AVO CT160. Protecting the meter.

[johnneyt]

I have just aquired a CT160 and have read with interest about the difficulties in sourcing a replacement meter and wondered if there was an easy way to protect the meter from damage. I read somewhere that I could connect two diodes in parallel across the meter terminals but would have to install a switch in order to take conductance readings. Is there a better way, I'm thinking along the lines of a simple fuse perhaps. I don't mind the inconvenience of opening it up to replace it should the need arise. I might add that I haven't attempted to use it yet for this reason !!

[glowinganode]

My CT160 movement died of natural causes rather than death by overload. The last valve I tested before the movement failed was an EF86, tested at a few mA's. No other components had failed, the 10K shunt was still within tolerance.
I attribute the failure of the movement more to environmental factors such as shock, fatigue or corrosion.
Rob.

[johnneyt]

So generally speaking, what is causing these meters to fail? Is it associated component failure or setup mistakes and how can I prevent it?

[Dekatron]

These meters are sensitive meters, 30uA and are therefore easy to burn when overloading but they more often break due to old age and corrosion.

Another usual problem is the glue used when glueing the moving coil to the axis. When these meters are overloaded the needle swings heavily forwards or backwards hitting the stops, even though they are spring mounted stops the relatively sudden stop can tear the needle from the moving coil or atleast snapping of the superthin copper wire used in connecting the moving coil to the axis. I have seen this in many meters I have opened and checked when they were dead. I've only been able to glue the needle back on one meter as the thin wire was still connected, all others have had the wire snapping off.

They are also prone to corrosion and wear over the years as the rubber glue used for the glass dries and moisture can enter the meter house that way and the small needle bearings (and other parts too) corrode making it harder for the axis to move. Sometimes the metal parts around the moving coil are so corroded that the corrosion touches the moving coil scraping of the thin wire making it open circuit.

Another problem that AVO was aware of is small metal swarfs sticking to the magnet which the moving coil moves around making the coil "stick" in certain positions, this is usually also the case after the rubber glue for the glass has dried making small metal particles enter the housing - and from metal particles already there from the production.

If you want to protect your meter in an CT/VT160 my best advise would be to install a meter OP-AMP amplifier that is limited so it can't overload the meter. Those meter OP-AMP amplifiers sold are not protected for overload so you'll have to build your own circuit to do that.

There is one company that I have used which can refurbish your meter if it needs to be done, Herts Meters UK and one company that can make a new movement that will fit but which will need a meter OP-AMP amplifier as they do not have any meters with the correct FSD and internal resistance last time I checked with them - Benchmark Instruments UK.

[GMB]

Originally Posted by johnneyt

but would have to install a switch in order to take conductance readings.

I don't understand this comment.

I fit a pair of diodes in parallel with all meters - checking that they don't alter the fsd reading. I often use Schottky diodes as they have a lower voltage drop.
You could add a fuse in series with the combination for best security.

But as said above, meters do also fail due to fatigue and corrosion etc.

[David Simpson]

To protect the 30uA meter, and the CT160 in general , fit a 2A anti-surge fuse in the 3pin mains plug, fit fuse holders in series with A1 & A2 and fuse at 100mA, and fit a fuse holder in series with the "grid" supply & fuse at 50mA. This will also protect valves & prevent cooking the 10Kohm -ve grid volts pot.
If you're not testing EL's & KT's etc, then reduce the A1 & A2 fuse size.

Regards, David

[johnneyt]

Originally Posted by GMB

I don't understand this comment.

I fit a pair of diodes in parallel with all meters - checking that they don't alter the fsd reading. I often use Schottky diodes as they have a lower voltage drop.
You could add a fuse in series with the combination for best security.

Thanks Everybody for your replies, the comment about the switch relates to the advice I read somewhere else where two diodes across the meter were recomended with a switch to take them out of circuit when taking conductance readings but no mention of why it would be necessary. Do you have a recomendation for Schottky diodes ( specification ) I will try fitting those plus a fuse as you suggested in conjunction with Davids good advice regarding fuses in anode and grid supplies. The 100mA Anode fuses I will fit in place of the removable links on the valve socket panel and the 50mA on the grid supply wire to the selector switches underneath.

[Dekatron]

I've heard about the switch and diodes but I can't find a reference to that information - if you have that reference please post a link here.

The only reason for the switch that I can see is that the balanced bridge in the CT160 will be off if the diodes are in the circuit, but I can't test that as I do not own a CT160 any more. I had one that I repaired and sold as I got hold of a VCM163 that is my current repair project.

[Kat Manton]

Hi,

Mine worked, then a few years later when I needed to test a valve again... it didn't.

As the CT160 is supposed to be sealed tight when closed, maybe make sure all the seals are intact and sealing, bung a bag of silica gel in there and keep it closed when it isn't in use. That may help prevent the meter winding going o/c due to corrosion.

Regards, Kat

[GMB]

You need to choose diodes such that they don't impact on the meter reading - then there is no reason to switch them out.

In the very old days this was done with metal rectifiers and later germanium diodes and I think it was hard to avoid some non-linearity being generated (an example is the Master Test Board that protects it's meter like this).

The forward voltage characteristic of silicon diodes are ideal for protection as there is a fairly well defined minimum voltage to get any conduction at all. Actually it's usually rather more than a meter would like, hence the lower drop type diodes like Schottky diodes are sometimes better - it all depends on the meter resistance. The diodes need to be powerful enough to not blow before either the fuse blows, if that is what you add, or to be able to sustain a worse-case fault indefinitely.

[pmmunro]

It is not easy to protect a moving coil meter with a fuse unless the resistance of the fuse is allowed for in the calibration of the instrument. (See http://www.cooperbussmann.com/pdf/28...e536281e0a.pdf)

The nominal cold resistance of a 32mA IEC 60127 (20 x 5mm) fuse is given by Bussmann as 40 ohms, but there is no tolerance stated. It would seem reasonable to expect that the actual resistance will vary too widely to allow any random fuse to be fitted without calibration.

Avometers before the Model 7, (1936) used calibrated fuses to protect the movement. (See older thread).

The concensus seems to be that the failures of these meters are most often not due to overcurrent but more to unfavourable storage conditions.

As these movements may be up to 50 years old there has been plenty time for unforseen deterioration effects to take place. The time of their manufacture was, by virtue of the rapid introduction of new materials, a time of experimentation, although it probably wasn't seen like that at the time.

One view, which collectors and restorers are unlikely to share, is that even movements which have failed so long after manufacture were a good engineering product as the have now outlived any reasonable life expectations. I doubt however that the original designer would have had this approach.

PMM.

[G4XWDJim]

What about meters on the Mk IV VCM which look to be just the same. Mine works fine and has done for the dozen times I have used it in each of the 30 odd years I have owned it but should I be thinking about doing something to protect it, other than using it in a clean dry environment. I do occasionally send it full scale, oops, forward and reverse during my testing and experimenting which a quick hand on the controls minimises. I have a brand new boxed spare which I bought from Anchor Surplus many years ago which I'm keeping, 'just in case'. Should I worry about that one also.

Yours fretfully,

Jim

[Dekatron]

Very early models of the AVO MKIV are not protected by any diode across the meter, later models by two diodes and models in between have one diode across the meter. If yours have one or none you can add one in the opposed direction from the one installed withou having to do anything else to the tester. These diodes are mounted on the CIRCUIT SELECTOR switch near the end.

Your tester and your spare meter can be protected somewhat by a bag of silica gel inside the tester and box or in their vicinity. Check that the glass is tight against the black bakelite, and not loose, so no moisture can enter the meter - otherwise I recommend you to re-glue the glass.

Your spare meter should be stored with a wire across the connections so the meter movement will be dampened, this so it will be protected if you drop it or hit the box hard. The needle will not swing so hard if it is dampened.

You can also test your meter as I described in this thread, only compensating for the lower FSD current drawn by the MKIV/CT160 movements of 30uA: https://www.vintage-radio.net/forum/...ghlight=vcm163 . The series resistance will be 1.2V/30uA = 40 Kohm minus the meter resistance of 3250 Ohm which then equals 36750 Ohm (33K + 3,3K + 270 + 180). Testsing the meter for FSD, series resistance and also current at the 1mA/V and SET AC position is good to do as you will then know how your meter behaves. Testing the rest of the resistors and capacitors in the AVO MKIV and other models is also good as they degrade over time and from over current/over voltage.

[David Simpson]

Fitting low amperage fuses in the anode & grid supplies to the valve under test wont upset the bridge characteristics of the CT160. I know I've banged on before - but these VCM's are what - 40/50/60 odd years old now. An excellent AVO idea - but they function under AC conditions. The voltages selected are at best - a nominal reference to their DC equivalents. If you want a true representation of a valve's function at certain voltages - then do as I've done - draw up a tabulation of selected voltages measured with a HP True RMS VVM/ AVO DMM/ AVO8Mk3/AVO8Mk5. If you're pairing up o/p valves - then the CT160 is ideal for drawing up Ia/Vg graphs.
Dont get me wrong - I love my CT160, but I've hybridized it to an almost un-reconisable state. Its only 10 years younger than myself & I'm almost 65. No way can I afford a replacement with internet sales prices at exhorbitant levels. So basic safety improvements are very worthwhile.

Regards, David

[glowinganode]

Originally Posted by David Simpson

Dont get me wrong - I love my CT160, but I've hybridized it to an almost un-reconisable state.

Care to divulge further?
Rob.

[David Simpson]

Rob, Very remiss of me, but I know I've not put out pics of finished projects in the past. Got them at full Megs on disc, but just havent got round to reducing them to less than 200K for Forum use.
Shall do that for the CT160, in it's wooden cabinet. In fact, the woodwork took about four times as long as the electronics. Apart from the additional fusing, the main improvement is the fitment of a lovely big 6" meter for -veVg.
Back in the 60's I had to change a number of RAF CT160's meters after they'd been whanged hard to fsd. Then there was the inevitable "glass tappers" - guys who thought that a good rap with a pinkie or knuckle would produce an extra couple of mA/V.
Some say the Hickock was the best VCM. I guess the 163 is probably the best, but I do love my 160.

Pics to be posted soon, Regards, David

[AlanBeckett]

Originally Posted by David Simpson

... but just havent got round to reducing them to less than 200K for Forum use.

You don't need to now
Alan

[qcda275]

Just upload them to photobucket (but not imageshack) and post the URL of the album.

[glowinganode]

Thanks David, look forward to seeing the photos.
I've often thought of making a new case for it and making it free standing like the '163. The CT160 takes up a lot of bench space, though the case does a fine job of protecting the innards.
Re. the diodes, peaks of a few volts occur accross the movement, see https://www.vintage-radio.net/forum/...ht=ct160+meter
These peaks seem important to the calibration, the fitting of clamping diodes mat affect this. Perhaps someone could make some tests on an un-modified movement, and note the effects of fitting diodes at high anode test current.
Rob.

[Kat Manton]

Just upload them to photobucket (but not imageshack) and post the URL of the album.

It is preferred that images are reduced to a sensible size and uploaded here; they load faster and we're not all using playing-field sized monitors...

(Otherwise I or another moderator retrieve the huge images from photobucket etc., reduce them to a suitable size and attach them to your post for you. This can get boring if it has to be done too often...)