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Old 2nd Dec 2017, 8:15 pm   #1
Join Date: Sep 2008
Location: Londonderry, Northern Ireland, UK.
Posts: 58
Default Can the voltage at the PF terminals of an Avo Model 7 Mk2 be used as a diagnostic?

A while back I had either an Avo7 or an Avo40 with dicky AC side, it's fixed and I don't remember the cause but during the investigation/repair I had the thought that a DMM across the PF terminals would surely see virtually the full working output voltage of the transformer.
Am I up a gum tree?
I have just got a Model 7 that reads low on AC V. I have had a play, first with 1 and then 2 Avo M200x's across the PF terminals.
I didn't take note of the actual indicated voltage on the 1st DMM but it was over 0.5V, adding the 2nd caused a 3mV drop in the reading given by the 1st and there was no discernible change in the reading given by the Model 7.
Putting 100V across the the Model 7 on the 100V range gave a reading on the Model 7 in the high 80's and the DMM's across the PF terminals showed around 1.149V ( one is TRMS and the other average? responding).

I am curious if measuring the voltage across the PF terminals has any diagnostic value?

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Old 2nd Dec 2017, 8:40 pm   #2
The Philpott
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 1,685
Default Re: Can the voltage at the PF terminals of an Avo Model 7 Mk2 be used as a diagnostic

I am very interested in people's opinions of this.

On occasions when i have had a avometer reading ACV too low, if it's an under-read to the tune of 60% it has been a switching problem (current going through DC parts of the meter that it should not be??) and if it's an under-read of the order of 10 to 25% it has been down to a lazy rectifier. Often when the rectifier is clapped out it can be seen to be quite corroded on the outside.

Last edited by The Philpott; 2nd Dec 2017 at 8:55 pm.
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Old 2nd Dec 2017, 11:39 pm   #3
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,166
Default Re: Can the voltage at the PF terminals of an Avo Model 7 Mk2 be used as a diagnostic

I think I'm correct in saying that the current transformer only operates on the AC current ranges plus the 10 volt AC range. Looking at the circuit diagram, the P.F. terminals are indeed directly across the CT secondary (with a total of 40 kilohms resistance in series) but the CT secondary is shunted by the instrument rectifier and the movement. The rectifier will present a non-linear load, and hence measuring the actual voltage at the P.F. terminals may not be particularly informative.

I'd add to Dave Philpott's suggestions that low readings on AC ranges can be due to the single diode S.A.1 across the moving coil being leaky - I've encountered this on at least one occasion. This protection diode is only there to cause the movement to move and trip the cut-out in the event of AC inadvertently being applied to the meter whilst set to DC, so you can cut it out completely and it won't affect the accuracy of the instrument.

“The place where optimism most flourishes is the lunatic asylum” - Havelock Ellis

Last edited by Phil G4SPZ; 2nd Dec 2017 at 11:45 pm. Reason: Correction & clarification
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