AVO CT160's & MK3's & 4's VCM's

[David Simpson]

I appreciate, Moderators, that it might be tedious & time consuming to gather up all previous & present thread posts regarding this family of sibling valve testers, and put them into just one "Sticky" listing. Similar to the VCM163 & Mullard Card's threads. However, it would hopefully reduce the amount of repeat queries posted time & time again, many without folk first perusing the "Search "facility.
Don't get me wrong, I'm not criticising individual existing or new Forum members, but there is a huge wealth of info on all aspects of fault, repair, maintenance & calibration of these valve testers already accessible. I realise that the old AVO company may be partially to blame for producing somewhat confusing circuit diagrams & conflicting technical info. Folk often refer to the "Manual", but there are at least 4 technical manuals for the CT160 for example, to my knowledge. AVO's own civilian one, a REME one, an RAF one, and an American Navy one. And of course the VDM - Valve Data Manual. Originally they would have been available in Hardback format or loose leaf AP-like format. These days any of them can be downloaded for free in pdf format from the internet. VMARS, BAMA & other sites for example.
As folk ken, I have(and a few others) a strong affinity for CT160's & VCM's/Testers in general. However, they are reaching pensionable age, and the costs of some spares on the internet are horrendous. Not to mention internet wheeler-dealers, over recent years, forcing up prices of complete testers to several hundred pounds or even over £1000! So a one-stop Forum thread would hopefully alleviate owner's worries that their repair efforts might result in serious(& expensive) damage to their valuable AVO Tester/VCM. Hey - even that AVO description confuses some folk. A CT160 is described as a Tester, and the MK's 1,2 3 &4's as - VCM's - Valve Characteristic Meter. A CT160 was primarily designed for valve testing on-site by semi-skilled servicemen(although it can produce jolly good readings for studying a valve's "Characteristic Curve" - Ia/Vg), or in the field. VCM's were designed for interior workshop bench use only.

Regards, David

[Dekatron]

I am glad that you put up the request here that I emailed you about yesterday about a sticky thread on this subject, it will definitely be a good idea to at least put links to some places like Frank Philipses where a lot of AVO material can be found and also putting other repair information in the thread.

I think it would be nice to have a separate thread to discuss repair information in to try to do some lists of where to start the repairs/checks before you damage your tester beyond repair and then transfer the finished lists to the sticky thread when people agree that they are finished, or more or less finished as it would take the load from the moderators to modify the sticky thread to often!

Those lists will hopefully help a lot of people from making the same mistakes that others have done before them.

/martin

[David Simpson]

As Martin rightly says, the ideal solution would be a single source of all the accumulated info both from this Forum & elsewhere.
For example :- Frank Philipses as Martin says, then there is - the Duncan's Amp Pages article on CT160's back on 26/08/06, Martin Scobie's CT160 article, an excellent article on RV2 by Mike Phelan back in March 2005. More recently another excellent article in Radio Bygones No153 Feb/March 2013 on MK3's & 4's, by Euan MacKenzie, which includes Martin's super revised circuit diagrams. Then, looking back, myself & Martin have banged on(sometimes excessively) in Forum posts over the years. My apologies for repeating myself at times. Also several other keen Forum VCM enthusiasts have added excellent info.
All we had back in the 1960's, when I serviced & repaired RAF CT160's, was a dog eared old AP/Manual. We accepted, without question, AVO's info & instructions, and their tabulations of expected voltages. Now in the 21st century, 50 years later, with access to modern test equipment, we can see that CT160's & MK 3's & 4's weren't all they were made out to be. But thanks to those folk credited above, we can use & maintain these testers to a high standard.
I admit that my software skills are pretty minimal, but if some younger & skilled Forum guy could devise some sort of website or Forum listing where all available info could be compiled & listed, then as Martin says- it would benefit greatly all AVO Tester/VCM owners for years to come.

Regards, David

[ms660]

Just been looking at the EMER manual for the AVO testers (CT160 etc) and it says that the mean grid voltage is 0.52 of the selected grid bias voltage, eg: 1.56 volts when 3 volts grid bias is selected, would the 0.52 figure have been arrived at to take into account the curve of the valves characteristic or some other reason?

Lawrence.

[Dekatron]

Have a look at the document I wrote some years ago after having figured it out, you find it in this thread, there you will see where the 0.52 figure comes from: https://www.vintage-radio.net/forum/...ad.php?t=86262

It is just a way to fit the scale to the law of the potentiometer circuit used in the AVO valve testers, nothing more.

[David Simpson]

What Martin is trying to explain, Lawrence, in basic terms is the DC resultant voltage which appears from the decay between the 10mS active un-smoothed pulses on the Grid. Yes, a scope shows two pulses, but they are from separate sources, and only one is in anti-phase with the +ve waveforms on the Anode & Screen. Were it a true FW rectified grid waveform then the resultant DC component would be nearer root2 over 2 which = 0.707. However, HW rectified pulses result in a DC component nearer 0.5. But once again AVO confuses us in referring to the grid voltage in DC terms for calibration purposes. This is because there is no multimeter on earth that can measure instantaneous voltages at any point on a positive or negative going 50Hz waveform. In AVO VCM's a valve only conducts (grid controlled) peak electrons for 10mS at 50 times a second. And it was only their range of multimeters which AVO recommended using 60 odd or more years ago.
As can be seen in my Standardised Valves thread last year, AVO VCM's can get jolly close to an analogue DC valve tester or a software driven one. So AVO engineers got their P - P, RMS, & Crest Factor(Calculus) maths right, all those years ago.

Regards, David

[ms660]

Thanks for the link, it's in the 4th attachment of Post#1 in that link if anyone else is looking for it.

It was the jump from 0.5 in the patent to 0.52 in the manual that confused me at first and prompted the question.

Lawrence.

[Dekatron]

Which ”jump” do you mean?

Is there any possibility that you mix the 0.5 factor used in half-wave rectified AC conversions with the 0.52 factor for the potentiometer scale? You can read about the AC/DC RMS & MEAN conversions here: http://c0418683.cdn2.cloudfiles.rack...M_Glossary.pdf

[David Simpson]

Here is a simple picture of the basic rectification process without any filtering. The titties in (b) are what a valve in a VCM sees under test, and the dotted line (I) is what a multimeter can only measure for calibration purposes.

Regards, David

[ms660]

Interesting, I didn't realise that the anode voltage was rectified......

Lawrence.

[Dekatron]

In early AVO VCMs the anode voltage is pure AC, in later models a rectifier/diode was used to half wave rectify the anode voltage but in the early models the valve under test itself works as the half wave rectifier. The measurement circuit with the 200 Ohm anode resistor only sees a half wave rectified voltage.

[ms660]

For the avoidance of any doubt I was referring to the online copy of the EMER manual that has the No:1, No:3 and the CT160 valve testers in it.

Lawrence.

[David Simpson]

Aye Lawrence, that's just a basic page out of an electronics principals bewk, NOT the AVO Manual.
Martin, I keep saying that the T/F's HT Sec winding's P - P voltage is apparent on the Anode, but obviously basic valve theory applies to the current waveform as any valve conducts only under +ve half cycles. Thus - simple ohm's law tells us that a small voltage is dropped across the 200ohm Anode resistor when the valve draws Ia. Its that sma voltage which is used by the VCM for testing purposes.
In my eagerness to help folk understand, I'd assumed that that was taken for granted. I don't want to get involved with the pedantics, or is that antics, of basic valve theory or basic electronics mathematics. The Internet can source hundreds of articles on valve theory & basic RMS maths, and so on. Bolloxed if I'm going to be driven into going off-topic, lets just stick to some way of rationalising all the available info on CT160's & MK3's & 4's for our fellow Forum members to use.

Regards, David

[David Simpson]

VCM MK3 :- Thought I'd put my money where my mouth is -
Food for thought - - After a busy past year of valve testing, the Coarse & Fine Backing-Off pots were getting a wee bit sticky. Removed panels & found a wee note written on the framework - "Refurbished Nov/Dec 2013". Jesus, has so much time flew by!
So as well as cleaning the pots, thought I'd check the cal. But more along the lines of Euan MacKenzie, than the old AVO Manual. Euan's safety warning about delving inside to open the link or adjust cal pots is well worth noting. But why fanny about opening the link - replace it with an accessible switch, that's my thought. Also - use a right angled ratchet screwdriver for RV4 & RV6. Also changed the 1/6A diode for a BYW96E. Needed to check that C1 was the correct value & rating. It was, but what a job removing it & its perished pink rubber sleeve. So its now mounted elsewhere. Thankfully, Cal is fine, but thought I'd delve further into the circuitry. Well worth noting Euan's remarks on the Electrode Tag/Terminal strip, and the colour coding of the wires.
I found a simplistic diagram of how a valve is basically tested on Page 6 of the old Manual(Figure 1). So I've added some additional simplistic info.

Regards, David

[David Simpson]

Time spare today to witter on about VCM's, due to bad weather. Need the diversion anyway after 7 days of heavy gardening, fencing & tree pruning.
Studying the circuitry of the MK3, it would seem that AVO decided to source their combined Grid voltage waveform from the 130V Sec winding & the 125V tapping on the Anode/Screen Sec winding. As opposed to the 55V & 66V Sec windings in the CT160. The MK3's Backing-Off Voltage Source is the 30V Sec winding.
I reckon that a more accurate calibration of Set AC can be obtained by also studying the 125V, the 130V & the 30V. All easily accessibly with the L/H panel removed. The 30V & 130V connections are at the bottom left of the transformer, & the 125V can be read straight off the Anode Switch which is very close to the opening. If the Backing-Off circuitry is getting spot-on 30V, then the subsequent mA/V reading will be spot-on for calibration purposes.
Best time of day for calibration is when there is no cooker, washing machine or tumbler dryer running in the house, thus your workshop/shack has a pretty steady mains supply.

Regards, David

[David Simpson]

Autumn has started, and the Test Equipment sub-forum is awash with queries about AVO VCM's & CT160's.
Folk are using 21st century DMM's, when clearly AVO in their manuals advised, back in the 1950's/60's, use of Mk7 & Mk8 Analogue multimeters. Folk are still unable to grasp the fact that AVO Valve Testers & Valve Characteristic Meters(and Hikoks, Mullards, Taylors, etc.) all use full peak to peak sinewaves & half sinewaves for the voltages applied to anodes, screens & grids. NOT DC, and not even the correct RMS equivalent voltages. Yes, AVO manuals, for calibration purposes, list expected voltages, either AC or DC. But that doesn't alter the fact that valves under test are only conducting for 10mS every 20mS - - i.e. 50Hz mains pulses. If we all had super-human eyesight, and used a high-powered magnifying glass we could see the meter's needle vibrating at 50Hz.
Folk - start looking at the electrode voltages with an oscilloscope, apply the maths(root 2, etc.) And be exceptionally careful - full P-P AC is on the meter's terminals(anode potential) = 240V x approx. 2.8 which getting on for 700V!
I'm still living in hope that someone with 21st century computer skills tries to consolidate the vast wealth of info which has accumulated(see "Search") on this Forum over the past few years, regarding AVO's range of VCM's & Testers. Then hours of repetitive answers could be avoided.
Why, some folk might ask, are they called VCM's ? Valve Characteristic Meter - - once all the basic tests for continuity, insulation, gas, etc., have been carried out we need to establish how much anode current flows for a chosen voltage on the grid. Then we need to know how the current changes for a small change in Vg. The valve's "characteristic" is a plotted graph of the slope or curve of a number of readings of Ia & Vg. All the mA/V Pot is - is a wee grid voltage potentiometer, compared to the much larger Vg pot. Meter readings for Ia & mA/V are a good indication of the valve's ability to amplify a signal, basically. Note - with anode & screen voltages set, ONLY the Vg Control(and to a much lesser extent - mA/V) controls Ia. Not the "Corse & Fine" Ia controls on a CT160 or the "Backing Off" controls on Mk 3's & 4's, for example.
I'm not criticising folk for asking important questions about their AVO valve testers, but I would encourage them to use the SEARCH facility more often. Also, as this is a VINTAGE Forum - I suggest that folk adopt a vintage(1950's/60's) approach to testing/servicing/calibrating their VCM's & Testers - as was the mind-set of AVO's engineers back in the 50's & 60's = purely Analogue.

Regards, David