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.

 

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!

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.

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-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.

Troubleshooting an Hameg HM203-6 oscilloscope - no trace

Right after fixing the high voltage being too high (-2500V vs -1900V) on the Hameg HM203-6 oscilloscope, the trace disappeared. Note that I was able to see the trace at the restored -1900V cathode voltage and I was after the ineffective un/blanking control.

HV was there. Un/blanking signal was there. What could have possibly gone bad, now?

When operating in XY mode (or component tester mode if your scope has it), there is no retrace or blanking: you should always get at least a dot somewhere on the screen (be careful with phosphor burning!). I was not getting a beam, even in XY mode. So something was blocking the electron beam into the CRT: either missing acceleration or blocking grid at wrong voltage. That is where I concentrated my efforts.

I knew that BF199 was bad. Even with a new one nothing happened. BF440 was good as well as the optocoupler. At least they tested as expected on the DVM at low voltage.

Don't forget that components may (will) show a different behavior or value depending on the voltage applied. That's why I changed three 68 pF 2 kV capacitors that have been pinpointed as being prone to failure in online forums.

When all voltages were measured as expected I moved on to in-circuit component testing. The device MUST be powered off and unplugged! I take no responsibility for whatever damage to animated or inanimate beings you may cause.

Remove socketed components as they can be tested individually. Then proceed with this methodology:

• resistors should exhibit the stamped value or lower, because they are in parallel with other resistors and alike: replace if value is too high or too low;
  
• diodes should conduct in the A>K sense and exhibit an open-circuit or high impedance on reverse polarization: replace if forward and reverse voltages are too low for the diode type, or if it's open

I did not need to test capacitors as I found two open resistors and one shorted diode. I mark them in red in the schematic diagram excerpt below. See the thread on eevblog for the whole story.

A part of Hameg 203-6 oscilloscope circuit diagram.

So the Hameg HM203-6 symptoms were:

1. trace too bright and not going dim/off with intensity control plus
2. not un/blanking 

They were caused by:

1. broken high voltage regulation, which probably lead to
2. un/blanking failure and
3. blocking grid control failure 

Components replaced:

• one 741 op-amp IC
  
• 3 high voltage 68 pF capacitors (perhaps not needed)
• one BF199 transistor
• two resistors
• one diode 
  

Cost about 5€(2023)  and 10 man/hours. Troubleshooting value: invaluable.

Troubleshooting an Hameg HM203-6 oscilloscope - high voltage on CRT

I used the Hameg HM203-6 scope to test the Tennis-for-Two emulator I quickly built in November 2023. I also carried it to the exhibition venue as a backup. When it returned home I felt the urge to play with this analog oscilloscope that was lucky enough to be extracted from my pile of "probably to-be-fixed stuff".

So, what was wrong with it?

1. compressed traces, about 0.7x shorter on both axes, and too bright;
2. unblanking not working properly (it's the circuit that blanks the CRT electron beam when it returns to the left);
   
3. rusty contacts.

Too high HV symptoms on a Hameg HM203-6 oscilloscope.

 

Hameg's were built when SMD was not a thing yet (mine is from 1988) and they use common (...back then...) parts. Therefore tracing the fault doesn't require a microscope and a replacement component can be found.

For the compressed bright traces I posted a request on eevblog and few fellas responded with very useful tips. Actually they noticed the problem with unblanking! The verdict is: High-Voltage is too high. It makes sense: higher voltage means faster electron beam that both gets less time for full deflection and hits harder the phosphor. I had checked the 150V line for deflection circuitry, which was fine. Sure enough the -1900V was at -2550V.

[Measuring kiloVolts is an interesting journey in itself and there will be a dedicated post.]

On eevblog was suggested that IC502, a 741 op-amp, might be faulty: "check that the voltage between inputs is few milliVolts max." I measured 455 mV, replaced the IC, -1900V was restored and traces "uncompressed". Considering the cost of a 741 I will not look any further for faults in the HV regulator, leaving a deeper inspection in case the problem arises again.

There is still the issue with unblanking. Something might have been damaged running at -2500V. There are three suspects: 68pF 2kV capacitors, the optocoupler, a couple of transistors. The latter two components are socket, most likely because they were prone to failure.

As for rusty contacts, they probably need some exercise, the most annoying being the "INV" button on both channels.

Tennis for Two emulator

The secret project that went live on November 17th, 2023, was an emulator of the Tennis-for-Two game from 1958. It has been built for MuPIn - Museo Piemontese dell'Informatica - that held an exhibition of tennis (retro)videogames for the ATP Finals 2023.

Several videogames were on display and could be played, like the original Pong from 1972 up to a Wii.

There was a panel describing what should be the first videogame in history, the Tennis-for-Two developed to let visitors (taxpayers) of a USA laboratory interact with their technology. When I saw it I searched online for a modern emulator, found a 2008 project and I knew I could build it with parts I had at home. 

So who visited the exhibition in the second weekend could play an emulator or Tennis-for-Two on a Tektronix oscilloscope.

Fun fact: people spent most of their visit-play time at Tennis-for-Two and Pong games.

The video above shows one of my tests on my Hameg analog oscilloscope, shot with the smartphone that didn't want to focus on the CRT trace. Please refer to the link above for the circuit diagram and firmware (ATmega168 and up).

PCB design: unfortunate position for a microcontroller IC

The last PCB I designed includes an ATtiny85 microcontroller. Since it will not need many updates I did not include the ICSP header for reprogramming. The first time I needed to swap out the '85 I realised I forgot to leave some room for pulling the IC from the socket. HAH!

I usually insert a small flat screwdriver under the IC and gently lift it up. In the configuration above it is impossible to get in from the left and just slightly easier from the right paying attention to the resistors.

Lesson learned.