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HP 500B Frequency Meter.

May 25, 2013

This arrived filthy inside, with two broken tubes and a perished cord.
The range is from 3Hz to 100KHz at “better than” +/- 2% accuracy. I found that it is indeed better than +/- 2%. Here it is after cleaning up, re-stuffing the two power supply reservoir electrolytics and replacing all the paper in oil caps:HP 500B Frequency Meter
Here are the original caps. One cap (the flat one) looks as though it has already been replaced but looking at a picture in the manual, it seems that all these caps are original. One of the two series connected power supply electrolytics had dried out completely, I re-stuffed them both, the cardboard sleeves make it easy to do this because they cover the results of cutting the aluminum cans open. Many of the caps had cracked cases and you can see that a chunk of the case had fallen away from one of them. I have never seen caps in this state before (and I have quite a bit of experience), this combined with the perished power cord suggests to me that this unit had spent much of its life in a harsh environment:
HP 500B Failed Capacitors
Here it is after repair showing the 60Hz self-test:
HP 500B Frequency Meter 60Hz self-test
And here at 15KHz:
HP 500B Freqency Meter, 15KHz
There is a resistor board that is used to correct each range thereby avoiding the need to use precision capacitors. I spent quite a bit of time working with this using a Tek 180A temperature stabilised crystal time marker as the frequency source:
HP 500B Calibration Resistor Board

I think it is worth describing the circuit action of this instrument, as I am not an E.E. I found it both novel and fascinating:
HP 500B Block Diagram
Refer to the block diagram above and the oscilloscope traces below:
The signal is amplified and applied to a conventional Schmitt trigger, the output of which is rectified and differentiated to apply a negative pulse (t1) to the A side of a multivibrator. This turns the A side of the MV off causing a positive pulse at the A plate that is applied to Phantastron run-down circuit, starting a linear run-down. At the start of the run down, the screen grid of the Phantastron rises and stays positive until the end of the run-down. This positive pulse is applied to the B grid of the MV (the constant current generator in the diagram) and is limited against a reference voltage by a clamping diode resulting in a controlled current pulse. The Phantastron run-down is stable and predictable in rate and duration and at the end, the screen voltage drops*, cutting off the current pulse (t2). This action results in a constant current pulse which is stable in duration. At the same time, degenerative action of the MV cathode resistor combined with the grid diode clamp causes the pulse to be stable in amplitude. One pulse is passed through the meter circuit for each input cycle. Since the meter has a large capacitor across it and the meter itself acts to discharge the capacitor, the more frequently the capacitor is charged, the higher the average charge voltage and the higher the indication on the meter.
The traces below were recorded on a Tek 547 with a 1A4 plug in:
Top, MV A plate.
Second down, Phantastron screen pulse.
Third down, Phantastron run-down.
Bottom, MV B current pulse.
HP 500B Oscillogram
I Believe the Phantastron is one of a number of extremely clever devices invented by Alan Blumlien just before or early in WW2 and so named because what it could do was, at that time, fantastic. Such triggereable timing and logic elements included multivibrators and were essential elements of radar and of course code breaking computers.
*It is fairly easy to see how the run-down commences, when a positive pulse is received at the screen grid, the tube starts to conduct, discharging the timing capacitor. The run-down termination process is not so obvious: The level at which the discharge ceases is determined when the potential between the anode and the suppressor grid becomes so low that the anode cannot draw through the suppressor grid the electrons that have passed the screen grid and so the remaining electrons return to the screen grid causing the potential of the screen grid to suddenly drop and so the tube cuts off ending the discharge.

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2 Comments
  1. Great restoration job,good article,fine business ! W.Priddy KA4FKU

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