DC Power Supply Ammeter replacement

[wireman]

I would like some advice on what would be involved in replacing the Ammeter in a Maplin DC Power supply (XG91Y).
The supply works but the meter failed many years ago.

I understand basic electronics but have only attempted very limited repair to electrical items in the past.

Here is an image of the same model..

https://www.picclickimg.com/d/l400/p...wer-Supply.jpg

I have much newer supplies so it may not be worth fixing this. I'm assuming I would have to find the same meter movement.

[Radio Wrangler]

You either need an appropriate movement that will fit the scale and case, or else a full matching meter.

Either way, it'll be hard to find. As it's the ammeter, the movement doesn't have to have the same sensitivity as the original, likely the shunt resistor can be changed to work with a range of meter sensitivities.

David

[Ed_Dinning]

Hi, it looks like a conventional moving coil ammeter. Ideally you need one of a similar size and shape. It can then be shunted (low value resistor in parallel) to get the correct sensitivity.
It would then be wired in series with one of the output leads

Ed

[wireman]

Thanks for the replies, I will look inside to see what the dimensions are.

There must be more than a simple shunt because of the selectable current range and dual-scale dial (I realise the image is small so hard to see).

[ortek_service]

It wouldn't surprise me if the actually used the same (micro)ammeter for both meters and just fitted different scales / used external series / shunt resistors - I once bought some surplus new voltmeters, that actually needed an external resistor as were 100uA? Micro-ammeters.

It is worth checking the 'Ammeter' one, to see if it is actually faulty, or something else.
- Can you measure it's coil resistance, when using a multimeter across its terminals (might need to desolder one-side, to eliminate any shunts across it giving misleading results).
And this will also probably make the meter move a bit, if it isn''t open-circuit.
Otherwise, does tapping the meter / shaking PSU a bit cause pointer to move?
If it isn't seized, then sometimes the spring contact has come desoldered, and can sometimes just be soldered back onto original joint.
So could be worth taking meter out / apart, to see if there might be an easy and cheap fix.

Otherwise, Maplin would have re-badged these so probably find them under a brands. But getting original spares might be difficult, so would have to try and find a similar size meter.
Or, if you're not bothered about originality, then you could upgrade it to use a cheap LED / LCD module, that should be lots available from the Far East.

Rather than trying to get one working correctly as the ammeters, if the original meters are identical, just with different scales, then could swap them / scales over, and just fit a DVM module on voltmeter side (maybe with suitable resistor divider to drop down to typical 200mV full-range

[WME_bill]

I see the scales appear to be 3Amp and 6Amp. The meter itself will certainly not be that. Very common is between 1 and 5 ma. Then a shunt is fixed across the meter coil, usually internally across the terminals.
As it is in series with the output line and you say that there is output, the fault must be either the switching Hi/Lo range, or the meter movement itself.

A quick test of a meter movement which is useful when rummaging through a box of miscellaneous meters at an auction or rally is to short the terminals. I use a penknife blade or a short length of bare copper wire. Then holding the meter dial up, quickly rotate by twisting the wrist. If the meter is sound, the needle will swing slowly when shorted, and oscillate wildly when not shorted or if open circuit.
You do it with an Avo or any working meter to try out the method. A gentle action is quite enough.

Otherwise, find a meter which will fit (pointer length), and make up a shunt resistor to get the range right.
A 1ma meter coil is frequently around 100ohms, = 100mV, so the shunt for 3Amp will be a few milli-ohms - a length of 20SWG copper wire will do, even though the temperature coefficient is very poor. I can let you some resistance wire to experiment with.
Otherwise, I might have a meter to try fitting if you give me the pointer length and meter barrel/ body diameter.

It always amuses me to see meters advertised on ebay or elsewhere which tell you the full scale calibration of the dial, but nothing about the full scale coil current and coil resistance.

I endorse what Ortek Service says, though be careful measuring the resistance of a coil with another meter, as the current may be far too high for the coil and burn it out. A DVM measuring 100 ohms passes 100ma - a bit tough for a little 1ma coil. Better feed a voltage from your power supply unit through a resistor (say 10k / 50k) , adjust the voltage to give full scale and measure the voltage across the coil with your DVM or Avo. Then a simple Ohms Law calculation. And you have proved the coil is sound as well.
wme_bill

[Bazz4CQJ]

Originally Posted by ortek_service

I, and just fit a DVM module on voltmeter side (maybe with suitable resistor divider to drop down to typical 200mV full-range

What I was thinking, especially if you are not too experienced, is rather than replace the meter, simply add on an inexpensive combined digital voltage/ammeter, and just wire it in to the outputs and sit it on the top of the psu. This is a typical offering from anyone of loads of sellers https://www.ebay.co.uk/itm/Digital-L...8AAOSwvR5aSQVw.

Generally, these things are surprisingly accurate and the resolution provided by a digital display can be useful at times.

B

[David Simpson]

I'm guessing, Wireman, that the ammeter's perspex window is approx. 1.5" by 3", and going by the location of the zero adj., the whole body of the meter behind the front panel is about 3" x 3" square. A common enough size. A replacement meter should be easy enough to fit behind the front panel. If not, a portion of the front panel could be cut out. The moulded enclosure for the meter movement could well be circular, and obviously smaller than the frontage.
Any chance of you removing the PSU's casing & giving us a picture of the inside, particularly the rear of the meter. Would also give you the opportunity to remove the meter & carry out the recommended tests.

Regards, David

[wireman]

Yes, I will try to look inside and take photos/measurements, either this evening or tomorrow afternoon.

[buggies]

There was another thread on an XG95D and I reverse engineered a similar XG90X and attached a (non-waranteed) circuit to post https://www.vintage-radio.net/forum/...7&postcount=20.
I did not find the ammeter rating - but the Hi-LO (600mA/1A) switch selected one of two presets in series with the meter across the 0.2 ohm sense resistor.

EDIT:Post crossed with Neutrino

[wireman]

Very intersting thread, which led me to this:

http://www.lodestarelec.com/01-dc_po...ply.html?CID=1

By the way I do have a circuit diagram.

[wireman]

Attached are some photographs.

The meters are sandwiched between a plate and the frame. The meter circular part diameter is 50mm, screw to screw is roughly 47mm and the square faceplace roughly 62mm.
Meter needles do move if you rotate the power supply.

It would take a while to work out how to dismantle further.