A minor repair reveals a footnote to history

January 07, 2014 // By Graham Prophet
This little alarm clock very nearly qualifies as a family heirloom; it has performed uncomplainingly for around 35 years, and I would not have thought to dismantle it if it had not developed a slight fault. And in the course of attending to that fault I noticed something about it that had escaped me for all the time I have known it.

The clock is branded “Metamec” - prior to venturing into the digital domain, this company sold mains-powered analogue clocks using synchronous motors. The digital clock is as simple as you could expect it to be; 12 hour display with am/pm indication on red 7-segment LEDs, single alarm set-point, and deriving its timebase from counting 50-Hz mains cycles. An interruption to mains power, and therefore timing, is indicated by a flashing display. A particularly sophisticated feature – for the time it was manufactured – is the inclusion of a touch-panel to activate the “snooze” function.

Just lately, it would enter flashing-display mode without any mains outage having taken place and, given its long service, it seemed worthwhile fixing it. I wasn't in any doubt as to what I was going to find when I undid the retaining screws. I expected to see a pretty rudimentary linear power supply with electrolytic capacitor(s) totally dried out after all these years. My assumption was that the ripple on the DC rail had reached the point that the chip was reading the dips as a momentary power outage, and changing mode accordingly.

And that is exactly what I found, as you can see in the picture  above; replacing the electrolytics restored normal operation, and that's the end of the repair story. It was such an open-and-shut case I didn't even bother to put a scope on it to see how bad the DC really was.

Having taken it apart, I was of course curious to find out what components had been used: it turns out to be completely standard. There is a single 40-pin dip, which is a National Semiconductor MM5387AA. Despite being long obsolete, the data sheet is still out there, archived on the web in multiple places. I had thought that by 1978 it might be NMOS, but the data sheet says it's PMOS, perhaps because the direct drivers for the LEDs were