Revox B77 REAL time digital counter conversion - part 1 - the mechanicals..

marcochezzi

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Revox B77 digital real time counter, Part 1 - the mechanicals..:

So since I am firmly locked away due to the C virus as many of you are, I have to keep myself entertained such that my sanity remains at fair levels...

Some time back, and with the prior purchase of 2 Revox B77?s, I had ventured down the path of a real time digital counter design of my own for one of the two units. There is no functional value to this modification apart from my desire to challenge myself to madness once more, plus I like the look, kind of brings a really pretty machine out of the 70?s era, which I believe it well deserves..

From a physical 5 x 7 segment led time count electronics, I already have that circuit which I designed many many years ago, basic cmos gating stuff, I?m too dumb for microprocessors, and in fact, it is already built and waiting on the shelf in the garage to go into the B77 pending some timing dividing electronics which can only be implemented at the bitter end, but that?s the ?easy? part, the challenge in this build are the mechanicals..

Now look, I am fully aware that one can purchase a screw in digital counter kit for this very machine from Studer themselves in Europe land, however this is not a real time counter, ie hrs:mins:secs, it just a 0000-9999 counter to fit in and take the place of the original mechanical counter, and for me that will not do..

There are options, you could purchase a later scrap Revox PR99 and carry over the parts as most would fit, but scrapped PR-99?s are hardly being tripped over locally, and frankly if they were available, I?d get that running instead of the B77..

Or... you can test your desire to challenge your marriage stability status and manufacture your own....

Now, sincerely, for me it is very important to maintain the aesthetics of the machine as much as possible, I mean, obviously things will change, but visual change must be limited as much as possible..

Just some old theory for the budding engineers out there, there are two principle ways that I know of to get a real time source.

The first one is some form of time code recorded onto the tape in its own track, normally nestled on the unused portion of tape between the 2 audio tracks.. now look, if you have the ability to recreate that, including the 4th head required to write and read the code, then please do not bother reading on because anything beyond this information is rather pointless and obsolete and you can consider yourself truly elite...

The second option is to receive a real time countable pulse stream directly relatable to tape travel, not reel travel, tape travel, because that is a fixed distance per second on all tape machines, which then translates to a fixed pulse count per time traveled, which then though clever grade 2 mathematics becomes a one hertz real time count reference pulse..

So you need a rotary encoder solution driven from a pulled that is directly linked to the tape path, something like the one I designed for my Technics RS-1500, or the idler wheel as standard on my Otari.

For those that are not electronic boffs (ps.. I?m no boff either..) but want to know a little more basics, a rotary encoder is like your linear digital vernier caliper, except that is operates in round round mode and not left right mode...the principle of operation is the same, you have a scale made up of a fixed number of masked and clear equally spaced segments, the "density" of which defines measurable distance resolution..

The scale is on the vernier shaft body, somewhere in the sliding head 2 optocouplers or magneto couplers are fixed, spaced out of phase with each other are slid along the length of the scale. Each masked to clear transition is picked up and serves as an trigger for pulse creation by one of the optocouplers.. the second optocouplers also creates a pulse stream exactly the same as the first, albeit out of phase with the first by some 90 degrees if I am not mistaken..

All that out of phase functionality creates is a way through some simple flip flop digital circuitry to tell whether the count must go up, or down, so in a sense it gives the movement a sense of identifiable direction..

This is no different in a rotary encoder solution, your masked/clear scale would be round instead of linear, with the scale being rotated around it?s axis, the rotation being picked up by once again two out of phase optocouplers, the pulse frequency defining the rotational speed of the scale disk, and the phase difference between the two optocouplers pulse steam defining rotational direction, ie clockwise (assume play and fast forward - count up trigger) or counterclockwise (assume rewind - count down trigger)..

The scale could be etched glass, printed clear plastic, or as simple as a machined slotted disk, all that is important is that the disk slots are perfectly equal, and that they effectively are able to create functional operating scenario for the optocouplers to read correctly..

This image provides a simple enough explanation of the waveforms generated and the phase solution we need..

egZUz0D.png


This image describes the gate solution for up / down count trigger:

7VMXVYd.png


Back to aesthetics, The B77 has a nice clean uncluttered tape path look, that is what I really want to maintain to the best of my limited ability, here below a picture of a previously engineered real time counter solution (not mine) fitted in place of the B77 tape splicing block, new pulleys and with all due respect to the original design an absolutely horrible tape path modification makes this look like a bears arse, not the solution I am looking for, but I bet one of the two pulleys incorporates a rotational encoder disk I refer to above....

AzT7GhL.png


My plan is to minimise visual impact, the counter itself will be on the left below the supply reel, as in the PR99. It will have limited functionality as in count, reset and return to zero, I honestly do not require anything more complex than that. The existing mechanical counter window will be masked with a Revox label probably engraved on an aluminium plate, neatly recessed into the panel, but that's for the future when the world opens up again..

So now that we have the theory out of the way, we need to examine the B77, and it?s mechanical design challenges that need to be overcome in order to create the correct solution, in a pretty way... again, I do not want a bears arse solution...

So read on if you wish, there will be plenty pics of many failures, but if you enjoy spanner stuff, you might well enjoy this too, but, remember its not all done yet, this is mechanicals only, the implementation of the electronics gathering dust on my garage shelf will only be once the component supply stores reopen for business, as I need a few transistors I do not have at home....
 

marcochezzi

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So why is it complicated, or is it..??

This image provides a visual of what is going on under the headshell cover, which is where I want to incorporate the signal pulley..

Mz7wQCZ.jpg


In order of left to right you start with the supply tape tension arm, inlet roller guide, this is a bearing based roller, 3 heads, capstan and exit tape guide. The exit tape guide is fixed, it is not a bearing roller, this has to do with the damping of tape takeup reel flutter..

So my intention is to adapt the takeup reel exit guide into a roller for the signal pulse, as the cavity is already there, and is visually clean..

However, those 2 roller looking things are fixed directly to the headblock plate,which is this part, which is screwed down to the chassis.. (ignore the circled item, that was detailed for another reason..)

TvThAZu.jpg


Here the supply reel guide with the bearing:

vESBMS4.jpg


Here the takeup reel guide with just the fixed spacer..

aLiMTjJ.jpg


If you look closely at both of the spacers you will see a bronze nipple, this is problem number one, because that nipple is required, in the correct fixed position, to hold the headplate cover on, the cover clips onto those nipples, so if you are going to make a pulley of sort, that part needs to be somehow re-engineered to the same position, however fixed, where everything below that would be allowed to rotate freely..

 

marcochezzi

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So the mechanicals section 1.1 - THE FAIL - because I am not clever enough to get things right first time, and I have no blueprints to work from, so everything is being invented on the fly so to speak...

Ignoring the nipple thing, lets put that away for the time being, my idea was to recreate the outlet guide as a pulley..

This necessitated a pulley of sort, and a bearing block which would be fixed to the headblock, almost a "hub" of sorts..

So I came up with this, found some reasonably small bearings:


2iylh3J.jpg



and machined a stainless steel pulley:


sS4Fj7U.jpg



and machined a bearing hub out of aluminium..


TKNL1NF.jpg




 

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Located the position for this pulley bearingblock affair on the headblock and drilled locating holes to suite..


dEPaQ0p.jpg


the pulley slides through the bearing block..


E2witXh.jpg



and from the back..


nIAjr34.jpg



 

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An encoder wheel was machined (not yet segmented), this slides over the pulley shaft and was located in place by a small 3mm grubscrew..


Kk5LsaI.jpg
.

You will notice on my previous post that the top of the pulley is hollow, apart from keeping unsprung weight down, there was a reason for this, the famous brass nipple..

So the blessed nipple had to somehow be kept in place, above of and centered to the pulley, this was necessary as without this there is no way the headblock cover plate was going to locate, and I did not want to damage the plate by machining additional alternate mounts anywhere to it..

So again from stainless I machined a cap to encase the bearing block / upper pulley. The intention was to screw the cap to the side of the bearing block, which I never go to, but for pics sake this is what I started with..


P3Ojmbm.jpg



That's the bearing & pulley cap, its upside down at the moment, the hole in the center bottom is where the nipple would eventually bolt to..


Milled the sides of the cap to allow an opening for the pulley such that the tape can get to the pulley, machined and fixed the nipple in place..


MCC6sY7.jpg



Here the three or four pieces together, imagine if you will the cap being drilled and screwed to the bearing block, just unfortunately never got there..


0awE1o0.jpg



and everything neatly in place on the headblock..


fNCykjL.jpg



another view:


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and a shot from the bottom..


iHu7C89.jpg





 

marcochezzi

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So what went wrong....??

So no matter how hard I tried, I could never get play out of the bearings, which resulted in the pulley wobbling about a few tenths, which is not acceptable..

I remachined the pulley out of aluminium, then plastic, then aluminium again, probably around 5 or 6 times, and still just could net get to within spec, the shaft was either oversized, or undersized..

Then I remachined the nipple holder cap thing out of aluminium which included a seat on the bottom allowing the cap to be bolted to the headblock, and not the bearing block, this worked fine, but the play thing on the pulled dogged me endlessly..

Then the crunch, I had posted this build on facebook in the Studer group if I recall, to which I received a kind reply that I might well run the risk of the pulley not turning because of:

A: unsprung weight potentially still being too high

B: No enough tape tension on the roller with bearing drag adding to the mix..

C: Air cushion on the roller during fast forward or rewind high speeds...

Damn, that threw a curveball, so I packed the entire affair up for a while as I no longer had the time due to business commitments..

Then Covid came along, and we dried up, and here I am at home bored, so why not finish this little affair - properly...

Time to dust off the old tools...
 

marcochezzi

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So fresh time, fresh ideas, this time I must make it work..

Air cushion problem suggestion - can it be true..

So on my working garage unit which you can see against the back wall to the left side of the Jeep, I removed the fixed exit roller guide slug, and replaced it with a bearing equal to the one on the inlet guide..

I know that there "could" be an impact on audio as this slug acts as a drag buffer of sort, but this is debatable..

Put the machine into play, the bearing spun as would be expected..

Put the machine into fast forward and then rewind, the bearing started up, but as reel wind speed grew, the bearing stopped!!! so yes, at speed there is air cushion over the slug or bearing, created by the tape movement, and the thing just stops, so that is no good as if I place a pulley there..

Damn...

But, the supply guide bearing does not stop, that continues to roll, properly..... brainfart moment....

So there the logic, the supply side has a tension arm, which directly influences the tape tension on the supply bearing, the exit side has no tension arm..

Plus, I received feedback that the counter roller on the PR99, which is in the same position as mine, has grooves repeatedly machined along its diameter, effectively getting rid of the air cushion effect..

Plus, the PR99 has an exit tension arm, which increases tape tension on the exit roller, and once again acts as the original exit slugs "flutter" damping...

Bingo...
 

marcochezzi

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So...

Scrap everything to date, and I do mean everything...

New plan..:

1. Create a new pulley optical wheel system that has virtually no bearing play..
2. smaller bearings = less drag - but that was to com later, yes this was still not a perfect solution..
3. Manufacture a takeup reel tape tension arm
4. The nipple thing, just better..
5. Get rid of the blessed air cushion..

So, in no particular order, to the above, but in terms of actual, the tape tension arm was to be created.. Naturally I immediately scoured what I could as in how the arm looks / works on the PR99, with precious little success, apart from the fact that the arm mechanism is mounted to the inside of the faceplate, unlike the supply reel tension arm which is mounted to the chassis. The PR 99 takeup tension arm mounting screws can be seen dovetailed into the front cover, but hidden from view once you have some reels on the machine.. I did not want this, so some clever engineering was the order of the day..

For absolute starters, I needed to create the takeup reel tension arm curved opening on the faceplate. This curved opening is not an exact missor image of the supply arm curved opening as the takeup reel arm travels through a different arc.. I must add it was a bit of a thumbsuck but I studied images of the PR99 faceplate and drew measurements as carefully as possible, to remain as accurate as one possibly can given the circumstance..

To cut the arc in the plastic faceplate I needed a start point, end point, and radius, so the only way to do this cleanly was to graft the plastic faceplate onto my small mill dividing head, which will allow for the required curvature..

First thing to do was to identify the centre of the curve radius relative to the faceplate..??


Jy1lpHA.jpg



Then transcribe that central point to the back of the faceplate..


pGGkgIo.jpg



Then machine a suitable aluminium "pad" which can be located in position on the back of the faceplate, this allows fixing of the affair to the dividing head chuck.. started with an old "foot" I had made some time back and machined to size and functionality requirement..

Before..:


ifBzLGP.jpg



After..:


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I had to mill a slight angle to a portion of the pad as the back of the faceplate is not completely flat and just happens to have a angled section running right through where the pad needs to go..


Z9QGEdC.jpg



This "pad" was then contact adhesive glued in position to the back of the faceplate , and left under clamping force overnight to "hopefully" hold up to the cutting loads on the faceplate, this was a risk taken, but I did not want to have to drill through the faceplate to locate this pad..


Uun4DAu.jpg
 

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Ok, so next morning and its fixed firmly in place..


htjp6rR.jpg



The stub of the pad located on the dividing head three jaw chuck, I used the "inside" jaws internally  :roll:, as this provided both clamping of the pad perfectly well, plus the extended chuck jaws on the perimeter gave good flat support to the faceplate, so I had no risk of the faceplate shaking about in the area requiring cutting..


LIh9xTJ.jpg



Top location shot just to make sure I was following the transcribed to masking tape curve radius as expected, and in cutting position..


LsfcH2v.jpg



Removed the masking tape and started the cut, 8mm width, as per the supply side width..


EsT3ZLN.jpg



And the end result, praise the good Lord nothing jumped out of place during the cut... looks factory... very happy indeed...  ;) ( Remember this cut is not supposed to be a reverse image of the supply path curve..)


fLM0IPt.jpg





 

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With the faceplate fitted to the machine, there is precious little space between the inside of the faceplate and the upper side of the pinch roller arm, something like 4mm only, and I had to get the tension arm mechanics between that, so I removed 1,5mm panel thickness off a defined area on the inside of the faceplate, every extra mm or part thereof which I can gain counts..!!


EbChoQo.jpg



Good, with that done I can focus on the tension arm itself..


First port of call was to reduce the thickness of the upper section of the pinch roller arm , as this runs at an angle so I needed to benefit space, this is not a loss good people, that arm is incredibly strong and therefore the exercise well worth the cut..


hMfL5Q7.jpg



GovWxtG.jpg
 

marcochezzi

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Next was to manufacture and position the tension arm and related mechanicals in this regard.

Unfortunately I do not have a series of photographs here, but it was a reasonably uneventful process of machining a base for the arm on the lathe, and then transfer to the milling machine to finish cut the curves and seats such that it locates nicely on the machine chassis in the correct position.. even though this part is unseen, it need to look pretty as well..!! :thumbs: :thumbs:

The baseplate end result was this..:


UBTWWd4.jpg



sbDV56H.jpg



Gtg2N8Y.jpg



I then machined a brass bush, which is wrong, because I needed bronze, but I do not have and the suppliers are closed until Cyril give the thumbs up, so beggars can't be choosers...


8CBedeU.jpg



Lightly pressed the bush into place and then machined a "bolt" for the arm itself...


P346rTT.jpg



The tension arm radius position was then drilled into the machine chassis, not the closeness of the capstan solenoid, not much fat here...


ETDE8Da.jpg



The tension arm baseplate was temporarily superglued in position to the chassis as I needed to drill and tap locating screw holes in position..


MhPAHn5.jpg



1AnmXPQ.jpg



Regarding the arm itself, clearly my phone went flat because I did not have any production photos, but to small detail, I cut a plate out of 2mm plate steel, and silver soldered the plate to the "bolt" I had made, Silver solder is damn strong, it will not come loose..

On the other side is the physical tension arm rod itself, this is an original piece "borrowed" from the supply reel tension arm of the donor machine as 1. its stainless (non magnetic) and I do not have stock in that regard, and 2. its a ground piece finished nice and smooth and I did not want to manufacture something that comes into friction contact with the tape itself that could cause surface damage to the tape..


mYttNX7.jpg



2zY27Ga.jpg
 

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Tension arm mechanism fixed in place to the chassis, the red circle depicting the spring mount spot..


hUeJpOx.jpg



Machined a small spring retainer to mount to the above...


cNTvcEn.jpg



And that in place as it should be.. Here I took the donor machine supply tension arm spring as I truly had no starting point regarding actual tension arm functionality. I positioned the spring at the same angle and location on the takeup tension arm as is on the supply tension arm, and then tested to see if the unit functions as designed, if too low, then spring can be replaced or re-positioned accordingly, no biggie...


XMa3anu.jpg



And as luck would have it, it worked perfectly, just don't know if I can add a video here so a picture will have to suffice...


VdN5hjD.jpg



You can see from the exit tape path that the "footprint" on the outlet tape guide has increased, which is exactly what was required..


Good - easy stuff finished now.... :roll:
 

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Right...

The pulley....

My nemesis...

Grief this part made me old, mark my words...

Ok, so I need to note that to get to the end, I had to redo the process below some 4 or 5 times, each time a little different, each time a little more frustrating, each time a little closer to the end result, I did not take photos of all that was done, I was too busy crying at my constant failures, and I did not want to cause liquid damage on my phone...

So I changed the process of hanging the solution on the head block baseplate altogether..

The idea behind this was to make available a longer pulley solution in terms of where the bearings were positioned, therefore if a little play was present, it would not amplify itself by virtue of a hanging hub system as originally designed..

So first thing to do was manufacture a small fixed shaft at the correct length. Note the picture below was using the 2nd bearing cap cover I had designed, this was no longer required, just at the time I did not quite figure this out..

Note as well that since the shaft is fixed to the chassis and no longer the headblock, the end of the headblock was milled open in order to facilitate removal.

Contrary to what might be imagined this does not weaken the headblock, the only reason for the now removed portion was the location and holding down of the exit tape guide assembly, the headblock is plenty strong with significant ribbing on the underside between the 3 mounting screws, which all fall way before the milled area is reached....

Naturally, in order to locate the shaft, the chassis needed to be bored, this required very accurate measurements as the shaft also holds the blessed headcover brass nipple, and if that is out of place, the coverplate cannot fit, point....


9ngDzJC.jpg


 

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and while on the subject of brass nipples, needed to make another one in order to get it to fit into the threaded end of the pulley shaft..

On the top, new, below the original..

4In3aLy.jpg


Don't know, just was not having much luck with the pulley again, remember I am trying to make these thisngs as lightweight as possible, and so all to often I found myself on the fine line of gone just that little too far..

Made this, BAM..!!, snapped in half on the lathe right at the bitter end when I "went too far.." on a later cut trying to remove weight.. note the fat tailend as this was still using the original "heavy" bearings I had on hand, by heavy, they were 6mm ID, 15mm OD & 5mm thick..

WycQjCX.jpg


That hurt.. its a small piece but its literally hours of machine work..

Still unhappy about the weight of the bearings, given the pulley was spinning most of the bearing mass, also the bearings were noisy, so I made a plastic pulley with bronze (I found some) bushes.. Not happy, in fact I think its crap, so scrap that..


dSROMXJ.jpg


Then I made another aluminium one with bushes instead of bearings, yep, you guessed it... BAM.. right at the bitter end again...


qicBwU7.jpg



Damn, looks like I was right out of luck...
 

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This was starting to take on a sense of delusion, I literally wanted to throw in the towel, I was at the stage of having had enough...

Then I "discovered" these smaller bearings somewhere in my stash, beautiful, small, featherweight and very very silent.. 5mm inner, 4 mm depth, 10mm outer


UmgocLT.jpg



So I had to press on, which meant a rethink and redo of the pulley shaft.. naturally I stuffed up.. The one on the left being the 6mm diameter shaft originally built, the one on the right the 5mm shaft, but just a little too short, naturally discovered only afterwards, so in the sin bin it went and back to machining...


zXIDT7E.jpg



Which netted the final functional device, on the left the two bearings, the 100th  :roll: aluminium pulley machined for the new smaller bearings, with the pulley face having grooves machined in 1mm intervals, 0,5mm deep x 0,3mm wide, in order to cancel the air cushion effect.

The new third shaft, machined such that it fits from the top of the chassis and not the bottom. The shaft bottom fits into the black plastic cap to the left, the 2 small plastic washers effectively fit into the mounting holes drilled into the chassis, and the flat large black washer fits over the black plastic cap such that when this entire affair is bolted down to the chassis, no nuts and bolts touch anything being steel brass or aluminium, thereby further reducing noise transfer from bearing shaft to chassis..


6XPXq4i.jpg



Here you see the insulative function from the bottom (back) of the chassis, the large flat black plastic washer would then fit over this affair, and the unit tightened down securely insulated from the chassis..


l1npGNU.jpg


All that was needed now was to trasfer everything over from the functional machine to the new machined chassis with new headblock, I removed everything including the physical individual heads and rebuilt to the new chassis..

Got to love how modular and really simple these machines are to work on, a testimony of Studers intellect and vision...


V31WOxD.jpg





 

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And............................

Drumroll.........................

.......................


..... it works!! So help me it works, pulley functions 100% correctly, tension arm works perfectly, tape runs clean and straight, audio is bang on, wish I could add a video so we can see the blessed things spinning but please believe me it works...

Pics below of the pulley nestled in place, with and without the headblock cover plate attached.. I'm so frikking happy I'm going to make a drink with 70% alcohol hand sanitizer and celebrate....


J5q2eyU.jpg



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Right, so using my Iphone and a strobe based rpm calculator app, I was able to identify the pulley revolution at 3 full turns per second.

So the next (technically last) mechanical bit of machining left to do is cut the slots on the sensor wheel side of the pulley

Using a 3mm cutter, I was able to cut 12 slots for the optical coupler.

Therefore, since I have 3 revolutions per second during play x 12 pulses per revolution = 36 pulses per second played / recorded.

I can now electronically divide the 36 pulses by a factor of 36 , and get to a 1 pulse per second solution, which is effectively my real time counter clock signal, done, chuffed..!!

Here the pulley arse side up on the dividing head this very morning, ready for the slotting..


xeD7B6h.jpg



Almost done.. this is generally where things go pear shaped with me..  :pray:


x27sZBr.jpg



And done..!!!!!


SUaW0FV.jpg



Here you can see her clearly in place..


EjxcJfg.jpg


 

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And yes, the sin bin, because there has to be a sin bin, all of that to the left, just to get to one of those on the right......


tGX0bWN.jpg



Admittedly there were more parts destined for the sin bin, but I could not find them, they had been thrown somewhere during one of the plenty fits of rage, and quite probably Max Verstaffie ate the bits anyway...



This mechanical side to this project was mammoth, COVID 19 lockdown allowed me the time to keep pushing, had lockdown not been here I doubt I would have done this till goodness knows when..

So for now the machine remains uncovered but perfectly functional in the garage, I could take a short video but I am unsure how to post the video via imgur.com..

Electronically the counter is built, I need to mill an opening for the display, I need to purchase components such that I can build the divide by 36 divider.

I have the optocouplers but I need a few small discrete components to make them work correctly and convert to digital, I will need to machine a bracket of sort to house the optocouplers in the correct phasing, on the machine chassis

I hope that you have enjoyed this thread, please drop questions if you wish..

I am happy..!!

 
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