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.