AVO CT160. Supply set function does not work

[grahamt]

Hello all, new to this forum so please forgive any errors.
I have acquired a CT160 valve tester. The instructions say to set the supply voltage using the SET position and setting the voltage tapping under the flap, however when I try this the meter does not deflect to right as its supposed to, in fact it moves slightly to the left! Any ideas?

[Pamphonica]

Edit: just re-read your post. Am I right that the meter is not deflecting to the right AT ALL? Not just a bit off. If so, ignore the rest of this post (for now!).

Your meter may (but not necessarily) have lost some magnetism and become a bit less sensitive. There's a resistor shunted across the meter but don't just adjust that as it throws out other bits of the circuit.
To check if it's the meter at fault, substitute another (analogue) meter such as an Avo 8 on 50uA range for the inbuilt meter and see what that reads. If you use a digital meter you may get odd results as it's measuring an odd pulsed waveform. The meter movement in the CT160 is 30uA full scale, so the set point is at about 82%x30uA = 24.6uA.
I did do a "quick fix" on one of my CT160s that might help overcome this lower sensitivity, if indeed that is the problem. It involves modifying both the internal "swamp" resistor and the external shunt resistor. If done correctly the rest of the circuitry is not impacted by the mod. There is a slight change in damping.
https://www.vintage-radio.net/forum/...ad.php?t=50786
Good luck!
-Jeremy

[Pamphonica]

Possibly check out the rectifiers that feed the various bits of the circuit. If one is open circuit it could produce odd effects.

[David Simpson]

Graham,
Your query has fallen amongst a flurry of CT160 thread enquiries. Its imperative that you follow Jeremy's advice. And, as a new CT160 owner, study all the recent thread posts as well as the AVO Servicing Manual.
If you cannot acquire the needle onto that black "Set AC" mark on the meter scale - firstly check your incoming mains voltage, then secondly(if your comfortable & experienced in electronics circuitry) measure the 55V secondary winding. This needs to be spot-on for valve testing procedures.
However, if you are new to vintage electronics & valve testing in general, then my repeat advice is to get your CT160 checked out & calibrated by a recommended qualified person. There are one or two amongst the Forum &/or BVWS fraternity. I'm not ringing my own bell here, as I'm too far away in the wild north of the UK to be involved in offering such valuable services. But "value" has a meaning here, as CT160's & their AVO siblings cost a fortune these days, as does genuine AVO replacement parts.

Regards, David

[grahamt]

Hello.thanks for valued info.i am new to the ct160 but over the years I have acquired knowledge on various aspects of electrical/electronic equipment so have got the “have a go at anything”attitude. I have dismantled my ct160, using a fluke Dmm I have checked all resistors on the control board,all seem ok.
I removed the panel meter and tested its resistance. This is 1622 ohm. Not what I expected. I dismantled the meter, the actual movement is 974,there is a wound bobbin inside the case in series with the movement (is this some sort of needle damper), this measures 648 ohm. Total 1622. The whole inside of the meter looks “not right”almost home made,maybe a previous bodge. Does anyone know if a suitable movement was used in any other instrumentation that could be transplanted into the ct160. Any help appreciated.
Graham

[David Simpson]

Hello again Graham,
It sounds as though your meter has been bodged as you say. Any chance of a picture or two?
As I've said in other thread replies, genuine AVO replacement meters are like rocking horse manure, and I've heard that they can cost £200-ish !
Advice given in another recent CT160 thread is to use an AVO8 multimeter on the 50uA DC range, to check the current being applied to the meter. I can assure you, this works fine, as I used this method a few years ago for a while whilst waiting for an op-amp kit for one of the CT160's I owned at the time which had a duff meter. There are a number of threads related to substitute 30uA meter manufacturers - check "Search". I used Benchmark of Ludlow. Possibly cot you a £100 or so these days.
Living in Chester, you are not far from Golborne & other major BVWS vintage radio Swapmeet venues etc. Once the virus emergency is over, I'm sure you could arrange to meet a BVWS &/or VRR&R CT160 repair/calibration expert.

Regards, David

[grahamt]

hello david, thanks for reply
does anybody know what the meter movement resistance is?[the actual movement bobbins etc not included]. I ask this because by chance last week I did a pre virus isolation shack clearout and threw all unwanted into my trailer, after I dismantled my ct160 meter and wondering if the same movement was used in any other I remembered that I threw out a sanders vswr amplifier [a huge contraption by todays standards]which had a large meter on its front panel, I would have normally taken trailer to the tip but due to virus tip was declared closed a few days ago. the meter is of the same era [1962] as the avo, I dismantled same . it will fit into the ct160 meter housing, fixing centres are the same however the resistance measures 2600 ohm, movement 1950,bobbin 650. avo bobbin is also 650 , do you think this may work?. the fsd on the meter scale is 100 microamp. advice appreciated.
graham

[David Simpson]

Basically Graham, for a meter which has a fsd greater then 30uA to work in a CT160, you would need to use an OP-AMP. There is a mass of info available via "Search" on adapting meters by using op-amps.
Best bet is to put out a "Wanted" post on the Forum. I know that Forum/BVWS CT160 experts have sold them in the past. Best to ask them. Gerry Horrox of Crowthorne Tubes used to sell op-amp kits, but I think he's now retired.
As I've advised on a number of occasions over the years, as a temp measure, an AVO8 makes an ideal substitute using its 50uA scale, but obviously just approx. 2/3 of its fsd. And, if a CT160 meter's scale plate is scanned, and with a bit of jiggery pokery using Sellotape & re-scanning, the resultant scale fits perfectly over the AVO8's glass & the first 30uA of the scale. Also, the analogue AVO8's internal resistive circuitry is more compatible with the CT160's. A modern DMM like a Fluke etc. isn't & gives winky wanky floating readings.
As Jeremy has already advised you, different points on a CT160's meter's linear scale must correspond to a certain number of microamps, otherwise the calibration is all to pot.


Regards, David

[Pamphonica]

Gerry Horrox stopped selling the meter kits some years ago, when the supply of meters dried up.
But the meter "mod" for less sensitive meters is so simple it can be made up on veroboard and hidden neatly inside the CT160.
The thread here may help:
https://www.vintage-radio.net/forum/...ad.php?t=46825

[David Simpson]

Just a thought Graham - living as you do in Chester, you are very close to RAF Sealand which housed 30MU. Now closed, sadly, I believe, but it once would have housed the biggest concentration of RAF CT160's. There must be hundreds of retired avionics technicians living nearby. Perhaps an advert in your local paper would put you in touch with someone who could mentor you.
I used to visit 30MU when I was stationed a bit further south at RAF Cosford, down the A41. The factory floor, so to speak, for just ARC52/PTR175 was about 100 foot wide by several hundred feet long - massive.
Re your CT160 problems - for Gods sake don't fanny about with the existing metering circuitry. If things seem too complicated, electronics-wise, or regarding fitting an op-amp - - seek practical hands-on help. A decent CT160 could be worth £500 - 600. A phooked one - just a couple of hundred quid, if that. This also applies to anyone on the Forum who is presently having CT160(or VCM MK3/MK4 siblings) problems.
As we said in the RAF( & Actresses said to Bishops) - If in doubt, keep it out, finger-wise.

Regards, David

[ms660]

RAF Sealand, been there, went for a job interview, waited ages for conformation, didn't hear anything so I assumed I wasn't suitable so I got fixed up with something else, the week after I started I got a letter about RAF Sealand…..saying I got the job, for various reasons I couldn't back out of the commitment I had since signed up to.

One thing that stuck in my mind at RAF Sealand was the clean workshop area, and the no expense spared look about the place.....not Fred Dawes style at all....

EDIT: Isn't the internet wonderful:

https://www.americanradiohistory.com...=%22sealand%22

Lawrence.

[grahamt]

Hello all and many thanks for valuable info.

Having read many threads etc. about this instrument I have decided not to muck about with it too much. I have emailed Benchmark Ludlow to see what they can do. As said in my last post I recovered a meter movement from a junked VSWR amplifier but it was 100 ua fsd. What I did do was to temporarily hook it up to the CT160 meter connections and set the instrument to "set", with previous bodged meter it would not move, however this 100ua meter moved straight away albeit only approx. 1/3 across the scale, this confirmed to me that the original bodge was the problem,and not resistor etc circuity. I will see what Benchmark comes back with. as for RAF Sealand we live about 3 miles away, when I was a kid we lived about 1/2 mile from RAF Hawarden [MU47], when they vacated [well before Sealand] we used to play in the empty workshops etc. as for Sealand there was acres of instruments there. I often wonder what happened to it all [sigh]. anyway thanks to all for valuable guidance.

Graham

[David Simpson]

Aye Lawrence, I'm sure you would have enjoyed working at Sealand. I had several RAF chums who worked there & they were jolly happy. When I was an electronics instructor down the road at Cosford, I used to pop up to Sealand now & then, to see how recently qualified technicians & fitters were settling in. They too enjoyed their work. Chester & west Cheshire & nearby north Wales & north Shropshire is a lovely area. As a boy in Liverpool, I used to visit Chester with my parents now & then, and loved the place.
Right enough Graham, best be safe than sorry. In recent years, CT160's have become a cult item in audiophile(some say audiophool) circles, hence the mercenary speculative prices being asked on internet sites. However, they are all about 60 years old & may have had a busy military life in all three services. Trouble is, they are just a VCM MK3 squeezed into a confined military spec clamshell case, with just one(sometimes overworked) complicated mains transformer, instead of two. Their delicate 30uA meters let them down. Hence, when I used to service & repair them in the RAF(Jesus - 50 odd years ago), meters were regularly needing changing.
Its over ten years ago since I sent a duff 30uA meter to Benchmark, so that they could take measurements & get a good scan of the faceplate. So fingers crossed that they can help you. However, other VCM enthusiasts have had success with other meter manufacturers. So its worth checking out their past thread posts.

Regards, David