72-100 voltage

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I recently encountered an unusual problem on two different 660 scanning auger systems within a period of a few months where the 72-100 multiplier voltage would drop off after a period of time.

On one system the 97 SED preamp would fail after a few minutes of warm up time, resulting in no image. On the other system the 96A V/F preamplifier would fail after 20 to 30 minutes, resulting in no AES data.

In both cases I measured the multiplier supply voltages at the preamp and the voltages were there when functioning and not there or unstable when in the failed mode. So easy enough, that should be a problem on the 72-100 board. To verify that the problem was on the board I swapped the 72-100 board with the other one. For those of you not familiar with the 660 scanning auger system or the 72-100 electron multiplier supply, there are two 72-100s on a 660. One is for the CMA AES analyzer and the other one is for the 97 SED preamplifier. The boards are identical and can be swapped out by physically moving the boards and changing an address switch.

Swapping the 72-100 boards produced the same result. That meant that the problem was not on the board, so the next most likely thing was the SHV multiplier supply cable. Those cables checked out fine with an ohmmeter but to be sure I also swapped the cables out and the problem was still there.

So as unlikely as it was, the only thing left was the 72-100 mother board.   I pulled out the mother board looking for a bent pin or something that could explain the problem and did in fact find that one of the 1K ohm filter resistors was out of spec and looked like it had been running hot. Moving it slightly caused the resistor to break in two.

Here is a picture of the motherboard that shows the resistor:

72-100 R1 R2

72-100 R1 R2

And here is the schematic that shows both resistors:

72-100 mother board schematic

72-100 mother board schematic

Replacing the defective resistor solved the problem. What was interesting for me with this problem was that in over 37 years of working with PHI surface analysis systems I had never seen this problem before, and then I saw the same problem on two different systems within a short period of time.

If you have an intermittent SED image or AES data problem on your 660 (or 4000 series) scanning auger system, keep this solution in mind as something to check.

4 thoughts on “72-100 voltage

  1. Interesting.. I just had a similar issue two weeks ago with an old 32-100. I was having s/n issues and replaced the multiplier (was running at about 3keV so it was time). Everything was better but still had some funny issues. After checking everything and being confused, I finally pushed on the resistor between the HV supply board and the SHV connector. It just pulled apart. I replaced it with a metal film resistor.

  2. That resistor has the look of an old Ohmite brand 1/4 watt carbon composition resistor. A very good manifestation of a rather poor type of part. They are inherently unstable over time and even the best brands drift a lot as they age and without a lot of derating in the design they also get delicate over time. Best practice is to replace it with a metal film equivalent (or even carbon film) of an equal or higher power rating. Be sure to pay attention to voltage rating especially with carbon film parts and if the circuit involves frequencies higher than a few megahertz use a non inductive type as carbon composite types are inherently low inductance. As someone who has restored more than one old radio I can tell you that end-of-life for carbon comps usually comes at around 30 to 50 years for medium to high quality parts in a conservative design. A lot of these beasts are living on borrowed time and the fact that they went 37+ years suggests that the 660 was exceptionally well designed

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