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Curiosities: Inside a lab-grade power supply from 1970s

2017-08-22 01:17:11,  In: Electronics, Curiosities

Some time ago I was looking for an used adjustable power supply unit. Finally I had a possibility to purchase a 300W adjustable (0.1-30V, 10A max) power supply unit for a relatively lower price. The only thing about it was that it was a unit from early 1970s, transformer-based, 25kg of hardware. I finally got it, brought home and decided to restore it.

It is a ZTR-1/71 power suply unit made by Inco in Poland. In 70s and 80s, Inco (Full name: Zaklad Produkcyjny Aparatury Elektronicznej INCO) was known manufacturer of measurement devices, lab equipment and all "special" production e.g. for military, intelligence or security offices (also devices for communication, location of radio stations or radar applications), so I expected quite interesting things inside. It is still possible to get their old catalog on Silesian University library's site. As always, the first thing to get is a service manual, in all older lab equipment the manual contains not only schematic and disassembly instructions, but also information about typical maintenance actions. Let's look at the front panel. It has a power switch, power light (using a small incandescent bulb), ammeter, output connectors and 4 dials for selecting current limitation (100mA, 1A or 10A) and 3 for decade voltage setting (0.1 to 30.0V). I found that sometimes under heavy load when voltage is decreased, the overcurrent alarm may buzz for a fraction of a second. According to manual, this is normal behavior. I opened it and cleaned.

After removing screws it was possible to lift the top cover off. Here is the photo of inside:

In the right part, a large transformer can be seen. Many wires are coming from it as transistors are used only in the final stage of voltage regulation - the coarse voltage is set by selecting transformer's windings making the transistors produce less heat. Near the transformer there is a fan. Yet, it's a 230V AC motor with blower pushing air into large aluminium plates, fortunately it is turned on only when 10A current is selected. In a service manual they recommend periodically checking it and lubricating its bearings with a drop of oil!

This picture shows a bottom of the unit. Two things can be spotted there: First is a part of rectifier made of a powerful green diodes. The second thing is a battery of capacitors having 16000uF. I measured them and... they are still in a good condition, although they have been made in November 1973.

You probably noticed that this power supply unit has an ammeter on it. This thick coil is its shunt. Usually they were made using a ribbon-like conductor to minimize inductivity, here it's coiled.

After removing rear panel, you can see the heatsink from the rear. Each transistor is mounted on its plate and they are connected on sides using panels. To minimize interferences, short connectors, even using emitter resistors are used.

This is something interesting. We can see the speaker, which is a part of overload alarm circuit, a blower and heatsink. In the plates there are a few transistors missing. Why? Service manual explains these spaces. This particular revision was made using newer technology, in which ADP672 transistor...

...drives 4 2N3055 transistors. So we have 5 plates, each one for one transistor. However, this is not an initial version built long before 2N3055 became accessible. What was used instead? 12 TG72 germanium transistors. Yes, 12. Yes, Germanium. You can see photos of this version in this forum post, because Ge transistors have different voltage drops, transformer's windings seem to be wired differently too. This is more modern version which uses silicon transistors for regulation and I think that with more modern regulation circuits, a good 3A more current may be squeezed from this transformer.

This picture shows general view on internals of the unit. Knowing that the motor is literally the same one as used in typical hand-held hairdryer you can see how big (and heavy) the transformer is.

This is probably an auxiliary rectifier built around 4 DMG3 diodes. Although 2N3055 are Silicon, these diodes are still Germanium diodes. These diodes have a "3D" manufacturing code which means, in notation used by Polish manufacturers these times, April 1970. Notice that these heatsink-mounted diodes have been manufactured usually in two types: One had anode on package's chassis, another with cathode. With these types it was much easier to construct bridges using single heatsink.

This is a brain of the power supply unit - a regulation board. It consists of some poorly-looking electrolytic capacitors (yet I haven't found any problems with the unit), metalized MT resistors (popular these times in Poland and most Eastern block countries), some native, some imported from USSR, and mostly Germanium semiconductor technology. An interesting thing is the box enclosed with styrofoam walls and pressboard cover - part of the circuit responsible for voltage reference and error amplifier must be in thermal consistency for exact characteristics, so it is not only shielded from environment, but kept in still temperature using a germanium transistor as heater, while another one maintains characteristics. This is a pure thermostat set to maintain temperature needed to get component's characteristics proper.

The last photo shows additional shunts for detecting overcurrent in lower-current ranges. Notice that some resistors have been made by OMIG plant, known from their quartz oscillators and integrated generators. They still exist and they make quartz oscillators and filters.

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