Computing stuff tied to the physical world

Flash from the past

In Hardware on Jan 23, 2012 at 00:01

A while back, Jeroen mentioned on the forum that he had an ancient GM5655 Philips oscilloscope, and since he lives in Houten we thought it’d be fun to put it side-by-side with my new Hameg scope I keep generating screen shots with in this weblog (ad nauseam for some, I suspect…). Here’s his vintage 1955 hardware (from a web image) – just imagine how attractive that must have looked to geeks back then:


It’s single-channel unit, with a sweep generator which can go “all the way” up to 30,000 Hz. The horizontal deflection can also be driven externally, making this scope X-Y capable. Vertical sensitivity is up to 60 mV/cm, horizontal up to 100 mV/cm. No positioning, no magnification, nothing. This is as basic as a scope can be.

Jeroen dropped by a few days ago and we had a go at it. Unfortunately, we couldn’t get it to work. The horizontal deflection appears to be broken (not just the sweep generator, since it also didn’t respond to external signals). Most probably one or more of the “tar capacitors” has failed. It’s always them capacitors with old equipment…

Neither of us felt confident enough to go about messing with this thing while powered on (vacuum tubes, and especially the CRT, run on high voltage). But the least we could do is open it up and marvel at how things were constructed back then. No semiconductors and no printed circuit boards – pretty amazing, if you think about it.

It’s a pretty scruffy unit, once you open it up!

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Note the big tar caps and the “flying” construction, especially around the rotary switches:

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Here’s the other side, with what looks like just the vertical input circuit:

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Here’s the top view, with 6 vacuum tubes in total:

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The bottom view, with one more tube and what looks like a bunch of electrolytic power supply caps to me:

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All this was manually assembled, so each unit had to be tested and debugged, since it wouldn’t take much to hook up things incorrectly. The invention of the printed circuit board not only creates a mechanical platform to hold everything in place (especially for low-voltage semiconductors), it also acts as a self-documenting area during assembly, since all the spots are already marked with what goes where.

Amazing stuff – I’m tempted to get a (somewhat more modern) analog scope off eBay, just for the fun of it :)

  1. Now THAT will take some debugging – where’s my trusty AVO ?

    Note the mains transformer position – dictated by getting the stray magnetic fields as far away from the actual Cathode Ray Tube as possible. Mu-metal for screening is expensive!

    Any idea of the purchase price mapped to today’s equivalent?

  2. Connecting old equipment to 220V (240V these days) can be a recipe for disaster. Old capacitors need to be reshaped first (‘formeren’ in Dutch) to restore the electrolyte composition. A series resistor is placed between the cap and an adjustable power supply. The voltage is slowly increased until the leak current stops decreasing. This process can take days with big capacitors. See also this page:

    • Fascinating link (in Dutch) – lots of pictures of stuff not just being repaired, but actually rebuilt to match their original looks!

    • Superb site, even after google translate had a go at it! I’m rather tempted to show that to my father, but I know what would happen… The old Philips Ovaltinies radio would come down from the attic again!

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