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Rebirth of an Old Friend - Weir Minireg PSU

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awemawson:
I've had this Weir MiniReg 402 power supply for YEARS and it has been unbelievably useful. 0-30 volts at about an amp, regulated voltage or regulated current. My first job was with Mullards in Southampton and they had the enlightened notion that anything technical that was being scraped could first be offered to staff - it was working but slightly battered, and was mine for £5. That was a lot of money to me then (1968-9) as my first weeks wage was only £7 !

It has served me well, but a few years back the meter gave up the ghost. (Upper hair spring came detached from the armature) so in recent times I've always checked it's voltage with an external meter. Then the PSU gave up totally - no output. I got as far as finding a circuit for it, but then it just sat by my desk patently awaiting attention for at least six months.

Now I have an eBay search set up for these PSU as they are very handy, but most don't have the multi-turn voltage setting pot - just a single turn. However one popped up as 'spares or repairs' with the multi-turn pot. I won it for £13 intending to use it as a doner to get mine going, but when it arrived it was fully working  :clap:

This shamed me into opening up my original one and doing a bit of fault finding. The meter issue was a badly tangled hair spring that was detached at the armature.  Deadness was only a dry joint on the PCB and soon fixed, but what about the meter?  It is a fairly standard 0-1 mA  45 mm square meter but actually not that common unless bought direct from China. CPC had a 0-500 micro-amp one in the same case size so I bought it intending to change the scale plate and put a shunt in it to bring it back to 1 mA sensitivity.

What they actually sent was a 45 mm square 0-30 volt DC meter comprising a 0-1 mA movement with an internal series resistor to give the range.

Time to pull a few bits apart and see what I can assemble from the parts:

awemawson:
Now although very similar the two meters differed in a few important details. Firstly the scale plate mounting holes were in a different location, then the scale plate 'notch'  was different, then finally the rear terminals were threaded 6 BA sockets on the original and M4 studs on the new one.

Old scale plate was re-drilled and it's notch re-formed to approximate the new one. I then put it back together and hung the meter on flying leads to check it's calibration. I had to alter the series scaling resistor on the small rear PCB from 27 k to 22 k to bring the scale in range of the pre-set calibration resistor.

Then how to mount the PCB that sat on the back - those M4 nuts would short other tracks on the PCB. I ended up putting a sheet of insulation on the mounting screw and bringing the meter connection out on flying leads and back onto the rear of the PCB to solder tags on the M4 studs. Seems to work :thumbup:

As you can see from the last picture, I've accumulated a few of these Weir PSU's over the years  :lol:

John Rudd:
Circuit design typical of 60-70's technology....simple but effective... :zap:

I have a couple of 0-30v 3A dual ch power supplies with digital meters that I use for test/experimenting with, they can be connected to give me 0-120v at 3A if I needed it..

awemawson:
They are definitely 'old technology' but they work !

The series pass regulator transistor is an OC28 PNP Germanium T03 that was even going out of fashion when I started dabbling  :clap:

That circuit is actually more complex than it looks at a first glance.


Later edit: Working it out, if I got it in 1969 and it was 'old' as far as the lab was concerned then, it's WELL over 50 years old - quite amazing  :bugeye:

PekkaNF:
That circuit diagram definately looks old school - the time that componenets were relatively expensive and design was relatively cheap. Analog designers too a pride in making high sprited circuits (usually pretty innovative, but not always clear, because componenet count was a thing). Series regulator et.al. looks textbook, but then there are some biases and down to floor stuf that I don't understand completely. Maybe aim was to get very close to zero on voltage and pretty low current too.

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