsewhere. The NTE-110MP is not a rare part; I just chose the vendor that I did because they were the lowest cost. If I wish to spend a bit more, I'm sure I can have them in a timely fashion. If I had known the people I ordered from were going to be this slow, I probably would have gone somewhere else to begin with. Oh, well... live and learn.

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

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