Avionics reverse-engineering: 60's Hughes telemetry unit - Page 1
Forum section: electronics / projects

author: D Straney
author_rank: Frequent Contributor
timestamp: August 16, 2025, 12:12:20 am
content: When developing anything that flies,telemetryis important. If your test vehicle suddenly takes a nosedive into the ground, you'll want an external record of speed, control surface positions & commands, engine status, etc. so there's some hope of figuring out why. I was lucky to find this Hughes Aircraft telemetry unit from the late 60's: everything in here could be implemented in $20 worth of parts from Digi-Key today, but it's a goldmine of fascinating historical electronics.The "AIM-4H" referenced on the label seems to be anold missile variant that never left development(scroll up a few lines). Considering how many different projects Hughes Aircraft was running, though, I'd be surprised if this wasn't a fairly general-purpose unit. This was also not something that would be used on an "in-service" missile - no need to have it broadcasting unnecessary data and announcing its presence. So just to forestall some uninformed "you're giving away military secrets!" responses and be clear about the context: this was aprototyping toolfrom over half a century ago, which any EE student could build now with parts from a Radio Shack if they still existed. That's not to diminish the amount of work that went into this, though - it's full of cutting-edge miniaturization for the time, like op-amp ICs and switch-mode power supplies.Looking inside, we can see a variety of different parts:1. Battery & Power SupplyThe telemetry unit gets its own internal power (in case the external power fails, I assume) from athermal battery. From a little bit of reading, these seem to have a small amount of fuel inside which burns in a controlled way when activated, producing heat and melting an electrolyte which enables electron exchange to start happening. (This is different than theRTGs used in spacecraft, but as in addition to radioactive decay as the heat source, those usethermoelectric generators- same principle as thermocouples.) The run time sounds like it's short but they are apparently very stable in storage (inert until triggered, so none of the self-discharge of a normal electro-chemical battery), so they were popular for some niche applications like this one where something has to sit for a long time, and then work reliably.The output from the battery is passed on to the internal power supply:This looks like a switch-mode power supply, which uses a transformer on the back side and several stud-mount power devices. From the single battery input, it generates at least 1 digital-logic voltage, and a set of positive & negative analog supplies.With the way it's put together I couldn't map out the circuitry in the power supply, but my best guess is that there'ssome kind of simple self-oscillating circuit like a resonant Royer oscillatordriving the transformer's primary winding; multiple secondary windings are then separately rectified and provide the different output voltages. There's probably linear post-regulation on each of these output voltages, as a multi-output supply can only keep one output regulated by controlling the switch timing. The larger board in the front has a lot of tantalum capacitors and diodes, which are likely the rectification and bulk storage for the output voltages. Counting the stud-mount devices, there's 5 transistors and 1 diode, which is enough for 2 switching transistors and 3 linear regulators.2. Input signal conditioningThe signals to be monitored all enter on the two external connectors.They're then fed by a complicated but very neat wiring harness to these 3 boards at one edge, that all provide scaling and input protection for the external inputs.Starting from the outside-most board...If you look carefully here, you can see multiple channels - not perfectly identical, but with common themes. Each one has a series & shunt resistor (protection and pull-down, plus a voltage divider), sometimes a filter capacitor or two, and a pair of diodes that clamp the signal between 0V and the supply voltage of the data encoder (which we'll be looking at later).The other boards contain more circuitry:There's small metal cans labeled "SL7160"......and a larger wide, flat metal can labeled 2809BG-003:I couldn't find anything matching these part numbers, so I cut open one of each can.The smaller one contains a simple op-amp:The larger one contains a hybrid circuit, with an op-amp, 2 JFETs, and a resistor network:The JFETs & resistor network form a high-input-impedance differential buffer stage which buffers the positive and negative inputs before they reach the op-amp. By about a decade later you could buy single-chip op-amps with JFET inputs such as theTL08xto achieve these high input impedances, but when this was built these weren't yet available, and so taking a custom-packaged hybrid-module approach made the most sense.I mapped out a couple of representative op-amp channels - they're all pretty much identical, maybe with different resistor values:The top one uses the large-metal-can hybrid and larger input resistor values (no bad effects with the JFETs' almost-zero input bias currents). However, they both perform the same function of: (1) receiving a differential input from the external connectors (a sensor or status output of some kind), (2) applying scaling to it, and (3) doing a differential-to-single-ended conversion to remove any common-mode noise picked up on the wiring. There's also a positive offset added by R9/R10 & R13/R15: I'm guessing this is to make a bipolar signal fit the positive-only 0-to-VAA input range of the data encoder.3. Data encodingNext comes the real "meat" of the telemetry unit - the so-called "multicoder", which does the multiplexing and modulation.This is a separate unit made by a different company, which I couldn't find any info on. It does the main work of cycling through each input channel one by one, creating a properly timed and modulated output signal to represent the state of each input in turn. That modulation could be digital (pulse-position, pulse-width...) or analog (frequency, phase, amplitude...). I'll look inside this box in the next post, but for now know that it takes 32 analog inputs and produces a single time-multiplexed output. You also might be able to see from the wiring that many of the input channels are redundant - apparently not all 32 channels were needed, so all the unused channel inputs (almost half) are connected as duplicates of other inputs.4. VCOs & Radio transmitterThis data now has to be sent to an antenna somehow. The first photos of the outside of the telemetry unit showed a flying BNC cable, and that comes back to this stack of electronics next to the multicoder and battery:At the top of the stack are a couple boards, which contain 2 channels of additional input-signal-conditioning circuitry. I guess there wasn't room on the other signal-conditioning boards and so the extra circuitry had to be crammed in here.Below these are 3 miniature VCO modules - some cute tiny little guys. We'll be looking inside one of these in a future post too.Finally, at the bottom of the stack is the radio transmitter itself. This holds the RF oscillator, final modulator, and power amplifier all in one.According to the label, using it is pretty simple - it only needs 3 connections, for power, ground, and modulation input. The antenna output is on the fixed coax cable that extends from the telemetry unit. Don't worry, we'll also be looking inside this module as well.5. Overall setup & connectionsSo, now we've seen the general overview of how signals enter, are scaled and/or offset, are muxed/encoded by the data encoder, and then are somehow fed to the radio transmitter. But what are the details? Why are there 3 separate VCOs above the radio transmitter?Tracing the circuitry on the two boards at the top of the radio-transmitter stack, the overall picture becomes clearer:The top section is just another mostly-normal channel of signal conditioning, although single-ended for some reason. Ignore that part.Notice how the 3 VCO outputs are connected together, and then connected to the radio transmitter's input: it looks like they must have finite output impedances, and are getting summed. The "set" wire from each VCO is also connected to a different-value resistor: these must be the center-frequency-adjustment connections.So, we have 3 separatesub-carriersbeing transmitted on the radio channel. One of these sub-carriers is modulated by the multicoder, and so contains the majority of the telemetry channels, time-multiplexed (VCO 3). A 2nd sub-carrier is modulated directly by one of the signal-conditioning channels (VCO 2), and the 3rd sub-carrier is modulated directly by the circuitry in the middle of the schematic (VCO 1). VCO 1's channel seems to be a sum of 3 different external inputs.Why the 3 sub-carriers? Why not feed these 2 additional channels to the multicoder and have them muxed along with the rest of the channels? The benefit of having these 2 channels on their own separate sub-carriers is that their readout is continuous: you don't have to wait through the full muxing period of the multicoder to get a glimpse of their value. These are probably higher-bandwidth signals like control loop outputs, then, rather than slow-changing DC values like temperature or battery voltage.Next I'll be showing off the inside of the multicoder module.

author: magic
author_rank: Super Contributor
timestamp: August 16, 2025, 04:52:57 am
content: The monolithic opamp isLM101A.The chip in the hybrid reminds me ofthis, but it matches neither the published schematic nor the actual circuit exactly.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: August 16, 2025, 07:58:32 pm
content: When I see these old electronic assemblies, I am always beyond-amazed at the amount of labor going into these units which is staggering, always assembled with great skill and meticulous care.Just look at the wire dressing! There must be perhaps 100 different wires, and all are perfectly bundled, routed, terminated and secured.The glass-bodied diodes all have strain reliefs.And judging by the handwriting on the boards, many of the components, and certainly the wires, must have been hand soldered.True craftsmanship!

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: August 16, 2025, 08:07:30 pm
content: I visited your Flickr page.What an amazing photo collection you got there.WoW! Just wow.

author: Haenk
author_rank: Super Contributor
timestamp: August 17, 2025, 09:48:19 am
content: Quote from: UnijunctionTransistor on August 16, 2025, 07:58:32 pmWhile true, these devices probably did cost as much as a luxury car, for exactly this reason.

author: Simmed
author_rank: Frequent Contributor
timestamp: August 17, 2025, 10:04:27 am
content: what about this gold shiny thingy ?power regulator ?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 17, 2025, 04:21:36 pm
content: Does this use the IRIG standard? Can you make it work and telemeter some stuff? I guess maybe not after de-lidding that chip...

author: KE5FX
author_rank: Super Contributor
timestamp: August 17, 2025, 05:47:08 pm
content: Never heard of a thermal battery before. The temptation to see if it still works must be overwhelming!

author: Stray Electron
author_rank: Super Contributor
timestamp: August 17, 2025, 10:35:03 pm
content: Quote from: KE5FX on August 17, 2025, 05:47:08 pmI still have two of them from a project that I worked on back in the 1990s and they're built like the Rock of Gibraltarand I have NO doubt that they're still viable.Eagle Pitcher's website:https://www.eaglepicher.com/technology/battery-chemistries/thermal-battery/You can click on Products and find thermal battery data sheets on the next page.

author: D Straney
author_rank: Frequent Contributor
timestamp: August 17, 2025, 11:01:57 pm
content: Quote from: Alex Eisenhut on August 17, 2025, 04:21:36 pmGood question - maybe! Those two channels will be dead with the op-amps I opened, but the rest might actually work, worth giving it a current-limited supply to see what happens.Quote from: Simmed on August 17, 2025, 10:04:27 amStud-mount power transistor, such asthese. Mostly an older 70's-80's package style that's not common anymore.Quote from: UnijunctionTransistor on August 16, 2025, 08:07:30 pmThanks, yeah even I'm surprised sometimes at how many photos I've built up over the course of a few years, hah. And yeah half the reason I love seeing inside these things is the wiring jobs.

author: D Straney
author_rank: Frequent Contributor
timestamp: August 19, 2025, 03:51:52 pm
content: Multicoder ModuleLet's look inside the data encoder, or "multicoder" module:Removing screws and popping off the lid gets a view of the label, finally:Code:Multicoder, P.D.M.T.T.C. Dynaplex Div.Mfg. Part No. 306B2K1S2F20-M1Ser. No. 2825Mfg. Date 4069HAC Spec Cont. No. PS30985-004I couldn't find any information at all on this "T.T.C." or their "Dynaplex Division", but wow, Dynaplex is some great 50's/60's naming along with "Raytheon" and all the "-tron" tube suffixes. My best guess is that the "PDM" is short for"Pulse Density Modulation", which is similar to bothDelta-sigma modulation, and frequency modulation. It's a fairly easy way to encode analog values over a digital channel with a few simple building blocks, and so better suited for late-60's parts in a compact package than doing a full analog-to-digital conversion to a binary word, and then managing bit timing, word synchronization, etc. Decoding is as simple as low-pass-filtering the bitstream.(I'm more familiar with the more FM-like form of PDM, where fixed-width digital pulses are generated by an oscillator whose charging time is controlled by the input signal; this is easy to do with a 555 timer. But apparently in the more general sense, PDM can be 1-bit delta-sigma modulation.)This module, when opened, consists of a stack of boards inside the box, with a lot of silicone-like transparent potting material. Luckily the potting was soft and easy to peel off, but you'll see some still stuck in some places in the upcoming photos (before I removed the last bits). Here's the stack of 4 boards:The photos do not capture just how much of a tiny, skilled-hands-on-work "ship in a bottle" construction this module is. There's tons of tiny soldered wires running everywhere, and the circuit board material is extra-thin.Let's walk through the boards in order, starting from the side closest to the terminals:Input muxThe wires from the input channel connections all go directly to this board. Besides the resistors, there's 6 ICs: 3 of them are unlabeled, but 3 are marked with "MEM2009". The "GI" logo is forGeneral Instrument, an electronics manufacturer that later introduced the famous PIC microcontroller series. Their IC business was spun off in the late 80's to become the Microchip Inc. we all know and...not "love", but "have mixed feelings about".The MEM2009 is a 6:1 analog multiplexer, so to make the math work, the un-labeled ICs must also be MEM2009s, arranged in a 2-level hierarchy. 5 of the muxes covers up to 5*6 = 30 channels (vs. the 28 actual channels), and then the last mux selects between the outputs of these 5 muxes. So this board, given a set of probably 6 digital inputs, selects one channel out of the 28 input channels.This board is smaller than the others because it has to share space with the...Power supplyThis was originally inside a tiny shielded can connected to the enclosure. The 28VDC power input for the multicoder travels here, and is converted to presumably 5V for the digital logic and possibly some bipolar analog voltages. The circuit is very simple, with only 2 transistors (probably in some kind of Royer-like configuration again) driving a transformer in the metal can, and a bunch of diodes and passives. There might be some crude regulation of one of the rails through a zener connected back to the base drive, or something like that - this is the one board in this module where I think I could trace the circuitry without losing my mind.Analog (small)Underneath the "input mux" board is this analog board. Only about half the board is used, to leave space for the power supply sitting above it:(I need to take a better photo of this board, it looks much better with more of the potting removed)There's a few discrete transistors here, and the 2 metal cans with "RAY" on the top, which areRaytheon RM709 op-amps.There might be an integrator here, considering the large, red 2%-tolerance capacitor and the trimmer at one corner, plus the op-amps. This board either has:1. The oscillator that times the scanning through the input channels, or2. The pulse density modulator (which is equivalent to delta-sigma modulation).Digital logicThe next board is all ICs, plus a lot of resistors and diodes:I don't know the identity of the 8 ICs here, as none of them have labels (possibly dissolved as a side effect of the potting process). My guess, though, is that these are flip-flops and logic gates used to generate the digital sequence for controlling the input mux, that counts through the 28 channels one by one. The diodes and resistors are probably some simple discrete OR and AND logic that helps set the count thresholds.Analog (large)The final board in the stack is another analog board, which like the other one could be either:1. The oscillator that times the scanning through the input channels, or2. The pulse density modulator (which is equivalent to delta-sigma modulation).Here too, there's a 2% capacitor and an op-amp in a metal can.Here most of all, the photos do not do it justice. The density of the board, for the electronics packaging available at the time, cannot be properly captured. Resistors and diodes are stacked on top of each other like a pile of firewood, tiny wires run everywhere... I'd love to trace the circuitry on this board but it is just not happening non-destructively (and this entire module is far too nice to let anything happen to it). I sure hope the technicians who put this together were paid well, as they did an amazing job, and I can only imagine how long this took for each unit.Anyways, hope you enjoyed this look inside. There's still the radio transmitter and the VCO modules to examine.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: August 19, 2025, 06:51:26 pm
content: “Enjoyed” this teardown is the understatement of the year.The enjoyment provided by watching this vintage-electronic-porn is similar to that experienced by a pubescent teenager watching a Playboy magazine centerfold.Well, I am exaggerating.But just a little.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2025, 07:32:21 pm
content: The 2N1482 TO-5 transistors in the power supply are still available, but about $43.00 ea at Mouser on special order in quantity 100.I looked them up to verify that they were silicon.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: August 19, 2025, 07:40:30 pm
content: D Straney, you probably loved what Jim Williams did for artwork on his living room wall. I had the privilege of seeing it in person more than once.

author: D Straney
author_rank: Frequent Contributor
timestamp: August 20, 2025, 03:50:06 am
content: Yes this teardown is rated NC-17: the "NC" stands for "Nice Circuitry".Quote from: AnalogTodd on August 19, 2025, 07:40:30 pmBig fan of those Autonetics boards all laid out - that's super cool you got to visit Jim Williams.Now it's time for the...Radio transmitter moduleWe've got our Microcom transmitter here from Horsham, PA, broadcasting on 230 Mhz.Removing the transmitter from the rest of the unit, you can see that the housing is a brass can slipped over the outside, and screwed down to the base. I had to drill out one screw head, but the rest came easily. The big cable in this photo is the output coax.Hmmm ok, this might be an issue.Luckily, the goop on the right was easy to scrape out, and the foam in the rest was hard but very brittle, and some excavation with a knife took care of that (seemed very similar to thefoam I found filling a radar frequency multiplier).Going in the DC-to-RF direction, first after the cover is the the top deck:This mostly holds power wiring and DC biasing resistors, as far as I can tell.On the other side of the same brass piece is the middle deck, with all the small-signal RF stuff.There's a crystal (19.2417 Mhz), plus a couple transistors and so many creatively-arranged 3D passives. Considering the gap between the crystal frequency and the 230 Mhz operating frequency, I'm guessing one of the transistors is a high-output oscillator (using the crystal) to maximize the harmonic content in its output. The output would then get fed through some selective filtering to pick out the correct harmonic, and probably boosted by the other transistor acting as an amplifier (due to the small amplitude of the correct harmonic).I don't know whether this transmitter uses frequency or amplitude modulation. In the amplitude case, the control-voltage input might modulate the bias point (and therefore the gain) of this 2nd transistor; in the frequency case, there would probably be a varactor included in series or parallel with the crystal (don't remember piezo equivalent-circuit theory & all the different circuit topologies off the top of my head) to tweak the resonant frequency of the oscillator's entire feedback network.Wait, is that something in the middle?Yes, it's a little cordwood power assembly that seems to be a voltage regulator of some kind.Anyways, the modulated RF signal next goes to the bottom deck, where the power amplifier lives. This is the chunkiest piece of brass as it has the most heat to conduct away into the telemetry enclosure.The white circle is the stud-mount power transistor, a Motorola MM1558, which provides the final amplification stage. The passives around it seem like pretty straightforward input- and output-matching plus biasing. The output of this power amplifier goes into the shielded cavities visible here, which have multiple filter stages (probably to clean up the power amp's output and cut out all the harmonics) before exiting to the antenna-coax cable.Interestingly,this questionably-trustworthy websitelists the MM1558's ft as 175 Mhz, which is well below the 230 Mhz output frequency. I'm not quite sure what to make of this - maybe the data is wrong, or maybe the final frequency multiplication happens at this stage by driving the power transistor hard to produce extra harmonics, and the antenna filters are selecting the appropriate harmonic. Seems like an inefficient way to do it, but I'm not intimately familiar with the exact devices and tradeoffs available at the time, so who knows.

author: D Straney
author_rank: Frequent Contributor
timestamp: August 20, 2025, 04:40:50 am
content: And finally, to finish things off, there's the:VCO ModulesIf you remember, there's 3 of these, producing 3 subcarriers. Like the radio transmitter, these are also made by Microcom, and are labeled "Micro-VCO" (accurate name: they really are tiny, look at the size of my fingers for reference). The 6 wires enter on one side through an open cavity that's filled with hard potting material, as strain relief.I cut into the case of one of them on all 4 sides around the wire entrance, and luckily there's no potting on the inside - the internals slide right out.The insides consist of two ceramic substrates, glued back-to-back.Connections between the two sides are made by plating the edges in a few places. These plated edges seem to be then soldered together:I didn't know that SMT ceramic caps even existed in 1969, so this is some real boundary-pushing miniaturization here.As with many other hybrid circuits, the resistors are printed directly onto the ceramic (black sections), although they have surprisingly non-rectangular outlines - I don't know if this was intentional, or the process control just didn't need to be very tight.The black dots on top of each ceramic cap might be a soft adhesive meant to cushion them against the inside of the metal enclosure, and keep them from bumping up against the walls when vibrated. The transistor packaging is also strange - if you click on the images for the full-size versions on Flickr and zoom in, you can see the little 3-terminal carriers that must hold the active components. Rather than having bare dies mounted and wire-bonded straight to the ceramic like other hybrid circuits, each transistor seems to be placed onto a little ceramic rectangle, bonded to terminals at the edges, and then covered with potting so that it can be soldered rather than wire-bonded to the main ceramic PCB. I've opened a good number of hybrid modulesand weird RF thingsat this point but haven't seen anything quite like this before.

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2025, 09:10:57 am
content: Absolutely stunning - thank you for all that work.The amount of R&D that went into this is also amazing. No wonder America got to the moon in 1969, when they had this technology!QuoteIt is probably an RCA power transistor. I have a 1972 data book with these in it. I actually used that book in the mid-1970s.QuoteThey are/were also used in atomic bombs, which sit in storage for decades and then need to work reliablyYou have a capsule with the battery plates but no electrolyte, and when it is flooded it becomes a battery. They were also used in guided artillery shells; the G (around 10000G) would fill the battery. I think these are all the same thing.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 20, 2025, 01:08:02 pm
content: Quote from: peter-h on August 20, 2025, 09:10:57 amNo, you're thinking of the batteries that used a glass ampule of electrolyte that broke by the setback forces upon firing and the liquid electrolyte flowed into the battery plates and created power. They were used on the VT (Variable Timing) proximity fuses on bombs and artillery shells in WW-II and most importantly on proximity fused anti-aircraft shells.A thermal battery is different. It contains a mix of potassium perchlorate and iron power that is ignited by either a mechanical primer or most often by an electrical squib. When the mix ignites it melts a solid electrolyte that then flows into the battery plates and creates power. Read the article on the Eagle Pitcher webpage that I linked to earlier.The really interesting thing here is that the perchlorate iron mix does not generate any gas so the entire device is completely sealed and remains sealed even after firing so the shelf life is fairly indefinite and there are no combustion products that are vented and that might damage any nearby mechanical or electrical devices. Again go look at some of the data sheets on the Eagle Pitcher website. You'll see that the runtime for most of these batteries is measured in seconds. But it's long enough to power a missile to it's target or to electrically power an emergency ejection seat in an aircraft, or to guide a bomb to it's target, etc etc etc. In the OP's case a thermal battery was used to power the telemetry circuits in a test missile (an AIM-9M).The same perchlorate - iron power mix is also used for the emergency oxygen generators on commercial aircraft but instead of melting an electrolyte it heats a compound (I THINK Barium peroxide) that releases oxygen.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 20, 2025, 11:58:36 pm
content: Fuze and battery stuff, here's a book I recommendThe Deadly Fuzehttps://www.amazon.ca/Deadly-Fuze-Secret-Weapon-World/dp/08914108721960s cutting edge electronicsMicroelectronic Packaginghttps://www.amazon.ca/Microelectronic-Packaging-George-Sideris/dp/0070573433

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 21, 2025, 04:57:26 pm
content: Quote from: D Straney on August 20, 2025, 04:40:50 amSMT was actually the first kind of packaging for ICs. There definitely were surface mount capacitors in the 1960s, but they (or at least the ones I've seen) don't look like that. Those look modern and mass produced. Are you sure about the date?Like no one would raise an eyebrow if that pictured item was from the 1980s, but it's certainly pushing it even for the 1970s, seems to me.The kind of '60s surface mount caps I've seen are in Tektronix gear like the 1S1 sampler. Or the 1S2 as well? I forget now.I have no pictures on hand as my junk is in storage, and a bit too far behind other junk to get easily...The 1S1 also has, apparently, embedded parts in the PCB itself although I've never noticed, it mentions it in the manual briefly.edit: I don't think they called them SMT in the Tek stuff, but leadless capacitors, to minimize lead inductance.

author: D Straney
author_rank: Frequent Contributor
timestamp: August 21, 2025, 05:49:03 pm
content: Yep, familiar with flat-pack ICs: it's kind of funny from a modern perspective, seeing boards full of SMT ICs along with comparatively-huge through-hole resistors. And yes I 100% get the doubt, but all the date codes I've seen on the contract number on the label, the transistors, the ICs, etc. are all '68-'69.For what it's worth,this late-70's digital radar computer board, from roughly 10 years later, has modern-looking SMT caps.Wasn't aware of the SMT caps in the Tek samplers, but that makes sense - sounds like they existed but were just very specialized/expensive for a while. That old 60's-Ghz-sampler stuff is fascinating. Looks like the7A21 ('71?) has some SMT caps.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 21, 2025, 06:23:03 pm
content: I think the 1S1 only has one leadless cap, I can see it in some pictures but I'm not sure anymore. It's the 1S2 that has little metallized dielectric wafers in the sampling pulse generator IIRC.Here's the posthttps://www.eevblog.com/forum/projects/ceramic-capacitors/

author: peter-h
author_rank: Super Contributor
timestamp: August 21, 2025, 08:08:59 pm
content: QuoteThere is a YT video out there showing a well known woman taking apart the Saturn 5 guidance computer. That was built by IBM and lived in a ring structure around the rocket - I don't recall which stage, or maybe they all had one. Apparently no drawings of this computer / control system have survived, unlike the much more famous guidance computer used in the CSM and the LM which is still able to run code.That Saturn 5 guidance computer used the same flat pack ICs. So I think that package goes back to the earliest integrated circuits. It was hermetically sealed. They tested the seals by filling it with nitrogen or some such and then doing a leak test to see if any of it came out.Edit: I think it is this

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2025, 08:19:09 pm
content: This is from memory (> 50 years ago), but I believe that the original flat-pack IC packages included ones with weldable flat leads.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 21, 2025, 09:52:41 pm
content: In the 1S1 it looks like a slab of dielectric with metalized contacts, C424 but nothing special is mentioned in the parts list, just the p/n 283-0072-00.In the 1S2 they look a bit more polished, discs with contacts, to couple the snap-off diode's super fast edges.[ Specified attachment is not available ]

author: D Straney
author_rank: Frequent Contributor
timestamp: August 21, 2025, 10:57:43 pm
content: Very cool - looks like fairly primitive single-layer slabs, like in the thread, interesting to see that!I have vague memories of seeing SMT caps in early stuff so went poking around, and here's some reference points:ATC patent from '74 on a multi-layer capacitor:https://patents.google.com/patent/US3838320A/enSMT capsin alate-70's bugprobably made in the USSMT caps in a late-70's bug, made by Soviet-controlled BulgariaThe Russian R-105 radio (design updated in the 70's) has some nice little ceramic modules with peanut tubes and SMT ceramic caps mounted to themSo considering that SMT caps like that ended up in Russia in the 70's, when they were lagging the US at the time in electronics, I guess it's plausible that they were super-high-end specialty just-introduced in the US in the late 60's.Oh wait, one more, I knew I'd seen this somewhere - a board from the Saturn LVDA with some SMT ceramic caps!Surprising how far back that goes.Or maybe most of the telemetry unit was late 60's parts from similar models, and they only assembled the whole thing with newer VCOs a few years later. Who knows.

author: RJSV
author_rank: Super Contributor
timestamp: August 21, 2025, 11:00:02 pm
content: Never heard of that:'Snap-off Diode'.Quick search revealed the diode type can issue a sharp pulse, Upon turn-off, or fast turn-off, which sounds fascinating, (and useful in function generators).

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 22, 2025, 12:01:22 am
content: aka step-recovery diode.

author: KE5FX
author_rank: Super Contributor
timestamp: August 22, 2025, 05:15:59 am
content: Or "Boff diode."