Cold Cathode gauges

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Chuck Sherwood
Posts: 36
Joined: Sat Jun 05, 2021 3:57 pm
Real name: Chuck Sherwood
Location: Naperville, IL

Cold Cathode gauges

Post by Chuck Sherwood »

I love ion gauges. In fact I think I got into high vacuum because I like playing with gauges. I am pretty sure I bought a hot cathode ion gauge before I had a system to put it on.

Originally I started out with BA gauges like the OLD granville Phillips 260 and 270.
Then I moved on to cold cathode gauges and I have found them to be very useful tools.
Here is an example of a couple GPH-320 gauges. The one on the bottom is old and the one on the top is very old.
The HV cable is different and the sensor connection is different as well. One nice feature is the sensor can be opened up on the top to pull the electrode out for cleaning.
IMG_2755.jpg
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The age indicators are the cord styles and the name plates. Beware cold cathode gauges will shock the crap out of you because some approach 3 KV.

These unit are functional but not very accurate. They can be powered up and even indicate pressures in the microns without instant death that would happen with a hot cathode gauge. This allows for a quick test to see if the chamber is coming down and you can watch the pressure rapidly decrease as the turbo spools up. In my experience they indicate lower than actual pressures most of the time but I know what they are telling me.

Next is an old Varian 860 series. This 863 has some set points associated with the red movable indicators. Seems to work okay and tends to read low as well. In particular it will drop into the 6s long before a hot cathode gauge. Probably much more common that the previous ones and sellers seem to think they are made of gold.
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The ones on the left are reasonable modern Alcatel FA series. In my experience these are the most accurate but not easy to find surplus at a reasonable price. Both of mine were DOA and require extensive repair. Both of these suffered the same fate. The voltage regulators died and took out every semiconductor in the unit. Fortunately the ICs are socketed and pretty generic parts. The voltage regulators are standard 3 terminal regulators. The FA111 had a couple burned out transistors associated with the set points too. There are calibration pots inside and if I knew how to adjust them, I would make the gauge read in Torr instead of mBar. I tweaked one pot to make them track a known BA gauge. I have not found schematics, but traced out some of the circuit to verify the parts were just op-amps.
chuck
Chuck Sherwood
Posts: 36
Joined: Sat Jun 05, 2021 3:57 pm
Real name: Chuck Sherwood
Location: Naperville, IL

Re: Cold Cathode gauges

Post by Chuck Sherwood »

Here are the parts I replaced in the Alcatel FA111 unit.
The voltage regulators and 741 are plainly marked generic parts.
The transistor is also marked and data sheets are available, but standard parts in the US have the flat on the opposite side! I used a generic PNP transistor because it is a simple emitter follower amp for the adjustment pot.

The 14 pin dip smelled like a quad op-amp but I needed more info so I traced out some of circuit board. Mapping the IC pin numbers to a generic quad op-amp I came up with this composite schematic.
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I was pretty confident the IC was a quad and I replaced it with a LM324.
alcatel ccg.PNG
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There are two pots in the unit which I did not adjust in the first unit. I tweaked one in the second unit to make them track.

Beware there is lots of high voltage inside here!!!!

Cables are impossible to find so it might be time to change that one out to a HV BNC.

This was a very satisfying repair and I use this gauge often.

chuck
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