MSU HV Base Debugging Guide

Removing a Base from the Wall

Before removing a base it is suggested that you write the following information on a shipping tag. (Shipping tags are available from the Physics Stock Room.) Suggested information includes the row, column, base number and cable number.

After you have found the correct base and verified its poisition and cable number, check for simple problems. Are the signal and ribbon cables plugged in? Assuming they were, you now want to extract the base. Find a long flat screwdriver (there should be two in the back of the darkroom) and if you are not immediately replacing the base, a cork. Corks are also stock items. It is a good idea to insert screws into the corks in order to pull them out of the hole when you want to replace a base.

Unplug the signal cable and tuck it in the pull-ring of a neighboring base. This helps prevent the signal cable from being "eaten by the wall." (See the section on finding a lost signal cable if you lose one.) Disconnect the ribbon cable and unscrew the screw in the back of the base. Now pull the base out and insert either a cork or a replacement base. Try not to shine your flashlight down into the hole as you remove the base. The neighboring PMT's are still under voltage. While they are less sensitive to red light, that is not synonymous with insensitive.

Examining a Base For Trivial Problems

There are a few common failure modes which are readily reparable at the experiment. If access to the LGD is not a problem (i.e. you are not rushed) it is typically worthwhile to make these repairs, plug the base into the wall and see if it now works. On the other hand, if you are in a hurry and are replacing the base anyway, it is suggested that you fix the problem then send it to the resident expert for a check-up. Common field-fixable failure modes are:

• The base contains a yellow capacitor with "Mexico" written on it. Check the resistance between the leads of this capacitor. If the meter reads less than 2M Ohm then this cap definitely needs replaced, and this may have been the problem. (In reality this is such a common failure mode, that these capacitors are routinely replaced even if they are not shorting out at the time.) Replacement capacitors are BNL stock items.
• The signal wire is broken in the back of the base. Plug a lemo connector into the back of the base and wiggle it. If it is excessively loose, the signal wire is possibly broken off inside the base. Remove the two small screw on that fasten the square housing to the base. Pull the black aluminum housing off of the base. The housing may be snug, thus requiring a some force to separate the two. Tighten the lemo connector with a wrench and give the wires a good tug. There should be a 1000 Ohm resistor between the signal pin and ground. This is to eliminate any static charge that may accumulate on the signal wire. If the connections are bad, apply some solder paste and re-solder. Now put the base back together and check that between the signal pin and ground you see about 1000 Ohms and between the signal pin and the signal pin on the front of the base you see less than 1 Ohm. The signal pin on the front of the base is located in the outer ring of pins near the top of the base. It should have empty sockets on either side of it. If the resistance is large (i.e. no contact) go to the next section.

• Check the resistance between the signal pin on the back of the base and signal pin on the front of the base. If this is small (typically less than 1 Ohm) then the signal cable is pobably okay. Next press the spring loaded socket. If the black signal does not move with the socket the connection is suspect. Pull gently on the signal cable, if it pulls out then obviously it's broken. You may want to defer to an expert as it gets tricky from here. But if you choose to continue: remove the two screws near the front of the base which are accessible from he back of the underside. You will often have to push these screws out with a pair of tweezers. Now press in the socket and remove the white or black plastic head. Use a screw driver to remove the clip-ring in the very front of the base. Now the socket should now come out through the front of the base. Using pliers and a the two extra hands you carry around with you re-solder the cable to the signal pin. Be careful because the white plastic sockets are low temperature plastic and melt easily. Now carefully put it all back together and test for continuity.

Examining a Base For NON-Trivial Problems

These problems are typically not user-servicable, send them to the expert. But FYI, here's brief list of phenomenological failure modes:

• The base is on at full voltage and uncontrollable (i.e. HOT). This is typically a problem with OpAmp 2 in the control circuits.
• The base is giving nothing out (i.e. DEAD), the cap is okay and the signal wire checks out. This may be many things, OpAmp 1, power transistor problems (VT problems), a bad connection in the Cockroft-Walton chain, or a dead custom chip.
• The base seems to give okay signal but you can't get it to change its setting. This may be caused by a dead custom chip or if the internal address is incorrectly set. The way the address is set is by attaching jumper wires to from the 8 pins in front of the custom chip to ground or +5V. The jumper wires form a binary representation of the address with the most significant bit on the left-hand side as you look from the back of the base forward. It is generally wise the desolder one of the legs of the electrolytic capactor which is installed above these jumpers and move it out of the way. In order to figure out exactly what jumpers you will need to set look at several bases and understand how they get their current addresses.

** 12 Jan 95 JG