Successfully transferred the circuit from breadboard to PCB and installed, tested and calibrated, works like a charm, no more worries about battery leakage, especially as its a piece of equipment that does not get used all that often and judging by the state of some of them offered for sale on eBay is pretty the same for most owners. This one wont be suffering from battery corrosion in the future
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You can just about read the heater voltage on the HP 3466A (5.385vac) in the background while the F27 (1.504vdc) on the left reads the output from the DC to DC converter and the F27 (6.66vdc) on the right reads the half wave smoothed DC input to DC to DC converter.
Winner winner.... reading VAC, VDC and resistance all on the same jacks without having change input jacks just can't use the original Heathkit RF probe which would shared the DC i/4" socket with the DCV input, and as I don't have a probe, it would be safer to convert to a more conventional way of using a multimeter.
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If anyone else is interested in doing a similar thing, the schematic below is a copy of the hand drawn battery replacement that Heathkit engineers designed and fitted to a lot of meters in the factory prior to shipping according an ex Heathkit employee. I did not use this method however, just dc to dc converter 5-6V in and 1.5v out and fed it with power taken from the same source as the alleged Heathkit drawing via a IN4001 and 1000uF 25VDC cap.
![](https://i.imgur.com/LKtTQqt.jpg)
![](https://i.imgur.com/CFw8ht4.jpg)