14 March 2024

Blinkenlights project

I spotted the dusty board shown in the picture on the right, carrying a keypad with large buttons and a good deal of neon lamps. Considering the 29 lamps it might become a weird clock, but I chose to make a blinkenlights gizmo. 

The circuit is simple, the board was certainly hand drawn and it has a comfortable 25+25 pins 0.1" pitch edge connector.

I chose to reverse engineer connections with my picture-over-picture method rather than probing with the continuity tester:

 

Front, back and numbered neon lamps.

Each lamp leg was mapped to a connector pin and everything went into a final table that looks like this:

The result of reverse-engineering the PCB. Click to zoom-in.

Two lamps ("q" and "1") go directly to the connector, probably because who drew the PCB didn't have time to look for a better routing layout. I think their "anodes" (neon lamps don't have proper anode and cathode) were tied to another "anode" pin, or at least that's what I did when looking for a connection layout that makes sense. In the end I came up with four groups almost ten of lamps, marked as A_ in the table. Lamp cathodes are connected to K_ pins.

Wait a minute. 4 anodes, 10 cathodes: that's the layout of a 4-digit multiplexed Nixie clock! I can drive the blinkenlights gizmo with one of my Nixie clock boards and a bit of custom firmware! How cool is that? It will need a heavier multiplexing strategy than 1:4 of a clock because in each "configuration" there can be more lamps "ON" for each anode. And you cannot have two lamps in parallel "ON" at once for some physics of Neon gas :)

Some cathodes "cross" just one anode, others three or four anodes. Well, that adds some extra randomness to the light show.

03 March 2024

Tektronix VX1405 power-up sequence

Could I refrain from powering up the Tek VXIbus mainframe? I do have a rather equipped "lab" and this was the chance to use several power supplies at once.

I needed 5V at few Amps (estimated 3 to 6), +12V at 2A and -12V at 0.5A. Without rearranging the whole desk I had to limit the +12V at 1A, but that's easy: unplug fans!

Since I cannot get an automatic proper power-up sequence, I opted to implement the most logical one: 5V goes first, then +/-12V and as last the second (internal) power supply with the remaining "service" voltages.

At 5V only without any plugin it draws 3A. With the command module that increases to 3.7A. 

Without fans the +12V sits at 0.5A and -12V a fraction of 0.1A.

If the command module is installed, at this point it shows two red dots. Then I pressed the power button on the rack that controls the left over PSU and in a couple of seconds I got a green "Ready" sign. See the video below, you can hear me pressing the rack button.


At this stage it is possible to keep the rack powered with a retrofitted external power source and test the command module and plugins. This test would require too much room for my small lab, so it will remain documented for the future repairman.


29 February 2024

VXI bus, Tektronix and more

If you could publish just once a month, would you miss the opportunity to post on the unique day of a leap year? Here's my February 2024 sum up.

A Tektronix VXI bus mainframe VX1405 with arbitrary function generator (HP E1445A), two channel oscilloscope (HP E1428A), waveform analyzer (Tek TVS625) and a command module (HP E1406A).
Tektronix VX1405 and plugins.
It turns out that I spent the whole month after a new "toy" I stumbled upon. It's a technology from 1990's that puts together many computer-controlled instruments into a rack. They are controlled via HP-IB (GPIB) and connected with a VXI bus. You can mix the plugins depending on your needs: signal generators, digital (storage) oscilloscopes, voltmeters, ... The configuration shown here has: arbitrary function generator (HP E1445A), two channel oscilloscope (HP E1428A), waveform analyzer (Tek TVS625) and a command module (HP E1406A).

First of all: the thing shown in the picture weights more than 20 kg. It is bulky, heavy, noisy and power hungry (500W). The whole troubleshooting endeavour is described on eevblog, so I'll try to summarize what I've done.

The baby did not give any sign of electronic life. I initially checked the command module and removed the NiCd battery pack with the obvious leak, fortunately contained within the pack. Still no signs. So I checked power supply voltages and +5V, +/-12V were missing.

Therefore I moved my attention to the power supply unit in charge of those voltages and tried to troubleshoot it: a switch-mode high-current configuration without a schematic diagram. It is a very packed circuit. I understood that there are some components that are failing since increasing the temperature the PSU tries to start up. I located the fault in the AC side of the switcher, most likely in a custom part/board so there are very little chances of a succesfull repair. Even the service manual says that PSU are black boxes that must be replaced as a whole.

In the end I gave up: I have learned enough of switch-mode power supplies that now I want to see a working unit on the oscilloscope. I might try to retrofit an external PSU to confirm the plug-ins respond to commands.

Also in February 2024 I received a bunch of Yaesu FT-23R and alike handhelds. Out all of them I got one working with the battery eliminator. I've always dreamed of owning one and now I feel 17 years old when I hold it. It's amazing how heavy and current hungry it is!

30 January 2024

Aging carbon resistors

4k7 ohm 5%, right?

Here's a post to mark January 2024. I've been doing very little in the lab because of work, flu and mostly cold temperature in my cave.

Today while trying to tidy up the desk I picked up a bunch of new old stock resistors I bought together with a drawer box at a flea market. They have a brown body and they are marked as 4k7 ohm 5% tolerance (yellow, violet, red, gold bands).

In order to sort them out I did as usual: confirm the color code value with a DVM reading. They all returned a high value, well above the expected 5% tolerance range, actually more between +10% and +20%.

I have read warnings about carbon resistors increasing their value with time, and I have met some while restoring valve equipment. But I never had "NOS" specimen.

4k7 now is 5k4!

These resistors cannot be trusted and will be used for artistic projects. I got quite a few of them with the drawer box: I will save time and avoid trying to sort them.


 

26 December 2023

Luce di cortesia per il baule dell'auto

Arriva l'auto nuova. E' dotata di sensori di parcheggio, retrocamera, frenata assistita, sensori di collisione, sensore di pioggia, sensore luci, cruise control, navigatore, sedili riscaldati, specchietti motorizzati, il profumo di auto nuova. Ha anche le ruote, i sedili ed un volante, certo. Fai un bel giro. Poi ti capita di fare la spesa e scegli il supermercato con ampio parcheggio coperto dove puoi lasciarla lontana dagli sportelli altrui.

Arrivi con le borse, apri il bagagliaio e.... buio. Nero! Sbirci dentro per cercare l'interruttore della luce di cortesia ma non c'è. Non c'è nemmeno la luce! Hai speso tante migliaia di euro per accaparrarti il frutto del lavoro di tanti progettisti e LORO si dimenticano di mettere un piccolo, economico ma utilissimo sistema di illuminazione nel bagagliaio! Tra l'altro, parere personale, apprezzeresti molto di più la luce nel bagagliaio degli specchietti retrovisori regolabili elettricamente. E costa pure meno. O sbaglio?

Ma oggi la soluzione c'è. Si tratta di installare una "luce LED notturna con sensore di movimento" che viene normalmente proposta per l'illuminazione negli armadi. E' alimentata a batteria ricaricabile entrocontenuta ed è leggerissima. Io l'ho fissata sotto la cappelliera, dapprima con i magneti in dotazione, e poi affiancati da velcro incollato con supercolla ("At...ck" o "Bos..k"): un lato sulla luce, l'altro sulla superficie interna della cappelliera.

E signori, questa soluzione non fa solo da luce di cortesia per il baule. Dato che si può spostare, può seguirci nel cofano motore se dobbiamo controllare i liquidi o accompagnare la lettura di un libro/giornale senza scaricare la batteria dell'auto (che se poi non parte son dolori).

La batteria dura almeno un paio di mesi e si può ricaricare mentre si è in viaggio usando un cavetto USB adatto (la mia barra LED è arrivata con il suo cavetto).

Spesa totale? 5 euro più dieci minuti del proprio tempo.

Nota sull'utilizzo di questa versione. La barra luminosa si controlla con un singolo pulsante: alla prima pressione resta sempre accesa, con un'altra pressione (fa un lampeggio) si accende quando rileva movimento ed è buio, con un'altra pressione (fa tre lampeggi) si accende quanto rileva movimento indipendentemente dalla luminosità esterna. Con la quarta pressione si spegne. Tenendo premuto il pulsantino si attiva il dimmer per regolare la luminosità. Quindi per l'utilizzo nel bagagliaio, partendo da spenta, premete due o tre volte il pulsantino.

Per i curiosi, l'auto in questione è una Suzuki Ignis del 2017. La stessa auto prodotta nel 2020 ha la luce di cortesia nel bagagliaio, ma questa soluzione fa più luce!

23 December 2023

Battery of Braun 5514 shaver

I have a cordless hair&beard "styling kit" from Braun, model 5514, that doesn't hold the charge anymore. I use my soldering iron more often than this shaver so I have no idea how long the stand-by period is. Still, loosing the charge in one week is not acceptable.

There is a good video on YT showing how to open it. You need a thin and a thicker plastic card/tool to pop open the top cover and a small straight screwdriver. 

There are two batteries wired in series, in my case they are marked as "SUPPO HS-AAA0.75" NIMH 1.2V. The guy in the video shows the same model.

Now, if you search for that string you can buy a replacement at 1/3rd of the price for a new tool. But those batteries are just AAA NiMH rechargeable cells with solder tabs, with a capacity of at least 750 mAh (so if you buy them of 1000 mAh that's fine, it will take longer to reach full charge).

Then it's a matter of cutting off old batteries and soldering in new ones.

Someone might mumble that it should not be so hard to replace a battery. This tool is designed to be used in wet environments so it is waterproof. This means that it's already gold it is not sealed. Also the motor in the shaver creates vibrations and it's better to have batteries (and everything else) held together as strong as possible.

09 December 2023

High-Voltage High-Z DIY probe

As a self-assignment for troubleshooting the Hameg oscilloscope, I needed to measure kiloVolts. The world agrees that you need to arrange a resistive voltage divider using an high impedance (10 Mohm) voltmeter. 

For example if your voltmeter has an input impedance of 10 Mohm you need a 90 Mohm resistor in series to obtain a 1:10 ratio:

 (kV)------/\/\/\/\/------[Voltmeter]------(GND)

            90 Mohm         10 Mohm

I looked for suitable resistors at home and I found 4x 10 Mohm and 5x 5.27 Mohm, all 10% or more. So theoretically I could reach 66 Mohm which gives about 1:7,6 ratio. The chosen voltmeter can measure 500V, that's up to 3800V "input".

Since it is hard to accurately measure high resistances and their value may change with voltage I decided to determine the ratio with an experiment.

Given that the input impedance of an instrument might be unknown or different from the spec sheet, I empirically found the ratio of different voltmeters I own. First measure a DC voltage V1 using the voltmeter alone. Then measure again the same source adding the big resistor in series, you get V2. V1/V2 is the ratio of that resistor using that voltmeter at that voltage range.

A high voltage probe with very high impedance is built and inserted into the plastic tubes of felt-tip markers.
A high-Z probe for HV.

I repeated the experiment at higher voltages, like 80V and 300V, to observe that the ratio increases 1-2% with applied voltage (300V vs 10V). I settled to 1:8.02 for my setup.

I completed the build with a label on the probe that reminds its usage and characteristics.

IMPORTANT. Whenever you work on live circuits that use high voltages prepare the measurement setup when everything is off and unplugged from the grid. Then put one hand in the pocket, reconnect power and press "ON". Before doing any adjustment switch off the device under test and unplug the AC cord. If you are unsure, let someone expert do it.

Why didn't I aim for the 1:10 ratio? First and foremost because I needed "now" to measure that voltage. Second because I wanted to use those high value resistors. Third because it's hard (and expensive) to get a perfect 1:10 ratio, so I would have used a calculator anyway: any ratio is good, then! Fourth because I had an excuse to use one of the vintage calculators from my collection.