Avometer 8 MK 5 Low readings - UK Vintage Radio Repair and Restoration Discussion Forum

IZ2EWQ_Giuseppe · 1st Jun 2019, 8:25 am

Dear friends, good afternoon.
This is Giuseppe IZ2EWQ, from near Milano.
I bought an Avometer 8 Mk 5, serial number 01355 8V/2/77 and, during the test, I found an error of - 30% on all the DC/AC ranges;
The meter appears in very well conditions, and seems never used because external haven’t dings or wear and internal the printed circuit solders and rotary switches contacts pads are bright, shiny and very clean;
So everything is ok, but testing the microammeter alone, I see that it needs 53 uA for total deflection (instead of 37,5 uA as expected); It’s resistance and swamp resistor is 2666 ohm, tested with digital multimeter.
The meter moves correctly without stuck and hair springs are in right position.

Have you an idea how can I repair it ?

I will appreciate your help.
Thank you very much.

pmmunro · 2nd Jun 2019, 6:27 pm

Giuseppe,

The information you give is very useful and exactly what is needed for diagnosing the problem.

The moving coil and swamp resistance figure is correct so the most likely cause of the low reading is a loss of magnetic field strength in the magnet. (I'm assuming that the pointer and moving coil move freely).

At the time when this instrument was made there were many repair workshops who had a magnetising rig and could have remagnetised the meter for you. Now, at least in the UK, there are very few left who do this kind of work. Perhaps it is different in or around Milan.

Remagnetising is probably the best solution if you are able to get it done. Some people have reported that they have found ways of adding small rare earth magnets to increase the field strength, but this is for earlier movements with block magnets, not centre pole magnets as your meter has.

Again in the UK, used Avometer Model 8 Mark V meters are still quite common and frequently offered for sale on eBay and elsewhere, and these can be a source of spare parts as they're not too expensive. It's best to use a complete movement. The scaleplates were selected from a range of 10 or 12 types to suite the "scaleshape" of the movement, i.e. it's linearity characteristic. The type number of the scaleplate should be visible at the bottom centre of the scale.

I'm sorry I can't be more encouraging, but it does look as if your meter has been in a strong magnetic field or someone has tried to dismantle the magnet assembly leading to a loss of magnetic flux.

PMM

1st Jun 2019, 8:25 am	#1
IZ2EWQ_Giuseppe Retired Dormant Member Join Date: Oct 2012 Location: Milan, Italy. Posts: 2	Avometer 8 MK 5 Low readings Dear friends, good afternoon. This is Giuseppe IZ2EWQ, from near Milano. I bought an Avometer 8 Mk 5, serial number 01355 8V/2/77 and, during the test, I found an error of - 30% on all the DC/AC ranges; The meter appears in very well conditions, and seems never used because external haven’t dings or wear and internal the printed circuit solders and rotary switches contacts pads are bright, shiny and very clean; So everything is ok, but testing the microammeter alone, I see that it needs 53 uA for total deflection (instead of 37,5 uA as expected); It’s resistance and swamp resistor is 2666 ohm, tested with digital multimeter. The meter moves correctly without stuck and hair springs are in right position. Have you an idea how can I repair it ? I will appreciate your help. Thank you very much.

2nd Jun 2019, 6:27 pm	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: Avometer 8 MK 5 Low readings Giuseppe, The information you give is very useful and exactly what is needed for diagnosing the problem. The moving coil and swamp resistance figure is correct so the most likely cause of the low reading is a loss of magnetic field strength in the magnet. (I'm assuming that the pointer and moving coil move freely). At the time when this instrument was made there were many repair workshops who had a magnetising rig and could have remagnetised the meter for you. Now, at least in the UK, there are very few left who do this kind of work. Perhaps it is different in or around Milan. Remagnetising is probably the best solution if you are able to get it done. Some people have reported that they have found ways of adding small rare earth magnets to increase the field strength, but this is for earlier movements with block magnets, not centre pole magnets as your meter has. Again in the UK, used Avometer Model 8 Mark V meters are still quite common and frequently offered for sale on eBay and elsewhere, and these can be a source of spare parts as they're not too expensive. It's best to use a complete movement. The scaleplates were selected from a range of 10 or 12 types to suite the "scaleshape" of the movement, i.e. it's linearity characteristic. The type number of the scaleplate should be visible at the bottom centre of the scale. I'm sorry I can't be more encouraging, but it does look as if your meter has been in a strong magnetic field or someone has tried to dismantle the magnet assembly leading to a loss of magnetic flux. PMM