Hello,
This is my first post here. I've been watching many EEVBLOG episodes, and Dave's enthusiasm has rubbed off on me. His video blog has re-kindled my interest in electronics, and I am trying to learn and have some fun. To that end, I am attempting to repair a power supply from 1976. It is a home brew job, and I believe it was part of an electronics course offered by Westinghouse in the mid-1970s in Pittsburgh, PA.
The power supply looks to be a fairly low-current (1 or 2 amps max) zero to 25 volt DC unit, with adjustable voltage and current limiting. There is an analog voltmeter, a power indicator lamp, and a current limit LED which is unfortunately lit 100% of the time. It has an on/off switch, voltage, and current controls (both are multi-turn wire-wound pots) plus two banana jacks for the output.
The circuit has a transformer, a full-wave bridge rectifier made of discrete diodes, and 2300 microfarads of filtering. There appear to be voltage regulator and current limiting circuits, but that is about the limit of my circuit knowledge. The active devices include a 2N2905A, a 2N3638, a 78MGT2C which appears to be a regulator. There is also a TO-3 case pass transistor with the markings "466 068001 6621" which I have not been able to identify yet. Testing it with a diode tester shows it to be an NPN type, and the B->E and B->C junctions test OK, biasing in only one direction.
I have attempted to sketch a schematic (see attached) but I don't know if anybody besides me would be able to read it. It's my first attempt at drawing a schematic from a piece of hardware. I could also take some photos of the board if that would be helpful. I tried in vain to find a schematic for it online, but the power supply is sold old that it's probably long forgotten. There are two devices that I am calling RF chokes, but they might be something else. They have some color bands, but the final band is a blue "T" shape. They look like resistors, but have what appears to be wrapped glass case. Any help identifying these buggers would be great. They measure about 15mH on an LCR meter, but the colors on the bands don't seem to correspond with anything I can measure.
The supply puts out proper voltage, and the variable voltage control works. I know that the limit LED should not be lit all of the time. It is very likely that this supply was connected to a load beyond its limits, as it was used in an educational lab setting. I have tested all of the passive devices in-circuit, and I didn't find anything obviously wrong. I could use some help with the following: 1) identifying the pass transistor type 2) identifying the devices with the blue "T" and 3) suggestions on what to test next to get the limit circuit working. If anybody can identify the circuit and perhaps point me to a real schematic, that would be great of course.
Thanks for listening,
Zapped