Advance VM78 sensitive AC millivoltmeter

[Phil G4SPZ]

Yet another piece of vintage test gear back in working order. I’ve had this neat unit sitting on a shelf for a while, waiting for me to get a round tuit. A recent fault in my ‘other’ AC millivoltmeter prompted me to look at this Advance meter in more detail.

The first issue was the batteries, as this unit is battery powered only. The original batteries were Mallory TR-233R 4 volt and RM-3 1.4 volt mercury cells, long since unobtainable, housed in a small internal compartment and arranged in a series string but with several intermediate tappings to create several + and - supply rails. There was insufficient space within the battery compartment to fit modern cells easily, but there was plenty of room elsewhere in the cabinet for an alternative battery pack. I used nine 1.2 volt AAA-size NiMH rechargeable batteries housed in two 4-cell and one single-cell holder, wired together and mounted on a piece of perforated board. I didn’t wish to drill or modify the chassis or cabinet, so I used double-sided adhesive pads to secure everything in place.

I ran through the setting up procedure as detailed in the manual, and the meter came to life after a fashion. The zero wouldn’t settle, the meter pointer swinging wildly about even with the terminals shorted, but this was quickly cured by thoroughly cleaning the fuseholder contacts and the ends of the 60mA fuse. Everything quietened down nicely then, and all the ranges appeared to work. However the FSD was about 5% low, even with the overall gain control RV44 at maximum. The transistors are mainly germanium PNP types so I was a bit worried at first, however I was able to adjust RV36 to achieve the correct 2.2 volts at the collector of TR7, although the voltage across the associated resistor R41 was only about 0.4 volts instead of the specified 0.6 volts. Then I spotted that someone had previously replaced R41, but with the wrong value - it was 560 ohms instead of 680 ohms. Once the correct value had been soldered in and RV36 re-adjusted, the voltages all returned to normal and there was adequate gain to enable the meter to be accurately calibrated. Its claimed accuracy is +/-3% of FSD but I achieved better than 2% on most ranges, and more importantly it is absolutely linear between 10Hz and 100kHz, and it is also very versatile, having twelve ranges covering 300 volts to 1 millivolt FSD and a dynamic range of over 110dB.

The cabinet looked rather sad, due mainly to numerous sticky marks and residue from sellotape, labels and Dyno tape over the years. Why do people think it’s OK to plaster test equipment with stickers? I can understand calibration labels... anyway, most of these came off with various solvents, apart from the top of the case where some clot had at one time glued something in place with what appeared to be Evo-Stik, which defied all attempts to remove it. So I have added a label of my own to cover the disfigured area, based on a photograph taken of the maker’s name and model legend on the front panel. I cropped this in MS Paint, resized and printed it out onto matt photo paper, cut it to size, covered it with 2” wide clear adhesive tape, trimmed the corners and stuck it down with Pritt Stick.

This meter is easy to work on, and at 45 years old has retained its accuracy commendably well. In my view, there’s little point in trying to restore test gear to showroom appearance, and this unit has some residual damage around the front panel where pieces have cracked away at the top corners of the plastic bezel, probably due to rough handling, but the finished article is cosmetically quite acceptable and a really useful addition to the workshop.

Phil

[Phil G4SPZ]

And here is the case, before and after I fitted its new label.

Phil

[Kevin Hoyland]

Hello Phil.

I always enjoy your Threads regarding repairing Test Equipment, Keep the good work up.
Kind Regards
Kevin.

[unitaudio]

Maybe not quite showroom fresh but very tidy looking indeed. Excellent stuff.

Regards,
Paul

[camallison]

Quite often, it was the fashion to stick traffolyte "Asset Labels" on the top, so that may be what you have now covered admirably. Although I see a Ferranti label - is that a cal sticker or asset sticker?

Colin

|A dolop of margarine over the evo-stik and left for ages (weeks) often softens it. Same applies to most self stick labels too.

[Phil G4SPZ]

Thanks for all the kind comments, chaps. It's a cal sticker, Colin, though the details that were once written on it are illegible now. Anyone know who "CHD Calibration Laboratories" were?

I suspect that stickers were affixed in some cases in an attempt to make the equipment less desirable to those who would try to liberate it! Organisations that were really serious about deterring theft would engrave equipment in highly visible places. It didn't stop it walking, though. I have several Avometers with the owning company's name moulded into the front panel, one engraved with a soldier's name rank and serial number, and a 1930s radio marked "Property of London Rentals"!

Thanks for the margarine tip, Merlin, that may come in useful sometime.

Phil

[AC/HL]

Could be Chadderton?