Yet another AVO

[sparkymike]

The latest find refuses to show any volts/resistance or ua. on the dial. Needle rests around the 5 volt mark on the 0-100 scale, so before I make a start I am thinking that it could either be corroded probe sockets,faulty cutout,dirty selector switches or the circuit just before the meter itself.
For starters, what is the method for checking just the meter?
Mike.

[Sinewave]

Have you got a micro amp/milli amp source?

When meters are unresponsive I connect direct to the moment.

[See_Mos]

Set a DC voltage range and rotate the meter so that the inertia moves the needle, is it free to move?

Set a digital meter to low ohms and apply direct to the movement terminals, the needle should move.

Set the DVM to a higher range and measure the resistance at the movement terminals.

One of mine did have intermittent cutout contacts, cleaning with a piece of thin card soaked with switch cleaner fixed it.

[sparkymike]

I have a Farnell bench power supply and can go down to 0.3 volt minimum.
0.3 amps output I believe.
Mike.

[The Philpott]

Re Post No.3, When and if putting a DMM across the movement, Set it to a high resistance range before moving to lower resistance ranges. Careless use of a low range may apply too much current (with possible reverse polarity) and risk damage to the movement. This is the reason an auto-ranging DMM should not be used for this particular test.

First things first though, if the needle will not zero by use of the screw on the front of the facia, the coil springs in the movement should be examined for distortion or tangling. Distortion is death, mere tangling can (sometimes) be rectified.

Dave

[sparkymike]

Hi Dave, will take all points on board.
On closer inspection it does look like it has been got at in one or two areas.
There is a bridge rectifier that does not look original, fixed on to the upper Paxolin board and wiring to and from it look a bit iffy. (In other words not professional as AVO would have done it.). So far all resistors and coils read out ok. Do the reverse direction switches ever go open circuit. ?
If so, heaven only knows how you get at them !! At a quick glance I guess the movement has to be removed in order to get at the item.
I will not get much chance today to carry on as yet another funeral to attend.
Another last Wednesday !!
This is what you get when you outlive your old friends.
This particular was an old friend Rex Garrod, whom was inventor and engineer of some fame.
He produced a TV program in the 80's all about how things work. You may remember it.
He built a water powered clock that was on the front at Felixstowe for many years.
Mike.

[The Philpott]

The name certainly rings a bell.
The REV MC switch is quite simple, and yes, access is after the movement is removed. I once (by chance) encountered a fixing screw for REV MC that had stripped out (they are quite short on, for example, a Model 8.)
Dave

[sparkymike]

Photo of the bridge rectifier and wiring. (It looks better in photo than with the eyes.) Where would this normally be sited, or does that single diode with the red cap do same job.?
Mike.

[The Philpott]

Model 8 (early) copper oxide rectifier.
Dave