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Old 23rd Jan 2019, 3:17 pm   #1
Bobdger
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Default AVO Type D 48A

I have just been given an old wooden box with a non legible plate on it. when the outside of the box was cleaned the model and ser number could be read.
Adpat 48A Ser n 18025 - 1144
on opening the box I found a type D avo meter with current shunts, current transformer and a hi voltage adaptor. The high voltage adaptor enclosed within a wooden box that is very well made the unit has only 2 connections and is rated at 3600 volts. The meter movement scale card has a date of 1949.
When I started to clean the outer box I found under the user instructions for a type D a as new plate with instructions for a type 48A. The meter needs an square type battery about 3" long for the ohms range. the voltage range works but is out of calibration by about 20%. I have not checked the current ranges yet. Time to find a circuit.

Bob.
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Old 23rd Jan 2019, 3:38 pm   #2
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Default Re: Avo type d 48a

The serial number will be appreciated if you write it on the data survey thread.

Thanks
 
Old 23rd Jan 2019, 5:54 pm   #3
Superscope
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Default Re: Avo type d 48a

Hi Bob,

As you have probably gathered, the Model 48A would have been the correct Meter for that Box.

Nice to have the Brass Plate on the Box.
Many surviving Boxes have lost their Brass Plates and many like yours have also lost their original Meter.

The Model D has obviously evicted the original Meter.

Regarding the 20% out of Cal, is the Model D reading High or Low?



Ian
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Old 23rd Jan 2019, 7:33 pm   #4
Bobdger
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Default Re: Avo type d 48a

Hi Ian
The readings are high.
Thanks.
Bob.
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Old 23rd Jan 2019, 7:51 pm   #5
Superscope
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Default Re: Avo type d 48a

Hi Bob,

That would imply that you have an open circuit Shunt Chain.
If you check the Current Ranges, you will probably see the same thing. Reading High around 20%!

The Shunt Chain is the name given to the Series Resistance Winding's which sit in
Parallel across the Moving Coil Meter Movement.

Normally, the Current feeding the Meter movement is split between the Movement itself
and the Shunt Chain.
If the Chain is open circuit anywhere along it's length, then there is more Current available
for the Moving Coil and it reads high. 20% high would suggest this is happening.

I don't have a Schematic of the Model D, but it shouldn't be to difficult to track down when you get one. It narrows this particular fault down a bit.
The problems come when the Resistance Wire is broken but you can't find the ends of the break. Repair becomes interesting to say the least.


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Old 24th Jan 2019, 1:12 am   #6
pmmunro
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Default Re: AVO Type D 48A

Sorry these copies of my Model Drawings are a bit rough but they should help.

The shunt board information is a bit better.

PMM
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Attached Files
File Type: pdf Model D DC circuit.pdf (200.4 KB, 44 views)
File Type: pdf Model D AC circuit.pdf (217.3 KB, 47 views)
File Type: pdf D Switches.pdf (219.6 KB, 39 views)
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Old 24th Jan 2019, 1:13 am   #7
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Default Re: AVO Type D 48A

And the inner view of the lower shunt board

PMM
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Old 24th Jan 2019, 8:56 am   #8
Bobdger
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Default Re: AVO Type D 48A

Thank you PMM.
This evenings job with "electronpusher" after the Haggis and neeps. is to open the Avo up and see inside. I will report back.
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