Delta D015-1.5 lab power supply (1965)

Delta Electronics is a Dutch electronic equipment manufacturer renowned for their quality power supplies. When I studied electronics, their power supplies were all over the place in the labs. Their history goes back to (at least) the early sixties. This D015-1.5 is a sample of their history. The type number indicates it has a voltage range of 0-15 V at 0-1.5 A. Both voltage and maximum current can be set. A 10-turn helipot allows for precise setting of the output voltage. It has a nice sixties-style meter switchable between current or voltage.

On the inside, it uses silicon transistors, very modern types for the time:

• 2N3055 as series regulator;
• 2 2N3053's as driver and current source for the reference zener;
• 5 2N3703 small signal transistors in the differential amplifiers;
• a number of silicon rectifiers and zener diodes.

I was given this power supply by a friend. It came from a laboratory at the company where he is working. It had been sitting in his attick for decades and was a bit neglected. First, the front was quite stained. The meteal case was bent a bit and the meter front had broken off. Apparently, this instrument had had a great fall. I discovered a few interesting testing and callibration stickers on the back, revealing some of the history of this instrument. The were on the metal cover that protects the power transistor. These must be from the inventory control department of the lab where this instrument came from. The first one was dated 1982. The glue holding it was dried out, so it was easy to pry it off. From under it came a sticker dated 1967, excitingly old. There is yet another sticker under this one, but the glue holding it is hard and strong so I won't be able to get the 1967 sticker off without damaging it. I decided to leave it.

I glued the meter and cleaned the stains. Then I opened the case and made some of these photographs. Fortunately, somewhere in a corner of the Net there was a manual, so I was confident I could fix this one. I connected the power supply to mains and switched it on. Checked voltages and surprise! Everything was working.

There was some room for improvement, though. The output stopped at 14.5V and the limiting current control was flaky. At both ends of its range, the output voltage suddenly dropped. I suspected the potmeter was corroded, or worn. Looking in the box of wirewound pots, I couldn't find a 100% suitable replacement. I decided to take it apart and see if I could fix it. It turned out that the resistance wire looked very good. But at the ends you could feel a sort of click. The multimeter confirmed that at both ends of its range, contact between the wiper and wire was lost. This was caused by the wiper of the pot hitting one of the rectangle shaped nuts that hold the ribbon around which the resistance wire is wound. These were a bit misplaced so the wiper was lifted off the resistance ribbon. I don't really understand how this failure has originated. The nuts are made of metal so they should make contact. Maybe simply the nut has developed a thin layer of nickle oxyde breaking the contact. It is highly improbable that the nuts had moved at the impact of a drop that had damaged other parts, as the nuts are simply too light. Maybe a bit of contact spray might have done the job, but I avoid that stuff whenever possible. I loosened the nuts a bit and shifted the nuts and the resistance ribbon so that the wiper always was in direct contact with the resistance wire. After this, the current limit control worked fine.

In order to correct the range of the voltage control, I had to adjust one of the trimpots on the main PCB. After this, this almost 50-year-old power supply was working fine. It was redundant in my workshop but it found a useful place in another friend's hobby workshop.