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HP 211A Square Wave Generator

August 31, 2011

Another ebay acquisition. The notes stated that it worked but there was a burning smell and that the frequency was low by about a half. After a quick exchange of messages to assure myself that the power transformer was OK, I made an offer and it was accepted. When it arrived, I opened it up (always the first thing to do with a new acquisition). A burnt resistor was immediately apparent. I checked the value and it was OK. I also checked the associated tube which was the multi-vibrator grid clamp that controls the basic frequency by limiting the negative excursion; this acts to minimise frequency variations due to tube ageing e.t.c. I could find nothing wrong, even the wiper of the setting pot was in good solid contact. So, I carefully powered it up using a variac and all went well including the supply voltage coming into regulation at about ac 110 v. Checking the frequency, it too was very close and the wave shape was excellent. I concluded that the frequency setting wiper most likely would have been the culprit, if it had lost contact, the clamp tube would have been conducting heavily and the frequency would have been low. Vibration during shipping must have restored contact! I took no chances and lubricated the pot using Deoxit fader lube and exercised it. It is a really nice generator, I prefer it to my Tektronix 106.

The range is 1Hz to 1MHz. There are two outputs from a push-pull output stage, one 75Ω having a peak output of -7 v and one 600Ω having a peak output of -27 v. The 600Ω outut can be made electrically identical to the 75Ω output by paralleling it with a 86Ω resistor thereby providing two balanced outputs. The 75Ω output has a risetime better than 20nS.

If you look carefully at the photo above, you can see the burnt resistor between the pot and the clamp tube in the centre.Here it is demonstrating the rise-time of the HP 150A scope with a 152A Yplug-in, using the 75 Ω output loaded with 75 Ω at 1MHz; the time scale is 50 nS/cm:

Since the original post, I acquired a second example, also in excellent condition. Lots of generators are useful to the job of regularly checking that all the scopes remain functional! This acquisition moved me to do the job of replacing the X1 and X10 range timing caps in both units. Old caps do tend to be leaky and that affects timing. On both units, the overall frequency accuracy is quite good but not to be compared with modern gear.


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  1. andyprigge permalink

    nice generator, if you don’t mind me asking how much did it cost?
    regards andy

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