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.

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.

Texas Instruments Consumer Circuits Data Book 1978

I bought this book at Marzaglia fair 2024. I have never owned a data book, so this was a chance to fill that gap.

The index looks very interesting. You can see how many specialised integrated circuits were designed and, probably, produced. 

My eyes stopped on "Timekeeping Application" and "Game Chips". That's something today you can do with one microcontroller, you can even dare to squeeze the whole index of this book into a single firmware.

Zoom in on these two pages from the index (click on them) and see what you needed to build a not-so-complex application back in the 1970's.

For your convenience and for search engines, here is an OCR extract of the two images above.

CB TRANSCEIVER APPLICATION
SN76514N	DOUBLE BALANCED MIXER	399
TP0235	FREOUENCY SYNTHESIZER	408
SN76515P	DOUBLE BALANCED MIXER	414
TP55I04	PLL FREQUENCY SYNTHESIZERS	416
TP55114	PLL FREOUENCY SYNTHESIZERS	416
TP55106	PLL FREOUENCY SYNTHESIZERS	416
TP55116	PLL FREOUENCY SYNTHESIZERS	416
TP55116N	PLL FREOUENCY SYNTHESIZERS	416
TMS1022NL	CB CHANNEL CONTROLLER	427
TMS1023NL	C8 CHANNEL CONTROLLER (GERMAN BAND)	451
SN16880N	LOG. STEREO LEVEL DETECTOR/INDICATOR	452
SN16889P	LINEAR LEVEL DETECTOR/INDICATOR	456
SN16921NM	SCALE DRIVER IC	461
TIMEKEEPING APPLICATION
TMS3879NC	R.C.TIMER WITH TIME CONSTANT MULTIPLIER	465
TMS38348N	MOS/LSI CLOCK CIRCUIT	471
TMS1951NL	DIGITAL RADIO CLOCK	483
TMS1952NL	DIGITAL RADIO CLOCK	492
TM1191943NL	DIGITAL RADIO CLOCK	302
SN76825P	16/23 STAGE COUNTER CIRCUITS	514
SN76835P	16/23 STAGE COUNTER CIRCUITS	514
SN76810P	MONOSTABLE TACHODRIVER	520
TMS1121	UNIVERSAL TIMER	530
LOW COST GAME CHIP
TMS1965N1	6 GAME EUROPEAN SYSTEM	547
UNIVERSAL GAME CHIP
SN76423N	AUTOMATIC RANDOM ENGLISH	555
SN76424N	WALL GENERATOR	559
SN76425N	SYSTEM REGULATOR AND SYNC GENERATOR	562
SN76426N	DUAL CHARACTER GENERATOR	566
SN76427N	WALL BALL GENERATOR	569
SN76428N	VIDEO GAME LOGIC	573
SN76429N	HOKEY,TENNIS/HAND BALL GAME LOGIC	577
SN76430N	SYNC GENERATOR/COLOR GENERATOR VIDEO SUMMER	581
SN76431N	COMPLEX CHARACTER POSITION CONTROLLER	586
SN76440N	SPACE WAR GAME LOGIC COMPLEX CHARACTER	590
UNIVERSAL GAME CHIP
SN76432N	HOKEY/TENNIS/HANDBALL CHARACTER 592
SN76442N	RACE CAR/ROCKET SHIP/UNIVERSAL MAN CHARACTER 597
SN76443N	COMPLEX CHARACTER GENERATOR 598
SN76444N	ROCKET SHIP/HOKEY/TENNIS 599
SN76445N	GUN FIGHTER/UNIVERSAL MAN CHARACTER 600
SN76446N	EXPLODING ROCKET CHARACTER 601
SN76449N	EXPLODING HELICOPTER CHARACTER 602
SN76460N	ZERO TO WIN AT 20 DIGITAL 603
SN76462N	SCORING 603
SN76483N	SPACE WAR OBSTACLES GENERATOR 608
SN76484N	SPACE WAR SWITCHING LOGIC 612
SN76477N	COMPLEX SOUND GENERATOR 615
TELEPHONE APPUCATION
TMS3833NC	ADAPTATEUR POUR CLAVIER TELEPHONIOUE 625
TMS3851N	C-MOS LOGIC CIRCUITS 631
TMS3858N	TELEPHONE KEYBOARD ENCODER 634
TMS3EI46NC	PUSH BUTTON TELEPHONE DIALLER 652
TMS38.59N	PUSH BUTTON TELEPHONE MEMORY 662
TMS387I NC	PUSH BUTTON TELEPHONE ADAPTOR PULSE DIALING SYSTEM
TMS3872N1	TELEPHONE FREOUENCIES GENERATOR 681
TMS3872-1NL	TELEPHONE FREOUENCIES GENERATOR 681
TMS3831N	TELEPHONE KEYBOARD ENCODER 689
TMS3878N	FREQUENCY DISPLAY 698
MICROPROCESSOR/MEMORIES
TMSI000NL	ONE CHIP MICROCOMPUTER 711
TMS1000/1200C	ONE CHIP MICROCOMPUTER IN- C-MOS 725
TMSI000	SERIES MOS/LSI ONE-CHIP MICROCOM. DATA MANUAL 743
TMS1024	MICROCOMPUTER I/O EXPANDER 787
TMS1025	MICROCOMPUTER I/O EXPANDER 787
TMS1117NL	MICROWAVE OVEN CONTROLLER 790
TMS1976NL	CAPACITIVE-TOUCH KEY BOARD INTERF. CIRCUIT 815
TMS2501JC, NC	64X5X7 STATIC USASCII CHARACTER GENERATOR 857
TMS4103JC, NC	64X5X7 STATIC USASCII CHARACTER GENERATOR 858
TMS4710JL, NL	COMPLETE ASCII CHARACTER SET GENERATOR 5X7 CHARACTER. 8X8 BLOCK 859
TNIS4027-30	NH 4096 BIT DYNAMIC RANDOM-ACCESS MEMORY 868