AVO CT160 Restoration - Meter

[Mtuckey]

Hi guys, I've just joined this forum as it seems to contain a wealth of information and experts regarding Avo test gear.

I recently purchased a CT160 in unknown condition from a friend. He bought it years ago as is from an Australian military auction. I believe this piece to be ex naval.

As far as I know the tester hasn't been turned on since it was last in service. I have gone through all the components within the tester, checked all resistors and caps to make sure they are within tolerance, changed out any that aren't (thankfully not many).

Now comes the meter. I removed it to find one of the joints in the rear has been glued together with epoxy - little strange. I then made the controlled current circuit as described in the following article from Dekatron - brilliant stuff by the way.

https://www.vintage-radio.net/forum/...ad.php?t=58088

And have returned with the following findings:

Movement Resistance: 3041 ohms (standard 3250)
1mA/V mark on green scale: 22.54uA (standard 22.2)
Red centre line of SET~: 27.6uA (standard 27)
FSD: 30.6uA (standard 30)

It seems almost within spec, but that movement resistance is concerning.

I did an additional test that's outlined in this page:
http://www.jacmusic.com/Tube-testers...dex-meters.htm
That involves setting the needle at certain levels and turning the meter 90 degrees onto its side and seeing if the needle holds its position. I found the same results throughout the meter range so for this example I'll use 50% FSD.
If the meter is at 50% fsd, and i rotating it 90 degrees to the left, the needle drops 2mm or so, setting the meter back upright, those lost 2mm hold. Doing the same again but turning it onto its right side doesn't seem to change the needle position much, but can correct the misdeflection caused by turning it to the left.

Im thinking it needs to be rebuilt?
What do the experts out there recommend?

Thank you all for your help in advance.
Mike

[Dekatron]

You can probably use your meter without rebuilding it, but you'll need to add two resistors to get it to work properly. Having it rebuilt means that you won't have to add those resistors and you'll also have it checked for other faults or problems. It is always a question of how much you want to spend and if you can live with the meter as it is knowing that it isn't perfect.

The first thing you need to do when checking the balance when turning the meter sideways is to make sure that you have adjusted the zero position with the meter needle moving towards the left while zeroing, that will ensure that the hair springs have been adjusted as intended during zeroing.

I wouldn't worry too much about those two millimeters if they are consistent and if the meter works in the intended slanted position.

You should also check linearity to see so that the deflection is linear and so that there are no sticking positions along the scale.

With the original values of 30 uA FSD and 3250 Ohm internal resistance you get some 97.5 mV volts drop across the meter, with the 10 k shunt in place that means that a total current of some 39.75 uA flows through the meter and 10 k shunt. Now since your meter needs 30.6 uA and has a lower resistance that results in 93.0546 mV, the difference needs to be dropped by a series resistor with the meter of some 145.27 Ohm ( (3041+145.27)*30.6 uA = 97.5 mV). Now you need to adjust the 10k shunt so that the correct current flows through it as your meter needs a little bit more, instead of 9.75 uA through the shunt it only passes 9.15 uA with your meter so it needs to be a little bit larger, some 655.74 Ohms should do it ( (10000 + 655.74) * 9.15 uA = 97.5 mV). With these resistors added the total is still 2452.8 Ohm and 39.75 uA flows through the meter and shunt.

[John_BS]

Mike:

The CT160 is not a "precision" instrument in the way that, say, a good DVM might be regarded. Your meter is only about 2% out, and was probably only manufactured to around 1% accuracy.

That said, as it happens you can tweek the 10k shunt (which is always connected in parallel) to bring the meter//shunt combo to precisley the design spec (39.7uA) by adding 240 ohms in series with the 10k, i.e. make it 10.24k.
The parallel combo "as designed" had a resistance of 2.44k and your meter+shunt will now be 2.34(5)k so if you add 100 ohms in series you will have a pretty much perfect meter!

John

PS if it's not sticking or showing any signs of hysteresis, I'd leave the meter alone.

[David Simpson]

Mike, I'll echo what John says. For a mc meter that is possibly getting on for 60 years old - yours is in pretty good spec. Definitely not needing a rebuild. Maybe a slight tweaking of the resistances as John & Martin rightly advise, but nothing else.
AVO didn't make the most robust Tester/VCM meters in the world, and your CT160 may well have had a rigorous military career. As did some of the RAF ones I worked on in the 60's. The earlier CT160 meters - lying horizontal in the bottom clamshell, were identical to the one used in the VCM Mk3's & 4's fitted at a steep slope.

Regards, David

[Mtuckey]

Ah fantastic news!
Thanks for the replies gents.

[Radio Wrangler]

If the results on a valve you test are 2% out, will it even be noticed? Besides the errors in other areas such as mains wave distortion can be even larger.

If you wanted to use it to match valves accurately, then the meter characteristic doesn't matter as long as it isn't sticky.

Avo did 1% movements for their larger meters, but not in this size. Springs and magnets both age, so the truly high precision analogue stuff used servo-controlled pointers.

It was done as a military valve tester, was it worth putting that valve back into that military radio? Would the soldier/sailor/airman be able to use it for reliable communication?

David

[David Simpson]

Thinking back to my days as a Junior Technician in the RAF - in the Wireless Bay we didn't fanny about with our CT160 if some Comms or ILS, etc. equipment came in with a fault - if a valve was suspect we just quickly shoved in a brand new one. Electronic Centre storerooms were full to the brim with heaps of new spares. It was only when sets came in for scheduled servicing, possibly one or twice a year, when valves might be subject to scrutiny in a CT160. More so in 3rd line MU's.
As David rightly said, errors in the AC waveform would have more bearing on the Gm of a valve. Particularly on a Naval vessel, using engine driven alternators for 115 or 230V AC 50Hz supplies. The CT160, nice as it is, is just called a Valve Tester. ie. a Red/White/Green scaled Go, No Go, tester. Although it can be used to produce reasonably decent Ia/Vg(Gm) Graphs. AVO's other valve testing eq. were called Valve Characteristic Meters(VCM's), and were designed for civilian workshop or laboratory use. They wouldn't have lasted 5 minutes in a military environment. At 103MU we once had a Mullard Hi Speed in for assessment - totally useless due to dust, grime, sand, salt, etc. ingress into the card reading mechanism in particular. Hence the advantage of the CT160 with its almost waterproof sealed clamshell casing.

Regards, David

[Mtuckey]

Hi all, I posted here a year ago when I first started this project, after a busy year I have finally got a chance to finish working on it.

Overall:
All resistors and caps have been checked and replaced if required and all switch contacts have been cleaned.

Meter: As described above.

My previous post regarding the meter is here, I added the resistors as recommended by dekatron.

Diode conversion:
https://www.vintage-radio.net/forum/...ad.php?t=74490
Has been carried out as per this guide, as well as the addition of adding 50mA grid pot fuses and diode + cap protection for the meter.

Now I have reached the point of the diode conversion guide where Mark talks about adjusting R6 while having a high current tube installed etc.

So I turn the meter on for the first time using a variac set at exactly 240v. I notice that in the mains calibration, the meter seems to be reading about 8v high.
I move on with the test (known working 6L6 inserted), all is going well until I reach the "Test" part on the circuit selector. The meter needle vibrates around the bottom of the scale and I hear a buzzing noise. At this point I'm not sure if the buzzing is coming from the relay or is just the sound that's being emitted by the vibrating needle. The overload light doesn't come on at all.
I tried a different valve, and then no valves with all selector switches set to the 0 position, the same thing happens.

I have triple checked all my diodes are wired correctly, and everything else inside the unit looks ok, I'm not sure where to go next other than looking at the relay?

Kind Regards, Mike

[Dekatron]

First I would remove the meter from the tester and use a DMM across a 3250 Ohm resistor in its place while doing tests until you have found the fault so you don't burn the meter.

Then I would check the cut-out relay to see if there could be high contact resistance across its electrodes.

Then I would trace the different circuits to see if there is a short between any of the wires going to the valve panel, that might show if there is a short between them that can be the cause for this faulty behaviour.

[Mtuckey]

Thanks Martin, thanks gives me some direction!

[Mtuckey]

Ok, I have spent some more time on this and have seemed to have narrowed the problem but not yet found the cause.

Relay contacts are clean, and I manually triggered the relay and its buzz is definitely not what I heard earlier. Relay seems to be working fine at this point.

Meter has been removed and replaced with equal value resistors.

All wires leading to the top half of the clam shell have been disconnected to rule the sockets/multi selector switch out.

With all knobs on the unit backed off to zero or its equivalent, with a multi-meter across the "meter terminals" its reading 0VDC and 3mV AC - seems normal enough?

Now, as I increase the anode current controls both the AC and DC volts across the meter resistors rise progressively all the way up to 8.96V AC and 7.3V DC with the anode current controls set at max. This only happens under the "Test" condition of the circuit selector, with the electrode selector set at C/H, A1 or A2.

I have replaced the 8uf electrolytic cap that's in there, so i know that's good. (sorry I don't have the schematic in front of me to give the correct number).

I'm assuming that these voltages across the meter are too high? Without a working meter for reference I'm kind of in the dark.

I have looked around the meter closely and prodded with a wooden skewer, the voltages across the meter seem solid, and not caused by any connections touching by mistake or anything.

[G4_Pete]

Hi
Apologies if this has been mentioned already but if you search "CT160 meter protection" on this board you will find articles on Capacitor plus diode protection for the CT160 meter movement. I have done the mods to mine , it fits on one small piece of pcb bolted onto the back of the meter studs so quite neat and saves the meter from accelerating up against the stops as it can if things go wrong with the valve or you set the wrong settings.

Pete

[Mtuckey]

Hi Pete, thanks for your reply.
I have meter protection installed already, an 8uF film cap and back to back diodes. During my trouble shooting process i removed them as a test, made no difference to the problem I'm experiencing.

Kind regards, Mike

[Mtuckey]

Originally Posted by Mtuckey

With all knobs on the unit backed off to zero or its equivalent, with a multi-meter across the "meter terminals" its reading 0VDC and 3mV AC - seems normal enough?

Now, as I increase the anode current controls both the AC and DC volts across the meter resistors rise progressively all the way up to 8.96V AC and 7.3V DC with the anode current controls set at max. This only happens under the "Test" condition of the circuit selector, with the electrode selector set at C/H, A1 or A2.

Is anyone able to tell me if these voltages across the meter under these conditions is incorrect?

Kind regards, Mike