Latest Acquisition and it's mint.

[timhoward]

further investigation finds that the 2 securing release pillars for the galvo are actually terminals, connected to both the galvo and the internal resistor banks. tomorrow will try and see if connecting 2 PP3s will get some life into it. if i read it right, maybe someone has modded it to use PP3s instead of the original battery, necessitating the removal of the meter to replace said batteries. weird.

What you are trying to reinstate is along these lines:

Hopefully that will set your bearings. (EDIT The middle button should say G and the bottom button should say B, sorry)

For testing purposes you could swap out the Galvanometer with a digital multimeter.

Here it is working....observe that with the DECADE set to 678 ohms, and a resistance of 678 ohms between X1 and X2 we get 0v in the middle:

For academic interest only, see this old thread of mine - it's exactly the same concept but yours is ready to roll, wheras a post office box needs a power source and a galvonometer/voltmeter connecting up.

Post Office Box experiment | on ElectriciansForums

Find Post Office Box experiment Advice and Help. How-to Post Office Box experiment in the Electrical Engineering Chat advice boards on ElectriciansForums.net Est.2006 | Free Electrical Advice Forum. Post Office Box experiment on ElectriciansForums.net Electrical Advice Forum.

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[telectrix]

RESULT.....Connected 2 x PP3 batteries to the galvo and got deflection. then added the 4.5V battery in the original position in the cabinet lid. a (nominal) 10K resistor then connected to X! and X2. Dials operated to give an almost Zero reading on the galvo, ( 1 division from absolute 0 ) and the setting showed 9.6K. Will try several different resistor values later. I have an organiser full of them, from 0.1 ohms to 10 Megohms.
The PP3s must be a requirement of the updated galvo.

 

[xmetal]

The original picture looks similar to the PO ohmmeter 18a 'megger'. That used a 9v battery.

 

[timhoward]

The original picture looks similar to the PO ohmmeter 18a 'megger'. That used a 9v battery.

I know what you mean. The 18A is a beautiful machine. British Racing Green....
Very similar in function, it has an additional ingenious trick to help engineers find a broken (as opposed to shorted) cable location. You join broken to good at far end, plug headphones in, find a quiet spot and run some maths.

The 'upgrade' (still sold today) isn't quite as aesthetically pleasing....

Any collector of electrical junk should keep an eye on "GPO Tester" results on ebay. There's even a brand new complete 18C for £50 at the moment. (but my wife said "No" and I have enough bridge-based testers!)

 

[DPG]

I know what you mean. The 18A is a beautiful machine. British Racing Green....
Very similar in function, it has an additional ingenious trick to help engineers find a broken (as opposed to shorted) cable location. You join broken to good at far end, plug headphones in, find a quiet spot and run some maths.


View attachment 109477

The 'upgrade' (still sold today) isn't quite as aesthetically pleasing....

View attachment 109478

Any collector of electrical junk should keep an eye on "GPO Tester" results on ebay. There's even a brand new complete 18C for £50 at the moment. (but my wife said "No" and I have enough bridge-based testers!)

That green one is very nice. Is it a military spec one?

 

[timhoward]

That green one is very nice. Is it a military spec one?

The first edition manual says September 1965. It's stamped GPO so I think it was just prior to the plastic revolution and built to be hammered and last.

 

[telectrix]

Bit premature with previous post saying result. readings are widely inaccurate e.g. a 6.8k resistor measures at 4.9k. a 3.3. k reads as 2.9k. could it be that the machine is reconfigured for 6V. the galvo has 2 x PP3s to operate the electronics.

 

[timhoward]

Bit premature with previous post saying result. readings are widely inaccurate e.g. a 6.8k resistor measures at 4.9k. a 3.3. k reads as 2.9k. could it be that the machine is reconfigured for 6V. the galvo has 2 x PP3s to operate the electronics.

Things to try:

-What does it measure with a dead short of low resistance between X1 and X2?
-try the resistors with an MFT to see if they are on the edge of their tolerance bands (gold 10%, silver 5%, none 20%).
-a normal multimeter on DC volts in place of the (originally) removable galvo.

The machine voltage actually doesn't matter, it's all about the calibrating the galvo so it reads zero when the bridge is balanced.

 

[telectrix]

it does read 0, but the values on the dials are way out compared to multimeter, both digital and AVO8. will try the dead short as suggested then investigate further.

 

[telectrix]

connected a very short piece of flex , measured on a low ohms meter at 0.020 ohms. Bridge says 0.8. across the terminals. Time to have a poke around inside I think..

 

[timhoward]

connected a very short piece of flex , measured on a low ohms meter at 0.020 ohms. Bridge says 0.8. across the terminals. Time to have a poke around inside I think..

The non-original part is the galvanometer. When the Galv says 0, stick a multimeter on volts across here and see if THAT says zero.
It looks like it can be adjusted.
The point of these devices is that the galv doesn't need to be super accurate, it just needs to be able to be zero'd and it feels as though that is the issue here. Does that black knob in the middle calibrate it?

 

[telectrix]

will try that today. the black knob does not appear to do anything but I'll have another look at it. also noticed that the 2 toggle switches are up and down instead of left and right, not that it makes any difference to the operation, but it's a bit confusing.

 

[telectrix]

did some trials this morning using a 680 ohm resistor. on the multiplier dial, the x10 and the x100 give sensible readings but other ranges are wildly out. time to check internal connections and maybe a clean up of the dial contacts.

 

[timhoward]

As this thread seems to be turning into an appreciation society of Wheatstone Bridge testers, I thought I'd post this here...
I was caught red handed looking at the listing and I've been expressly told I can't buy this by the boss.

But if it stays at this price - someone really should!

Vintage GPO BT OHMMETER 18a tester | eBay

Find many great new & used options and get the best deals for Vintage GPO BT OHMMETER 18a tester at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

 

[DPG]

As this thread seems to be turning into an appreciation society of Wheatstone Bridge testers, I thought I'd post this here...
I was caught red handed looking at the listing and I've been expressly told I can't buy this by the boss.

But if it stays at this price - someone really should!

Vintage GPO BT OHMMETER 18a tester | eBay

Find many great new & used options and get the best deals for Vintage GPO BT OHMMETER 18a tester at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

Seen that already. I remember the dodgy photos and carpet