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Old 20th Dec 2017, 8:17 pm   #1
rsedwards1
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Default AVO Valve tester CT160

Hi everyone,

I'm sure there are people on here who know the best option for my valve tester. I haven't used it for over 20 years, now back in the workshop the panel meter coil is corroded and open circuit. I'm sure a similar replacement meter will do, as it's only for indication. Any info on this would be welcome.
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Old 21st Dec 2017, 11:10 am   #2
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Default Re: AVO Valve tester CT160

Just search the forum as this has been covered many times. No easy solution I'm afraid.
Rob
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Old 21st Dec 2017, 12:02 pm   #3
David Simpson
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Default Re: AVO Valve tester CT160

Just wondering - how could the meter mechanism become corroded & o/c - as the whole tester is contained within a military spec, gasketed, damp-proof aluminium clamshell case ?
If the meter is phooked, then that's at least £200 worth of rocking horse manure flushed down the drain. Only a 30uA meter of the same resistance & identical scaling would do as a replacement. "Only for indication" is completely wrong.
News of such a catastrophic injury to a CT160 doesn't bode well for the state of the rest of it!

Regards, David
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Old 21st Dec 2017, 12:24 pm   #4
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Default Re: AVO Valve tester CT160

Quote:
Originally Posted by David Simpson View Post
Just wondering - how could the meter mechanism become corroded & o/c - as the whole tester is contained within a military spec, gasketed, damp-proof aluminium clamshell case ?
..................
News of such a catastrophic injury to a CT160 doesn't bode well for the state of the rest of it!

Regards, David
I sympathise.

AFAIR from my marine equipment days, usually the last thing one should do is to keep electronics 'sealed' because it stops it drying out. Gaskets and other seals are helpful in rendering it splashproof, but inevitably some damp air gets in there. Then on a cold day you get condensation all over everything inside the enclosure. That then needs decent air circulation so that it can dry out reasonably quickly.

Not an easy aspect of enclosure design.

Martin
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Old 21st Dec 2017, 12:46 pm   #5
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Default Re: AVO Valve tester CT160

As the temperature of the day changes the thermal mass of the cabinet and contents lags it, there are also barometric pressure changes. So the air in a cabinet is always 'breathing' This fetches moist air in sometimes, and provides colder surfaces inside for it to condense on.

Military equipment also got chucked in unpressurised aircraft, so if not vented, the pressure variation would be a huge stress on a case. The gear I now design has to be tested for survival under sudden decompression. I can't afford sealed areas of any significant volume. The testing is quite dramatic.

Nowadays we have things like goretex stuffed vents which pass dry air and exclude water vapour, but they are relatively recent things.

I'm awfully disappointed with the failure rate and vulnerability of these avo movements. Avo were supposed to be good at meters!

I wonder about making some tooling to handle their coil formers so they can be re-wound? some reassembly jigs would be needed. Wire ought to be available and might benefit from modern varnish on the reliability front. The people who rebuild mechanical watches ought to find the movements crude and clunky.

David
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Old 21st Dec 2017, 12:51 pm   #6
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Default Re: AVO Valve tester CT160

Problems with meters seem to be a common issue with a number of the Avo valve testers. I have a VCM163, which is not sealed, and that's had corrosion problems (inside the meter) too. As said above, original meters are a king's ransom. Alternatively, you can use non-original meters by using an op-amp to drive the new meter, giving true indications, and there have a been a couple of circuits published to do just that.
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Old 21st Dec 2017, 3:08 pm   #7
David Simpson
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Default Re: AVO Valve tester CT160

Folks, there is nothing in any of the CT160's(or its smaller sibling - the CT446) manuals available online or held by owners which says they must be left un-clammed whilst in storage.
There was never any such technical instruction given to us RAF guys who serviced & repaired them back in the 60's. Yes, perhaps those CT160's used by the RN aboard ship needed special precautions, and perhaps this is a poorly treated ex Navy one ?
Every RAF base would have had two or three, and the MU's would've held dozens in reserve.
I don't ever remember seeing evidence of moisture damage to the innards back then, or more recently since joining the vintage fraternity. Most likely, at sometime in the past, some silly tit with a screwdriver has been delving into the innards, or someone with a big pinky has been "glass tapping". My views on glass tappers & forceful pot twirlers, CT160-wise, have been well documented, forum-wise.
Internet prices for meters - £200 +, RV2 Pots - £200 +, and complete working CT160's - - £500 to £1000 ! Speculation has gone silly.

Regards, David
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Old 22nd Dec 2017, 3:13 pm   #8
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Default Re: AVO Valve tester CT160

A questions that occurred to me after my last post. My 163 has two meters and one is fine and the other has had problems. With the latter, the coil has a lower resistance than specified and needs an op-amp driver to get a corrected reading from it. Additionally, I opened it up and found lots of white, crystalline deposits growing on the 2BA screws that are the posts for the connections to the outside (see attached).

If we make an assumption that both meters are the originals, I think it tends to suggest that who ever made the meters might have had a "dirty" production environment. Trace contaminants got on to some meters that caused chemical and then electrical effects that didn't show up for years, but were then very severe. This is one of the reasons why I always wince when somebody suggests using anything that has chloride in it for a cleaning job. Contamination may not show up quickly, but that does not mean it never will.

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Old 22nd Dec 2017, 5:44 pm   #9
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Default Re: AVO Valve tester CT160

Regarding the two meters in the VCM163, they should have different resistances. If you look at the corrected schematics I drew here: https://www.vintage-radio.net/forum/...81&postcount=7 you can see that one should be 2.4k and one should be 1.5k, both 50uA FSD. The 2.4k one has an extra internal compensation network of resistors and an NTC to compensate for the temperature change in the coil and surrounding.

If your meters differ a lot from these two resistances (at 20 degrees Celsius) then you could either have a damaged movement coil, burned windings shorting together resulting in a lower resistance is quite common, or high resistance due to the corrosion seen in your photos making a high contact resistance where the solder lugs are screwed onto the screws.

I have seen this corrosion on almost all meters in the VCM163s that I have come across and it is due to moisture as these meters are not sealed and moisture enters them when they are not stored properly. You can clean the screws, washers and nuts in an ultrasonic bath and then polish the solder lugs with relay cleaning strips.

All of the meters that I have seen the insides of have a moulded print at the back that says that they were made by an Italian company and not AVO. I know I posted some photos of the insides of some meters here on the forum but couldn't dig them up right now.

These meters are also susceptible to corrosion between the different metals used in the moving coil assembly making them erratic, sometimes they work but when they move fast they act like open circuit or change their resistance drastically due to this contact corrosion. It looks to me like there is a combination of aluminum and other metals in the parts connecting the coil to the hair springs and needles, and on at least one meter I could measure an open circuit between these metals but when shorting the coil with hair fine wires to the hair springs it all worked but it was not easy to get a good balance any longer due to the small solder blobs and wires - at least it showed that the contact problem was between the metals and not a dead coil!

I have put a big bag of silica gel in all my VCM163s and also store them in normal room temperature.
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Old 23rd Dec 2017, 6:40 pm   #10
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Default Re: AVO Valve tester CT160

Quote:
Originally Posted by David Simpson View Post
Folks, there is nothing in any of the CT160's(or its smaller sibling - the CT446) manuals available online or held by owners which says they must be left un-clammed whilst in storage.
There was never any such technical instruction given to us RAF guys who serviced & repaired them back in the 60's. Yes, perhaps those CT160's used by the RN aboard ship needed special precautions, and perhaps this is a poorly treated ex Navy one ?
Every RAF base would have had two or three, and the MU's would've held dozens in reserve.
I don't ever remember seeing evidence of moisture damage to the innards back then, or more recently since joining the vintage fraternity. Most likely, at sometime in the past, some silly tit with a screwdriver has been delving into the innards, or someone with a big pinky has been "glass tapping". My views on glass tappers & forceful pot twirlers, CT160-wise, have been well documented, forum-wise.
Internet prices for meters - £200 +, RV2 Pots - £200 +, and complete working CT160's - - £500 to £1000 ! Speculation has gone silly.

Regards, David
David, I am sure that when these meters were made it was not expected that they would still be be in use 50 or 60 years later. I doubt very much that any meters corroded within the early years. More likely it would be when the valve tester was put away in less than ideal storage conditions. However it is easy to see how 20 or 30 years of slight damp in the air could cause the corrosion to the aluminium parts but I agree it is less easy to see how it could cause the meter to become open circuit. Herts Meters can fix most things but not if it is open circuit..
Nick
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Old 23rd Dec 2017, 7:07 pm   #11
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Default Re: AVO Valve tester CT160

Quote:
Originally Posted by Fourlegsgood View Post

David, I am sure that when these meters were made it was not expected that they would still be be in use 50 or 60 years later. Nick
But isn't it true that there are many older meters around which do not have comparable problems? There must be loads of HRO receivers with S meters that are 70-80 years old and I don't recall anyone ever coming on the forum to say they've had a problem with one. It is very hard to avoid the suspicion that the meters in various Avo valve testers have common issues that make them 'vulnerable'.

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Old 23rd Dec 2017, 9:59 pm   #12
David Simpson
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Default Re: AVO Valve tester CT160

Aren't AVO's range of VCM meters( & AVO 8's & 9's) fitted with phosphor bronze movement springs? Virtually corrosion resistant.
Back in the day, I can remember replacing CT160 & CT446 meters a few times. But they had suffered mechanical damage - mainly broken glass. Thus ingress of particles into the movement. One or two had O/C coils, but that was no doubt to currents far in excess of 30uA. Back to fsd whanging again.
Nick has hit the nail on the head. Those testers I worked on in the late 60's were less than 10 years old. Some now are at least 60 years old. During their service life they would have been well looked after & kept in a damp-free environment. But as the services have shrunk, many hundreds have been sold to ex Gov shops etc. over the last 20 to 30 years. Some were just probably sold as metal scrap.
The main bulk storage/distribution for avionics & test equipment used to be RAF Stafford. I once remember visiting a large civilian scrapyard in Stafford, back in the early 70's. Tons of 50's/60's stuff that us lot would now weep over, was stored in open sided sheds, exposed to the elements.
Not everything received a warm safe welcome at Birkett's in Lincoln.

Regards, David
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Old 7th Mar 2018, 11:12 pm   #13
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Default Re: AVO Valve tester CT160

David,

Only just noticed this thread. I have a picture, the magnet is what corroded and put force on the coil. When I tried to move the meter it was attached to the rust. Moving the meter rubbed the winding to open circuit.

Robert
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