Avometer 8 Mk4 repair and restoration - UK Vintage Radio Repair and Restoration Discussion Forum

Sinewave · 26th Dec 2017, 6:16 pm

Some time ago I acquired a Universal Avometer Eight, Mark 4. Unfortunately it didn't work at all.

Every so often when I had time I pulled out the drawings and followed the circuits to try and find what was stopping it from working. As it were completely non operational, I was rather hoping of some common fault which would be quite obvious to find.

But, I couldn't find a fault, so that then took me to look at the movement. So I removed the movement from the meter to take a good look and make some checks. I connected my loop calibrator to it to supply the 37.5 micro amps to see the needle move the full scale deflection.

And much to my surprise, the movement works. I needs a few more micro amps to get the full scale deflection, so perhaps the magnetic shunt needs adjusting, but this is something I haven't done yet.

I then placed the movement back, connected everything up and thought I'd have another go at tracing the fault and much to my surprise the entire meter is working again. So perhaps I'd disturbed a poor connection on the reverse moving coil switch when disconnecting the movement from it.

I have mentioned this in another thread which was really for something different, so I thought I'd make this thread so that we can go into more detail and get more out of this.

I'd like to bring the accuracy of the meter back into specification, so I'll need to find what I should be adjusting. But before that I need to see if it's out of calibration on all ranges or if it's only certain functions and ranges, by making cal checks on all ranges.

26th Dec 2017, 6:21 pm

Ah, it's only 5% out, good enough for most (all?) repair work.

The Philpott · 26th Dec 2017, 9:04 pm

One possibility that comes to mind regarding the actual meter movement:

If the insulation between the layers of wound wire has become corrupted, a short may have occurred between 2 sections of wire in different layers.. the designated current for FSD would not generate the required deflection as the current would be effectively flowing through a shorter coil than intended. If for example the current was running through a wire 'path' of 19 inches instead of 20 inches the deflection would fall short. (I hope this makes sense)

Sinewave · 27th Dec 2017, 8:38 pm

Quote:

Originally Posted by merlinmaxwell

Ah, it's only 5% out, good enough for most (all?) repair work.

Well, that's on this FSD test, but I really need to see how linear the inaccuracy is to see if it's across all ranges.

Quote:

Originally Posted by The Philpott

If the insulation between the layers of wound wire has become corrupted, a short may have occurred between 2 sections of wire in different layers.

Yes that makes sense, but I hope it's not the case with this one. If it doesn't go faulty again, then hopefully it's alright.

Phil G4SPZ · 28th Dec 2017, 10:07 am

I think the first thing you should try is adjusting the magnetic shunt to see if you can get 37.5uA to generate a full-scale deflection. If not, the magnet may have lost strength - Dave has tried adding tiny neodymium magnets to boost a tired main magnet.

Do check the basics like hairsprings and zeroing first.

Jon_G4MDC · 28th Dec 2017, 12:00 pm

One of my 8s had a magnet that lost it's voodoo.
Even without the shunt it was under reading by 20%.

With nothing to lose I wound a solenoid of 16SWG enamelled wire (I forget how many turns perhaps 20?) and zapped it with a 12V car battery. Just a glancing swipe with the end of the wire on the +ve terminal.

After that I had a movement which read 20% the other way! Today it has the full shunt and an additional magnetic bypass on it and it works well enough.

Sinewave · 28th Dec 2017, 8:07 pm

Has anyone had any luck with 'magnetizers' to re magnetize the magnets?

The Philpott · 28th Dec 2017, 9:50 pm

As Phil says, the magnetic shunt may be able to compensate (for some inexplicable reason I thought you had already tried this- I blame Christmas booze for this unfounded assumption)

The later shunts I have tweaked on a Mod 8/III are razor blade shaped, secured by a screw and spring washer, and quite a small change in position gives a significant amount of compensation. I imagine the later Mod 8 units are similar.. The earlier meters with the horseshoe magnets have less scope for adjustment. As Phil mentions i managed to resolve an early meter with 7% shortfall totally outside the scope of the adjustment mechanism, by aralditing a neodymium magnet in a suitable place. I would think on a later meter you have better chance of success without resorting to this.

i don't personally know anyone who has tried remagnetising. Likely to be rather specialized due to the precision involved.

26th Dec 2017, 6:16 pm	#1
Sinewave Octode Join Date: May 2016 Location: Oxfordshire/Bucks borders, UK. Posts: 1,604	Avometer 8 Mk4 repair and restoration Some time ago I acquired a Universal Avometer Eight, Mark 4. Unfortunately it didn't work at all. Every so often when I had time I pulled out the drawings and followed the circuits to try and find what was stopping it from working. As it were completely non operational, I was rather hoping of some common fault which would be quite obvious to find. But, I couldn't find a fault, so that then took me to look at the movement. So I removed the movement from the meter to take a good look and make some checks. I connected my loop calibrator to it to supply the 37.5 micro amps to see the needle move the full scale deflection. And much to my surprise, the movement works. I needs a few more micro amps to get the full scale deflection, so perhaps the magnetic shunt needs adjusting, but this is something I haven't done yet. I then placed the movement back, connected everything up and thought I'd have another go at tracing the fault and much to my surprise the entire meter is working again. So perhaps I'd disturbed a poor connection on the reverse moving coil switch when disconnecting the movement from it. I have mentioned this in another thread which was really for something different, so I thought I'd make this thread so that we can go into more detail and get more out of this. I'd like to bring the accuracy of the meter back into specification, so I'll need to find what I should be adjusting. But before that I need to see if it's out of calibration on all ranges or if it's only certain functions and ranges, by making cal checks on all ranges. Attached Thumbnails __________________ Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.

26th Dec 2017, 6:21 pm	#2
Guest Guest Posts: n/a	Re: Avometer 8 Mk4 repair and restoration Ah, it's only 5% out, good enough for most (all?) repair work.

26th Dec 2017, 9:04 pm	#3
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,104	Re: Avometer 8 Mk4 repair and restoration One possibility that comes to mind regarding the actual meter movement: If the insulation between the layers of wound wire has become corrupted, a short may have occurred between 2 sections of wire in different layers.. the designated current for FSD would not generate the required deflection as the current would be effectively flowing through a shorter coil than intended. If for example the current was running through a wire 'path' of 19 inches instead of 20 inches the deflection would fall short. (I hope this makes sense)

28th Dec 2017, 10:07 am	#5
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: Avometer 8 Mk4 repair and restoration I think the first thing you should try is adjusting the magnetic shunt to see if you can get 37.5uA to generate a full-scale deflection. If not, the magnet may have lost strength - Dave has tried adding tiny neodymium magnets to boost a tired main magnet. Do check the basics like hairsprings and zeroing first. __________________ Phil Optimist [n]: One who is not in possession of the full facts

28th Dec 2017, 12:00 pm	#6
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Avometer 8 Mk4 repair and restoration One of my 8s had a magnet that lost it's voodoo. Even without the shunt it was under reading by 20%. With nothing to lose I wound a solenoid of 16SWG enamelled wire (I forget how many turns perhaps 20?) and zapped it with a 12V car battery. Just a glancing swipe with the end of the wire on the +ve terminal. After that I had a movement which read 20% the other way! Today it has the full shunt and an additional magnetic bypass on it and it works well enough.

28th Dec 2017, 8:07 pm	#7
Sinewave Octode Join Date: May 2016 Location: Oxfordshire/Bucks borders, UK. Posts: 1,604	Re: Avometer 8 Mk4 repair and restoration Has anyone had any luck with 'magnetizers' to re magnetize the magnets? __________________ Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.

28th Dec 2017, 9:50 pm	#8
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,104	Re: Avometer 8 Mk4 repair and restoration As Phil says, the magnetic shunt may be able to compensate (for some inexplicable reason I thought you had already tried this- I blame Christmas booze for this unfounded assumption) The later shunts I have tweaked on a Mod 8/III are razor blade shaped, secured by a screw and spring washer, and quite a small change in position gives a significant amount of compensation. I imagine the later Mod 8 units are similar.. The earlier meters with the horseshoe magnets have less scope for adjustment. As Phil mentions i managed to resolve an early meter with 7% shortfall totally outside the scope of the adjustment mechanism, by aralditing a neodymium magnet in a suitable place. I would think on a later meter you have better chance of success without resorting to this. i don't personally know anyone who has tried remagnetising. Likely to be rather specialized due to the precision involved.