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Belar AMM-4 Frequency Monitor

R

RadioMan203

OK folks, here's the problem: I've got a first generation Belar AMM-4 frequency monitor that uses a 3 MHz temperature compensated crystal oscillator (TCXO) time base. The problem is the oscillator drifts so much the unit is unreliable for long term use. Through a couple of different sources I picked up a couple of alleged 12 MHz TCXOs and built a TTL divider similar to the one already in the AMM-4 to bring the oscillator signal back to 3 MHz to inject into the board. The problem with the cheap China oscillators is that they drift worse than the old one in the unit.

I have had very little luck finding a suitable replacement oscillator. I've tried all the usual suspects like Digikey and Mouse and they have nothing close. I checked with a few crystal manufacturers and I would have to pay for a run of 25 units just to get one made - obviously not a economical route.

I would hate to lob the unit in the trash. Does anybody have an idea on I can repair this unit without it costing a small fortune?
 
Call Belar and ask Arno what - if anything - can be done.
Also, if you have a GPS receiver which outputs a 10MHz reference, that ought to be stable enough... divide it down to what you need. Use >good< logic stuff, if any of the gates have jitter, it will appear as frequency shift.
 
Pete Partenio said:
Bill

get your xtal here: http://www.icmfg.com/

I had them make the xtals for the I95, TFT reciters. those things start to wander around like a lost dog when they get old.


Pete
Click to expand...

They were the first ones I called as I usually get new rocks for Belars through them. The only problem was since it wasn't a stock item I would have to pay for a run of 25 just to get one - approx $650. At that point you have to question placing that much money into a $1200 unit that's possible 6 to 10 years old.
 
I would have to pay for a run of 25 units just to get one made
Click to expand...
Strange, in '05 we had them make 12 crystals for Marti STL's and RPU's. Couple examples. 945.750 & 450.98125. Not stock items.
Even had them do the aging process. Cost for each crystal? $19.95!
 
Crystals are an entirely different thing. The AMM-4 uses a temperature compensated crystal oscillator and not a "plain" crystal. I had single crystals made from them without any problem, but I guess they must have a subcontractor make the oscillators.
 
Pete Partenio said:
Bill,
Have you checked the caps by the rock?
Click to expand...

In my vintage of the AMM-4 there are no caps near the oscillator. It's a TTL output feeding some J/K flip flops to divide down to the 1 MHz reference. I may try calling them to see if they have the newer 6 MHz oscillator from the newer design and just build up an additional circuit to handle the extra needed divider.
 
I never found a good quality replacement TCXO and I wasn't brave enough to get a price from Belar, but I checked the master clock with a frequency counter and adjust it after the unit's been on for awhile and it will only drift about 1 or 2 hertz after an hour warm up.

It's sort of ironic that you need another frequency counter to align what equates to a frequency counter.
 
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