Dial-A-Level lives again

[@CuriousMarc]

And here was George’s reaction to the video: “Hi Marc: After viewing the professional video describing all the technical aspects of the Vintage Dial-A-Level liquid level control, I was impressed, and emotional, seeing how well you did this. You also described, and showed in detail how the circuits are arranged and work. I sent the link to our son who is also well versed in electronics. Thank you so much for doing this, I am honored. Best, Expo Instruments George Rauchwerger” To which I responded that the honor was mine, being in the fortunate position to merely have to admire what others have so nicely invented and engineered. Anyhow, a class act he sure is! [Edit] And I eventually got my beer with George! He told me the fascinating story of how his parents escaped from Hungary to the US, and survived through farming hard work. Then he learned electronics and became an inventor. Found his market with the wineries, making customs industrial systems to measure wine levels in tanks. He his still busy inventing today…

[@Miata822]

What a wonderful tale. We all hope our work will live on and add something to the larger human story.

[@ahndeux]

The workmanship and craftsmanship is awesome. The wire harnesses was neatly laced together with nylon straps. the layout is clean. You don't see this level of quality and workmanship in today's throwaway SMT electronics world. I'm sure George is proud of what he invented.

[@tesla242]

That´s real engineering! fascinating that you could get in contact with the designer!

[@rallymax2]

He’s the American dream realized. I’m so glad you came across the box.

[@little_fox9695]

So cool you actually got in contact with the inventor himself :D

[@CuriousMarc]

That is the nicest feeling when you meet the inventors at the end of or even during a restoration. It gives real meaning to the whole effort. People are more important than things! It happened with the Xerox Alto, the IBM 1401 and the Apollo Guidance Computer. At least one of the Alto guys (Dan Swinehart) actually became a fervent patron and is sometimes found commenting on the videos!

[@theLuigiFan0007Productions]

@@CuriousMarc I really like the design of the control logic in that device, simple but reliable and effective. It's great you are able to connect with inventors and learn the story behind these devices.

[@MistahHeffo]

Inventor: Wow, how did you find it!! What container did you want to measure?? Marc: Container?? No, I want to make this help control an Apollo Gyroscope

[@MrCarlsonsLab]

Great stuff Marc! Enjoyed every minute :^)

[@CuriousMarc]

Thanks for stopping by...

[@garbleduser]

I REALLY want to see you on site, helping with a restoration! Or even a long distance collaboration. You two are the best vintage tech preservationists I know! The nostalgia, warmth, entertainment, and knowledge that I get from you effects and inspires me on daily basis. Thank you!

[@ikocheratcr]

The capacitive method of measurement fluid level, is used on car engines to report engine oil level. It uses a *two* capacitors, one that is always under oil, as reference of what it is to be in oil, and the other one that does the measurement itself. It is necessary to have one "capacitor" under oil as reference, because the oil dielectric changes with time (it gets dirty) and temperature.

[@compu85]

I had a similar response when I emailed a Baltimore MD company that made a serial protocol analyser which hooks up to a PC parallel port... I couldn't find the software. A few hours later I got a note from the CEO with a link to their FTP!

[@ShainAndrews]

Of course Marc would get a response... from the inventor. George as a stranger on the internet, I thank you!

[@MadScientist267]

I wish I could have run across an explanation like this a few years back... I was working on trying to detect ice formation and was exploiting the idea that ice is an excellent insulator, where water will readily conduct when ions are present... I only sorta got my version working, and while it was capacitive just as this is, I had problems with repeatability and stability both because I was going about the whole thing wrong. I never realized why at the time and ended up going an entirely different direction, but thanks to you Marc, I at least understand what was wrong with my approach. While this video obviously "didn't help me" for that, this is what I love about your channel... You cover things that are kinda hidden away in history but still could have very useful application that is timeless. You never cease to amaze and amuse. Thank you!

[@Ddub1083]

The explanation was in their patents. Thats what getting a patent means... publicly disclosing how the device works.

[@MadScientist267]

@@Ddub1083 You're missing the point. Congrats.

[@borisvladimir7151]

Imagine being the inventor of a very specialized tool to measure some water level or something like that thinking what you did would stay buried for ever ... and 50 years later receiving an email from some dude asking you questions about how to use it, how to fix it etc. And then when you ask him in return what is the size of the glass of water he's trying to measure the dude tells you something about interfacing with a gyroscope for AGC and there is this giant rocket and this LEM and he will go to The Moon.... Mr CuriousMarc, I have only one thing to say : I LOVE YOU !

[@warrenrexroad1172]

It might have been a minor part of this video, but I'm so happy to learn about Excess Solutions. When Weird Stuff and HSC Supply went out of business I thought I was out of luck for a good surplus shop in this area. Apparently I just hadn't looked hard enough!

[@666Blaine]

Next week we'll fix the Dial-A-Yield on this thermonuclear warhead I found at a yard sale.

[@AsbestosMuffins]

Man those Dial-A-Bombs are such a PITA, just when you think its working, kaboom

[@hernancoronel]

CuriousMarc does not need no stinking Arduino! He can get a big box, fix it, contact the inventor, reverse engineer it and at the same time shoot this excellent video with a great story to go along! Thank you, thank you, thank you!

[@ckm-mkc]

All this is so Silicon Valley. This is exactly why I moved here 20 years ago - the wealth of knowledge is insane and happy to share. It's changed in recent years, but the spirit is still there.

[@JohnJones-oy3md]

11:05 - I didn't realize that the TO-92 package dates back to the mid 60's. Very interesting. BTW, love the sense of humor that you interject into these videos.

[@SeanBZA]

One of many styles of package, but won out over the others because it was low cost. The others were a ceramic cup with epoxy fill and assorted styles of epoxy filled cans, which are still made for military use. There are a whole host of JEDEC standard cases that have vanished from common use over the years, and also a whole host of similar surface mount cases that replaced them and went obsolete as well.

[@72polara]

I am sort of in the habit of looking up patent numbers. Always interesting to see what exactly is patented in a device. I looked up the number on a bottle of machine oil, thinking it was for the oil, but it was for the design of the plastic bottle.

[@soulrobotics]

From the 70's until now, i always connect my ammeters and miliammeters in new projects at the very first start. I can tell by Marc's reaction: " wooop, oh, oh ,oh, aww, it's ok !..." That he was from that old school also. We have stayed away from the magic smoke... Well,..."that" smoke....

[@sefarkas0]

When Marc gets the gyro hooked up it will finally verify he is off his rocker.

[@TheRealColBosch]

"Strange, the needle pegs itself at 'Hewlett-Packard fanatic' every time I turn it on."

[@jaybrooks1098]

Pfft

[@gavincurtis]

True dat. No more rocking once the gyro is operational.

[@msylvain59]

10:50 That capacitor in the middle is the biggest silver-mica capacitor I have even seen !

[@arthurharrison1345]

It may be a film capacitor. At least one manufacturer at that time made them with the same dark-colored coating you find on mica types.

[@KarlAdamsAudio]

Hard to be sure, but it did seem to have that characteristic 'dog-bone' shape that I associate with silver-mica caps.

[@SeanBZA]

@@arthurharrison1345 You want stable you go with encapsulated silver mica. I have a 100n 2kV silver mica capacitor, it is about the size of a half brick, with screw terminals, and an 11A rating at 2MHz. funny enough used to go through the town called Mica, where the material was mined from for a good part of the world production at one time, and you still can see the massive deposits of it there.

[@acmefixer1]

Yeah, it's not a silvered mica, it's a film capacitor. I was fooled by that look once before, but not anymore.

[@dant.3505]

@@acmefixer1 yes I agree if you look at the side you will see the "-" polarity mark that gives it away as a film and not silver mica.

[@projectartichoke]

It's so satisfying to see long-lost electronics from the 70s with hand-drawn PCBs come back to life. That was the period in which I really became interested in electronics and those old devices are all very special and they still have things to teach us.

[@cocusar]

pretty cool and amazing to know you got a response from the company!

[@SeanBZA]

That capacitive measuring has so many applications, from fuel level in aircraft, to the level of fuel and oxidiser in rockets, because the sensor is very simple to integrate into the tank, and is there with no moving parts at all. Only thing is, the measurement needs to have the fluid have a defined level, so your fuel gauges in rockets only works when you are either under thrust, or use ullage thrusters to settle the fluid. Otherwise the range will vary from empty to full, as blobs of the liquid move around the tank and impact the sensor. You even use it in microphones, where you use capacitance change, and either applied or stored charge, to make electret microphones work.

[@Nighthawke70]

And you know, there was one piece of flight test gear that could have prevented Apollo 13's near-disastrous oxygen tank explosion while on the way to the moon; the O2 tank temperature readout. It was only calibrated to reach the maximum level while monitoring the tank of about 80F. But the ill-fated tank was cooked to around 800F, destroying the Teflon lining on the wiring. All the while the gauge was only reading 80F and everyone pretty much ignored it, shutting it off later on after the O2 slush was purged. This was fixed later on, along with a lot of other subsystems and checklists, standardizing voltages used on ground test gear and flight hardware to meet the Cape's higher than factory voltages at the launch pad.

[@DrKlausTrophobie]

These days two dial-a-level go into a box smaller then a 1/4 liter. Including analog output, selecting Levels/Min/Max/Hystereses with 2 or 3 buttons, small LED Display and even temperature measurement included.

[@markocebokli6565]

A very similar two SCRs circuit was used in older photo flash units, to turn off the light, but there the second SCR conudcted only shortly, until the cap charged.

[@johnopalko5223]

You made George's day.

[@opros7]

That flipflop with an SCR is very simple, elegant and perfect for the job. The only failure i see is that both could get a pulse at roughly the same time (min & max pulse very close together). And even that was being taken care of since the min gate is ac coupled with C10 and the cap between the SCRs is probably large enough to extinguish one within time. Thanks for showing the schematic.

[@SeanBZA]

Yes, and also was common as industrial control for power, using 2 SCR devices to do DC switching, with one device being used to control power, and the other being used to couple through a pulse to turn it off. SCR used for being the most rugged semiconductor ever invented, capable of handling massive power for the time. Only thing is slow speed, so your original motor controls were sub 1kHz in operation, making them very noisy, but still many are still running in train traction applications driving DC motors.

[@graemedavidson499]

Noah broke it.... hurriedly went on to make a large boat afterwards...

[@77leelg]

I bet you made George’s day! That was an obscure but very interesting instrument.

[@michaelhyde9971]

Great to see this old stuff put back to use. I wish we had a place in the UK to find equipment like this. Keep up the good work.

[@Zeem4]

I remember places similar to this existing in the past, my favourite was Anchor Supplies in Nottingham, which sold all kinds of weird military surplus. I once bought an Amiga 2000 from there, but I remember seeing all kinds of electronics and even an armoured personnel carrier for sale. I'm not sure it's there any more, but in the meantime, it seemed to turn into a place selling coats and shoes and stuff.

[@edgeeffect]

"mjtronix" on E. Bay have loads of groovy military surplus... bit on the costly side though.

[@ahmedalshalchi]

Brilliant .... Thank you Marc and George ... You've just sparked a new idea for new electronic project for commercial use !! ...

[@bborkzilla]

The use of the SCR flip-flop is very reminiscent of the "Thyratron Rings" used in Colossus, which also used this sort of circuit (albeit with tubes) to produce the shift registers that were used to simulate the mechanical wheels in the Lorenz SZ-40/42.

[@haroldsmith45302]

Good work! And interesting. Thank you. Since the density of a gas increases with pressure at a given temperature, and if the permittivity of a gas increases as a function of its density, perhaps this system could also be used to sense and control the pressure of a gas in a containment vessel, assuming that the capacitance ranges of the instrument and sensor were made suitable, and controller dead band made sufficiently narrow.

[@milantrcka121]

Awesome! I relate to 70's + electronics - mag tape recorders, servos, measurement electronics, etc. I would like to visit the surplus shop. Alas, I am in So Cal.

[@acmefixer1]

Same here. There was supposed to be one place up in Van Nuys, quite a while ago. I don't know what happened to it.

[@milantrcka121]

@@acmefixer1 Yes. Was there once some three decades ago... Awesome place but a fright on account of a possible earthquake whereby being buried in all manner of electronics equipment all piled up on rickety wooden shelves.

[@graemedavidson499]

Similar to hieroglyphs the symbols on the back were higher-or-lower glyphs

[@MLX1401]

Old handrawn THT boards are so pretty! Also, I like how the cable cutout in (the bottom of) the heatsink looks like there could be a tiny steam engine rolling through at any time :-)

[@PeterCCamilleri]

A fascinating look at a classic instrument. It is so cool that you were able to converse with the inventor.

[@jadney]

Seems odd that you have to choose either min or max settings and alarms. There would be many applications where one or the other would be the one normally controlled, but the other would be a definite alarm/fault situation. Seems like a trivial expansion of the concept, and simple to implement since they've already done 95% of the work. Interesting to see 2 capacitors used for an AC only multiplier. I never thought of that; just another thing that's obvious once the Egyptians explain it to you.

[@sibsbubbles]

Must say those concentric aluminum knobs are superb.

[@forbiddenera]

"Why?"..thats the best button I've ever seen on any piece of equipment ever 👌@0:16

[@nutsnproud6932]

Thanks for the video. A very interesting piece of kit. Best wishes.

[@RinoaL]

2:52 HAH! thats Liquid Flame from FFXIV, i wondering why my volume in the game went up as i was watching that and realized "wait a minute, im not in that zone.. oh thats coming from this video! woah" wonderful little song!

[@WakenWanangi]

THAT'S where that music is from! I kept thinking that track sounded so mmorpg-ish but I couldn't place it lol.

[@MarcelHuguenin]

Hi Marc, so wonderful to see you research and reverse engineer such an old device, which is indeed very nicely made btw. Great to see the inventor got in touch with you, he will be proud of your work and video I guess. Looking forward to the next episode. Thank you!

[@gwyllymsuter4551]

I find this way more enjoyable than eevblog. So much in depth detail. Only wish curious Marc and his compatriots published their archaeological findings in electronic form, for us nerds to examine more closely. 😁

[@CuriousMarc]

Usually I put the details online on my web site at www.curiousmarc.com (usually setting up the page when someone asks for it), Ken writes it up on righto.com, and Carl blogs it on rescue1130.blogspot.com.

[@gwyllymsuter4551]

@@CuriousMarc I'll have a little bo peep and get some data on this beast. Looks fascinating

[@CuriousMarc]

@@gwyllymsuter4551 And here is the web page, just for you: www.curiousmarc.com/instruments/dial-a-level

[@ingresswizard9044]

So awesome the inventor was still alive and responded to you. Hopefully he gets a kick out of your channel and the videos!

[@tomsmith3045]

This is a great story! The design itself is interesting, and so was the discussion with the inventor. Analog electronics are great, when they're done well.

[@larryscott3982]

It’s a good visual example of 70s electronics. Imagine what it would look like if it was an HP.

[@heyitsandrew2209]

This is fascinating. As an amateur electronics tinkerer I love learning about stuff like this. I'm new to it all. This is like the analog version of some of the Arduino projects I've been learning from.

[@lucashinch]

That would be excellent case for a Hi-Fi receiver/amplifier. I recently repurposed the case from a 70's era "HEATHKIT oscilloscope calibrator". Original styling is conservative and contemporary. with a similar sky blue cabinet cover and a pastel white/grey body it was well suited to become a DIY headphone amp/pre amp . internals were replaced with 2 Ch ins' and outs' and everything between. It's also a receiver for aptX Bluetooth and a DAC via TOSLINK . Listening is through Sont MDR-V600 or as a pre amp to CARVER PM600 then to a pair Klipsch F1. Reusing certain higher quality parts and components can yield some amazing results regarding audio system building.

[@MrJohnBos]

Very nice video. I love electronic treasure hunts like this, I always learn a lot. Thank you.

[@williamsquires3070]

You can also control two separate devices by removing the shorting jumper from the back, then finding some SPST relays with N.O. contacts. Wire one end of the relay coil(s) to the outputs from the SCRs, and wire the other two together and hook that to the positive supply rail for the internal reed relay. Now you have one set of contacts that turn on when the “min” output turns on, and the other when the “max” output turns on (of course, the other will turn off.) This should work with a thermistor-based temperature probe, though you might have to replace the feedback capacitor with a resistor whose resistance matches the AC impedance of the feedback cap at the oscillator frequency.

[@SpinStar1956]

Yup, you have to do a video with George; we'd like to meet George and hear some "war" stories from the past! Great video as always!

[@nicoras8803]

Three prongs are used for measure ring resistive temperature sensors very accurately. It should be very simple to convert to RTD.

[@surgeon942]

Awesome story, it was golden age of engineering. Really inspiring. Hope to see George in next videos

[@rwdplz1]

That's awesome George is still around!

[@largepimping]

It would be so much easier to just use an Arduino and a couple of simple sensors... but I'm SOOOOO glad you didn't! Your videos are so fantastic, thank you for continuing to make them.

[@CuriousMarc]

But it’s actually not. All the work is in the panel, the display, the pots, the contact blocks, integrating the power supply, the high voltage isolation. The electronics is the easy peasy part, just 5% of the thing. People forget what it takes to make a complete project, and the electro-mechanicals involved, which is most of the work.

[@largepimping]

@@CuriousMarc well, ok fine... For ME it would be easier!

[@rasheverak]

I absolutely love the '70s-style hand-drawn-no-silkscreen PCBs. Shame it's so hard to duplicate these days.

[@briangoldberg4439]

Thank you for teaching me about the role of the ancient Egyptians in the invention of the triode. I feel enlightened ;)

[@LMacNeill]

That's an ingenious piece of circuitry! So elegant!

[@phillip5245]

I wonder if Protovision still operates out of Sunnyvale, CA?

[@Nighthawke70]

You know, that sketch of that ground test equipment sparked an old memory. Back in the mid-80's the folks and I went to Huntsville Space and Flight Museum (now U.S. Space & Rocket Center). Well, I somehow wound up in their boneyard that contained a massive amount of gantry hardware, along with, guess what? Old ground test gear rigs with lots of dial, gauges and switches. That one diagram I THINK was one of them attached to one of the skeletons that was once a gantry. This being 20+ years ago, I don't think it could have survived much longer in the elements.

[@eluder25rs]

"YOU'RE MY ONLY HO" 🤭

[@RealEngineer]

This channel is KZbin gold!

[@kins749]

Love it, great to hear George still going strong after all of these years

[@RickBaconsAdventures]

Came for the awesome old electronics, but I LOVE THE JOKES in this video "YOU ARE MY ONLY HO" had me rolling on the ground

[@dosgos]

George - that is a clever box!

[@donpdonp]

This video reminds me that Marc has forgotten more about electronic theory than I will ever know.

[@flp322]

That's why we watch his channel :)

[@jeremiefaucher-goulet3365]

So.... Georges is Egyptian? So awesome you got in touch with the inventor. What were the odds??? Small world.

[@irgski]

The “old” is “new” again! Excellent reverse engineering!!!

[@edgeeffect]

This is the great advantage of fixing vintage equipment instead of current stuff... when I E. mail ST and ask for advice on programming their microcontrollers in assembly language, they think "this guy's a loony!" and just ignore me. ;)

[@termm-O-vision]

This device has some strong "Apollo 13 current meter" vibes to it. Great video.

[@justus4293]

I just learned about a reed relais five hours ago in my apprenticeship... coincidence? Awesome Videos, learning lots ;-)

[@casparberends2719]

Nice video. This exact system has been used in BMW motorbikes to measure how much fuel is in the tank.

[@arenaengineering8070]

An interesting device. Thank you for the video. Like from the Republic of Belarus. 🇧🇾👍

[@SenileOtaku]

A patent from back when they actually meant something, and actually related to quantifiable items

[@boonedockjourneyman7979]

Patent drafting is an art. As technology changes, drafting changes. Patents from “back when” typically describe simpler material than patents “from today.” If a modern patent is troublesome to understand then get some help. There are lots of places scattered around for free help. Reddit and Stack Exchange are both good. Saying things like “patents today don’t mean anything” simply identifies you as someone who has allowed themselves to be left behind.

[@RoamingAdhocrat]

I love this. Hope you get that beer soon!

[@kevinreardon2558]

Thanks George!

[@CEverett55]

Why not just use a capacitor with a really crappy thermal coefficent, instead of a NP0 use a NP1500. Im sure you could adjust the op amp gain to make it super sensitive to changes in capacatence caused by temperature. Just a thought.

[@acmefixer1]

Good idea. There are some larger value disc capacitors that are very temperature sensitive. One problem is that they are also microphonic, so if you tap on one it might trigger the alarm. 😱😱

[@FrozenHaxor]

@@acmefixer1 Yeah, especially in gyroscope that has spinning motors.

[@emmanuelr6698]

Extraordinaire, merci !!!

[@micarifamily1]

Very interesting and informative thank you!!

[@A3Kr0n]

Dial-a-level reminds me of Bill Gates, Paul Gilbert and Paul Allen's Traf-O-Data for highways from the 70s.

[@HorochovPL]

Dial-a-level sounds like "dial eleven" - "it goes up to eleven" reference?

[@CuriousMarc]

Actually it looks like the level can be set past 100%!

[@FF177-]

Spinal tap was a decade later, but maybe he was ahead of his time

[@rarbiart]

If you need that extra push over the cliff!

[@thebitcoinimist]

"... These go to alevel. "

[@nicksantos43]

I have a gyro from a Mk46 torpedo that is around the same vintage as the IRIG gyros but not as advanced. They are spring-start with an electric sustainer motor, been trying to figure out what would be the best way to power the thing. I'll be interested in seeing you go over the power requirements of the IRIG (if you haven't already). From my research my gyro requires 73 volts at 400hz and I am wondering if that is similar to the IRIG?

[@kimh9736]

I like that music, it reminds me of IMAX NASA movies and From The Earth To The Moon. I always enjoy your explanations and circuit diagrams, you make it look so easy. Keep up the good work.

[@64bittz93]

Those absolutely wild looking traces tho

[@electrofan7180]

I guess that George can tell us a lot of stories from those days.

[@zh84]

8:53 What are the prominent disc-shaped components with cross slots in their centres?

[@derkeksinator17]

trimmer potentiometers, it's a resistor you can set the value of by turning said discs.

[@esepecesito]

Variable resistors. "Trim-pots" Today much smaller, with a little screw, typically light blue. In those old ones, you could even see the carbon trace and the physical cursor.

[@zh84]

@@derkeksinator17 Thank you. The trimmers I saw when I was learning electronics in the 80s were considerably more compact! :-)

[@zh84]

@@esepecesito Yes, those are the kind I remember.

[@CuriousMarc]

@@zh84 Yes trimmers. Specifically to ensure that the min and max control levels scale correspond exactly to the 100% markings on the front panel. The 3rd one calibrates the alarm knobs, which does not have a separate cal for the min and max.

[@acmefixer1]

Cool! Looks impressive, with the meter. But if everything is working right the meter needle will never vary off the middle half a degree C. So now you have to somehow change the temperature into a variable capacitance. I'm patiently awaiting the next episode... 🤗🤗

[@spehropefhany]

He must be in his eighties by now. Very nice.

[@djayers]

The transistor oscillator, isn't that the classic square wave generator? I'd imagine given all the harmonics it'd be tricky for something like balancing capacitors. But fair enough, if it's worth patenting.

[@beamer.electronics]

Yes, you're right it's an astable multivibrator and produces a square wave. But, by the time the SW goes through the C2/T1/R1 a sine-like wave is produced. This type of oscillator is temperature, supply voltage and ageing sensitive? And, if Q2 and Q3 are too closely matched it will not fire-up at all - due to a failing race condition.

[@rpavlik1]

That's a pretty clever operating principle. No idea if it's any good for your oven-controlled gyro, but certainly a neat gizmo to learn about!

[@RobSchofield]

Great fun!

[@roberthayes6329]

More Please!

[@janbos1975]

CuriousMarc. I want to learn Electrical Engineering. Are there specific textbooks that you would recommend, that you and the other members of your team found helpful to them in their journey to where they are today?

[@shadowking6008]

i just noticed the ffxiv background music being played atm XD

[@Quasam]

Screws from compaq/HP desktop.

[@aserta]

Neat "complicated" but simple, variable switch.

[@MVVblog]

Hot & Cool at the same time!

[@darrenerickson1288]

Actually the patents were a smokescreen. This was really all about helping to control the Stargate, and the hieroglyphs were your National Treasure clue to that.

[@mcsniper77]

You could shove 2 probes in the ground and use this to control a timed pump to water your plants.

[@BAD_CONSUMER]

Had no idea you were in the Bay Area... I used to go to HSC before they closed. I heard Excess purchased their inventory.

[@CuriousMarc]

Yes, some of the HSC stuff has just re-materialized at Excess. Yay! We can finally rummage through it again and get that odd Xerox Alto connector, missing screw or weird capacitor value/voltage combination right off the shelf to keep our restorations going. Also the collection of vintage ICs has also reappeared there. I could have continued my HP 98035 restoration without interruption if I had known.

[@UpLateGeek]

Wow, George has a great memory considering they stopped selling the product more than 35 years ago! I can barely remember the details of projects I've worked on 35 days ago!

[@KX36]

Your opamps don't need markings if the ubiquitous opamp of the time was the μA741. ;-) The patent drawing even uses "μA" as a designator for opamps. (Yes, I know there were other opamps at the time, including others with μA in the name)

[@bprosman]

Could have been a uA709

[@CuriousMarc]

μA was the generic prefix for a long list of National Semi (or was that Fairchild?) linear circuits like op-amps, comparators, power regulators, etc..., so it does not imply 741. It just stands as a shorthand for “op-amp” in the patent. And obviously you can see from the recovered schematic the op amps all have different pinouts. Some are compensated, some not. They also are fed unbalanced supply (+11/-6), which is unusual. No μA markings on them. Compensated ones can’t be 741s for sure. I’m still not exactly sure what they are.

[@KX36]

oh cool, thanks. I did mean it sort of as a joke. Imagine the "roll safe" meme. Sorry, I wasn't very clear.