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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Some weeks ago I bought in an auction for very little a job lot of electronic scrap from the Atomic Energy Establishment, Harwell. This included seven Mini-Instruments Mini-Monitor G-M counters, which had been worked to death over a quarter of a century. The tubes in all seven were either broken or missing. Some had no HT, and others had faults in the rate meter circuit. They had been designed to work from two of the obsolete PP6 9V batteries which are now only available from specialist dealers at fancy prices, so my first modification has been to fit an external 18V battery charger PSU.
So that's the first of my seven Mini-Monitors restored to good health. I fitted it with a Centronics ZP1401 alpha beta gamma end window tube, and adjusted the HT voltage set VR to 500V in the middle of the plateau. To count alpha the tube-holder plastic cap must be removed. (The cap is in fact the top of a Russian pine resin solder flux container.)
I'm going to upgrade the next one by taking the HT adjust trim-pot on the PCB out to a multi-turn VR on the back panel, so I shan't have to open the case if I want to change to a different tube.
Further iterations will include a pulse shaper to feed a USB data logger, so the pulse output can be directly graphed on a computer. I shall also add a variable uSv level alarm which did not come as standard on the Model 5.10 but wase later bundled onto the board in the 900 series Mini Monitor.
Out of the original 7 defunct units, I shall have 4 complete upgraded better-than-new Mini Monitors, and three Monitor PCBs for use in other projects.
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Mini-Instruments Mini Monitor G-M Counter Model 5.10 Refurbishment
