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Old 10th Mar 2018, 5:41 pm   #1
Stevie342000
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Default AVO Universal Measuring Bridge Type 1

I finally bit the bullet and bought one of these yesterday (it has not arrived yet) but I thought I would ask some questions on how I go about repairing the item (my assumption being that the old paper in oil capacitors will be shot).

I have lots of pieces of equipment that I need to repair and I thought this would be a useful piece of kit to have to test all those old capacitors. I know I'm making work for myself when I should taken it as a given and just replace them all.

Having searched the web and this forum I have all the information I need including the manual for me to repair the item - it was the first thing I did before I purchased it.

1. I assume I should be looking at replacing C1 - C9, C11 - C16 & C22, the others are the precision standard capacitors.

2. Should I be looking at any of the resistors? My thinking is the 20% tolerance is not as precise and may have drifted, will check the high values to make sure they are in spec - I have a digital LCR which I can use or an Avo Model 8 (note to self must look to see if Maplin have the BLR battery).

3. How do I clean the switches and with what? I'm not wishing to dismantle them.

4. Do I need to calibrate the instrument once I have changed the capacitors (and if so not sure how the instructions are not particularly clear)?

5. The reason I am not looking at most of the resistors is that they are precision mostly 1% or 2%, they will of course have a lifetime as well but they should not have drifted much, is this a wrong assumption? Again I am assuming that they will be pink bodied and be made by Paignton.

Thanks, I hopefully have a number of winters work to keep me going once this has been fixed.

Ah yes why do I not just use the digital LCR, well this instrument in my mind is far more useful for matching components. I assume if you attach a known tolerance resistor or capacitor this then becomes the standard against which you are measuring which means you can get really closely matched or tolerance values.

Oh yes of course it is me being a bit of completest freak and wanting to use period equipment on vintage restorations. Apart from which this is stunning to look at much like my BBC Test Equipment - TS10 & ATM 1 (both of which will be checked over with this piece of test kit before being put into service.

In addition I have a Scopex 4D/25 dual beam oscilloscope - but no idea of that works or not - it has been discussed here (I will need to go read that thread). I have the bits for lamp limiter as well as instructions.

Other than that I do not think there is anything I have missed, Oh yes the Heathkit AG-9U, needs repair as well. Then there are all the reel to reel tape recorders and other bits I want to build from new - including a valve stereo FM Decoder for the Quad FM1 and the Jason FMT-3 (Wireless World Article 1962).

The only thing I do not have is a frequency counter but I thinking of going modernist with that and getting a new one from China, a valve one would be big and probably expensive and solid state will contain unobtanium thus I am taking the supply source with the least resistance plus they are cheap, any thoughts or suggestions?

I have some experience in building valve equipment and about 20 years back I built a hybrid phono amplifier (an ETI project) which worked first time and looked like a rats nest as it was not in a case and it worked perfectly first time I powered it up (that is another one of my projects to revisit too). My knowledge and understanding is what has increased in that time frame and now it is time to tick items that I want to repair or build off my bucket list.

7. The lettering around the switches and dials looks like it needs to be filled in again - what should I use to do that? Correction that was the other earlier model I was looking at not this one - the front panel lettering is perfect.
8. What should I used to clean the outside so I can determine if that lettering is white or red, assuming they did not use any other colours on the front panel?

My plan as a starter for 10 is to build the lamp limiter, check the Universal Measuring Bridge over visually and then fire it up to determine if there any issues before replacing the capacitors, I do think replacing all of them except the standards is over kill but they are all going to be at least 50 years old and they will be near or at their end of life.

Oh yes I have variac as well 5 amp which I am going to include in the lamp limiter, it should power most of the equipment that I have. However I feel this Avo Universal Measuring Bridge is the linchpin as at least I can test all the equipment I have and replace what needs replacing to bring it all back into working order.

I have spent the last 10 years plus hoarding tape machines, Brenell (it is my opinion that the Brenell tape link is the same as the Jason tape link and appears about the time Jason goes out of business and Brenell bought the rights to produce it)and Jason Tape links (the article to build one of those from Kit form is in one of the tape recorder magazines on American Broadcast history), amplifiers and so forth building the system I want from new to bring this all back into working order.

I have for now called a holt to purchasing any more items the Brenell STB - 2 I got the other week was the last piece in the jigsaw, other than full studio tape machines by EMI or Leevers-Rich and honestly I do not really need those and it would be more overkill. The end!!!!

Last edited by Stevie342000; 10th Mar 2018 at 6:03 pm. Reason: Correction
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Old 11th Mar 2018, 2:38 am   #2
Ed_Dinning
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Default Re: Avo Universal Measuring Bridge Type 1

Hi Steve, is this the bridge in the tin box, or the earlier one?

I may have manuals at home for both of these so mail me if you need a copy.
The balance controls on both bridges will need cleaning but do with extreme care, use meths or similar and re-lube

Ed
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Old 11th Mar 2018, 3:20 am   #3
Stevie342000
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Default Re: Avo Universal Measuring Bridge Type 1

It is the later one in the tin can, duly noted about cleaning method. I will of course have a better idea of what you are saying when the item is sat in front of me, just trying to get ahead of the game.

I have a 20 plus odd page copy of the manual, I assume that there are no more pages than that? I assume you posted it on here or you got the same copy? Mine came off this site slightly undersized scan so does not print to margins on A4 not without a lot of messing about that is.

I am just taken it as a given that I should replace all electrolytic and the paper in oil capacitors but none of the standard resistors or capacitors.

I got lucky I suppose this one I am about to get is in spotless condition from the outside both the panel and the container. I know they do not come up that often have been looking for one for a while and just like I said got lucky it was relatively cheap as well about 50 all in.

Given that it was military grade and made to pretty much the same standard as BBC test equipment there is not much to go wrong and it is relatively easy to fix.

The hard to get components will be the meter and bridge balance resistors I would assume.

I assume you are familiar with this equipment and its repair? Are there any known issues I suppose is what I have been trying to find out?

I has been suggested in another thread that it is not very accurate when you compare a resistor measured on Digital LCR and when it is measure on the Avo Universal Measuring Bridge.

The parameters are not the same its like comparing chalk and cheese. However it is testing the components are more real world scenarios that is the conclusion I have drawn from what was said on that thread.

I got this kit so I could (possibly) match components to circuit value to a high tolerance from channel to channel, but the real reason was so I can do leakage tests to determine which capacitors in which bits of equipment I have awaiting repair need replacing. Plus it does a fair job of explaining in the manual how to make in circuit tests as well and as I have found out it is relatively easy and quick to use (having read the manual).

Last edited by Stevie342000; 11th Mar 2018 at 3:25 am. Reason: slight addition
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Old 11th Mar 2018, 5:09 pm   #4
WME_bill
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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.
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Old 11th Mar 2018, 5:46 pm   #5
Stevie342000
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Default Re: Avo Universal Measuring Bridge Type 1

I have just snapped the pictures of the item in question from eBay.

It is the clam shell metal case, it has a captive mains cord from what I can see.

What should be in the lid that is not there?

I have the 27 page version of the manual I got it off here and I think it was you that posted it from memory.

The one thing I do not know because it is not mentioned in my manual (unless my eyes have missed it on the schematic) is the various working voltages that should be present. I could figure it out but it is a lot of maths to do so.

I will go check out the other thread where someone was fixing one of these, I think they replaced the wax type paper in oil capacitors and possibly the electrolytic capacitors.

I am of course assuming that mine is the 12AU7 version, I have no pictures of the inside.
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Old 11th Mar 2018, 5:53 pm   #6
WME_bill
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Default Re: Avo Universal Measuring Bridge Type 1

Avo Universal Bridge, Suitcase style.
Yes, that identifies it nicely. The lid is just a duplicate of the base, with clips that fit over the top. Nothing in it. Not like the Avo Valve Tester CT160, where the lid contains the valve bases.
Voltages you can almost guess from the usual operating conditions of the !2AU7. Quite close enough, apart from the main HT rail. I seem to remember around 200v. But changing the smoothing electrolytic capacitors will resolve that.
Good luck. I do hope the slider resistor is unharmed.
wme_bill
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Old 11th Mar 2018, 6:23 pm   #7
Stevie342000
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Default Re: Avo Universal Measuring Bridge Type 1

Thanks Bill I sent a PM requesting a copy of the manual. I might take you up on the spool of resistance wire just in case. I sent one to Colin as well. Having just had a quick scan of various threads the last two digits are visible on the pictures I have of the serial number it ends in 74 and the type is UMB.

Thanks
Steve

Last edited by Stevie342000; 11th Mar 2018 at 6:26 pm. Reason: Add info
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Old 11th Mar 2018, 7:02 pm   #8
Stevie342000
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Default Re: Avo Universal Measuring Bridge Type 1

Thanks Bill for the manual, I agree not the spirit of things, which is why when I find something I usually post it on here or in other forums on line sort of undermines the gold diggers.

Duly noted get the power supply working correctly and change any grid capacitors and no more. It's the leakage test function that I want this item for mostly.
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Old 12th Mar 2018, 4:50 pm   #9
Ed_Dinning
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Default Re: Avo Universal Measuring Bridge Type 1

Hi Steve, Yes I have that version too; as has been said the slide wire is one of the main problems as well as switch contacts that need cleaning.

Ed
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Old 13th Mar 2018, 3:49 pm   #10
Stevie342000
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Default Re: Avo Universal Measuring Bridge Type 1

Well it has arrived but I am not sure which screws I need to remove to get it out of the case, help ?
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Old 19th Mar 2018, 11:44 am   #11
Stevie342000
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Default Re: Avo Universal Measuring Bridge Type 1

The answer is the 4 large nuts on the back, there may be others, will investigate further and report back. It's not in the manual either.
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Old 19th Mar 2018, 12:54 pm   #12
Stevie342000
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Default Re: Avo Universal Measuring Bridge Type 1

That was indeed the answer the 8 4BA cheese head screws were not the ones to remove they held some small metal parts which wedge the front panel in.

One of those is missing a screw and it arrived with that bit rolling about inside, only for me to add to the bits rolling around inside. They are now back in place as the 4 large bolts on the back did indeed enable me to remove the item from the case.

The large wire wound resistor looks to be in good order, the valve shield for V2 was the other part I found that was rolling around in the case, it does not fit very well.

I can find not date other than what I assume is the date of manufacture for the meter but not sure which digits I am looking at other than the last 3 digits are 69 6. Would that be June 1969?
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