Avominor Mod 1 refurb (general)

[The Philpott]

Having made a few avominor model 1 units behave normally i thought it would be worth sharing a few notes. (apologies for any duplication)

The movement is of course rather fragile, i have found the damping to be so non-existent that anything over 80% fsd will make the needle hit the ceiling before coming to rest. The second most common problem i found was the 100v bobbin being o/c. This of course then disables the 250v and 500v ranges as the bobbins stack up in series as you use higher ranges.

This bobbin is usually marked with red paint, and is the SECOND one up not the third.

The 2.5v bobbin is inboard of the others and is 2k.ohm.
5v is 8k (10k cumulative)
25v is 10k (20k cumulative)
100v is 30k.(50k cumulative, you get the idea)
250v is 60k

At the 500v range things are different- it employs two 50k bobbins which are necessarily wired in slightly differently from the others. These 50k bobbins are also, it seems, prone to going o/c.

I have done some transplants, but also been lazy and just piggy backed precision resistors across the dead units with good results.

For those wishing to retain bobbins, i note that after the defective winding is removed a loop of thick wire remains on the bobbin (usually green) Both ends of this 'saddle' poke up through the end of the bobbin, one is tinned for soldering to the terminal pylon and the other has the winding wound round and soldered to it and glued over with (shellac?)

The CURRENT bobbins are usually ok unless someone has thumped too much current across the 500ma terminals. The glass will be brown if this has happened!

Good choice for replicating the original 1.5volt dry cell fitment is easing 11 or 12mm bore thinwall general rubber fuel hose (unreinforced) over an AA with use of wd40, and using a brass 10mm nut (with 17mm hex) A short piece of 10mm brass studding is then wound into the nut to adjust the mod. to a suitable length, this studding can then be pin punched to secure if required.

Make sure the ohms adjuster is fully anti-clockwise before shorting the terminals to use resistance ranges. with a new cell this should give 80% deflection or thereabouts. (None of my ohm adjustment coils work properly, they are all dead noisy- tried resoldering the wiper on; no dice)

best tool for removing swarf from the concentrator is blu tac on a wooden toothpick (one of you lot told me that i think)

Regards. Hope this helps someone

[Phil G4SPZ]

Which model of the AvoMinor are you describing? Some serial numbers and pictures would be helpful. I have a small sub-collection of Minors including Universal and DC-only models, and none of mine have a 2.5 volt range. The basic movement sensitivity of the Universal AvoMinor is 2.5mA or 400 ohms per volt, so the cumulative total resistance of all the multipliers on the 500V range should be 200,000 ohms.

Otherwise, I agree with most of your suggestions. AvoMinors are a subject in their own right, as they are significantly different from conventional Avometers. Instead of using precision resistors to replace bobbin multipliers, I fit standard ones, either singly or in series combinations, selected using a DMM.

[The Philpott]

You're right, i have refurbed so much it has driven me round the bend. 5v not 2.5v

I meant: 5v- 2k
25v- 8+2
100v- 30+8+2
250v- 60+30+8+2
500v- (50+50) 60+30+8+2