UV-3R Plus fading display

I got a second-hand-new Baofeng UV-3R Plus and it shows a problem that has not been documented (or I use the wrong search keywords): the LCD display fades out. The longer it stays still, the more it fades out.

See this example about 10 seconds "in":

Fading display on Baofeng UV-3R Plus

 

Top line displays "RADIO" and bottom line is "108.000". Also the display is reacting slowly to any change at 25°C room temperature.

I am not an expert in LCDs and their hardware drivers so I ask if there could be a fix? Or what is the cause of this effect. Given the planned application of this radio and the long queue of projects I will not try a fix myself, unless I am directed to the right spot.

Sega Master System II power supply

I was testing some Sega Master System 2 vintage game consoles with a friend and we couldn't get a signal out of them. I used a portable TV with analog tuning to look for the RF signal and nothing. So we tried a different antenna cable, checked the wallwart output, cross-checked with a C64.... Then we found the original Sega PSU and .... OPS! ... SMS2 uses the "Japanese" polarity with (+) on the outer ring and (-) on the tip.

A cold shiver hit us: had we fried all three consoles? Were they already cooked (probably)? Regardless we tried with the right PSU and the signal was there on UHF channel 36 (about 603 MHz)!!

The good news is that Sega used a polarity reversal trick of some kind, but they forgot to put a "power on" LED. The required voltage is 9V.

Happy retroconsoling!

PABX Excelltel MK-208 recapped

New look for the MK-208

This was an easy fix, on an easy to rework PCB. I replaced the five largest electrolytic capacitors in the MK-208 PABX, even if three of them were still within spec. This 3€(2025) repair restored a crystal clear analog line so the PABX can go into service. I have also drilled free-hand some holes above the 7805 area to let warm air escape.

Phone to phone works. I am after at least one working fax machine and a computer with modem to host the other end. A 1980's BBS will be last.

Electrolytic capacitors in the power supply circuit.

PABX Excelltel MK-208 recap

The (Excelltel) MK-208 PABX (and alike) is a modern local telephone analog exchange for small offices. "Modern" because it was built after 2010, which is strange since plain old PSTN analog phones were already declining in favor of VoIP. "Small offices" because it features up to 2 incoming lines and 8 internal branches. There is a whole series of these PABX with different combinations of incoming vs branches line, and they all fit in the same case. 

MK-208 boards

Why did I buy such a thing? Because I want to be able to recreate an analog telephone line that can ring those old phones, run a BBS, transport a FAX, all natively without passing through VoIP codecs. This is going to be used in a public interactive exhibition. With 8 branches of the MK-208 there can be up to 4 simultaneous calls.

The PABX has a nice case and its weight might induce you to think it is metallic for heatinking purposes. As it will turn out, it's not.

The PABX powers up and the green status LED blinks. This is normal, whereas you would expect a solid light. I connected two phones, they rang and communicated but the line was extremely noisy. No, not the kind of noise caused by a rusty connector. It was a strong hum. Ouch. The hum/noise increased when I picked up the second phone. Ouch ouch. But there is hope: it might be a problem in the internal power supply, even if a poorly designed product would not the THAT bad (and the Internet would be full of bad reviews, which is not).

Let's look inside. The nice heatsink-shaped case is made of plastic, thick, but plastic. The power supply area is easy to locate, with the transformer and the regulator heatsink in good sight. Needless to say, there are three bulging electrolytic capacitors. This unit is 10 years old, it might have been in service non-stop for 8 years. Probably it gets quite hot as there is no fan and the slots for air cooling are very small (to my under experienced eye).

Close-up on failed capacitors.

I will replace the failed capacitors and those that were subject to too much heat. If this fixes the hum, I will also drill holes in the case right above the power supply area to provide a well needed escape to hot air. In between the two I should measure the temperature inside to confirm that extra holes actually help.

 

Composite video at variable refresh rate

I am currently troubleshooting the CRT monitor of an Olivetti typewriter that produces composite video at 70 Hz refresh rate. Weird? Indeed. Strange design choice, but I need to fix it to have a history-correct setup.

Since the ETV 2700 typewriter is quite bulky and takes up all my desk space I would like to have a small source of PAL composite video at 70 Hz to feed the CRT circuitry. I checked Arduino TVout library, which supports both NTSC (60 Hz) and PAL (50 Hz), but I see no way to fiddle with timings and even at 60 Hz it is already a stretch with very reduced resolution (I don't care).

Dead end? No, read on.

The clock on many Arduino-alike boards is set at 16 MHz, whereas the ATmega328 chip can be clocked up to 20 MHz. A program whose timers have been computed for 16 MHz at compile time, at 20 MHz it will run faster, therefore I will obtain a faster video signal.

So, this is the plan.

1. Write an ATmega328 with a demo program of TVout library, as in a normal Arduino-alike board
2. Test it against a known working screen that accepts composite video signal (probably your smartTV does)
3. Change the fuses of ATmega328 to use an external clock
4. Feed a variable frequency clock signal to get a linearly controllable composite video signal!

For a 70 Hz PAL I need a clock of 22.4 MHz, that the ATmega328P is likely to withstand peacefully (check online discussions about overclocking those Atmels). I will decide whether use a fixed clock source or some DDS with TTL output. In the latter case I will get a PAL signal with variable refresh rate from 1 Hz up to the maximum overclocking frequency of the microcontroller.

I need this tool because I also have a bare green phosphor CRT with control circuit that doesn't sync on the standard PAL. Who gave it to me mentioned some "trick" used by the manufacturer of that industrial system to do a form of vendor lock-in on spare parts or avoid clones, and a non-standard refresh rate now makes sense.

My first RIFA

Over the years I have repaired few devices and never found a RIFA capacitor. Until today when I finally met them in all their smelly glory.

H7864-A PSU label

They are inside the PSU of a 1980's computer, a digital microPDP-11. The exact model is H7864-A, in this case the compatible part by Astec AA12131.

I had already looked into a H7864-B that had non-RIFA capacitors. Or at least not with the usual square shape. It did stay powered without blowing. It should be noted that while -A and -B have the same voltage outputs (and probably the same pinout), their current ratings are different with -A being beefier.

A RIFA capacitor!

I tried plugging a -A and after few minutes it popped and smoked. Well done, Paolo: mission completed! So I picked another -A from the pile of microPDP-11's and here it is on the bench. It was easy to spot RIFA's since they are close to the AC input, so just follow the input line. According to Internet sources the H7864-A should mount 3 RIFA's, but I found only C101 (0.47uf X2) and C102 (0.22uF Y2), while C201 (0.1uf X2) is of a different brand, shape and color. 

 

C101 was kept in place with some kind of wax that should be removed to free the component. Heat, extra solder and flux and they will come out quite easily. Just note that the leads were bent, so you need to straighten at least one.

Do C101 and C102 really need to be there? They form the filter... I should not write that, but the PSU works without the filter. At least for a smoke test it will be fine.

 

 

 

Old Capacitor Wisdom

I've received a homebuilt oscilloscope from 1950's. Scuola Radio Elettra, an Italan company back then, was very popular and helped many people to get into the eletronics world. The instruction manual was 770 pages long and it is half builder's manual, half electronics student's book.

The scope does not show a trace on screen. I was told the instrument was OK, but it has been sitting unused for months, to say the least. I read through the manual looking for the troubleshooting section and came across some wisdom on old capacitors.

"The only components that require replacement with use are the electronic tubes and the electrolytic capacitors. If the oscilloscope has been left inactive for a very long time (months or years), it will be advisable to first turn it on for about a minute and then turn it off and let it rest for at least half an hour, then turn it on again for another 10 minutes. After this, let it rest for another half an hour and finally you can use it with peace of mind. This process serves to allow the electrolytic capacitors to reform and regain the efficiency lost during the long period of inactivity."

Even in 1950's when writing the original manual they knew of electrolyic capacitors "hobby" to change their value with time. I assume that "decades" falls into "years" of inactivity, so I will try the procedure reccomended in the manual. With the covers off, so that I see filaments glowing.

 

 

 

Sony walkman WM-B12 repaired

Now that my interests are expanding to retrocomputing, I feel the need to read those C64 tapes. Which means that, somewhere, a tape/cassette player needs to be fixed.

I began from my walkman Sony WM-B12. A rather cheap unit back then. I suspected the failure was in the rubber belts. That model is just two plastic shells locked together that come apart with the usual gently-pry-here-and-there method. The remnants of the two belts were just enough to suggest their original path. They melted into a sticky dirty goo that is non-conductive. Hint: the goo is quite very filthy, so work on a disposable surface, use gloves and protect your clothes. Seriously.

The goo came away with mechanical method, so I didn't need to use chemicals. Then I needed to understand which belts I needed: square, round or flat? The remnants suggested square about 1mm per side. Maybe less, but I needed a proof it can work, I don't need perfect sound (from a tape?).

The usual Chinese sources sell a bag of 1mm square belts of assorted lengths. They came faster than expected.

I did not take any measurement. I just picked two belts that seemed right. And they were indeed! Now I have a working walkman and something to show to teenagers. Their reasoning about how we built a "playlist" back then is very interesting. Especially when they realize there were no commercials.

I was not that lucky with a later model from Aiwa with autoreverse plus AM/FM radio. It needs a longer belt, probably round and thinner. I tried the 1mm square and the result is very obviously wrong. Another time, maybe.

 

If you still have tapes and a player do consider trying a repair. It's pretty easy. Just be prepared to have a wayback machine in your ears when you succeed.

FM antenna hack on a rented car

This Summer we spent 10 days driving around Greece in a rented car. Being high season I looked for cheaper alternatives to the big names. Using a local company (directly, not through a broker) I could save 20-30% and get a better deal on "options" like FDW, different drop off location, unlimited km.

 

Where is the catch? The car is not exactly new. We got a 7 years old Citroen C3 (we asked for it) with 75k km. Mechanically was OK, it even had cruise control and lane assist. No parking sensors and rear window wiper, though. Most important: it had an FM, MW, LW radio.

 

Too bad someone had hit a very low something and broke the antenna leaving an exposed PCB.

The result was that in FM we could pick whatever station was in line of sight. Greece is full of valleys and hills, so any station would last maybe 10 minutes max.

With a close inspection I noticed a pad with a hole on top of the board so we all started looking for a not-rusty wire: why not try a MacGyverism?

The exposed antenna fin PCB and my addition.

 

It is curious to note as in 2024 you don't find pieces of electric wire on the ground as it used to be (I used to carry 20 cm of hookup wire in my wallet and I should restore it). After 4 days of looking I spotted a venerable paper clip in a beach bar terrace.

In the hole it went and the FM band sprang to life. Well, at least several station could be heard vs. maximum one at any given time.

 

Mission completed.

Fairchild Multimeter Model 7050

I bought a Fairchild 7050 multimeter because it has slightly larger than usual Nixies. There was no indication of its state, but I could count the three tubes in the pictures. It looked a bit dirty as visibile in the picture below.

Picture by the seller.

When it arrived I tried powering it at half AC voltage, just in case, but nothing happened. There was no current draw as the power supply didn't sag a single Volt. As it was sold as a "low cost accurate 3-digit instrument" (quote from the manual), there is just one screw in the back keeping everything together. Also the AC cord is soldered to the board, there is no socket.

Once I could look inside I measured that the fuse was intact, so I moved to the ON/OFF switch and sure enough it did nothing: open in both positions. If everything else is OK, it will have to be replaced and shouldn't be too hard to find since it was a "low cost instrument" probably using very common parts.

I created a short across the switch and tried again the power-up sequence as before. At 120 Vac it wouldn't display anything, but there were no signs of smoke, overheating, smell, burns, exploded components, ... so we stepped to the nominal 220 Vac... whoa! It is working!

I did some tests and the readings agree with my other DVM's. Well, let's say they are in the ballpark but hardly within the 0.1% advertised in the manual. Thought there is no drift after the warm-up and nothing blew up after more than 60 minutes of doing something.

So, apart from cleaning, fixing the power switch and maybe trying to recalibrate it, this little beauty from the past should not present other challenges and is ready to go into service.

Fault on Atmel AVR ATmega

For the blinkenlights project two posts behind I chose to use an ATmega48 because the firmware is very small, while all my other (clock) projects would not fit. I noticed that it was not scanning all neon lamps and that the current consumption was periodically jumping about +100mA.

I determined that the problem was in the ATmega48 and probed all output pins with an analog voltmeter, looking for a swing that matched the current variations.

I found an output pin reaching +3.5V in the HIGH state, and it was in sync with the extra current draw. That digital pin measured 64 ohm to ground even with the uC out of the circuit. Culprit located.

I cut off the pin with broken control circuitry and used a different one with a simple jumper on the PCB. It's just a matter of updating the firmware.

Problem solved: no more I variations and all lamps light up as expected.

Why did I use an analog voltmeter? Because I could visually see the voltage change and estimate its value. A DVM would take too long to detect the change and I would not be able to correlate it to the current flow. I could have used an oscilloscope, of course.

Scheda supplementare solare Fondital Pegasus KC

ATTENZIONE. LE INDICAZIONI RIPORTATE DI SEGUITO SONO IL RESOCONTO DI UNA OSSERVAZIONE PERSONALE PER ILLUSTRARE UN POSSIBILE GUASTO, NON SONO DA INTENDERSI COME ISTRUZIONI PER RIPARARE QUESTO O ALTRO DISPOSITIVO ELETTRONICO E DECLINO QUALSIASI RESPONSABILITÀ PER EVENTUALI VOSTRI TENTATIVI DI RIPRODURRE QUANTO ILLUSTRATO.

Mi è stata portata la scheda supplementare di una caldaia Fondital Pegasus KC che andava in errore E41: "Mancata comunicazione fra schede supplementari".

La scheda del solare è stata sostituita dal tecnico (150€+IVA, marzo 2024)  e l'impianto ha ripreso a funzionare senza errori. Ma perché non provare a capire dov'è il guasto?

Blocchetto nero al centro: fusibile.

Il circuito è semplice: un alimentatore 220 V > 5 V lineare (con trasformatore, ponte diodi e 7805!), tre relè, un microcontrollore. Ho iniziato la caccia del guasto individuando l'ingresso 220 Vac. Il tester non mi dava continuità sul blocchetto nero visibile in foto. Stando alla mia ricostruzione (e alla serigrafia) si doveva trattare di un fusibile ad azione ritardata da 6.5 A.

Parentesi sul fusibile. Questo fusibile ritardato non salta quando si superano i 6.5A (1430 W!), ma ha un tempo di risposta che dipende dalla corrente. Per ottenere l'interruzione immediata devono passare 65 A (sessantacinque A). Non ci credete? Controllate il datasheet dei fusibili Bussman! Non so quanto tempo abbia impiegato ad aprirsi, ma la scheda non presenta segni di bruciature come mi aspetterei da un circuito del genere che gestisce 1500W o più.

Ho sostituito il fusibile con quanto avevo in casa ed ho alimentato il circuito con un trasformatore d'isolamento che eroga al massimo 30W, poi "si siede." Il risultato è stato promettente, con i relè che si sono mossi e il LED con lampeggio rosso e lento. Il LED comunica che il microprocessore ha rilevato l'assenza di comunicazione (RS-485) con l'unità centrale (corretto, dato che non ho tutta la caldaia a disposizione).

Il guasto è stato molto probabilmente causato da una perdita/infiltrazione di acqua dall'impianto solare dei vicini(!!) con corto circuito o simile. Proveremo ad inserire la scheda riparata per capire se il guasto era veramente così limitato, ed avere un backup in caso di necessità futura.

La scheda supplementare Fondital.

PS. Mi piace l'elettronica Fondital perché è ancora semplice e, in alcuni casi, riparabile. O quantomeno "comprensibile" senza uno schema elettrico a disposizione.

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.

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.

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!

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.

 

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.

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.