Datron 1071, penultimate installmentOnce I was able to see and probe the PSU circuitry, some worn out electrolytics were swiftly identified, along with a diode bridge which
had suffered badly from the excess current it had been asked to provide. There were also a bunch of tants which were replaced without
testing as some were grossly under-dimensioned for the voltages they were dealing with - a 47µF
6.3V tantalum capacitor on the main
5V rail being a prime example. The 350V 10µF capacitor in the centre of the picture had no measurable capacitance at all if I remember
correctly!
My eye was also drawn to the horrors of the main 5V rail's LM309K voltage regulator.
Quite a few 10xx series meters suffer from this problem. It would seem that Datron used an acid flux when they were soldering the
wires to the TO-3 packaged regulator and fitting it to the heatsink. The acid was not washed off and once the rubber insulation tubes
were added over the wires, the acid could sit undisturbed to do its evil work. This not only caused the rubber to appear mouldy as it
does here, it also slowly ate through the regulator's leads and even migrated along the package's leads to the internals of the regulator.
As you can see, it has even started eating into the anodised aluminium where it seems to have flowed from the rubber tubing onto the
heatsink.
Removing the heatsink to replace the regulator and wiring is made even more entertaining by the fact that there's also a transistor
on the heatsink below the regulator which is soldered directly to the PCB. The transistor is bolted to the heatsink with a nylon screw
and nut. As you can imagine, after some 45 years of baking on the heatsink, the nylon was good and crunchy! As was the hardware
on the heatsink right next to it.
In the end, I ended up replacing the majority of the PSU components. It may not have been strictly necessary for some of them, but
the end result was ripple free and accurate rails and a warm, pleasant feeling from knowing that I will never have to take this part
of the Datron apart ever again!
Next up, the final installment and a classic factory induced problem...