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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 5th Mar 2013, 9:37 pm   #1
Phil G4SPZ
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Default Avo 7 Mk II protection diode SA1

I recently encountered a 1950's vintage Avo Model 7 with a strange fault that I'd never seen before, in that all DC voltage, DC current and ohms ranges were perfectly accurate, but all AC voltage and current ranges indicated about 60% low.

This turned out to be due to the protection diode having gone virtually short-circuit in both directions. This diode is shown on the Mk II schematic, wired in reverse polarity across the + and - terminals of the 5mA bridge rectifier, but it's not fitted on all examples of this instrument. I snipped it out, and the AC ranges returned to full accuracy.

One other thing I now know is that the correct in-situ resistance of the CT secondary winding is 200 ohms, measured by placing the prods of another Model 7, switched to the 100,000 ohms range, across the secondary. In one direction, this action should also drive the pointer to about 3/4 FSD, and in the other direction, backwards against the end stop.

The purpose of the diode is to generate pointer movement if an AC overload is applied to the meter set on a DC range. Without the diode, the pointer would not move, as the moving coil would just sit there humming to itself and could be burned out. The diode rectifies one half of the AC potential and causes the coil to move, hopefully sufficiently hard against the end stop to trip the cutout. This diode also appears in Model 8s, but wired across the movement itself.

The wee beastie below is from the Model 7 and was wired across the AC-DC leaf switch contacts. I expect a germanium diode would serve as a replacement if desired, but we never make silly errors like applying AC to an Avo switched to DC, do we?

Phil
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Old 5th Mar 2013, 10:11 pm   #2
Alistair D
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Default Re: Avo 7 Mk II protection diode SA1

If you find a Germanium diode is not suitable I have a stock of BAT49 Schottky rectifiers that might do the trick. PM me your address if you want a couple. FOC.

Al
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Old 5th Mar 2013, 11:08 pm   #3
Phil G4SPZ
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Default Re: Avo 7 Mk II protection diode SA1

Hi Alistair,

Thanks very much for your kind offer, however I won't be fitting a replacement... having done a little more research, both in this old thread and in the service manual for the Model 7, where SA1 is described as a "surge suppressing rectifier", it is not at all clear what the purpose of this component actually was.

Given that it appeared relatively late in production and all Mk 1 Model 7s seem to survive perfectly well without it, I'm going to leave it out.

Thanks again anyway!

Phil
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