Test Equipment Anonymous

I took the opportunity to do a repair job on the Elenco XP-720 power supply that smoked when I was testing the Fluke D-802 multimeter that came in a while ago. I thought it would be fun to use the Fluke D-802 to troubleshoot the power supply that failed while I was testing it since it'd give me a chance to put the meter through more of its paces.







First step was to sanity check the D-802 on a good diode to make sure that function worked. The 34401A agreed as well. I attribute the lower diode drop on it to the fact I had been holding the diode under test and warmed it up. The next step was to go down the column of diodes in the Elenco's 5V supply rectifier.



All four checked out fine. So, the capacitor wasn't exposed to alternating current from failed diodes and didn't short out hard and destroy the diodes. I don't have any pictures of that test but I did do a resistance check on the capacitor and it wasn't shorted out.



I managed to get the board off the standoffs and shifted away from the case far enough to get in with my old high school soldering iron at a pretty shallow angle. I didn't want to chew up the tip on the good iron on this or go to the trouble of unsoldering all of the connections on the board to the external parts and then resolder them all again after changing out the capacitor so the old iron and board more or less in place was what I decided on. Some heat and solder wick later the bad capacitor was out.







The Agilent U1231A was the nearest meter with a capacitance function and 870 uF is not 4,700 uF by a long shot. So this capacitor didn't short out but it did vent and smoke and lose most of its capacitance. I'll stick it on the LRC meter later and see what its parameters look like. The replacement was within tolerance so I soldered it in and trimmed the leads.



I set the board back down on top of the standoffs but didn't clip it down on them in case more work on it was needed depending on how testing went.



And success. The power supply didn't smoke and the five volts came up the way it should. I haven't clipped the board back down or reassembled the case on it yet because I want to perform some loading tests and take some measurements in there while I have it open and everything's accessible but the preliminary results look good for both the XP-720 and the Fluke D-802.

Guaranteed if you’d snapped the board back onto the standoffs before testing there would have been additional problems and you’d have had to disengage the clips again. Nice job of thwarting Murphy!

-Pat

An account of a nice simple repair is strangely heartening.

Zenith wrote: ↑Tue Feb 28, 2023 12:41 pm An account of a nice simple repair is strangely heartening.

It is, it's nice to have a break sometimes when it's straightforward like this instead of a rabbithole like this friend's Nordmende radio that I need to dig into sometime in the next few days.

I decided to use the big 7.5 ohm rheostat as a dummy load to test out the power supply before putting it back together:





Connections taken off to various test equipment. The banana plug leads are for the current loop through the rheostat and 3478A multimeter, the spiked leads are connected to the 3457A multimeter which is observing voltage and the alligator clips go off to the Tektronix TAS 485 which I had just racked and wanted to being trying out.



Rheostat fully in circuit drew a current just over half an amp with the supply giving 4.97 V.



I loaded it up to 2.01 amps and the power supply sagged to 4.87 V but I didn't take it any further for two reasons. I was beginning to run out of space on the rheostat and didn't want to suddenly put a dead short on the supply by accident and second, the 3 A nameplate rating on the supply leaves no margin for error both with the supply where the rectifier diodes are rated at 3 A and the highest current range on the HP 3478A which is also 3 A. I didn't want to take all three pieces of equipment right to their limits unnecessarily.



The picture shows observing the DC off the power supply. I did play around with the TAS 485 a bit while I was drawing the 2 A current but this really is not the best scope for observing ripple.



The capacitor that smoked has capacity of 233.5 uF still out of the original 4,700 uF.



It makes up for the small capacity with huge ESR though. 29 ohms.





I meant to measure the replacement I installed before soldering it in but forgot to so I measured off the second 4,700 uF, 25 V capacitor in the package I bought. The capacity is there, the ESR isn't, and the dissipation factor and phase angle of the replacement are both far more respectable than what were measured on the faulty one.



So, after all that, I deemed it good and buttoned the Elenco back up and reinstalled it in its spot on the equipment shelves.