Test equipment to repair, part 1

I like old test equipment, particularly of the era where they had switched fully over to silicon transistors and achieved truly modern levels of performance but hadn’t gotten completely into cost-reduction — before they outsourced half of the design and production, back when companies like HP were seriously vertically integrated.

(imagine a box of random parts) HP 419A null meter. This is a null meter based around a chopper amplifier circuit which is very sensitive and was specified by HP in the calibration procedures for numerous equipment from the late-60s through the 1980s. I think it only truly became obsolete (from their perspective) after the impressive HP 3458A 8.5 digit auto-calibrating DMM was introduced. Mine was acquired cheaply, in mostly good condition — except the NiCd batteries had leaked their potassium hydroxide all over the place and the mercury cell was long since dead. It’s currently sitting in pieces in a cardboard box, I long ago finished cleaning the dirty parts off and I think I have all the replacements I need — I just need to do the tedious reassembly and see how it works. Also I’m going to replace the dead mercury cell with a CR2032 lithium and a low-quiescent-current regulator or series reference IC — It doesn’t even need to be exactly 1.35V, I think, it just needs to be there to provide a stable floating offset.

HP 3314A. This is in some sense the first notable arbitrary waveform generator, though it’s mostly easy to use as a simple function generator with sweeping, it has some awkward ability to construct arbitrary waveforms. And it doesn’t synthesize these with a microprocessor and a DAC in the modern way. No, it constructs them piecewise by integrating a digitally-controlled current source for various periods of time. I have two of these. One that fails its auto-calibration procedure on every powerup, one that sometimes fails when changing ranges, and has issues giving output with any consistency. I figure I can get one working one together out of both of them at least. It has that hefty late-70s monster feel of a half-rack 3U enclosure with a fan, 7 segment LEDs and lots of boards.

HP 5370A. This is the basic starting point of modern time interval counters and a classic time-nuts favorite. It uses some fairly complicated interpolators to exceed the resolution of conventional reciprocal counters. I have replaced the processor board in mine replaced with the Beaglebone-based replacement, HP5370 Processor Replacement Project, which makes it easy and reliable to connect to the network. However, mine is clearly a bit out of adjustment, and I need to go through the full adjustment procedure. This lead me to some other purchases including two 500 MHz scopes (a subject for another post) and the HP 8082A.

HP 8082A. This is a pulse generator with variable transition times as low as 1 nanosecond (and up to 5 volts into a 50 ohm load). Not bad for the late-1970s, and you’d still spend a lot today to exceed its specs. It has custom ECL ICs with beryllium oxide ceramic cases in the output stage! It mostly just works perfectly, which is fortunate, as the front panel slider switches are known to wear out and rely on gold plated PC board contacts to operate. Unfortunately it becomes terribly jittery above about 150 MHz (it should go to 250 MHz.) So far I’ve determined that it’s not the rate generator, because I’ve reproduced approximately the same behavior driving the external input with a reliable RF signal generator. Compared to other HP gear of the era, the service manual kind of sucks — has schematics and adjustment procedures, but it’s not really detailed about troubleshooting if things don’t work.

In the future I’ll talk about HP 3457A battery replacement and maybe fixing a 500 MHz Tek scope and some plugins (Hopefully — the 7000 series plugins are electrically brilliant but mechanically a lot inferior to their earlier work in my opinion.)

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