AVO CT38 Multimeter

[AndyGilham]

I have just acquired one of these military instruments for the Megger museum, it's quite a beast.
I wonder if anyone else has opened one of these? I have obtained the user manual, and read that there are 8 screws on the front panel holding the panel assembly to the case. However, as you,see from the photo, I have remove these and it's not separating.....even with reasonable force! It probably shifts a 1mm at most!
Am I missing something? Any advice?

Andy

[pmmunro]

Andy,

Congratulations on acquiring an "Ammunition Box Avometer" - now you're into serious Avometer collecting, but you have to get it working, and that may be a bit of a challenge.

Paragraph 48 of the manual states that 8 screws need to be removed but there are 12 screws, 3 at each edge of the front panel. I have one here that is loose in it's case as I haven't put the screws back yet and they all seem to have a corresponding fastening in the rim of the case. However, in your photograph they all seem to have been removed. Other than that there doesn't seem to be anything else holding the unit in its case. It could be that there's a perished sealing gasket sticking the front panel to the case rim. It might be worth puting a very broad lever between the panel and the case and applying some careful but sustained leverage for a time to see if this will release it. I'm thinking of a piece of steel plate, possibly most of the length of the front panel edge and about 200mm long with the edges beburred and the corners rounded. You'll have to judge if the retaining force is from some almost inelastic part or if it is just gasket adhesion.

I think Dave (The Phillpot) has one, so perhaps he can advise.

Inside it's a true work of art. I hate to think what they cost to make; the 1962 price list gives the price as £162 - 10s Nett, which I take to mean they weren't subject to purchase tax, probably because they were classed as industrial equipment. A Model 8 cost £24 - 0 -0d at the same time, so a CT38 cost the price of 6.77 Models 8s. Taking the final price of a Model 8 at £900, had the CT38 still been available, it could have cost £6094.

You will probably want to find a CT151 calibrator, 10S/16444 which goes with the CT38, another work of art, if you don't already have one.

PMM

[Sinewave]

Wow good find Andy. I've always wanted one of these. They just look the business don't they!

[AndyGilham]

Thank for the advice PMM. I removed the screws myself already, so I imagine you are right, the seal has maybe perished......some considered, Gentle force required! I'll give it another go at work tomorrow, using some steel strip/sheet.

The instrument came with the lid, still including the schematic lid, leads, shunts and probes.

I have seen a CT155 on eBay, wish I had purchased it then!

sinewave,
I saw this for sale on Facebook marketplace, and messaged the seller. He is a member of the Birmingham radio society. Once I explained who I was, he was happy to donate it to me for the museum. However, I did pay,for the postage and donate some money to their society.

I do seem to gain donation (it useful working for megger). I have another couple of old Evershed instruments coming my way.

[The Philpott]

That's an excellent piece, i'm glad you got it for the museum. I don't have one myself, saw one for sale recently but told myself i didn't need it. Those rotary selector knobs i recognise, from early 'scopes that my Father used to bring home.

Peter may be thinking of the Precision Avometer, which i DO have!

Dave

[pmmunro]

Andy,

I meant to ask if you had the lid. Like other AVO military equipment these often contain a number of useful items - and often seem to have gone astray. We can speculate on the reasons.

Is it possible that there is any documentation on the CT151 in your archives please?

I don't suppose that any correspondence from the sales department of the time has survived. It would be very interesting to know who exactly this sort of kit was designed for and who drew up the specification. It is clearly for military use as it has "CT" numbers.

The 1958 catalogue page states that it meets "Interservices Specification K.114 as laid down by the Radio Component Standardisation Committee of the U.K. Ministry of Aviation" and that "This instrument meets the requirements of the U.K. Air Registration Board".

I seem to remember some advertising material which had a contemporary British jet airliner in the background.

PMM

[Sinewave]

Yes, the lid would have stow away areas for the power lead and other accessories. I've got an Avo frequency generator of similar build and appearance with all of the accessories in the lid. Very lovely these are. Just she sheer size and weight and materials alone is just so different to how they're made today.

[The Philpott]

Sinewave; If you don't want to stay 'Anon' do you mind me asking-what is your name!?

Dave

[trh01uk]

I had one of the CT38 instruments. Unfortunately mine had an open circuit meter movement. Someone told me that this is a common fault with them, because the meter is very high sensitivity - 30uA I vaguely recall - so they are prone to this problem. Hopefully yours is fine.

Richard

[Sinewave]

Originally Posted by The Philpott

Sinewave; If you don't want to stay 'Anon' do you mind me asking-what is your name!?

Dave

Anthony.

or Avo Ant as they say at work!

[Radio Wrangler]

All the screws are out. The gasket between panel and case goes sticky.

I bought one of these in the sixties. There was a Birmingham connection. In the pages of Shortwave Magazine, they were being sold by Amateur Electronics (G3FIK) Alum Rock Road, Alum Rock. The address lodged in my memory. Was this where the stuff in my chemistry set (The tube of white crystals that did nothing interesting) was mined?

Circuitwise, it's rather nice.

There's a time delay switch in the HT line, so you power it up and the pilot light comes on, but it plays possum for a minute or so.

The 6AL5 AC range rectifier is in a rectangular box in the hatch in the panel. It comes out and goes on a scoddy short cable to use as an RF probe. The cable stows in the lid.

All told, a very good instrument. But hardwork to take everywhere you went. It was this schoolkid's second best toy - after the AR88!

David

[AndyGilham]

Originally Posted by pmmunro

Andy,

I meant to ask if you had the lid. Like other AVO military equipment these often contain a number of useful items - and often seem to have gone astray. We can speculate on the reasons.

Is it possible that there is any documentation on the CT151 in your archives please?

I don't suppose that any correspondence from the sales department of the time has survived. It would be very interesting to know who exactly this sort of kit was designed for and who drew up the specification. It is clearly for military use as it has "CT" numbers.

The 1958 catalogue page states that it meets "Interservices Specification K.114 as laid down by the Radio Component Standardisation Committee of the U.K. Ministry of Aviation" and that "This instrument meets the requirements of the U.K. Air Registration Board".

I seem to remember some advertising material which had a contemporary British jet airliner in the background.

PMM

PMM
My CT38 does include the lid, and a the leads...etc.

Ive had a look in the archive, i have some limited info regarding the CT155, a few part drawings, nothing significant, but there is a schematic if thats of any use?

I have approached our sales dept. regarding old sales data, but we have nothing at all. All the interesting information was that used up on London, during the vauxhall bridge days....i guess it was all destroyed when the factory shut down.

Andy

[pmmunro]

Andy,

Thanks for making these inquiries, it's very good of you to do this.

A circuit diagram of the CT 151 would certainly be good to have thank you.

PMM

[AndyGilham]

PMM,
Please find attached the CT155 schematic.

Andy

[pmmunro]

Many thanks Andy. Some interesting points about this circuit diagram.

But for your interest, and this forum, I wonder how much of this material might survive.

PMM

[AndyGilham]

Well, i managed to open the CT38 i have and took some photos of the internals. As mentioned earlier in the thread, it is quite an assembly in there! It took 2 of us to seperate the from panel assembly from the case due to the perished seal.
I believe this is the first time this beast has been opened and the smell was quite potent!

One question, in the bottom of the outer case was a large pad of a fibrous material, which instantly raised the 'asbestos' alarm bells. Since i was not sure what it wads i took my pics and got the instrument closed. Any idea what this is?

Andy

[AndyGilham]

More images

Andy

[trh01uk]

"Fibrous material" doesn't necessarily sound like asbestos. I've seen plenty of plenty of white padding in equipments, which looked closer to felt in the past. Its usually used to keep components in place - while of course, asbestos is only included if there is a strong heat source and something else needs shielding from it - very unlikely in a CT38!
If you haven't seen asbestos close up, then I can imagine the two might be readily confused.

Some photos of what you have got might help us identify what it is.

Richard

[Sinewave]

Some of the old Avo's used what looked to me like an asbestos rope gasket.

I used to see loads of asbestos in old switch gear used as gaskets, not just the flash pads.

[pmmunro]

Truly a work of art!! I hope your more cynical colleagues are truly humbled to see the quality of the workwomanship from the past.

It deserves a custom made transparent acrylic case and a prominent place in your museum. Maybe you need a second one to show the appearance in its case as well.

If you want to get it operational, I would suggest you test all the aluminium cased tubular paper capacitors and carefully check that the mains lead is the correct one and its condition. I think there may be more than one way the instrument end of the plug could be wired for different pieces of equipment and the wrong lead could be hazardous.

PMM