Another tool for planning LOS contacts

Following a previous post on tool for planning microwave (outdoor) activity, I would like to add a web service that eases the search of two spots in line-of-sight: https://link.ui.com .

Select a starting point in the map (single click), then "+ Add PtP" (Point-to-Point). Then drag the grey circle (receiver) around and pay attention to the color of the line: green means OK, red means NO-GO.

From the bottom of the page a slide-up appears which shows the terrain profile below the RF link and antenna heights. Don't forget to review antenna heights because the default value is 12 meters above ground!

Remember that the tool does not take into account buildings and vegetation!

Not all CDM324 are created equal

Since there is probably no way to use my three specimens of CDM-324 24 GHz radars, I decided to open one of them. Remember there is a well done teardown of a similar module on youtube, which I use as a reference.

The lid is glued and comes off sliding a blade all around the perimeter. Don't try to pull or lift, it doesn't work.

Surprise: there is no RF sponge in the lid vs. the module shown in the video. Also the PCB layout is slightly different, some vias are not present either.

Unfortunately there seems to be no identifiable physical sign that helps spotting different modules.

AS-IS CDM324 is unsuitable for 24 GHz QSO

I tried receiving the 24 GHz signal at 4 metres distance pure line-of-sight inside a room. The receiving end IF port was sent to an RTL-SDR receiver, while the transmitting end was modulated with a WBFM injected into the IF port or CDM-324 (a.k.a IPM-165).

The signal was quite weak, about +10 dB above noise and quite spread. Considering that I received an HB-100 with an HB-100 at 300 metres without the aid of extra antennas, I can state that the CDM-324 AS-IS is not suitable for WBFM (ham radio) operation on 24 GHz. This is actually the information I was looking for and did not show up in my searches.

Maybe there is a loose LPF under the hood, as shown in a video on youtube. It could explain with the signal with 50 MHz difference is weaker than the one at 25 MHz delta.

I will open one of my CDM324 and try to make it as similar to an HB-100 as possible.

Modulation through IF port on CDM324

Since the frequency difference of a couple of my modules falls within my SA/TV-test-set receive range, I could try WBFM modulation. I used the same modulator of my HB100 10 GHz transmitter, without decoupling-smoothing capacitor across +5V/GND. The audio was signal fed through the IF port.

Modulation did work, but I was probably overdriving something because the spectrum looked awfully wide and jagged.

CDM324 can be modulated on the +5V line too, so there are two ways to do it.

But .... it is more than 3 times as sensitive as an HB-100, so much more care must be taken on the electronics side.

First check with CDM324 couples

As I did on 10 GHz back in Spring 2017, I checked the frequency difference between my 3x CDM324 modules. Currently I have no means to measure a frequency at 24 GHz, so I trust that they are in the ballpark.

Test setup is as follows:

• put two radars in front of each other at 20-40 cm distance
• power them up and pick the IF output of one CDM324
• feed the IF output signal to an
• oscilloscope
• frequency counter
• spectrum analyzer

Samples 1-2 were visible on the 'scope and the counter, about 25-30 MHz apart.

Samples 1-3 were not detected on the first two instruments, but the SA put them about 70 MHz apart.

With the couple 2-3 I should be able to understand how they are spread around 24.125 GHz :-)

This result also confirms that there is no low pass filter on the IF output of these things, so they can be used with an external "receiver" to demod WBFM.

Size of CDM324 vs HB100

Three CDM324 24 GHz radars have arrived. This is the size comparison vs the 10 GHz counterpart HB100:

The CDM324 has no holes for attaching it somewhere, so you need a daugtherboard behind it which acts as a driver too.

I will try WBFM on 24 GHz

I know I haven't told you yet about my achievements on 10 GHz and plans for the future, but I want to leave a trace on the Web that I will do some experiments with 24 GHz (twenty-four GHz) radars. The reason I write this short post is that I found no reports of HAM WBFM experiments involving CDM-324 / IPM-165 or other InnoSent and similar 1.3 cm modules. Neither positive nor negative. Nothing.

I have ordered three CDM324 radar modules to play with. They are not tuneable, so I hope they will not be right on the same frequency! I will detect their frequency difference, test the WBFM modulation process and then move to distance without and with dish.

So, in case you are interested in these experiments, or you have already been there, please get in touch!

BTW, in case an InnoSent representative reads this: I would like to test IMS-944 module but it is not easy to buy as a private customer.

Zagreb Hrelic flea market

Hrelic, at last! In August 2019 I could spend a whole Sunday in Zagreb, which means I could visit the large open-air flea market (every Sunday morning).

Zagreb Hrelic flea market, August 2019.

It is no different from other flea markets, of course! Lots of clothes and books (99.5% in Croatian, obviously), mechanical stuff, fishing equipment, very little electronics. I noticed that there was almost no stuff from 1970's, because Croatia at that time wasn't as wealthy as it is now and their borders (Yugoslavia) were closed to many Western products. Most sellers place their stuff on the ground, so be prepared to bend if you spot something that catches your interest.

It took us two hours to walk through 90% of the stalls, but consider that only 70% of them were occupied. And by noon they started leaving! Therefore plan you timing accordingly, if you are serious about finding something unique (for you ;) ).

I bought a working Canon VHS videocamera/camcorder for less than 100 kn and a vintage 4.5V flashlight for 1 kn. The first item will donate the CRT viewfinder to a clock/display project while the flashlight will need some restoration of battery contacts before being usable again.

How to get there.
You can get there by tram and walk about 1 km on the Western bank of the river Sava. Or you can go by car, pass the used cars market and enter a large parking area (10 kuna/stay in 2019). Note: throughout the 1 km walk there are occasional "pirate" sellers, that you'll miss if you go by car.

It is all in open air. Hot in a sunny Summer day, freezing cold in Winter. Dress accordingly! I was lucky to be there in a cloudy August day, which was perfect.

You can buy food and beverages at few kiosks spread around the area, if you aren't looking for a modern cozy romantic restaurant :) I didn't look for toilets.


It was a positive experience and I will pay another visit when I have a chance.

LNB current consumption

My 10 GHz LNB draws 106 mA regardless of the voltage. And apparently it works down to 8V: battery power for /P is possible!

Actually there is a 5V regulator inside the LNB, so it is possible to bypass it and feed 5V from a USB power bank.

A second dish for 3 cm

I got a surplus 40 cm parabolic reflector for my 10 GHz experiments. It doubles the caravan set where I will mount the LNB and use in the receive chain. Since the caravan set can be mounted with a suction cup, it will probably stay on the roof of the car, while the TX dish has to be adapted to sit on a photographic tripod.


The picture depicts the way I found to attach it to the tripod: a removable vice. The tripod is lightweight, but on a field without vegetation I don't need to extend it fully, so the center of mass will be as low as possible.

Then I needed to see how it performs, depending on the position of the TX signal vs focal point. Here things got interesting. We have installed locally up in the mountains a WebSDR equipped with just an LNB pointing towards the Pianura Padana. It covers 10368.0-10369.0 MHz and receives signals beyond 300 km. It is at about 40 km from my home, slightly behind a building, into the Fresnel zone I dare to say.

The "red cover" TX was put into service. Its TX patch antennas are quite off the focal point, closer to the dish and on a side. The resulting beam, as seen on the remote RX waterfall is quite wide, with strong sidelobes. I could not get a null.

Second TX head.

Then a second TX head "white cover" was tried. It is shorter and closer to the focal plane, slightly lower than the focal point. The beam is sharp now! Because of the RTL-SDR AGC I cannot estimate the gain, but any movement impacts the signal strength, move few degrees and it is gone from the waterfall!


I am worried that dealing with two dishes will make beaming twice as time consuming, but at least the HB100 TX has a form of passive thermal stabilisation that keeps it relatively still in frequency once it reaches equilibrium.

Unfortunately the dish weight makes it difficult to make smooth movements on the cheap tripod, but that is what I have for now.

Last but not least, going from about 25°C (home) to full sunshine (35°C, not measured) resulted in 2 MHz frequency change downwards of the whole system. Good to know when arranging for real QSOs!

Thermal isolation for HB100 transmitter

The actual frequency of an "HB100 10 GHz radar doppler HAM RTX module" is very sensitive to external factors: supply voltage, proximity, temperature.

Voltage is easy to stabilize. Proximity too, and if the IF port it becomes less sensitive.

On the other hand, temperature has a devastating effect, both on short and long term. On the long term, the frequency will drift until the HB100 reaches a constant temperature. But even a quick breeze will produce a snake on the waterfall. So, in order to minimize the effect, I found my way to add short-term thermal inertia to my 10 GHz transmitter:

Transmitting end on 10 GHz, modulated on the IF port.

Variations on long term ("minutes") will still occur, but at a slower pace so that it's easy to do a manual AFC on the receiving side. And the trick is to let it reach a "working" temperature before use and avoid variations over the whole QSO.

Tools for planning 10 GHz "expeditions"

Activity on 10 GHz means portable operations. If you live near some mountains, it is a matter of finding a panoramic spot, clear of vegetation, open to possible correspondants. Besides asking for advice to "who has already been there", technology and Internet help a lot.

Street View on Google Maps allows to survey many places directly from home. You can get an idea of the possible presence of vegetation, parking place, pic-nic table or grass field for stuffing the equipment.

Once you're done you can run a coverage simulation with Radio Mobile online. You'll need some practice in order to get meaningful and reliable maps, but I am sure there are many tutorials to choose from.

Last but not least, the wonderful www.peakfinder.org website lets you name mountains/peaks surrounding any location of the world: without running a simulation you will know if two remote locations can "see" each other and stand chances for a 10 GHz QSO. This is also useful if someone will operate from home and wants to know if the portable station is visible.

Just another 10 GHz test transmission

Yesterday I wanted to test again my HB100 signal and the online receiver 30 km away. No dish on either side. This is the result of my WFM signal modulated with a square wave (which generates two parallel traces on the waterfall).

Three beacons and me on 10 GHz.

My transmitting setup.

I did not leave it transmitting for long given its strength on the waterfall and the frequency too close to beacons. Moreover the TX beam was 60 degrees off, at least according to the theoretical/datasheet radiation pattern. 

Simply amazing.

32 km on 10 GHz

Still one-way, but it is quite outstanding: my bare HB100 signal could be received by a bare LNB at 32 km distance with a 13 dB S/N margin. I could increase the signal by adjusting the TX direction, but the RX side is fixed on a remote location (remote SDR receiver).

Also I think that the strong wind was waving some vegetation into the path because the signal would disappear with heavy QSB.

What is even more interesting is that the bare LNB is receiving a 3 cm beacon 300 km away! At least now we know a remote site where we could try a super-HB100-DX!

Distorted readout on Tektronix 7603

I got to use my Tektronix 7603 analog oscilloscope over the weekend and started worrying.

I usually fire it for few minutes to check a signal and then off it goes for weeks. Not long ago, during longer operation, I heard a "clunk!" and with side-vision I saw the CRT display going blank and then back again.

Last weeks the "Trig'd" lamp on the timebase plug-in broke, but it triggers just fine.

Fast-forward to this weekend... when switching vertical plug-ings left-to-right I noticed the Tek was using a different font for readout:

Wait a moment! That's 1970's hardware, it cannot change font!

I am afraid I need to start troubleshooting from the PSU. This calls for a way to see PSU signals, that means I need another oscilloscope.

And dilemma: keep it or let it go? It will hardly sell as a single piece (to be re-capped, the least). I might sell individual plug-ins, 2x 7A18, 7A13, 7B80 (doesn't sweep), 7B53A. Or keep the beast since I can configure it to provide 4 channels at 100 MHz BW.

8,85 km on 10 GHz

I've made it! Today 2019-04-13 at 11:20Z my bare HB-100 WFM TX was received with a bare LNB at 8.85 km (5.50 miles) over a pure line-of-sight path. Then I added the parabolic dish on the RX side and the signal became very, very strong. BTW, nobody was aiming the TX side, so the signal could have improved a bit.

The current world record is 204 km (HB-100+dish to LNB+dish) and I am longing for it. :)

The Gunnplexer Cookbook (1981)

The Gunnplexer Cookbook was published in 1981 (and can be found online in PDF form, but I am not sure if it has been relased into the public domain). In 2019 we are playing with HB-100 and low noise SAT-TV LNB's rather than Gunn diodes, but I think that book contains good old wisdom that still applies nowadays. Temperature stabilization, a simple and mechanical way to estimate a frequency at 10 GHz (useful for HB-100 transmitters too) as well as other pearls to enrich one's knowledge of microwave bands.

Have a look at it if you are interested in GHz operations.

Welcome to GPS Week #0

The second GPS Week Number Rollover (do a web search for more info) has just happened. Your GPS device might think it is in 1980, or in 1999. The first symptom would be the long time to acquire a GPS fix. Then a wrong date is shown.

How's yours?

800 metres with HB-100

Since I have discovered that from home I see a parking space 600 m away, I have just completed two tests with HB-100 TX and TV monitor+LNB RX.

At the parking spot the signal was loud and clear with plenty of SNR, so I can go further with this setup.

Then I took a longer route home and I stopped 800 m away on the same direction, and the signal could be received through reflections, but very very weak. Well, the TV monitor instrument is not as sensitive as an RTL-SDR, but there was no clear path between "me" and me.

NB: no dish was used on either side!

While measuring the distance on Google Maps I noticed that if you zoom in on the drawn path the line is black on "clear" and grey when there is an obstacle: helpful!

HB-100 RF polarization

The test at 200 meters distance with HB-100 TX and LNB RX helped me:

• to confirm once again that the 10 GHz signal can travel long distance
• identify which position corresponds to horizontal polarization

Two years ago I could communicate one-way HB-100 to HB-100. I did it again but this time I had an LNB on the receiving end: plenty of signal to try a longer distance. Unfortunately as long as I am alone doing these experiments, it is either 200 m or 8 km.

Then, since the LNB has a known polarity, with no bounces off the room walls I determined (I am 90% confident) the polarization of the radiated 10 GHz signal. The rule is: polarization is given by the position of the line joining the two mounting holes. Incidentally it is rotated 90° with respect to the position of patch antennas.
So, when the longer side of HB-100 is horizontal, you should expect horizontal polarization.

HB-100 vs Voltage

Having built a DFCW modulator for the HB-100 controlling the operating voltage, I could observe that a difference of +130 mV in the supply voltage causes about +200 kHz shift. The higher the voltage, the higher the frequency.

Besides the lack of temperature stabilization, switching voltages affected negatively the resulting RF frequency, causing all sorts of short-term drifts. Most likely I will switch to FM-modulated CW (audio tone over the carrier) once I want to have a real QSO and not just a reception test.

HB-100 vs Temperature

My simple observation of a single HB-100 module is that the frequency increases as temperature decreases. 

I think it has gained 150 kHz while cooling of 15 °C, so -10 kHz/°C. That's just one test of one specimen, YMMV, even MMMV. In any case the temperature should be stabilized as much as possible, at least for short term variations, by adding thermal insulation.

I wonder if the relationship between temperature and frequency is deterministic, therefore either parameter can be inferred if the other is known: frequency-based thermometer, or temperature-based frequency estimation.

There is life on 10 GHz around here

I am really excited: I received HAM signals on 10 GHz!

I needed to add a T-bias to my RTL-SDR box and the longest part was drilling a hole for the pass-through capacitor.

Here's the quick setup on my crowded desk: can you see the LNB and the RTL-SDR?

 

What did I receive, beaming free-hand the LNB out of the window? Three beacons! And two of them were bouncing off the mountains surrounding Torino.  The recording has HB9EI/B ID signal at 147 km while both the LNB and the RTL-SDR were warming up, so it drifts:

Cool! Next days I will try QO-100 satellite, but I need to move everything on the other side of the house.

Finally measuring the frequency of my HB-100's

Yesterday I wired up the LNB to a TV-spectrum analyzer to check that the preliminary setup would work. Besides an occasional short into the coax that trips the power-over-line circuitry, the LNB noise is visible. Then I fired up my HB-100 transmitter, that has been sitting in a box for months: its modulated signal peaked at 621.45 MHz on the screen.

So, if the LNB L.O. is at 9750 MHz, my TX is at 10371.45 MHz. That's well within HAM allocation.

Then I measured the HB-100 I had used as a receiver, which sat at 694.70 MHz = 10444.70 MHz. This confirms the delta of 73 MHz I observed in May 2017 when doing my first experiments.

Last I measured an apparently untouched HB-100, my third and last sample, and it could be received at 676.35 MHz, meaning 10.42635 GHz. Knowing me, I think I had left this spare radar module untouched, as a reference.

So, if the assumption of 9750 MHz LO is correct, all my HB-100 modules fall within the 3 cm HAM band (10.0-10.5 GHz), even if I was expecting a "brand new" one to be at 10.525 GHz. Hypothesis: I bought off eBay HB-100 modules that were rejected because they did not meet the spec on the operating frequency, and luckily were too low. Unfortunately I haven't found someone that published a measurement of their frequency.

I need a beacon of known frequency to confirm these relative measurements. Fortunately there is a beacon in L.O.S. from home and QO-100 satellite few degrees away. It will not give me accurate frequency values, but enough "ballpark precision" to allow a fellow HAM to quickly tune on my TX frequency and complete a QSO.

Returning to 10 GHz

I am again doing something at 10 GHz.

During the journey to Montichiari HAM fair, fellow Pino JNS showed us how easy it is to receive the geostationary satellite QO-100 with a PLL-LNB and a small dish. The "last mile" is then done with the ubiquitous RTL-SDR.

Now my HB100 radar modules have a valid RX companion, so a 10 GHz RX-TX setup can be built and operated. It will be WFM or CW (A1A), but at 10 GHz and with very simple RF parts. The total cost for RF is about 30 euro.

I will initially try to receive QO-100 and a 10 GHz LOS beacon. Once I confirm the LNB is stable and spot-on frequency, I will tune my HB-100 modules into the 3 cm HAM band to a known frequency. Then again a short distance test, 200 metres "DX", and who knows what will be next.

Thinking out of the box Telecommunication test for millennials in IT

I've just done this at work, using some tricks that were daily routine back in the days of dialup. I wonder how a sysadmin half of my age would tackle this problem.

You are logged into a Linux server through a sort of corporate menu-driven shell proxy and you need to get a 10 MB binary file out of it for local troubleshooting purposes. The shell proxy and firewall do not allow your unprivileged account to ssh/sftp directly into the machine and you don't know its password either. There is a webserver but your unprivileged user has no write rights to its folders. You try to mail the file to yourself with the "mail" utility, but the test message you try before going all the way never reaches your mailbox into the organization. The server cannot do outbound ssh/ftp either.

So, how do you get the file on your local computer? Ask for further details and share your ideas in comments (moderated).

GPS Week Number Roll Over, 2019 edition

I discovered this detail of GPS raw signal in 2010, when my Garmin Geko 201 showed a wrong date. The guys behind GPS (not Garmin) implemented the transmission of a 10-bit weeks counter value that rolled over in 1999, so that some devices began showing a date that was 1024 weeks behind.

It will happen again at the end April 6th, 2019 (UTC).

(Somehow) The week counter has been upgraded to 13-bit, 157 years (but week #0 is still back in 1980-01-06). Probably most devices using GPS location data built in the last 19 years have a software way to handle the rollover, so you will not be affected. Probably. Hopefully. I know too many lazy developers to be 100% sure all my devices will not be affected.

Unless the device is tested with a fake GPS signal ahead in time, all we can do is to wait for the first full weekend of April 2019.

What's worse, the device might have an embedded lookup table "GPS week vs real week" that is 19.5 years long and would roll over any time in the future. And, in most cases, all these devices are unlikely to get a firmware update.

See you in week #0 :)

Weird behavior of ZS-042 RTC modules (DS3231)

First of all: all these ZS-042 RTC modules were paid about 1 USD each and come from not-certified sources. 

One night I decided to measure the 32K output of some DS3231 ZS-042 RTC modules. I wanted to see what difference would make if I retouched the aging register. This experiment lead to few unexpected discoveries.

1) Maxim DS3231 datasheet mentions 32K output and 32.768 kHz interchangeably. Alright, it is a digital device, so 32K equals 32768 if K=1024 as used for bytes. But we are talking of Hz, not bytes, so I think should be K=1000. Proof? Search for "768" in the datasheet.

2) There are two quite different devices that share 6 characters of the part name - DS3231 - and are distinguished by their suffix: M or SN. The former "M" is 5 ppm with MEMS resonator, while the latter "SN" is 2 ppm with TCXO. In simple terms, "SN" is better than "M". But you can't choose when buying ZS-042 modules and you will receive either model

"M", "SN", "M":notice the deeper blue PCB of "SN"

3) All my ZS-042 modules with DS3231M output a frequency close to 32.700 kHz, while those with DS3231SN are close to 32.768 kHz. The "M" would be loosing 180 seconds/day, 1.5 h/month, but none of them does! Actually I have one DS3231M in my longest running Nixie clock and it is within 5 ppm specification.

Measuring "32K" output of DS3231M.

So, let's forget DS3231SN that are most likely second-hand/pulls but legit. How can a DS3231M that outputs 32700 Hz be so accurate as I observed? If it is genuine, Maxim has a way to calibrate the chip once, before it is sold. If it is a fake, then it is microcontroller with proper firmware and calibration to count the correct number of interrupts. I could sacrifice one and decap it, but then I lack the microscope to examine the silicon die.

Conclusion. Buy the DS3231SN if you need an interrupt rate of 32768 Hz. Otherwise for hobbyist use, they are both worth it.

Timebase for homemade clocks, 1 pps

I am currently into a minimalist approach to building (Nixie) clocks that does not include a user interface. The are several reasons, like simpler and less buggy firmware, straightforward user interaction, less hardware, fewer holes to drill in the case. Since these are one-offs "products" without commercial intents, I can afford it.

I am after a simple way to get an accurate 1 pps or integer fractions of it so that the microprocessor can count the elapsed time.

The most elegant way would be to clock the AVR microprocessor with a purposely chosen XTAL, feed that timebase into a divider and use it as timekeeping interrupts. I've got plenty of 11059200 Hz crystals, but they don't divide well into suitable interrupt counts.

Another way is to take advantage of the AC line at 50 (60) Hz, but that means a special power supply (AC-AC) and additional circuitry to get DC voltage for the logic and the booster in case of Nixies. No, I don't want to use the direct-AC method!

How about a watch crystal at 32768 kHz and a 4060 divider? That would work, indeed.

But when you take into account both the additional wiring and the cost of each of these solutions, it turns out that modern RTC chips (at 1 USD shipped, already on breakout boards) are equally cheap, maybe even less complex to wire, output a 1 pps signal and offer greater upgrade possibilities if you ever want to, like battery/supercap backup, date, power loss warning. And their SOIC package is probably not too difficult to hand-solder either.

By the way, how to set the time on a interface-less clock? Well, it always starts at an agreed time, and you simply have to plug it in at that time. We are surrounded by clocks and "my" device doesn't aim at being the time reference standard.

Windows 10 1803 failed update week-long nightmare

In December 2018 I bought a laptop with Win10 (Asus, for the records, but I think it doesn't matter). It has been little responsive since the beginning, so I let it run all updates for 24 hours: at least it stopped using the hard-disk like crazy.

But it still complained that there were pending updates, the "Windows 10 1803", that had already failed three times to install. I don't know which version was running, probably it had arrived to 1709 (that is Fall 2017).

Microsoft forums already gave the workaround: skip the 1803 update and go for 1809. Alright, but how? As of January 2019, Microsoft-provided tools would not do it.

After a week of wasted evenings I was successful with:

1. downloading the [correct] 1809 ISO from the the Microsoft downloads site pretending not-to-be a Microsoft browser (do it from Linux or use the documented trick for Internet Explorer/Edge); it is a circa 5 GB file
2. burning the ISO into an USB pendrive (min 8 GB) with Rufus tool under Windows
3. going into the laptop BIOS and disabled Secure Boot
4. from the BIOS forced to boot from the USB pendrive
5. installed the update by choosing to keep existing files

Step #5 took few hours (overnight) and my files were not lost. Do yourself a favour and backup first, just in case!

I dare to say it is not for the unexperienced user; friends confirmed that there is no Microsoft-ish way out if 1803 update doesn't install: it would go on forever killing your HDD, CPU and Internet bandwidth.


Tip. If you want to control when Win10 downloads and installs updates, find the way to mark your connection as metered.

Good luck.

Taking control of HP 9403A

The HP 9403A odyssey might be close to an end. After tracing most connections on the "DVS Program I/O" connector to corresponding ICs, thus understanding if they are inputs or outputs (they all end up into an HP 1820-0107 level adapter), I begun to take control of the device:

HP 9403A: first signs of human control after 40 years.

I know which are control lines and which are data lines. There is no serial protocol, but just plain parallel negative logic. If I want to do something with the bulky piece of equipment I will need a microcontroller with lots of I/O lines, or some serial-to-parallel (open collector) converter.

HP 1820-0107 pinout

I have finally found some documentation about HP 1820-0107 custom IC from 1970's. It is (hidden) in the HP 3720A manual.

HP 1820-0107 as documented in HP 3720A manual.

That document describes it as "Interface buffer gate" and draws the chip pinout as follows:
Well, I've never seen a NOT gate with two inputs. I confirm Vin is a Vcc pin (+5V in my case). In a custom chip Vout could make sense, but why so close to the Vin pin?!

In the next page of the same document I see this:

which makes more sense considering two Voltage pins and the NOT symbol. Further down the manual, the schematic diagram the corresponding ICs (A6MC21, 25, 31, 33, 35) are drawn as interface inverting gates:

Alright, now this makes sense! It explains the presence of resistors on data lines in various places of the HP 9403A. But, most important:

1. the 1820-0107 is not a flip-flop of any sort (easier to reverse engineer the circuit)
2. the input pin requires pulling down to ground (which explains why I haven't found yet a common +V line on connectors towards the external world)

 Back to the PCBs now!