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.

 

Motorola MicroTAC GSM battery pinout (incomplete)

A colleague has given me two Motorola GSM phones, a d460 and an International 8700. According to timestamps on the battery packs, they were used in 1998-2001. 

All three battery packs had no output, of course, but helpful + and - signs mark where energy would enter the mobile phone. This allowed me to use an external power supply and confirm the devices work.

Why not trying to recharge their battery packs? These "MicroTAC" battery packs have connections both on the inner side, to the phone, and on the outer side, to the desk charger. I probed around with the multimeter and found the common ground between front and back.

With nothing to loose I set the power supply to 7.2V and started probing around: soon enough the Li-ion battery pack started taking current. What's even better is that it keeps the charge and powers the mobile phone!

For future reference, here is the incomplete pinout for Motorola MicroTAC SNN batteries:

Motorola MicroTAC battery pinout

The pinout depicted in the picture above should apply to these models:

SNN 4027
SNN 4058
SNN 4102
SNN 4132
SNN 4239
SNN 4259
SNN 4263
SNN 4283
SNN 4310
SNN 4314
SNN 4340
SNN 4346
SNN 4383
SNN 4458
SNN 4467
SNN 4612
SNN 4836

As usual, fiddle with these batteries at your own complete risk, even if you know what you are doing.

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.

T.I. Data Books

Here they are, on my desk. It started in high-school in the 1990's. These books were a good alternative read for nerds, instead of those brick-thick romance novels from a century back. 

Few Texas Instruments Data Books

That unmistakable combination of colors and shape makes them visibile from very far away. Their content is probably everywhere online, but I think the paper format allows a better overview of available components. If we were to design circuits like in 1970's-1990's, back again.

I am afraid this is just the beginning.

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.