Hewlett Packard 122AR Restoration

[MaxFrister]

I recently acquired an HP 122AR oscilloscope for $11.56 USD. 



This is an interesting machine.  It was introduced in 1958 and produced, I believe until the early 1970’s  We don’t see many contemporary tech products made for a decade or more…

This was an early scope for Hewlett Packard but already has many of the features we expect on a “modern” analog scope: dual calibrated display, chop and alt, auto-trigger, B-A display, XY display, etc.  It has at least one feature that surprised me: balanced (isolated ground) inputs.

It has an amazing 200Khz of bandwidth and was sold as an economical  scope, costing about $625 in 1959.  It was intended for low-frequency applications including audio; HP made a higher-bandwidth scope and soon introduced another.

This particular machine is painted a non-standard color and badged as “Ling electronics”.  Ling made (makes?) vibration analysis systems.


The 122AR has 25 vacuum tubes, numerous neon bulbs (used as voltage references) and 2 transistors!  Those lonely transistors serve an interesting purpose: they are the pass transistors for the 24V rail used for the filaments of the vertical amplifier tubes.

The 2 transistors provide a nice symmetry to my circa 1966 Tektronix 453 — it has many transistors and just 2 vacuum tubes (also in the power supply).




This machine arrived filthy inside and out.  I disassembled it and cleaned it by hand.   I know that people hose these machines out, but I was not that brave.  A quick spot check showed that electrolytics were gone.  The machine showed signs of a long, useful life: pencil marks from previous technicians and a variety of tube brands.  The last calibration sticker was 1986.

Despite its huge size (and weight), this was a difficult machine to work on.  Many components are packed around the selector switches or jammed in small areas:








Because there was no place to fit new standoffs I ended up “restuffing” the old electrolyic cans.   Fortunately most of the tubes were okay:



Last night, after many many hours of work, I was finally able to power on the machine:




It still needs some testing and a thorough calibration but I was relieved to see it power-up without smoke.  I’ll also need to make space in the rack.

[james_s]

That looks really good. I'm glad someone is out there rescuing this old stuff.

[MaxFrister]

Apparently I’m not done with this quite yet.  This machine was last calibrated in 1986 and contains 1 tube with a 1985 date code (the earliest is a 1942 tube).

In working through the calibration procedure, I found 2 problems:

The B channel amplitude changes 6.3% with 127V AC (limit is 3%).  The A channel is stable, so this is probably a tube in the B vertical amplifier.
I am unable to compensate the B channel in the 10V range.  I’m guessing this is unrelated to #1 and is some component in the B attenuator.

Next I’ll try swapping tubes between A and B and to look closely at the attenuators.