I built this and it worked well. Why the past tense? I passed it on to another forum member! When I was actually servicing in the Trade, it was very useful particularly with VCR power supplies. However when I left the Trade I had no further use for it as I no longer repaired ‘modern’ equipment ( well not very often anyway) and you don’t find zener diodes in vintage (valve) equipment.
Anyone contemplating building this will find that the specified display is not available (it wasn’t when I built it about 15 years ago). I just used an ordinary test meter switched to the appropriate voltage and did away with R3 and R4. Mind you there are probably plenty of other displays available. Once you’ve built the tester, don’t touch the terminals…there’s over 100V across the terminals with nothing across them. The current is very low and won’t kill but it will make you jump…..guess how I found out!