Restoration of an old Oltronix power supply

Work In Progress
P.3/5 2024-06-27

Turning it on

I continued reassembling everything and flipped the main switch, and to my delight, it powered on immediately! After some initial testing—turning the knob and setting the sensing switch to internal—I cross-checked the output with my bench-voltmeter. It was off by about 200mV across most of the 0-60V range (though a few spots were further off). Still, for such an old, inexpensive find, it was more than usable! While this was done with no load, it showed a lot of promise.

supply running for the first

However, during my next round of testing, things took a turn. I powered it off and back on, and... it tripped the breaker. Oops. I gave the supply a quick visual inspection, hoping to spot any obvious damage, like burn marks or charred components, but nothing stood out. I reset the breaker and tried again—same result. It seemed the power supply would need more attention to get it fully operational.

Feeling a bit discouraged, I took the supply apart for a deeper inspection, cleaning up any areas that were dusty, oily, or dirty. Again, nothing stood out as obviously wrong. Testing the main transformer's primary leads, I measured 0.78 ohms. That seemed a bit low, but given the size of the transformer, I figured it was just the winding resistance, which would likely increase under load.

measuring transformer

Moving on to the primary board, I began testing components by desoldering one leg at a time. A few capacitors were way out of spec, so I temporarily swapped them for newer ones. Most of the diodes tested fine, showing a voltage drop of around 0.5V. Despite my efforts, nothing definitive turned up. I did notice a deep scratch on the main board's copper plane, which could potentially cause issues, but I wasn't convinced. I bridged the scratch with solder just in case—it almost looked as if someone had scored the board intentionally.

After all that, I reassembled everything. Realizing that using a 10A breaker might be contributing to the issue, I decided to switch to a 16A breaker instead. I plugged it in, powered it up, and—success! The supply started up without tripping the breaker or blowing the fuse. A small victory!

One thing I forgot to mention: after the supply first tripped the breaker, I had to discharge the large input filtering capacitors before opening it up. The supply has a massive capacitor bank, and I waited about 15 minutes to allow the charge to dissipate before proceeding. However, when I measured the voltage, the capacitors were still holding 40V! I waited another 15 minutes, but the voltage only dropped to 28V. These capacitors were excellent at retaining their charge, especially at lower voltages, and to my surprise, there was no bleeder resistor installed. That was potentially dangerous, given the size of the capacitor bank and the deadly charge it could hold. To address this, I rigged up a temporary bleeder resistor and waited until the voltage dropped to a safer level (around 3V). Only then did I discharge the rest of the charge across the leads using a screwdriver, hearing a small pop as I did. After that, I continued with the disassembly and troubleshooting process. showing underside with caps In the picture above, you can see one of the two rows of the massive capacitor bank located in the bottom right corner.

Prev Page.3/5 Next
[1]
An unexpected piece
2024-06-25
[2]
A first look
2024-06-26
[3]
Turning it on
2024-06-27
[4]
The Manual
2024-06-28
[5]
Leaky discovery
2024-07-06

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