Latest Acquisition and it's mint.

[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

 

[nicebutdim]

Seen that already. I remember the dodgy photos and carpet

Now trying to figure out why my mother is selling a tester.

 

[DPG]

Now trying to figure out why my mother is selling a tester.

Early retirement?

 

[nicebutdim]

Early retirement?

Yeah, but that was about 25 years ago.