·Make sure you have a small watchman?s screw driver and a DMM with clamp meter.
Once a crate failure occurs the following steps should be taken to isolate the problem and bring the crate up again.
Inside the WAH switch on the crate and see the display. If the crate becomes ON and then goes off, it will give a message ?U?Voltage low? (? Could be a number from 0 to 7). Check the LVPS table to find out the DB corresponding to this U? Terminal. .In case the crate remains ON check the currents for all the channels of the crate using SLOW CONTROL and compare with the LVPS table. Find out the DB for which the current now is lower than the table. This shouldbe the problematic DB. Next go and verify physically .
In case the crate immediately goes off without giving any message(a very small possibility), observe the DB?s from close to seesparking while powering on. IN case the DB is sparking ,then it has to be removed physically from the detector and the PCB has to be got repaired by either Danny Padrazo or his assistant Phil. Also check if any spare DB is available with Danny. Reinstall the repaired DB and do calibration for all the chains.
2.FINDING THE PROBLEMATIC CHAIN
If some of the channels on any DB are off (one or both the green lights on either side of a chain connector is off) just press the reset switch. If both the lights come on, that particular chain is OK failing which the chain needs to be debugged. You may have to do this for all the chains on that particular DB. In case the problem persists for more than one chain or the whole DB goes off and causes the crate failure. Remove all the chains from the DB and then switch on the crate. Add the chains one by one till you are able to find out the faulty chain by the method of elimination.
3.FINDING THE PROBLEMATIC BOARD
To find out the problematic board in a chain, a DMM with a clamp meter is needed. If you can physically find some board hot, just remove it and then power on the crate. Other wise plug out power from 9(the number of boards can be less if the chain is small) boards from the chain. Power on the crate. If the channel trips remove 9 more boards. Again power on the crate. If the channel trips again remove all the boards. Start by adding one board at a time. Put the clamp meter on the power cable and monitor the increase in current as you keep on adding the boards. The faulty board would cause a jump in current. A normal board draws about 100-120 milliamperes of current whereas a faulty board could draw much more depending upon the number of damaged chips on that board.
Another approach could be to increase the trip current of that particular chain to such a level that the chain does not trip. (Turning the small blue potentiometers on either side of the chain, anticlockwise can do this). Then start removing the boards one by one till you notice a sudden drop in current. (Typically a full chain with 27 boards could draw up to 3-3.5 amperes current on the negative side, but in this case it can be above 4 amperes)
Replace that board with a board of the same type and then again measure the current and compare it with the current for that chain mentioned in the LV table. If it is still high there may be another board, which is bad. Repeat the above-mentioned steps until the chain is fully debugged.
HOW TO MEASURE CURRENT
There is a combination of a current clamp meter and a DMM, which is to be used for measuring the current.The following steps should be taken every time current in the chains is to be measured.
i)Switch on the DMM and put it on mV dc range.
ii)Hold the clamp meter and press the button on it making sure that the red light glows. (If it does not glow turn the zero adjust clockwise slightly and again press the button.) Observe the reading on the DMM. It should be as close to zero as possible. If it is high either positive or negative, adjust it to be close to zero with the help of zero adjust by moving it slightly, clockwise or anticlockwise depending upon the reading which is showing. It will never be zero but it should be close to it say + or - .0something.
iii)Open the clamp and put it around the wire through which you want to measure the current. Remember put it only around one wire and not all the three wires. Putting around red wire gives the positive current drawn by the chain and blue should give the negative current.
CALIBRATING THE DB CHANNEL WITH A CHAIN
Trace out the chain, which is connected to the channel of the DB. Plug in any spare connector in the chain into the dummy load (a small white PCB with 3 pin connector and two big pre-adjusted potentiometers, lying on the table near the PMD racks). Now start adjusting the small potentiometers (with the small watchman?s screwdriver) on the Db on either side of the channel by rotating them clockwise about 3 turns at a time, one after the other. Keep on doing this till the channel trips i.e. one of the LED?s on either side of the channel trips. Remove the dummy load. Reset the channel (If the crate trips power on the crate again). Make sure LED?s on both sides of the channel are on. To test put on the dummy load, the channel should immediately trip.