AVO VCM Mk3 - Meter switch

[JanWra1]

Hi guys,
I have an AVO Mk3 which I had to dismantle in order to clean it properly and get surface rust of all the panels before painting them.
While dismantling I missed to mark the Meter Switch so that the knob with the skirt could get back in the right position. All other knobs/switches are easy to put right as the have stops att each end of their scales, but and this is the problem- the Meter Switch just turns and turns with out any end stops. So how to get it right i.e. so the indicator on the skirt points at the right value?
Anyone have any advise on this?

Jan, Sweden

[David Simpson]

God dag Jen, scriva til "Dekatron" - - Martin Forsberg po Linkoping Stan.
Martin is the Svenska expert on AVO VCM's on this Forum.

Regards, David

[Dekatron]

He wrote to me but I can’t get to my documents as I am away working, and I can’t remember which position the knob should be in. A photo of the front and back of the switch with the knob in the same position should solve it nicely!

[ms660]

You could check the meter switch position by using a test meter set to measure Ohms.

Make sure the unit is unplugged from the mains:

Connect a shorting link across the meter terminals, set the circuit selector switch to Test, set the mA/V scale switch to Outer, connect Ohm meter between the shorting link on the meter and the anode of the backing off circuit rectifier on the power supply panel, rotate meter switch until the resistance reading on the Ohm meter reads 0 (zero) Ohms or thereabout, that position should be the mA/V position, fix control knob to suit, remove the shorting link from the meter when ready.

Lawrence.

[JanWra1]

Hi Guys,
Thanks Lawrence for the advice, did as you suggested, see enclosed picture, and got the following result:
11 positions gave no connection and the 12th indicated 441 Ohm. Is this the correct mA/V position.

Is this right or is something else wrong/missing

Jan

[JanWra1]

Correction

1 position is 441 Ohm
Next pos counterclockwise is 710 Ohm
followed by 910 Ohm, 930, 940, 940, 940, 940, 940, 940, 940, 940,

[ms660]

I used the last schematic in the link below, in the schematic before it the diode is connected the other way round so try connecting the probe to its cathode and see if that make any difference:

https://frank.pocnet.net/instruments...VCM_Mk-III.pdf

Lawrence.

[JanWra1]

Hi again guys

Connected the Ohm-meter to the other side of the rectifier and the result is that all positions indicated abt 200/210 Ohm. Tests were done both with V1 and V2 in place and removed made no difference whether the valves were in place or not.

Jan

[ms660]

I think the 441 Ohm resistance reading in the 1st set of results is most likely the correct one for the meter switch being set to the mA/V position, apparently there's a 750 Ohm resistor in series with the mA/V contact on the meter switch, unfortunately it's not shown in the schematic in the original manual, according to a quick calculation it would mean a 750 Ohm in parallel with a series combination of 200 Ohm + 760 Ohm which calculates to be 421 Ohm.

There's a modified schematic showing the 750 Ohm resistor in the link below, its circuit ref. is R39 in Figure 33:

https://nvhrbiblio.nl/schema/AVO_CT160_rht.pdf

Lawrence.

[JanWra1]

Hi Guys

Thanks Lawrence, the 441 Ohm position turned out to be the right one. My Mk3 is now working with a new meter from Portugal.

73 Jan

[ms660]

No problem.

Lawrence.