UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 7th Jul 2020, 5:53 pm   #1
RetroRichard
Triode
 
RetroRichard's Avatar
 
Join Date: Feb 2020
Location: Maidenhead, Berkshire, UK.
Posts: 12
Default AVO8 DC Current range error

I wondered if anyone could tell me if there is a company in or near Berkshire who would be expert in calibrating an AVO8 meter, or may have an answer to the problem described.

I recently decided to check the accuracy of my three old analogue AVO meters (AVO 8 MkIII, AVO MultiMinor Mk.4 & Unversal AVOMinor) against six different DVMs and a couple of cheap analogue meters.

The Volts and Ohms ranges agreed very well but I found an error in the AVO8 on the DC Current 1 A range, where the current in all of the other meters was indicating 200 mA (or extremely close) but the AVO showed 170 mA. I have the original manual and circuit diagram for the AVO8 and the meter element is shown as 37.5 µA full scale deflection (fsd) at 3333Ω. When set to the 1A range the current flows through two sensing resistive wire windings shown as 0.05 Ω in series with 0.45 Ω, and these are in parallel with the combined resistance of the meter element (3333 Ω ) and the rest of the wire sensing windings ( 9999.5 Ω ).

An indicated 170 mA on the 1A range would be a current in the meter element of 6.375 µA and calculations suggest that the nominal 0.45 Ω winding must be 0.075 Ω less than the nominal value. This small difference makes virtually no visible difference on the lower current ranges.

I haven’t tried to alter any windings; does anyone have any advice about this error, or information on repairs or knowledge of a calibration laboratory?

Regards

Richard
RetroRichard is offline  
Old 7th Jul 2020, 7:35 pm   #2
frsimen
Heptode
 
Join Date: Jan 2011
Location: Croydon, London, UK.
Posts: 767
Default Re: AVO8 DC Current range error

Hello Richard,

You could confirm that the resistance is incorrect by measuring the voltage across the input terminals when you are feeding in a known current and calculating the resistance from that measurement. It seems unlikely that the 0.45 ohms resistance would drop in value on its own.

Paula
frsimen is offline  
Old 7th Jul 2020, 9:18 pm   #3
pmmunro
Octode
 
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
Default Re: AVO8 DC Current range error

Richard,

Is it just the 1A DC range which has a low sensitivity, or are all direct current ranges affected?

The 50 microamp range is basic to everything, but if that reads correctly then the movement has the right sensitivity and the problem has to be in the circuits external to the movement.

If you let us know the relative sensitivity on all the direct current ranges, it should be possible to help you to an accurate diagnosis.

It's best to have as much information as possible before attempting to cure the fault.

PMM
pmmunro is offline  
Old 8th Jul 2020, 9:46 am   #4
The Philpott
Dekatron
 
The Philpott's Avatar
 
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
Default Re: AVO8 DC Current range error

A non-invasive test to try and flush out mechanical reasons for the discrepancy:
Grab the DC rotary selector switch at 12 o'clock and 6 o'clock positions (not easy being as it is conical) and rock it back and forth as much as the brass bushing will allow. Monitor the erroneous reading as you do this. Repeat this with the AC rotary selector.

I see no reason for a calibration facility to be involved at this point- thus far this would appear to be a gremlin- rather than a calibration issue.

Dave
The Philpott is offline  
Old 8th Jul 2020, 2:33 pm   #5
RetroRichard
Triode
 
RetroRichard's Avatar
 
Join Date: Feb 2020
Location: Maidenhead, Berkshire, UK.
Posts: 12
Default Re: AVO8 DC Current range error

Many thanks to all for those replies, I will have a go at finding the answers to those suggestions.
Richard
RetroRichard is offline  
Old 9th Jul 2020, 5:57 pm   #6
RetroRichard
Triode
 
RetroRichard's Avatar
 
Join Date: Feb 2020
Location: Maidenhead, Berkshire, UK.
Posts: 12
Default Re: AVO8 DC Current range error

AVO 8 Test
As suggested, I did a further test of the various current ranges and measured the voltage across the AVO 8 terminals, using a known good DVM for the voltage and another known good DVM and my AVOMinor (previously tested and showing very good agreement) for the current.

Please see the attached pdf file showing the circuit diagram and table of results and calculations.

The table shows that the sensitivity of all ranges below the 1A range is very good, with less than 1% error. In contrast the error in the 1 A range is close to 7% and implies that the sensing resistance that should be 0.50 Ω (comprised of two windings 0.05 Ω + 0.45 Ω) is in fact 0.4660 Ω . The fact that the meter current IM is correct for all the other ranges means that the calculation of IM for the 1A range is valid. This small difference makes no real difference to the lower ranges.
I will just have to accept the error as I am a bit concerned that an attempt to add 0.034 Ω to the 0.45 Ω winding will potentially do more harm than good.

Cheers
Richard
Attached Files
File Type: pdf AVO8_circuit-diag_2.pdf (162.7 KB, 40 views)
RetroRichard is offline  
Old 9th Jul 2020, 7:12 pm   #7
The Philpott
Dekatron
 
The Philpott's Avatar
 
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
Default Re: AVO8 DC Current range error

On a physical inspection, do you have a shorted turn in the 1A DC winding? Unlikely unless it has been unevenly wound onto the board, then suffered overheating.

Dave
The Philpott is offline  
Old 9th Jul 2020, 11:34 pm   #8
MotorBikeLes
Nonode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
Default Re: AVO8 DC Current range error

Why not get a pair of smooth jawed pliers, and just squeeze the wire a little. Keep checking. I think you can add that extra bit of resistance easily that way.
Les.
MotorBikeLes is offline  
Old 10th Jul 2020, 11:14 am   #9
The Philpott
Dekatron
 
The Philpott's Avatar
 
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
Default Re: AVO8 DC Current range error

That's logical- the 10A shunt typically looks to have been manufactured 'under value' then notched to trim it upwards, so the basic principle should apply to the 1A shunt also.

It's conceivable that the meter had this fault from new, we think of them as very high quality but even in the 'sixties a few would inevitably have leaked out of the factory with flaw/s.

Dave
The Philpott is offline  
Old 12th Jul 2020, 11:26 am   #10
pmmunro
Octode
 
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
Default Re: AVO8 DC Current range error

Les,

I would think that the squeezing method you suggest would be worth a try in a non-critical test but is there not a possibility that it would just be altering the shape and not the overall cross-sectional area? It could be that there will also be sufficient elongation to give the required effect however.

Another method of increasing resistance is to file a section of the shunt which will reduce the CSA and increase the resistance, carefully guarding that the filings don't go where they're not wanted, of course. Depending on their size, various instrument shunts were/are slotted or cut to trim their value. The printed shunts in later Avometers are trimmed by cutting printed tracks and laser trimming is standard practice for various film resistors.

PMM
pmmunro is offline  
Old 12th Jul 2020, 6:07 pm   #11
MotorBikeLes
Nonode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
Default Re: AVO8 DC Current range error

PMM, you just could be correct. In light of that, fresh suggestion.
A pair of crimping pliers. The thinner type, not the big "break back" versions.
These put a "U" in the joint, so increasing the length. If it should be overdone, a blob of solder somewhere will give a parallel resistance briging back down a bit.
Les.
MotorBikeLes is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:08 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.