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Old 15th Jun 2024, 8:19 pm   #1
factory
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Default Solartron digital voltmeter LM1440.3

Solartron were once well known for their digital voltmeters, the 2300 count LM1420 was quite popular in the 1960s & early 1970s, in this thread I will be repairing the 29999 count LM1440.3, quite a heavy voltmeter at 21kg, it uses a mix of transistors and three or four valves (depending on age), it's one of only a few vintage DVMs that have a range above 1kV (max 2kV).

I've had this DVM for a few years, it was found locally from the estate of a collector, along with two LM1420 and some other vintage DVMs from other brands.

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I've never attempted to power this up, one reason being I didn't have the unusual test lead, a big thank you to forum member to buggies for finding a test lead.

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I've now started to work on this DVM, first things are to check the capacitors with the ESR meter, none were found to be very bad, there are a couple of plastic cased ones from Plessey & TCC that show some signs of electrolyte leakage and will be changed before bringing it up with a lamp limiter.

Another problem that had prevented me working on this, was the ECC86 valve (used in the reference amplifier) that had gone to air, turned out once I knew the USA part number 6GM8 and spent the afternoon searching through the spares I actually have one , a NOS Tek one I got from Sphere in Canada along with some other uncommon test gear valves, the ECC86 was also used a few FM car radios just before transistors took over.

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David
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Old 22nd Jun 2024, 12:34 pm   #2
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Default Re: Solartron digital voltmeter LM1440.3

Turned out not to be much to repair on this one, apart from the cracked ECC86 and two dodgy plastic electrolytics, the only other problems were the decimal point neon for the 2kV range and the 10 ohm resistor for the input low earth/isolation switch.

Here is one of those electrolytics, part of the input circuit which uses electrometer valves, earlier versions of the LM1440 used another ECC86 in place of the 2N2484 that this one uses. The other electrolytic was a 50uF at 150V across the power supply for the decimal point lamps.

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The original decimal point lamps are Hivac 16L, I have a few NOS from a local sale a few years ago, the old neon failed to light with 100V from a Megger and was very dim with 250V, the new ones light below 100V, access was just about possible with some PCBs removed.

The low side of the input can be grounded via a 10 ohm resistor, or switched to isolated for floating measurements (up to 400V max), this resistor had been burnt by a previous owner and was open circuit, a new metal film resistor has been fitted.

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The mains fuse in these Solartron DVMs is a Belling Lee size 00, someone didn't have the correct 750mA slow blow fuse and they put a 2.5A in it's place, closet I have is 1A. The 750mA 00 fuse seems an uncommon type & I will probably need to fit a different fuse holder for ones you can actually obtain.

All the parts that have been replaced, the heater of the ECC86 is intact, suggesting it was OK last time this DVM was used, prior to being stored by the previous owner, the glass has cracked from pin corrosion.

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David

Last edited by factory; 22nd Jun 2024 at 12:56 pm.
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Old 22nd Jun 2024, 12:49 pm   #3
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Default Re: Solartron digital voltmeter LM1440.3

A few internal pictures of the LM1440.3, mostly taken before I started the repairs.

A general view from the top and bottom.

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The input section which has the electrometer valves and two chopper relays running at 50Hz, it also has the Muirhead Weston cell (unsaturated type) for setting the +/- reference voltages.

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The reference Zener diode, which is housed inside this tiny oven.

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Old 22nd Jun 2024, 6:27 pm   #4
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Default Re: Solartron digital voltmeter LM1440.3

Grand work David. It looks a well built bit of kit, I take my hat off to the, ladies I presume who wired these up. I wonder how long it took: hours probably.

The foam on top of V1/2 might be worth replacing. If it's not already disintegrating it probably won't be long before it does.

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Old 22nd Jun 2024, 8:58 pm   #5
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Default Re: Solartron digital voltmeter LM1440.3

It must have taken a long time, everything is wrapped around the connections, they even put loops in the solid wire between the connections for the card edge connectors.

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I'll revisit the foam now you've mentioned it, it's just about in one piece at the moment. I did clean out some bits of the screening paint, that fell off the electrometer valves.

For some reason I forgot to add some pictures of it being tested, seems to work quite well, the Spannungsgeber above is actually hiding a HP 6209B inside, with some precision resistors to set the output voltage. The second picture shows the decimal point neon after replacement.

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And finally I've been looking for something to replace the Belling Lee size 00 problem, I've found a Eaton/Bussmann fuseholder that should fit in the limited space behind the panel.

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Old 23rd Jun 2024, 10:12 am   #6
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Default Re: Solartron digital voltmeter LM1440.3

Beautiful. Love the old cal etc stickers. I've got a Bulgin fuse holder that may do, it's 3 cm long or 25mm minus the lug.

Andy.
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Old 23rd Jun 2024, 4:06 pm   #7
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Default Re: Solartron digital voltmeter LM1440.3

Lovely that you have it up and running again. Which is the Zener oven, is that top left in the 1st image in post #3?
Odd that they felt you needed a Weston Cell to calibrate reference voltages when they already have an ovenised voltage reference
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Old 23rd Jun 2024, 9:52 pm   #8
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Default Re: Solartron digital voltmeter LM1440.3

Quote:
Originally Posted by Diabolical Artificer View Post
Beautiful. Love the old cal etc stickers. I've got a Bulgin fuse holder that may do, it's 3 cm long or 25mm minus the lug.

Andy.
Good to hear I'm not the only one that likes to keep old cal & other stickers, they are all part of the history, this one was last calibrated in 1980, date codes on parts inside suggest it was made around 1970 or later.

The fuse holder they used is very short, the position of the transformer limits what can be fitted, as there isn't enough space behind for most fuse holders, also there will be a larger than normal hole to fit a replacement.
The picture below shows the limited space behind the transformer, but I forgot to take a better one showing the fuse holder itself.

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This is the same type of fuse holder, from another Solartron DVM I have.

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Is the Bulgin one like this 5x20mm one I have? The cap with fuse is from the DVM, the nut is the same size as the one fitted, if I can find some washers I could probably fit this one.

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I wouldn't normally change an undamaged fuse holder, but the tiny slow blow fuses haven't been made for years and aren't getting any easier to find.

Quote:
Originally Posted by Julesomega View Post
Lovely that you have it up and running again. Which is the Zener oven, is that top left in the 1st image in post #3?
Odd that they felt you needed a Weston Cell to calibrate reference voltages when they already have an ovenised voltage reference
The tiny oven is on the PCB in picture 5 of post #3, the picture 1 shows the mains transformer in the top left. The Weston cell & front panel cal trimmer pots are a feature of most of the early Solartron DVMs, but not many have an oven for the reference Zener.

Of course the markings (+/- 1.01905) on the front panel are no longer correct, the cell has lost a little of it's voltage, measured with the high input impedance mode of the 34401A, a 0.27mV difference (neither meter has seen a cal lab for decades), ignore the last digit on the 34401A as the Weston cell will drift up/down with the ambient temperature.

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Old 24th Jun 2024, 8:32 am   #9
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Default Re: Solartron digital voltmeter LM1440.3

I have no less than six Belling Lee fuse holders which extend approximately 17mm behind the panel, the nut being on the outside of the panel rather than the inside as in your example. I suspect they take short fuses. Given the lack of availability of short fuses I think I'll throw then away.
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Old 24th Jun 2024, 7:00 pm   #10
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Default Re: Solartron digital voltmeter LM1440.3

If you can't get a 5mm diameter fuse to fit the cap they will be for the tiny 00 fuses, they are 5/8" in length & 3/16" diameter or 15.9mm & 4.76mm.

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Old 24th Jun 2024, 9:48 pm   #11
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Default Re: Solartron digital voltmeter LM1440.3

Yup. A 4.7mm twist drill just fits the caps of my holders and a depth gauge shows the length of the fuse to be around 16mm.
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Old 24th Jun 2024, 10:14 pm   #12
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Default Re: Solartron digital voltmeter LM1440.3

Thanks for confirming that.

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Old 26th Jun 2024, 5:18 pm   #13
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Default Re: Solartron digital voltmeter LM1440.3

Found these, the fat one might fit, it's 3/4" 18mm approx from end of thread to lug.

Andy.
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Old 26th Jun 2024, 10:27 pm   #14
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Default Re: Solartron digital voltmeter LM1440.3

The fat one looks identical to the one with the unobtainable short fuses, that I'm trying to replace. Do any of them take a modern metric 5mm diameter fuse? The other one in the first picture may have clearance issues at the back, the Bulgin one is probably too long as well. Thank you for having a look though.

Some pictures to show the problem, I've only got 1/4" space between the fuse holder & the transformer. If only they had mounted the transformer 1/2" nearer the front, there was space for them to have done this, but they didn't.

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Old 27th Jun 2024, 11:17 am   #15
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Default Re: Solartron digital voltmeter LM1440.3

Apologies, the big one is one of the odd fuse types, however found this one, see attached that should do the job using spacer washes, which I have too. It takes normal 5mm by 20mm fuses. Hole dia is 12.6mm or 1/2", length - 18mm or a tad under 3/4". If it's any use, drop me a PM, Andy.
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Old 28th Jun 2024, 11:02 am   #16
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Default Re: Solartron digital voltmeter LM1440.3

When I worked in the Physics dept at the University of Bradford in the 70's we had one of those Solatron DVM's as our Lab reference. Used only to check the other moving coil meters in the teaching labs, and our own technicians equipment. Hardly used. I wonder what happened to that one ? The Physics dept is no more, as with a lot of the "old School". Physics was split into several disciplines , Nuclear Physics, Optics, Material science, Archaeology, Solid state Physics.. How times change
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Old 28th Jun 2024, 2:36 pm   #17
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Default Re: Solartron digital voltmeter LM1440.3

Hopefully it got rescued by someone that was interested in old test gear, apart from the LM1420 hardly any of these early DVMs seem to have survived. The mention of Archaeology, I was considering that as a career option before I left Scotland.

Quote:
Originally Posted by Diabolical Artificer View Post
Apologies, the big one is one of the odd fuse types, however found this one, see attached that should do the job using spacer washes, which I have too. It takes normal 5mm by 20mm fuses. Hole dia is 12.6mm or 1/2", length - 18mm or a tad under 3/4". If it's any use, drop me a PM, Andy.
That one should fit OK, it looks very much like one of those in the Eaton/Bussmann datasheet I found a few days ago, I've sent a PM.
https://www.mouser.co.uk/datasheet/2...el-2255807.pdf

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Old Yesterday, 11:17 pm   #18
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Default Re: Solartron digital voltmeter LM1440.3

Great work David!

Solartron equipment is so well built. This thread has inspired me to take a look at my Solartron LM1420.2 digital voltmeter. It has the add on print module at the back. I think it's the BCD version ie the EX 1418 unit.

I did have a quick look inside a while back and it is well engineered with good access for servicing. I think the standard cell will need replacing as it was measuring a little on the low side but I wonder what would be a suitable replacement.

When I can find the time it will be good to start work on renovating it.

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