Solartron 7150 Plus

[Zenith]

I already have one, which is currently on the operating table and under the knife. It does strange things when powered up, sometimes it powers up properly and sometimes it just produces strange patterns on the LCD display. What's shown on the LCD changes when the unit is tilted. I suspect it's probably just a bad connection. It also has the two dim yellow LEDs which make the display hard to read and which need to be replaced with white LEDs. The complicated and expensive IEC connector with filter, fuse and mains voltage selector has failed and needs replacing. It is a work in progress.

I bought a second one from synx508 at the Rugby swapmeet last Sunday. He assured me it works and he has an honest face.

I tried it and it does work.

I have a fairly new Brymen 867s and a cheap voltage standard based on a Maxim MX584KH, which are in agreement within their specified limits. The Solartron's readings came out about mid way between the two.

I have a few Weston Standard Cells I bought a couple of years back for £3 for the lot. I wasn't going to pay any more than a very small amount, because they are very easily damaged. Also they are made of highly toxic materials, cadmium and mercury, and you take on something of a responsibility to dispose of them as hazardous waste. They really aren't supposed to supply any current at all. Connecting them to a 20k/V meter will certainly degrade them. It's easy to imagine someone doing that somewhere along the way. I did check them briefly with a 10MOhm input resistance DMM. I gather that this is just about acceptable, and there's a procedure for measuring their internal resistance along those lines.

Anyway, the Solartron 7150 Plus has a 10 gigaOhm input resistance on its lowest ranges, which should have the standard cell providing 100 pA, and that's generally considered to be fully safe. So I tried it against the standard cells. A couple of the units have two standard cells. I think they for were some balanced arrangement.

DSCN4003.JPG

DSCN4005.JPG

The results in Volts are as follows, from left to right..

1.018572 &
1.018579

1.018493 &
1.018510

1.018461

1.018540

They have thermometers and the cases are supposed to be filled with an inert mineral oil, as a thermal buffer, to prevent rapid changes in temperature. They are temperature sensitive and there's a formula to correct from the voltage certified at 20C to the voltage expected at the current temperature. They were supposed to be left to settle for a time before use, if they were moved.

There's a fifth unit shown in a box with two standard resistances, which came in the same lot.

DSCN4004.JPG

That reads 0.915877 V. It's useless. When they were in use as a practical standard, they tended to drop very slightly in voltage with age and had to be sent off to a lab for calibration. The lab would have extra carefully managed and compared standard cells. If the voltage of the cells they were sent had dropped below 1.0183 V the lab wouldn't certify them, as they were considered too unstable.

An enormous amount of R&D went into Weston cells. They had to be treated very carefully and they must have cost a fortune to buy and maintain. It's a fascinating piece of technological history.

It was interesting playing safely with the standard cells. I suppose they'd be a good enough voltage reference which could be taken as 1.0185 Volts, although not particularly practical. I don't know where the Solartron fits in with this, as it could have drifted away from its calibration slightly. I suppose it's the most accurate piece of equipment, and the standard cells have drifted down very slightly. I believe when they were freshly manufactured, they had a voltage which varied slightly around 1.01865 Volts. There were two sorts of standard cells, saturated and unsaturated. It's interesting that the cells in the double units are quite close, probably because they were kept in identical conditions throughout their lives. There are probably all sorts of things you have to do when measuring voltages to a resolution of a few µV to avoid spurious results.

I don't think volt nuttery is a good thing to be drawn into and I've ventured onto a slippery slope.

[tggzzz]

I don't think volt nuttery is a good thing to be drawn into and I've ventured onto a slippery slope.

Saturated Weston standard cells were my gateway drug

Unlike unsaturated cells, saturated cells can be used as thermometers.

[synx508]

I've never calibrated the Solartron, it's as I got it from Stewart of Reading in that respect, all I've done is change the backlight LEDs because the yellow was useless. I sold two 7150s on Sunday, the other was a non-plus version that I've had for about 30 years. That older one was slightly closer to "correct" calibration than the plus, but IIRC they both drifted towards each other after a while, but that resulted in the older one being further from my two HP 3456As and the plus version being closer but never quite agreeing.

One thing I would've liked to have tried on the 7150 was to flip the LCD polariser, which I think would've looked excellent with the blue-green backlight LEDs, but I think it's glued and not easy to flip. I was planning to reduce the brightness a bit, too. It's strangely hard to photograph that green, it looks a lot more evenly lit in real life.

I am pretty sure I changed the AC inlet receptacle/filter on one of the Solartrons but I think it was the non-plus version, so it's probably worth keeping an eye or nose on that.

[Zenith]

I've bought from Stewart of Reading a couple of times and found him to be a straight shooter, which seems to be the general view.

The original yellow LEDs on the 7150 plus which is in bits, were almost useless. I found some white LEDs and played about with them with a bench PSU and 1K resistor and found they were very bright at 10mA. The problem is with it in bits and the manual putting the unregulated supply between wide bounds, it's awkward to get the ballast resistor right. Some it is bound to be a matter of taste, and trial and error, anyway. I don't want to wreck the LEDs and have to order more.

IIRC that mains inlet/fuse holder/voltage selector is still made but is very expensive - over £30 I believe. On the other one it's definitely suffered and was about to replace it with an unused looking one from the collection with socket and fuse, and hardwire the instrument to 240V. Oh, and finish it off with a blanking plate. Now I'm wondering if it might be possible to open up the old one and replace the dead caps. Generally those filters look as if they were sealed for good in the factory.

[bd139]

Just did my 7150 a couple of weeks back.

I bought a new filter from Mouser which was fine. They sell new ones. Yes they aren't short of £30 but it's that or burn the house down so I figured it was a good trade off.

I did use the genuine correct HP LED parts, which are still sold by Broadcom now (!). The display is barely visible piss yellow as expected. Don't do what I did

[Specmaster]

Just did my 7150 a couple of weeks back.

I bought a new filter from Mouser which was fine. They sell new ones. Yes they aren't short of £30 but it's that or burn the house down so I figured it was a good trade off.

I did use the genuine correct HP LED parts, which are still sold by Broadcom now (!). The display is barely visible piss yellow as expected. Don't do what I did

You are better off replacing the standard HP LEDs with high power white LEDs, they make the display so much more useable.

[AVGresponding]

Just did my 7150 a couple of weeks back.

I bought a new filter from Mouser which was fine. They sell new ones. Yes they aren't short of £30 but it's that or burn the house down so I figured it was a good trade off.

I did use the genuine correct HP LED parts, which are still sold by Broadcom now (!). The display is barely visible piss yellow as expected. Don't do what I did

You are better off replacing the standard HP LEDs with high power white LEDs, they make the display so much more useable.