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 Post subject: Edge display/stepper voltmeter crystal frequency reference
PostPosted: Oct Tue 23, 2018 1:59 pm 
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I recently finished refurbishing an old Electro Instruments Mark IV stepper switch/edge display digital DC voltmeter that uses one of these Bulova FC-100 crystal ovens. The same crystal oven is used in the Tektronix 184 time mark generator so I was able to conclude from that and other sources that it is a 10 MHz crystal. I have no schematic or any other printed information in regards to this meter and had to wing all the repair work, but the circuit appears to be similar to that of the Non Linear Systems 481 which also looks to use this same Bulova crystal oven. Of course I can find no schematic to the NLS 481 either, although Steve Johnson had some good pictures of the 481 that I could confirm the circuit similarities between it and the EI Mark IV.

I’m trying to analyze the circuitry of these meters for the sake of curiosity, and am at a loss for what this 10 MHz crystal actually does, or how its frequency is converted down to something that this device makes use out of. Removing it from the circuit causes nothing other than erratic operation and of course incorrect measurements. The voltage reference in the EI meter is a 6.2 volt Zener diode, if that is relevant. Any ideas?

Basic meter specs:
0 - 999.9 volts DC in 3 automatic setting ranges, auto polarity display.

EI Mark IV meter: Image

Bulova FC-100 oven stabilized crystal: Image

-Mark-

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Tue 23, 2018 4:30 pm 
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Unfortunately I have never seen any NLS schematics for these. I have the first two hardcover theory books published by NLS but they are theory and don't contain any circuit schematics.
Image

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Tue 23, 2018 5:10 pm 
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Steve, is there anything in those theory books you have in regards to the crystal frequency reference usage? I have noticed that with both Electro and NLS, this crystal oven seems to be limited to the “lower cost” and portable meter models, it looks to eliminate a good chunk of circuitry. There are downloads for the NLS 5005 and 1955 service manuals, but those models don’t look to use a frequency reference and no mention is made of one in their theory sections. I just don’t get how a 10 MHz time base plays in to these things. It’s keeping me awake at night. :P

The guts of the Mark IV:

Image

Image

-Mark-

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Tue 23, 2018 8:31 pm 
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I looked through them and found nothing on crystal frequency reference. Didn't even see anything mentioned in the discussions about calibration and accuracy.

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Tue 23, 2018 9:32 pm 
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Thank you for checking Steve. It's looking like I'll need to study vacuum tube op-amp integrators and do some experimenting in order to sleep again. :)

-Mark-

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 8:44 am 
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One of the many ways to measure voltage is using a voltage to frequency converter then measuring the resultant frequency, which requires frequency counter circuitry. Hence, the need for a stable and accurate frequency source.
Rick


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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 12:12 pm 
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That's not the case here. The principle used by these electromechanical stepper DVMs is basically a combination of successive-approximation and single-slope A/D conversion. The steppers count up until the reference voltage from the steppers matches the input voltage, one digit at a time

The frequency reference has something to do with timing the stepper pulses. If it is in fact a 10 MHz reference, how the heck does it get divided down to something slow enough to drive a mechanism? There doesn't appear to be enough tubes in these DVMs to do the frequency division, even by the monostable multivibrator method.

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 1:59 pm 
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I had a rack mount Stepper Voltmeter way way back in the day. I picked it up at Electronic Surplus in Cleveland, long since gone. We referred to it as Western Garbage (original name was Western Surplus). I never had a manual, but the one thing I do remember, well two actually, was that it was HEAVY ... and that it was amazingly quiet with the covers on... just a soft hum during use. Take the covers off and it sounded like a cement mixer gone berserk.

Having been involved in museum display design using relays, and steppers, back then, I found the whole concept fascinating since by that time everyone had hybrid or solid state VTVM's.

I never did have a manual for it, so no idea what the 10MHz reference is for. Guessing, perhaps it's got something to do with gating/clocking the A/D circuitry and/or timing the steppers. Just wildly guessing here. Perhaps the type of A/D depended on precision gating for accuracy in voltage measurement. I was preipherally involved in some early A/D systems being developed for video, and all those converters were clocked with a precision reference, either external or onboard. But by then the A/D converters themselves were solid state so who knows what happened with much earlier tube versions. But speculation is so much fun :)

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 2:22 pm 
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Saw one of these voltmeters for the first time a few years ago
and have been looking for one ever since. Fascinating concept!
I'm also into collecting and restoring old pinball machines so anything
with a stepper in it is of interest.
Steve

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 2:22 pm 
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The bread and butter for Electro Instruments was in operational amplifiers for analog computing, they were a competitor to Philbrick Researches for about a decade and were a successful company mainly through cold-war government contracts- the founder became a multi-millionaire in a few short years. That is what fascinates me about these types of early digital meters- there is more going on than meets the eye as far as the circuitry goes. But it quickly became a lost science as soon as HP rolled out their 405AR. I would have to say people likely couldn't get these stepper switch meters in to the dumpsters fast enough, as not many survive today.

Even the Bulova crystal oven is interesting if not something of a fire hazard- just one of those big canned crystals with resistive wire wrapped around it with a bimetallic thermostat and ceramic cap, all dipped in potting goo and running on AC line power. The thermal switch on mine does not work, but the crystal is fine. It would be easy to fix to get it to heat again but really not worth the bother as the meter only drifts a few counts going cold to warm.

Image

-Mark-

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 4:44 pm 
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Mark, I can offer some information on the Bulova oven. I have a couple Electro Instruments 8409
DVM's in working condition but they are about five years newer than your voltmeter and use
germanium transistors and Nixie tube readouts. They use a Bulova FC-100 oven just like your meter though.

This is a temperature controlled zener diode not a frequency source. It is used in the 10 volt
reference supply that is then used for comparison the the input voltage being measured.

I have never measured the zener voltage but I could if you really need it. You could check your
Bulova part. Pins 1 and 5 are from a 115 VAC source and heat the oven. Pins 4 and 8 are the zener voltage.

Do you really have 10Mhz coming out of yours?

Bob


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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 5:38 pm 
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The Electro Mark IV meter is indeed Zener diode voltage referenced, but the reference diode is a 1N429 that is located underneath the chopper. It is a 6.2 volt Zener that is temperature compensated. I currently have it replaced with a standard NTE 5013A as the original 1N429 tested open.

Zener reference: Image

All the reference material I have found in regards to the Bulova FC100, including the military parts description, lists it as "Frequency controller, Piezoelectric crystal, 10 MHz, Oven Stabilized." The Tektronix 184 time mark generator uses this same device, its manual also shows it as a 10 MHz crystal oven.

FC100 in the Tek 184 time mark generator: Image

I know everyone just wants me to sleep again :o I have a funny feeling I'm going to be firing up the active probe and doing math problems this weekend :( Seems like all of my projects go this way- I understand the circuit but can’t get it to work, or I get it to work but I don’t understand the circuit.

-Mark-

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 9:28 pm 
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Have you measured 10Mhz coming out of your FC-100?

I measure 2.591 volts DC out of my FC-100!

My schematic doesn't specify the FC-100 voltage but does say it is a temperature stabilized
zener diode for the reference supply. This reference supply is then chopped so it can be
either amplified or attenuated for comparing to the voltage to be measured. No need for a
high frequency or accurate frequency either.

My guess is that Bulova used this same temperature controlled oven for either a stabilized
frequency or a stabilized voltage. Too bad there is so little information on these devices.

So what is your FC-100 outputting?

Bob


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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Wed 24, 2018 10:33 pm 
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MarkPalmer wrote:
The Electro Mark IV meter is indeed Zener diode voltage referenced, but the reference diode is a 1N429 that is located underneath the chopper. It is a 6.2 volt Zener that is temperature compensated. I currently have it replaced with a standard NTE 5013A as the original 1N429 tested open.


All the reference material I have found in regards to the Bulova FC100, including the military parts description, lists it as "Frequency controller, Piezoelectric crystal, 10 MHz, Oven Stabilized." The Tektronix 184 time mark generator uses this same device, its manual also shows it as a 10 MHz crystal oven.



I know everyone just wants me to sleep again :o I have a funny feeling I'm going to be firing up the active probe and doing math problems this weekend :( Seems like all of my projects go this way- I understand the circuit but can’t get it to work, or I get it to work but I don’t understand the circuit.

-Mark-


Is the FC100 marked with a value in MHz/KHz? FC100 was a "series" of ovens, with a suffix designating the actual frequency. Looking at WEBFLIS/FEDLOG and two other databases, I can find FC100-series stuff with crystal frequencies from 10MHz all the way down to 100KHz. BTW, Bulova sold out/was bought out by American Time Products, which continued the ovens, albeit with a new/improved design, and a new part number series - FC101. Oak made a fit/form equivalent (per logistics data) in their PSA101 series, some of which are available on eBay, in the 100KHz configuration. American Time Products (FSCM 03040) is listed as being part of Power Control Devices (FSCM 09747). Federal Approved nomenclature for the FC100 is "OVEN, ELECTRICAL-ELECTRONIC COMPONENT" - NIINs under this description include some 329 products, including several HP, Tek and EIP ovens. Most have the "cancelled, no replacement available" character in the database.

I'm not aware of any industry standard pin-out, and have only three ovens (ex-machina), two from HP, and another from an old Nixie Freq Counter made by Systron-Donner for another company. All of mine are made by Northern Engineering Laboratories.

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Thu 25, 2018 12:51 am 
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Mystery Solved! I got home and did some measuring, and just as BobVB stated it does indeed check out to be a 10 volt Zener diode in the can. I guess the Bulova FC100 "series" can be a lot of things. It might have been nice if they had labeled it as a voltage reference in some way, the only markings on it are the FC100, 115 VAC, 55 degrees C. Now for the next mystery- why two Zeners in this meter, the 10 volt heated one and a 6.2 volt temp compensated one? Might be a topic for another day. Thanks everyone for your help!

-Mark-

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Thu 25, 2018 12:55 am 
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I love it when we all learn something!! :P

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Thu 25, 2018 2:49 am 
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It is so good when a mystery is solved. I just wish there was ANY Bulova documentation on
these ovens. Talk about obscure!

You asked why there are two zeners. My later EI DVM has two zeners in the reference supply too.

So the answer is obvious. LOL They used an inner bridge and an outer bridge with a zener in each bridge.

I have never before encountered this method but the inner and outer bridge scheme allowed them to come up
with an accurate and stable reference. I am not sure but it may be like a power supply that uses a pre-regulator
and then a final regulator. Remember this was back in the early to mid fifties and digital voltmeters were
just being developed. They started a revolution in digital instrumentation though.

I scanned three pages describing the bridge scheme but the document is too large to attach.
If you PM your email I could send a copy to you. Also your unit is all tube and mine from the late 1950's is transistorized
so they will be different.

Bob


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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Thu 25, 2018 4:02 am 
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Mark -

Is that the one that was on the bay of evil a few weeks ago? If so, nice score. I was looking at it, but managed to resist.

I have its bigger (and I suspect older) brother in my repair queue - an EI 1040 that I got a few years ago from the 'bay - horribly packed (like in the cardboard was basically folded up around the instrument and taped - no padding at all), looking like 15 miles of bad road and with the choppers and some of the steppers torn loose and bouncing around inside.

Image

Image

The brown powder is what's left of the sound deadening foam that lined the compartment, after drying for 50+ years then getting pulverized by the loose steppers.
Image

Yours looks very nice! If you're willing, I may want to pick your brain at some point - at least one (maybe more; can't recall at the moment) wire(s) are broken from the steppers that were floating around unmounted, and I can't for the life of me see any remnants to indicate which terminal it might have broken off. Unfortunately the meter is buried downstairs at the moment, and the wiring may well be different, but it's worth a shot as what you have is the closest I've seen to mine thus far...

The rest of the pics of mine are at: https://pmanning.smugmug.com/Electronics/Electro-Instruments-NLS-edge

-Pat

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Thu 25, 2018 1:51 pm 
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Hi Pat,

I saw the photos of your EI meter while I was working on mine, and wondered if you got anywhere with it. The one I have was listed on E-Bay for a little while, and I initially thought it was just a display section for a multi-piece unit. I bit on it once I discovered they made a few self-contained portable edge display meters, but wasn't totally relieved until I got it and confirmed that's what it was. One of the few times I gambled with a win as it wasn't in too bad of shape and totally restorable.

In its past life all six of the steppers fell off their rubber mounts, and it ran with them sitting at the bottom of the cabinet beating themselves to death for a while until one hit the main fuse holder and broke it shorting the unit out- this actually saved it's life. The original rubber isolators had turned from rubber to carbon, and the steppers broke free. Like yours, all the foam sound insulation turned to dust and fell all over the steppers, that along with sticking to the brown gooey glue dripping down made a huge mess. Step one involved an entire weekend of cleaning all of this mess up.

I was able to source new rubber isolators that are very close to the original dimensions, and got the steppers re-mounted. This was a relief. Fortunately, no wires broke off during all of this process. Restoring the op-amp board was just conventional vintage tube electronics work- replacing the black beauties and re-stuffing the electrolytic cans. Other then the blown 6.2 volt Zener everything there worked right from the start after being refurbished. I replaced a few lamps in the display, mocked it up on the bench, and it slowly came back to life after more stepper switch cleaning and cycling. I made some new sound insulation to line the cabinet with out of 3M foam board, and it's been working great since.

If you need any info, feel free to hit me up.

YouTube vid of it running: https://www.youtube.com/watch?v=7hWlwhlLgQ8

-Mark-

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 Post subject: Re: Edge display/stepper voltmeter crystal frequency referen
PostPosted: Oct Thu 25, 2018 4:18 pm 
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Cool, thanks. I got the isolaters from McMaster-Carr and remember taking photos, but obviously never uploaded them to the SmugMug gallery for the meter. I had to trim the threaded boss on the end that went to the stepper relay.

I got a copy of a supposed manual for it (after I'd tucked the instrument away, of course), but it's in rough shape and I need to really go through it and see how close to what I have it is. I've been badly spoiled by HP and Tek documentation - a comb binder of fuzzy prints just isn't the same. :lol:

It's nice to see the video of yours clacking away making measurements - I now feel more inclined to excavate mine and continue with it once the bench is cleared again. In the meantime, I'll look to see if there are any other relevant photos and upload them. I doubt I took a detail pic of where the wire broke off, unfortunately. That could make q quick comparison to yours easy.

-Pat

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