AVO Universal Measuring Bridge - UK Vintage Radio Repair and Restoration Discussion Forum

RobinBirch · 12th May 2008, 10:27 pm

Hi,
Recently bought this for not very much and found when it turned up that it didn't work.

It looked as though it had been dropped in that the case was slightly distorted and the glass window for the balance meter had come loose. The balance meter also read very very strange and no balance could be achieved. Externally it was filthy.

So. Many thanks to Colin Armstrong for the manual. I then bought a new set of caps for all of the tubulars and gave her a good external clean. The meter window was glued in place and the caps changed (except one that I found I had bought the wrong value for. I tested it and it wasn't too far out so it was left in pending a 0.02 uF at 500V passing my way).

Switched it on and I could get the centre zero to work - immediate massive improvement. However, on trying to balance the bridge the meter danced all over the place. Hum

Quiet think. Obvious really - bad contact on the balance resistor. Tried dribbling some switch cleaner in to no effect

. At this point things started getting annoying.

On first inspection the chassis looked really easy to work on - components relatively accessible and all of the metalwork bolted togeter with relatively clear access - except - if you want to get at anything inside all of the outside has to come off and it is all hung togeter with a lot of separate wires. To get the balance resistor out you have to release the entire chasis from the front panel together with the scale selector switches, two pots and two push button assemblies. Once you have done this and unsoldered several wires to allow everything to move just enough, you can get the balance resistor out. Not difficult but very very fiddly.

The balance resistor is a wirewound one about 8 inches long with a slider that supports the scale graticule. Took it to pieces and wiped the contact area with de-oxit and cleaned the slider up. Moved the wiper part into a beter position and put it all back together. This again was very fiddly, the whole thing is just the wrong size for my fingers.

Still, screwed it all back together, soldered all of the wires back in place and switched it on. Bingo, it works

Seems reasonably consistent when compaed to my fluke on both R and C. I haven't tried it on L yet. It has been running on my dining table for about an hour and seems quite happy - result -

So yet again, thanks to every one who answered my call for help. In principle very easy to repair just a very fiddly ******* to take to bits.

Robin

12th May 2008, 10:27 pm	#1
RobinBirch Hexode Join Date: Apr 2003 Location: Cotswolds, UK. Posts: 465	AVO Universal Measuring Bridge Hi, Recently bought this for not very much and found when it turned up that it didn't work. It looked as though it had been dropped in that the case was slightly distorted and the glass window for the balance meter had come loose. The balance meter also read very very strange and no balance could be achieved. Externally it was filthy. So. Many thanks to Colin Armstrong for the manual. I then bought a new set of caps for all of the tubulars and gave her a good external clean. The meter window was glued in place and the caps changed (except one that I found I had bought the wrong value for. I tested it and it wasn't too far out so it was left in pending a 0.02 uF at 500V passing my way). Switched it on and I could get the centre zero to work - immediate massive improvement. However, on trying to balance the bridge the meter danced all over the place. Hum Quiet think. Obvious really - bad contact on the balance resistor. Tried dribbling some switch cleaner in to no effect . At this point things started getting annoying. On first inspection the chassis looked really easy to work on - components relatively accessible and all of the metalwork bolted togeter with relatively clear access - except - if you want to get at anything inside all of the outside has to come off and it is all hung togeter with a lot of separate wires. To get the balance resistor out you have to release the entire chasis from the front panel together with the scale selector switches, two pots and two push button assemblies. Once you have done this and unsoldered several wires to allow everything to move just enough, you can get the balance resistor out. Not difficult but very very fiddly. The balance resistor is a wirewound one about 8 inches long with a slider that supports the scale graticule. Took it to pieces and wiped the contact area with de-oxit and cleaned the slider up. Moved the wiper part into a beter position and put it all back together. This again was very fiddly, the whole thing is just the wrong size for my fingers. Still, screwed it all back together, soldered all of the wires back in place and switched it on. Bingo, it works Seems reasonably consistent when compaed to my fluke on both R and C. I haven't tried it on L yet. It has been running on my dining table for about an hour and seems quite happy - result - So yet again, thanks to every one who answered my call for help. In principle very easy to repair just a very fiddly ******* to take to bits. Robin