HP 427A
Posted: Sat Nov 30, 2024 3:57 pm
This came from a swapmeet for £10. I didn't notice the flaking scale, which is something to look out for on all HP meters of this period.
There was a mains/battery option, but this one is battery only operation. The battery is a 22.5V EverReady No 763, which is now obsolete, but substitutes are made and cost $70! Someone had modified it to accept 22.5V from an external source via two Wander sockets, which are a 3mm version of a Banana socket. Wander plugs and sockets were used on radios in the 50s and 60s but are now less common. They can be bought from ebay and I think I have a few somewhere.
The first thing was to remove the covers and check it over for anything damaged, disconnected or missing. Then power it up with a bench power supply set to 22.5V and check it out.
It uses a regulated split power supply which was producing + and - 6.8V, which is in the required range.
The battery check was OK.
DC Volts produced about a half reading. It turned out that a push on connector to one of the boards was unplugged.
The DC Volts were more or less on track, apart from that when the needle was pushed to FSD it became stuck at around 90% FSD. It was caught in the flaking coating. These HP meters have a scale which is printed and covered in some sort of lacquer. With age and damp, the lacquer flakes, curls and breaks up. The meter had to be removed and the flaking coating dealt with. To avoid damaging the needle and contaminating the mechanism, the scale was removed. The flaking coating was removed with a soft bristled artists brush and a dissection needle and taken back to non-flaking material. The underlying printing has the permanence of dirt and previous experience shows it is very easily erased.
I decided to spray the scale with clear lacquer used to fix charcoal drawings. The scale is glued to the backing, which is unfortunate since if they could be separated there'd be no chance of spraying the mirror. Paper masks had to be made to shield the mirror. The top arc of the mirror has a radius of 54mm and the bottom one, 50mm. The scale was scanned using a flat bed scanner as a precaution. With the scale sprayed, the meter was reassembled and checked and then reinstalled.
The DC/Ohms infinity pot was erratic. It can be removed without too much dismantling. It was soaked in IPA for a few minutes, then exercised, dried out and replaced. The wafer switches were cleaned with switch cleaner and a cotton bud.
Next the ranges were checked roughly. AC Volts didn't work. There was next to no voltage across the AC metering circuit when the AC range was selected. After some head scratching this was traced to a 100µF 12V electrolytic, which was short circuit. Fortunately that could be replaced without removing the board.
All functions were checked and adjusted as per the manual against modern instruments. It just about performs to the specification in the manual.
The original battery has a capacity of 1,700 mAh, but modern replacements are too expensive to consider. It could be run on two PP3s with a capacity of 500mAh??. It could be run on AA cells in suitable adapters. It could be adapted to use an internal power supply, but it would need to be well shielded. The easiest option is to power it from a bench power supply using the Wander sockets which were already fitted by a previous owner.
It's a solid state version of a VVM (VTVM). I believe it was in production from the mid 60s to the early 80s. It would have been lighter and smaller than VVMs, and battery operation could have been a big advantage. It has an input resistance of 10 MOhms. The AC range is average responding up to 1MHz. It's a classic HP instrument.
It was interesting to play with but I doubt I'll use it much. It doesn't do anything that can't be done better with a DMM or bench DMM, or another analogue meter.
There was a mains/battery option, but this one is battery only operation. The battery is a 22.5V EverReady No 763, which is now obsolete, but substitutes are made and cost $70! Someone had modified it to accept 22.5V from an external source via two Wander sockets, which are a 3mm version of a Banana socket. Wander plugs and sockets were used on radios in the 50s and 60s but are now less common. They can be bought from ebay and I think I have a few somewhere.
The first thing was to remove the covers and check it over for anything damaged, disconnected or missing. Then power it up with a bench power supply set to 22.5V and check it out.
It uses a regulated split power supply which was producing + and - 6.8V, which is in the required range.
The battery check was OK.
DC Volts produced about a half reading. It turned out that a push on connector to one of the boards was unplugged.
The DC Volts were more or less on track, apart from that when the needle was pushed to FSD it became stuck at around 90% FSD. It was caught in the flaking coating. These HP meters have a scale which is printed and covered in some sort of lacquer. With age and damp, the lacquer flakes, curls and breaks up. The meter had to be removed and the flaking coating dealt with. To avoid damaging the needle and contaminating the mechanism, the scale was removed. The flaking coating was removed with a soft bristled artists brush and a dissection needle and taken back to non-flaking material. The underlying printing has the permanence of dirt and previous experience shows it is very easily erased.
I decided to spray the scale with clear lacquer used to fix charcoal drawings. The scale is glued to the backing, which is unfortunate since if they could be separated there'd be no chance of spraying the mirror. Paper masks had to be made to shield the mirror. The top arc of the mirror has a radius of 54mm and the bottom one, 50mm. The scale was scanned using a flat bed scanner as a precaution. With the scale sprayed, the meter was reassembled and checked and then reinstalled.
The DC/Ohms infinity pot was erratic. It can be removed without too much dismantling. It was soaked in IPA for a few minutes, then exercised, dried out and replaced. The wafer switches were cleaned with switch cleaner and a cotton bud.
Next the ranges were checked roughly. AC Volts didn't work. There was next to no voltage across the AC metering circuit when the AC range was selected. After some head scratching this was traced to a 100µF 12V electrolytic, which was short circuit. Fortunately that could be replaced without removing the board.
All functions were checked and adjusted as per the manual against modern instruments. It just about performs to the specification in the manual.
The original battery has a capacity of 1,700 mAh, but modern replacements are too expensive to consider. It could be run on two PP3s with a capacity of 500mAh??. It could be run on AA cells in suitable adapters. It could be adapted to use an internal power supply, but it would need to be well shielded. The easiest option is to power it from a bench power supply using the Wander sockets which were already fitted by a previous owner.
It's a solid state version of a VVM (VTVM). I believe it was in production from the mid 60s to the early 80s. It would have been lighter and smaller than VVMs, and battery operation could have been a big advantage. It has an input resistance of 10 MOhms. The AC range is average responding up to 1MHz. It's a classic HP instrument.
It was interesting to play with but I doubt I'll use it much. It doesn't do anything that can't be done better with a DMM or bench DMM, or another analogue meter.
