Avo 7 repair

[EF80TVVALVE]

Hi all,

I thought I would make a start on my Avo 7. Initial inspection it looks very clean and untouched. The battery tray is free from corrosion and the bakelite face and housing are in good order. The internal condition looks the same, untouched, clean and original.

I have fitted two 4.5v cells and the D cell and gave it a test. I have noticed that on both of the 10,000 & 1Meg resistance settings that the readings are off. 10,000 is not by much but the 1Meg setting is impossible to adjust to 0.

I also noticed that one of the windings on the paxolin strip looks a bit off, I'm not too sure if it has became damaged from misuse or if the wire has suffered from corrosion so I've attached some photos, although it's a bit difficult to get a good one.

This is the first time I have serviced one of these meters so any pointers will be appreciated, in the mean time I'm going to source a schematic and go from there.

[Refugee]

The wire on the centre terminal of the pot in the second photo looks like a dry joint.
What is that bit of wire with the fluff on it on the middle winding?

[Phil G4SPZ]

The attached circuit diagram should help.

Those windings look fine to me. You should see the state that some of them get into, yet still work! Sorting out the ohms ranges is quite straightforward, mainly a case of identifying and measuring the range resistors and replacing as necessary. It's also worth cleaning the P, Q and R pots gently with some switch cleaner. The R pot is difficult to get at though with the movement in place. In my experience, the back contact behind the Q knob can be responsible for all sorts of odd faults, so make sure it is thoroughly clean and making proper contact.

Good luck!

[The Philpott]

Problems with the 10kohm and/or 100kohm range may indicate the arrow shaped contact spike on the end of the Q adjuster has worked loose- it is 'pinched' into the hollow end of the Q shaft by a nut. The bronze leaf onto which this spike sits may be dull as well.
The R & P wiper blades could be dull and not making proper contact with their windings.

The rotary Q wiper blade or the Q winding could also be dirty- this would affect the 1Megohm range. Observe these areas first before moving on.

One other thing- i seem to recall that on some Model 7 and 40's one of the solder tag connections on the outside of the battery box is very prone to getting a dry joint- Two solder tags link direct to each other without a wire in-between, and when the screws are tightened up one of the tags may shift which starts a shear fracture in the solder blob. Impossible to see with the naked eye, prodding required!

Dave

[The Philpott]

Ah, Phil has prompted my memory-

The R adjuster shaft is brass, and and can tarnish thus not making proper contact with the springy leaf. It can be removed with the movement in place, but tweezers needed. Two out of three avo's of this vintage will have this problem; Vaseline stops it happening again.

Dave

[Phil G4SPZ]

Originally Posted by The Philpott

Impossible to see with the naked eye, prodding required!

Quite! I had a related fault where zeroing was intermittent on the ohms/100 range on a Model 8 recently, due to a marginally loose screw in the battery compartment. I thought it was me who had the screw loose, as it took me ages to find it!

[The Philpott]

These pictures zoom into the 'solder sausage' that i mentioned (This is a Model 7 Mk.II) The dome head screw that is just visible (sideways on) is part of the + (either knurled or hex nut) terminal for the 1.5volt supply.

Another trouble spot for lack of continuity- in this meter the other screws visible are cad. plated steel self tappers! In most cases the pointy end just about pierces the 1.5volt section of the battery box, and if there has ever been cell leakage the cadmium is no match for it- although the head of the screw may still be shiny.

Dave