A while ago I replaced the battery in my HP 3478a multimeter, this battery is in charge of keeping the calibration values inside an SRAM. Since I norally have the multimeter plugged into the wall anyhow I decided it may be a good idea to add a standby power supply such that the battery doesn’t drain that much (other than its internal resistance). So I did. There is a 78L05M generating a 5V rail, and I also added some super capacitors for good measure that on their own will keep the SRAM supplied for >1 week without the battery.
Just a quick and small project, I put an electronic transformer into an enclosure. It’s my favorite EuroBox enclosure, front and backpanel were 3D printed. Update 20.01.2023 I replaced the lamp driver with an old 12V halogen transformer I had sitting around. For further protection I added a polyfuse to the secondary side.
During full load some resistors on my VEB Statron 3224 started to smoke, so let’s investigate. The power supply is a fairly simple construction using a LM723 and a large pass transistor. The first two resistors are for clipping the maximum current on the 8.5V (4A) range. It’s the two large resistors at the top left of the ciruit board. The two resistors are 2R7 and 0R82 in parallel, giving an equivalent resistance of 0R62.
Replacing the old RIFA capacitors Older RIFA capacitors are known to crack, which results in moisture getting into them which subsequently lowers the internal insulation resistance which at some point results in catastrophic failure. I had a couple of those explode on me in the past. On the HP 3478a the RIFA capacitors are mounted before the mains switch meaning that they are live whenever the multimeter is plugged in. Which means that unless those have been replaced I would not leave the meter plugged in without supervision.
So I recently got a Keithley 196 digital system multimeter with 6 1/2 digits which was sold as “broken - parts only” for virtually nothing so I figured in the worst case I have something to salvage parts from. After it arrived I tried to power it on - nothing. Line fuse was okay, so I started checking the power rails, on the analog board the +15V, -15V and +5V were fine.