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HP 122A

October 19, 2011

A basic two channel ‘scope, it was the last of the oscilloscopes developed by HP in the 50s and the price was $850 when last listed in 1970. It was advertised as a “Dual-Trace Automatic Base Line Oscilloscope For The DC – Several Hundred KC Range”. The time base features a very nice automatic trigger that both triggers reliably and maintains the base line without a signal present, quite free from temperament. It has the usual nice to use HP controls, 3/16″ thick front panel and excellent scale illumination. Very few capacitors needed to be replaced to get it working. As received, it was grimy and greasy inside so I wonder if it saw service in a production environment or was stored in the back of a burger joint! Another nice feature is a “B-A” display setting allowing difference signals to be viewed, I use this feature a lot on all my scopes, I only mention it here since the HP 150A with 152A Y plug in does not have this feature. I guess that in the intervening 5 years, HP learnt to include this feature.
It was the first HP design to incorporate transistors which were used to regulate the DC heater supply for the front end of each Y amplifier, I understand that this was done more to prevent the traces perambulating around with varying line voltage than to reduce hum. The design was by this time quite basic, yet very sound. I surmise that the cost was partly kept down through using just 3 tubes in each Y channel. The penalty is bandwidth, since to get adequate sensitivity, the load resistors for all stages are large in value and there are no cathode followers, the result is flat to 200k, dropping off fast thereafter. Another area where cost control shows is that the Y system does not have chop mode blanking, so the chop mode is only useful at quite low frequencies where the brightness can be turned down far enough that the chopping is not (too) visible. It was a very good scope for audio applications and like all HP equipment, I like it. My loyalty to Tektronix is being challenged and by this, HP’s bottom end offering!

There are a total of 25 tubes + CRT, this can be compared to the Tektronix 310 which is a single channel scope having 30 tubes + CRT and a significantly higher specification at a price of $1050 when last listed in 1071. The compromise is clear; $1050 for a 3″ single channel scope versus $850 for 5″ dual channel scope.

Though I did not have to replace many capacitors to get it working, the start linearity is fairly poor, I would look at replacing the timing capacitors if I were to want to use this scope seriously.

While sorting it out, I experienced something quite odd: Initially, I had got it working while still filthy. I then washed it (simple green rinsed with distilled water and an overnight stay in hotel oven at 170°F). After that, the trace would shift while changing DC sensitivity on the B channel. This caused much head scratching, the input tube was not pulling grid current so what the heck? Finally, I allowed myself to take seriously the notion that something was going on with the input binding post so I disconnected it. Problem gone. I then removed the post and cleaned the base thoroughly, reassembled it and, problem gone. The front panel does have some corrosion and I surmise that the wash had caused aluminium salts to get onto the binding post mount, resulting in galvanic action between the binding post and the panel causing a DC offset.

The calibrator is a double neon relaxation oscillator, clipped both ways to produce a squarish-wave.

Here it is displaying two waveforms of unrelated frequency:

The Y deflection line has suffered from heat and cracking however, it works so I left it alone.

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2 Comments
  1. John Burgoon permalink

    Thanks for the post. That bit about the mysterious missing grid current was helpful to me, I have a Tek 454 at home that acts that way sometimes so now I wonder if I have a similar issue.

    • Hi. Yes, initially I thought that there was a problem related to the attenuator presenting too large a DC resistance to the grid, if there was any grid current present that could explain the issue, however this was not the case so I had to let myself consider “out of the box” as it were. Richard

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