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.
HP 427A
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Re: HP 427A
I wouldn't have that much patience with a flaking scale!
I like the concept of using an e-ink scale https://arnweb.nl/gitea/arne/EinkAnalog but I would prefer it if the e-ink display could have a cutout in it
I like the concept of using an e-ink scale https://arnweb.nl/gitea/arne/EinkAnalog but I would prefer it if the e-ink display could have a cutout in it
Re: HP 427A
Like most jobs, it takes a while to work out the problems of doing it. The next time, the problems have been worked out and it's much faster.
There is software for making custom meter scales. I haven't tried any. The other option is to scan a scale and print it. Failed meters on TE can be a real problem. A friend has an AVO CT160 valve tester (now very expensive - £600 and up). The meter failed. It's an AVO 30µA 3250R meter with a special scale. The meter can be rebuilt at horrific cost, or maybe a replacement could be bought - not easily available and also at horrific cost. There's also a fix with a Simpson panel meter which is in production. It costs over £200 and then needs the custom scale. Spending a lot of money on something you are going to open up and mess with doesn't appeal. I made up a board using an LM358 op amp and an NPN transistor as an impedance converter, so that a cheap, modern 10mA panel meter of the right size could be used. He had to print a scale for the new meter from a scan, which was fiddly. It works well, although I should revisit it and add meter protection, although a 10mA meter is much more robust then a 30µA one.
There is software for making custom meter scales. I haven't tried any. The other option is to scan a scale and print it. Failed meters on TE can be a real problem. A friend has an AVO CT160 valve tester (now very expensive - £600 and up). The meter failed. It's an AVO 30µA 3250R meter with a special scale. The meter can be rebuilt at horrific cost, or maybe a replacement could be bought - not easily available and also at horrific cost. There's also a fix with a Simpson panel meter which is in production. It costs over £200 and then needs the custom scale. Spending a lot of money on something you are going to open up and mess with doesn't appeal. I made up a board using an LM358 op amp and an NPN transistor as an impedance converter, so that a cheap, modern 10mA panel meter of the right size could be used. He had to print a scale for the new meter from a scan, which was fiddly. It works well, although I should revisit it and add meter protection, although a 10mA meter is much more robust then a 30µA one.
Re: HP 427A
I just used my 427A (Mains option, and no scaly flakes) to adjust the gain of the outputs of my distribution amp, over in this repair thread.:
That is exactly what the wigglies are good at -- gain / balance adjustments.
That is exactly what the wigglies are good at -- gain / balance adjustments.
Re: HP 427A
Wiggly things certainly have their place for adjustments such as you describe. However at rough count I already had nine before I bought this, including TRMS and RF millivoltmeters as well as a VVM. Then there are four portable ones including a couple of AVOs. So I'm not entirely sure what induced me to buy it.
It didn't cost much. It seemed like a good idea at the time. I hadn't had one before. It isn't very big or heavy. It was fun to play with. If the worst came to the worst, at least the HP screws and other bits and pieces are worth having. Ebay always beckons.
Yours looks to be in a better state than mine.
It didn't cost much. It seemed like a good idea at the time. I hadn't had one before. It isn't very big or heavy. It was fun to play with. If the worst came to the worst, at least the HP screws and other bits and pieces are worth having. Ebay always beckons.
Yours looks to be in a better state than mine.
Re: HP 427A
It is. It only required removal of the Eveready battery and calibration and adjustment. As noted, it has the mains PSU option, and is recent enough to have a C14 inlet, so no PH-163 exorcism required.Zenith wrote: ↑Sun Dec 01, 2024 1:08 pm Wiggly things certainly have their place for adjustments such as you describe. However at rough count I already had nine before I bought this, including TRMS and RF millivoltmeters as well as a VVM. Then there are four portable ones including a couple of AVOs. So I'm not entirely sure what induced me to buy it.
It didn't cost much. It seemed like a good idea at the time. I hadn't had one before. It isn't very big or heavy. It was fun to play with. If the worst came to the worst, at least the HP screws and other bits and pieces are worth having. Ebay always beckons.
Yours looks to be in a better state than mine.
Re: HP 427A
I rather like them. I think the best feature is they use about 2mA when operating unless you're on low ohms mode. Some nice efficient design in there. Also the input JFET leakage is offset by injecting current into the gate. Very clever. Gives the front end divider a stupidly high impedance.
Very useful still to this day when you don't need N digits precision or are watching for settling points.
Very useful still to this day when you don't need N digits precision or are watching for settling points.