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Old 28th Apr 2016, 5:44 pm   #1
Techman
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Default Rank Bush Murphy EHT meter

I got this little unit from a local ‘silent key’ junk sale of old test gear and other components etc. It cost 1 as there was very little interest and I was the only bidder. I could have come away with quite a number of AF signal generators by such as Marconi and Pye for 1 each, but I would have only wanted them to break for parts so they remained un-sold. I still came away with too much rubbish but luckily didn’t spend too much. I did get out-bid on the various lots of large air spaced variable capacitors, but then I am very tight and don’t go much higher than a couple of quid in most cases!

Sometimes it’s the simplest things that please and I have to admit that I had been intending to make some kind of multiplier to measure very high voltages with a standard meter for some time, but never got round to it, so I was pleased to acquire this little unit.

This unit being made by Rank Bush Murphy is obviously intended to be able to measure with reasonable accuracy, the high impedance EHT supplies on TV recievers, but would expect that it would have some noticeable loading effect. I connected it to the EHT power supply and it certainly seemed to read accurately, although this power supply is likely to have a lot lower impedance and therefore higher available current than a TV line output derived supply.

I’m wondering whether these EHT meters were made for Rank Bush Murphy service engineers only, or whether they were available for general use to anyone in the TV trade? Also, how they compare with other EHT probes made for use with AVO meters etc. I’ll have to open it up and have a look at the innards – I don’t expect to find a lot in there.

Pictures below:-
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Old 28th Apr 2016, 6:08 pm   #2
turretslug
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Default Re: Rank Bush Murphy EHT meter

I'm slightly surprised that there's no prominent labelling warning to make the chassis connection first.... Maybe it's from an era when such things were taken as a given.

I wonder what the loading is, I think of 50uA FSD as a "typically" good meter movement, so that would be OK for CTV use but 'scope control divider chains might be problematic. There were electronic Brandenburg 30kV meters around with a 60 Gohm multiplier but I expect these would have been rather more expensive.

The "usual" AVO probe encountered is a GRP tube with multipliers inside, it's probably fine with care but I doubt it'd pass muster as regards modern creepage/clearance expectation.
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Old 28th Apr 2016, 6:17 pm   #3
dazzlevision
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Default Re: Rank Bush Murphy EHT meter

Quote:
Originally Posted by Techman View Post
I’m wondering whether these EHT meters were made for Rank Bush Murphy service engineers only, or whether they were available for general use to anyone in the TV trade?
They were designed and made by Rank Bush Murphy and if you do look inside, you will see a clear plastic case used to house the EHT tripler components in their A823 solid state colour TV chassis.

They were advertised in RBM's "Service Skill" periodic bulletin for dealer's service departments, so I expect they were the main customers. However, RBM did by that time also sell parts to the general TV trade, so these meters may have had an outlet that way. They launched a components catalogue called "RSVP" - Rapid Sound & Vision Parts. I must have a look to see if any test gear was listed in these catalogues.
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Old 28th Apr 2016, 7:25 pm   #4
Lloyd 1985
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Default Re: Rank Bush Murphy EHT meter

I got one of these meters in a box of old test equipment a good few years ago, it's been a great little meter! Very handy on every set I've played with, also good to have an EHT stick rectifier handy so you can test before the EHT rectifier valves in some sets. I also used it on my own crazy EHT generator thingy, it was a scary thing that was, it died in the end though...

Regards,
Lloyd.
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Old 29th Apr 2016, 9:07 am   #5
mark pirate
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Default Re: Rank Bush Murphy EHT meter

These are really good meters, I have had mine for a few years now and find it indispensable.
You certainly got a bargain with yours!

Mark
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Old 30th Apr 2016, 3:15 pm   #6
Techman
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Default Re: Rank Bush Murphy EHT meter

Thanks for all the replies, all interesting stuff. I had a look inside and there's actually a bit more to it than I expected. It has that Perspex type housing as described.

Searching on-line a couple of days ago, I found a picture of one that showed a slightly different meter fitted that had 100uA actually marked on the front scale. I'm wondering if this has had a replacement meter fitted at some point in its life, the reason being that there was a sticker with a number on it on the front plate of the unit that was partially under one corner of the meter and took some 'picking' to remove - the sticker must have been stuck on there with the meter removed.

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Old 20th Sep 2018, 5:20 pm   #7
TonyDuell
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Default Rank EHT meter

I bought one of these meters this morning along with some other test gear. But mine is a little different to the one described above. It has the perspex-encased potted multiplier resistor assembly on the inside of the back panel. It has the meter on the front (I assume the original meter) and the resistor from the -ve (chassis) terminal to the case. But mine doesn't have a bit of perforated circuit board on the back of the meter. The only other component is a preset potentiometer wired as a shunt to the meter, mounted between the meter terminals.

Since it was suggested that the meter had been replaced in the above unit at some point, I wonder if the extra components were to maintain the calibration using a meter of the 'wrong' FSD

Last edited by AC/HL; 20th Sep 2018 at 7:33 pm. Reason: Post moved as suggested
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