[WaveyDipole]

I tested it on 240v today, first with a lamp limiter, and having been satisfied with that, then directly on the mains. I got approximately the correct voltage reading at the heater (6.8vAC), slightly high but in the right ball park. The DC voltage across the smoothing cap does seem rather high at 167v, but I had no indication that anything was being stressed. The valve heaters glowed red at about the brightness they should do.

I followed the calibration procedure and found that DC+ and DC- zero balance nicely and the voltage readings seem quite accurate on all scales. I used a known DC power source rather than a battery. The AC balance was also readily set and I could achieve equilibrium between the AC, DC- and DC+ zero settings, but the AC calibration control just barely allows me to set the correct AC voltage with the wiper at far left. I used a transformer rather than direct mains as per the instructions, and measured the output from this using a DMM and then tried matching that with the VTVM. It is by no means exact but is safer than prodding the mains directly.

The calibration controls seem a bit finicky and do seem to drift, although I might have allowed a longer warm-up time before making adjustments. The Ohms Adj control also seems a little temperamental about the infinity mark although I haven't squirted anything into the pots yet to clean them.

Both DC and AC voltage readings seem to compare well with the DMM, but the resistance readings are a little inaccurate (e.g. 4.7k reads 5.5k). This might be down to the fact that the pointer does not seem to fall back to zero when the probe is touched to the common lead after setting infinity. Naturally there would be some lead resistance of an ohm or two registering at x1, but at x1k it ought to be zero, yet it fell a little short of that by a minor mark or two. The battery reads over 1.5v on my DMM so should be good. I also noted the position of the switch on the probe and made sure that is was set to AC/Ohms for Ohms measurements.

The mains cable has now been replaced with a proper flex. I found a barrel type fuse holder, but installing it would involve cutting a hole in the case, which I didn't want to do. Installing an IEC connector might have been another option were it not for the fact that it can't be easily accommodated on the rear plate, and it would be impractical to mount it on the rear outer case, so I will leave it as it is. Once the unit is re-assembled it will be fitted with a fused UK mains plug anyway.

I guess the next step is to check resistors particularly in the AC and Ohms paths. A suitable capacitor is on order for C1 and will be fitted when it arrives.