V4 Differential Valve Problem

This post describes how to fix the problem of the V4 differential valve not opening on an AVC when the DIFFY PUMP ION GUN button on the AVC remote box is depressed.  Look at the photos at the bottom of the post for additional information.

The AVC (Auto Valve Control) has two DV6M thermocouple gauge tubes which are used to monitor the vacuum levels in the load lock and at the turbo pump. In the case of systems that have two turbo pumps, the second thermocouple gauge tube monitors the ion gun differential pumping turbo. The load lock TC gauge tube is located on under the load lock or in some cases as shown below, under the table top.

The AVC displays the vacuum level of the load lock thermocouple gauge at all times with the exception of when the DIFFY PUMP ION GUN button on the AVC remote box is depressed. In that case the AVC will momentarily monitor and display the turbo pump thermocouple gauge tube. If the vacuum level is sufficient then V4 will open. Monitoring the turbo pump vacuum is a built in safety feature of the AVC to prevent the chamber from being dumped in case the DIFFY PUMP ION GUN button on the AVC remote box is depressed while the turbo pump is off or not up to speed.

A not uncommon problem with the AVC unit is that the V4 valve will not open when the DIFFY PUMP ION GUN button on the AVC remote box is depressed.

The most common solution to this problem is that the turbo pump thermocouple gauge tube is not working properly or is not matched closely enough to the load lock gauge tube. Here is how you can test the ion gun gauge tube –

1. Press the PUMP INTRO button on the AVC remote and pump out the load lock until you have 5 bars displayed on the AVC remote.
2. On the back of the AVC (located on the back of the vacuum console), remove the TC1 gauge cable. TC1 monitors the load lock.
3. Unplug the TC2 cable and move it into the J1 connector where you just unplugged TC1.
4. Look at the AVC remote box. If the turbo pump thermocouple gauge is working properly you will see 4 bars displayed. After 2 minutes you should see 5 bars displayed.   If only 3 bars are displayed you can adjust the AVC so that V4 will open by following the steps in this older blog post – https://www.rbdinstruments.com/blog/auto-valve-control-adjustment-procedure/

If only 2 bars are displayed then the next step would be to try replacing the DV6M thermocouple gauge tube. There is only one TC controller (Hastings T6 called the hockey puck because of its shape) inside the AVC and a relay switches between the two thermocouple gauge tubes when the DIFFY PUMP ION GUN button on the AVC remote box is depressed. If the two thermocouple gauge tubes do not have similar offset and gain properties then the only solution is to replace the turbo pump DV6M thermocouple gauge tube or the intro thermocouple gauge tube in order to get them to match more closely. If the AVC reads 1 bars all the time then the hockey puck may be defective.

Usually though, the problem is that the K3  thermocouple relay inside the AVC is not switching. Or the contacts on the relay may be dirty. In the case of the older Blue plastic relays it is common for the plastic switch throw to be cracked at the top in which case the relay may close but not all the way and so the turbo pump thermocouple gauge is not read at all.    The RBD Instruments part number for the blue/yellow K3 relay is PN K3AVC24RE.

If when you swapped TC1 and TC2 only one bar was displayed, then most likely the relay I broken or the contacts are dirty. Here is how to address that problem;

1. The easiest way to get to the back of the AVC is to come in through the top. So, remove the wooden table tops and the aluminum ones that are towards the back of the vacuum console.
2. Close all valves on the AVC and also turn off the turbo pump (s).
3. Turn off the AVC control power.
4. Remove cables from the back of the AVC, including the power cord.
5. Unplug the air manifold cable and remove the gas manifold from the AVC (2 to 4 screws) The air-lines stay connected to the manifold and the compressed air that is connected the back of the vacuum chamber stays on..
6. Remove the AVC control and unplug the 40 pin remote box ribbon cable.
7. Place the AVC control on a work bench or table and remove the cover.
8. Inspect the TC relay K3 (on older AVCs there are only 2 relays and the thermocouple gauge relay is the left one). If you have a newer AVC with 3 relays, the TC relay is the far left one. The far right one is the up to air relay. If it is the old blue relay you can remove the two mounting screws on the back of the AVC to make the relay more accessible.
9. If the TC relay is the old Blue plastic type(RBD part number PN -K3AVC24RE),  then most likely the relay contact pull piece is broken at the top. If you remove the cover you can inspect the top part of the pull piece and if it is cracked then the relay needs to be replaced. (You can also just move a new pull piece over from a new relay rather than unsolder the relay).
10. If the TC relay is the newer style then most likely the contacts are oxidized and you will need to clean the contact with some fine sand paper or emery cloth followed by paper soaked in isopropanol to remove the residual small particles. (Or you can replace the relay with a new one).
11. To clean the contacts, unplug the relay from the socket and remove it from the AVC.
12. Remove the plastic cover from the relay (it snaps on so use small screwdriver to un snap it) then clean the contacts.
13. You can measure the resistance on the contacts with an ohm meter and manually move the relay throw to switch between the two sets of contacts.
14. Reinstall the relay.
15. Reinstall the AVC control and reconnect the gas manifold and all the cables.

That should do it!   If not then please contact RBD Instruments Inc. for more assistance. We can repair your AVC controller and also provide DV6M thermocouple gauge tubes.

RBD Instruments Inc. www.rbdinstruments.com 541 330 0723

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