Doubt with AVO 40 range 1000 ohm

Hello, yesterday i was testing an Avo 40, everything is correct in all ranges, the interior is clean, untouched and all the shunts seems to be ok.

The Avo is impossible to zero on the 1000 ohm range, The others are OK. I tried with another battery, tested the contacts, also tested the shunt board.

I can only get zero if i try with a 4,5 volts battery, (probably i will get zero with less voltage, perhaps 2 volts, i did not try with the power supply).

With the same battery i can Zero others Avo 40, 47, etc.


The resistor for 1000 ohm range is 15 ohm and the wire looks original. I have not desolder the wires to test the resistor alone, due to it is untouched and do not want to desolder the original joints.

Any idea?

[pmmunro]

Author,

The P and R pots need to be really clean to be able to get this range to zero. The wire wound track, the phosphor-bronze wiper and the spindle contacts all need to be thoroughly cleaned as do the battery contacts. A cotton bud with isopropyl alcohol is usually enough for the wire-would tracks.

The movement has to be taken off the front panel to get access to these pots. so the overload cut-out will have to be adjusted when you put the movement back.


PMM

Thanks, I tested for continuity the P pot yesterday, but i did not notice that that the spike of the P pot was little bit loose, so i disassembly all the parts and cleaned it perfectly.

Also, i took apart the movement, i cleaned the R pot and also the cut-out.


Yesterday i also repaired an Avo 47, curiously the 1000 ohm range was not working too, in this case the wire resistor was open.

Now both are working!!

Thanks!!! Peter

[The Philpott]

Postscript: If all the above remedies should ever fail, there are a couple of other likely possibilities:

1000ohm range difficult to zero- leaf switch problem.
1000ohm and 10000ohm ranges difficult to zero- Q pot problem.

Dave