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 Post subject: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Tue 08, 2021 7:12 pm 
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Joined: Mar Wed 27, 2019 1:56 am
Posts: 305
Location: Mountainside, NJ
Does anyone know what variable resistors in the FC71 are for? I need to align but cant find any info on what adjusts what? Only info I found is in manual and I don't have a color TV so its useless to me!


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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Tue 08, 2021 9:34 pm 
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Joined: Jan Sat 16, 2016 4:15 pm
Posts: 302
This manual?

https://bama.edebris.com/download/senco ... 1%20OM.pdf


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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Wed 09, 2021 12:51 am 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 3209
Location: Dallas, TX
So it looks like the pots have to do with input sensitivity. No info on adjustment for the adjustment of those pots in the manual. The exact sensitivity isn't that important anyway.

The frequency calibration would be something the Sencore service department could have done.
Using the color burst frequency as a standard wouldn't work today because the analog TV broadcast that would transmit it are no longer on the air. (The TV locks to the transmitted signal.)
You might try verifying the calibration using WWV, but you really couldn't adjust it anyway.

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It's not the Destination, It's the Journey.


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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Wed 09, 2021 3:54 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 10627
Location: Long Island NY
According to the manual (available on BAMA, https://bama.edebris.com/manuals/sencore/fc71) the calibration of the FC71 is stored on an EPROM and is unique to the reference modulator module in the unit. You would need to change both the module and the PROM to fix it. There are no user tweaks or adjustments.

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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Wed 09, 2021 11:27 am 
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Joined: Mar Wed 27, 2019 1:56 am
Posts: 305
Location: Mountainside, NJ
Yes I seen the eprom controls calibration but I believe it’s effected by voltage, there are several variable resistors onside that need to be set and I’m not sure how to check/ set them


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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Wed 09, 2021 11:48 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 10627
Location: Long Island NY
You do realize an EPROM is a digital memory which stores the calibration data in some form that only makes sense to the microprocessor in the instrument? To calibrate the instrument you need the software to write the new data into a new PROM unless the existing one is erasable or alterable. Each PROM was individually written for its associated oscillator module. Only Sencore ever had the software and hardware to do this; you can ask them but I think they will politely tell you they no longer support the instrument.

Unfortunately, by the mid to late 1980s many test equipment manufacturers had switched to calibrations and operating software in PROM firmware which are now inaccessible because the software and hardware that wrote the PROMs no longer exists. The chips themselves are often NLA too. If the instrument drifts or parts have to be replaced, there’s usually nothing you can do to bring it back into calibration. Some instruments have been “hacked” but that doesn’t necessarily get you back to factory new performance. If you need an accurate counter your choices are basically to buy a new instrument , or get an older one which does not depend on firmware and can be repaired and recalibrated by normal means.

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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Fri 11, 2021 4:35 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3664
Location: Monterey California USA
As above, but just to add, first of all the Sencore that made this is no longer in business. The new Sencore makes different products and they have more or less disowned all the older stuff such as this. As you will learn if you call or write them.

Originally the usual way to calibrate this was to send it back to Sencore. If you were stubborn, you could order a new EPROM and TCXO set, as they were made up at the factory as a matched set and would arrive pre-set, so to speak. And were $$$. My manual has a sheet in it that says the factory offered some sort of data on doing a field calibration yourself, but you would have to call them about it. You would need a temperature chamber, a NIST referenced frequency source (and it was not 10 MHz) and an EPROM programmer. They must have sent you some kind of EPROM map or software to program one against your graph of frequency changes. I should have asked when I had the chance but my own FC 71 is an orphan now. It is still within calibration for 2-way radio work but I don't like things that can't be calibrated and updated as necessary.

I hardly paid anything for my FC 71 but this is a reason to appreciate HP counters and high-end test gear. You can field calibrate it...

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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Fri 11, 2021 4:28 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 2237
Location: Roanoke, VA
Geoff Fors wrote:
I hardly paid anything for my FC 71 but this is a reason to appreciate HP counters and high-end test gear. You can field calibrate it...

I still use my Sencore FC45. One of the reasons that I bought it is that it uses a 10MHz OCXO that can be zero beat against WWV.

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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Fri 11, 2021 6:50 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4496
Location: Lexington, KY USA
Is there a problem with the present frequency calibration? How far off is the frequency as it stands? Is this stable? A small error that is stable can usually be lived with.

Ted


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 Post subject: Re: SENCORE FC71 Frequency Counter Allignmnet
PostPosted: Jun Fri 11, 2021 7:11 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 2518
These days WWV is hard to zero beat. The fading obscures the beat for accurate adjustment.

Granted, one who uses a Sencore counter may not be interested in the greatest precision but I found it desirable to acquire a rubidium standard to calibrate my HP. Every few months I dust it off and readjust the HP, which uses their high precision 10811 oscillator. That way I am good to the last digit, even on 10 second gate times.

It's very satisfying to read 600 MHz to the nearest cycle, even if there is no practical purpose served. But it's a hobby, and if I get my kicks this way, at least it keeps me off the streets.


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