View Single Post
Old 11th Mar 2018, 5:09 pm   #4
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
Default Re: Avo Universal Measuring Bridge Type 1

Avo Test Bridge.
It would be helpful if you would post a picture when you have received the instrument so that we can identify it properly.
The original Test Bridge of 1935-45 has a bakelite case, a rotary balance control and a single 6B8 valve. Unlikely to be yours.
Universal Test Bridge. Uses single 6SN7. Rotary balance control on horizontal panel. Manual 12 pages.
Universal Measuring Bridge typeI, Suitcase Style. Around 1960. Uses two 12AU7. Plessey MarkIV mains connector. For military use. Large flat front panel, used horizontally. Main balance control is a long linear scale. Manual is 38 pages, or a shortened version which I posted years ago of 27 pages.
Type 150/3. Vertical front panel, and rotary balance control. Uses transistors/I.C. 1970/80.

I had the Suitcase version. The problem is the linear slider balance resistor. It is a bit fragile, 11" long, with resistance wire wound on a 1/2" diameter paxolin tube. The wire fractures from wear.
That is the weak spot with this model. I had to rewind mine. A fiddly job, but not impossible. Value as rewound is not particularly critical, around 5.5k. I have suitable resistance wire, which came originally from a GPO damping resistor panel, with a dozen of bobbins of the wire on it. I can let you have a bobbin amply sufficient if you want it.

I would not haste to change components. Often changing components will cause more problems than your efforts will cure. I would change the two smoothing electrolytic capacitors, but leave everything else. Measuring voltages everywhere will tell you if there are any other problems.
Clean and sparingly lubricate all the switches, and in particular the slider for that linear balance pot. But do check the rubbing contact on the slider which goes along the resistance wire as well, and be prepared to replace it with a contact from say an old volume control.

For cleaning, I always use a mix of white spirit with some vaseline, applied with a brush or scrubbed with a toothbrush, as suggested years ago by Marconi. Or buy it in a nice squirting tin as Servisol / Electrolube if you want to pay for the convenience.

Cleaning the outside. I use meths or better surgical spirit (same alcohol, but without the sticky blue dye to stop you drinking it) or Isopropyl alcohol. But remember that common warm soapy water will clean almost everything with less risk of other damage than when using solvents. Philips Oscilloscopes you must use soapy warm water, as alcohol ruins the finish.

Search the forum here for details of retouching the lettering. Useful tips there.

I agree very much with your comments about chalk and cheese comparing analogue with digital measurements. There is little value in knowing a capacitor accurately to 0.01% when the original specification is +20%- 10%. Even the standards are only to 1% I think. The resistors of that era tend to go high.

Do please keep us informed how you get on.
wme_bill.
WME_bill is offline