AVO CT160. Meter over reading.

[Station X]

I'll relate the full saga of this as it may be relevant.

Turned on CT160 with selector knob set to "SET~". As the tester warmed up the needle moved to the right and settled to the right of the "~" section.

Switched off in preparation for adjusting the mains voltage tap and noticed that the needle had settled to the right of zero. Figured that perhaps the needle was bent and attempted, unsuccessfully, to set the needle to zero using the mechanical adjustment screw.

Removed front panel of meter. No sign of a bent needle, but there was a great deal of axial and radial play in the movement's shaft. Adjusted the front bearing until the needle would just move freely without sticking. It was now possible to set the mechanical zero correctly with the needle moving equally to either side of zero as the screw was adjusted.

At FSD the measured current was 27uA, the correct figure being 30uA.
At half FSD the measured current was 13.5uA.

Swamp resistor measures 680R.
Movement measures 2510R
Total resistance 3190R, the correct figure being 3250R

All I can think is that the magnet's flux has increased or the hairsprings have weakened.

Any other suggestions?

Thanks.

[Dekatron]

Unless you already know that the meter was correct before this happened you could try to measure what current flows through the meter and 10k shunt.

Does it have a magnetic shunt and has that moved?

/Martin

[Station X]

I have never previously measured the meter's FSD. It was repaired by The Hunts Meter Company about 20 year's ago and presumably recalibrated.

Previously it's been possible to adjust the mains tap so that the meter needle sits in the "~" section, but not any more as it sits to the right of the "~" section.

I could not see any magnetic shunt.

[Dekatron]

Since the movement had come loose I'd send it in for repairs at either Hunts Meters or Herts Meters so it could be checked properly.

Seems strange that it has a lower FSD now, they usually rise as the magnets gets weaker over the years.

/Martin

[Station X]

Sorry I meant Herts, not Hunts. Too many capacitor threads in these forums.

I'll get The Herts Meter Company to look at it assuming they still do this type of repair.

[David Simpson]

Graham, you're fortunate that the meter's FSD has fallen slightly short of 30uA, rather than gone beyond, when tested outside the meter. Gives you the opportunity to alter the shunt resistance a tad.
If you're now happy with the meter's mechanical zero, and the issue is just with getting true "Set ac" - have you checked the 55V winding to see that its voltage is spot on(with the pri. tappings adjusted to your mains voltage) ? Sometimes its a bit of a balancing act between RV3 & the wee slider sw to get centre of the black set ac zone(27.5uA), & maintain 55V.
After all - its the 55V derived grid -ve "pulse" that is critical for "Test" procedures.

Regards, David

[Station X]

Thanks David.

I had a think about this and came to the conclusion that AVO wouldn't have made a movement without some means of adjusting the FSD ie a magnetic shunt.

A bit of searching found this thread:-

https://vintage-radio.net/forum/showthread.php?t=101810

Where there's a picture in post #18, reproduced below, showing the magnetic shunt. However my own movement (second picture) has no magnetic shunt. It clearly had one at one time though as evidenced by the remnants of adhesive and an insulating sleeve on one of the needle stop screws. Why there's no magnetic shunt is a mystery.

I'll post a wanted ad to see if anyone has a scrap movement with a magnetic shunt. Failing that I'll make one, but the choice of material might be difficult.

[HamishBoxer]

Silly question Graham,has the shunt fell off and hiding somewhere in the tester?

[Station X]

I don't see how that could possibly have happened. Up until now the meter has never been dismantled in the presence of the tester and I haven't removed the screws securing the end stops.

[HamishBoxer]

Sorry,I was forgetting that the meter would have a case on it.

[Dekatron]

Since this meter has been serviced by Herts Meters previously they might have put together, or removed, whatever parts necessary to get it up and running. From discussions with them they use parts from old scrapped meters as there are no spare parts to be found elsewhere, so maybe your meter was fashioned from other parts last time it was serviced or someone before Herts Meters has been in there and removed the magnetic shunt. Since they also re-magnetized meters they might have been spot on with yours meaning that there was no need for the magnetic shunt. If you still have the receipt from the last service you might be able to read what they did to the meter at that instance. When I and some friends have used their services they have always included a detailed receipt of the service made to the meter.

The internal moving coil resistance you measured is a bit high, AVO specified a resistance of 1600 Ohm on the moving coil and that is along the lines that I've found on almost all original AVO VCM Mk III/Mk IV and CT160 movements that I've checked. However movements coming from the series of AVO Transistor Testers have a higher internal moving coil resistance more in line with the one in your meter, so maybe the moving coil has been repurposed from such a meter. You might be able to check this if you look carefully at the swamp resistor and try to discern if someone has removed some turns of resistance wire to lower its resistance.

There is however a less nice possibility and that is that the wire in the movement has been overheated and slightly burned which in some rare cases increases its resistance.

[Station X]

Originally Posted by Dekatron

AVO specified a resistance of 1600 Ohm on the moving coil and that is along the lines that I've found on almost all original AVO VCM Mk III/Mk IV and CT160 movements that I've checked.

That implies a swamp resistor of 1650R which is way different to the 680R I measured.

This post suggests 2707R for the meter and 543R for the swamp resistor:-

https://www.vintage-radio.net/forum/...ad.php?t=50786

Does this imply that in both cases the movements were repurposed?

[Dekatron]

AVO could also have changed the specifications over time to make just one meter movement for both VCM an TT. I’ve mostly checked meters on AVOs that were sold in Sweden by SRA, which I was told where original and which had never been serviced, only stored in military lockers for future use (they had been checked regularlyso they were within specifications though). I have however been contacted by people who have bought AVO VCM’s where the seller has told them that the meter comes from an AVO TT and those have had resistances closer to the higher values and som have even been missing the swamp resistor altogether (these didn’t even have the drill hole in the back shell for the screw/nut drilled out, just the impression in the bakelite). There were some of these meters without swamp resistors on eBay but they either had an AVO VCM scale plate or were sold as AVO VCM meter for either CT160 or Mk III/IV - I never got any answer from these sellers when I asked for details on the meter, so my guess is that they were in fact AVO TT meters.

[Dekatron]

Right now when re-reading the thread on the CT160 meters I did check figures I got from a friend in Australia and he had only seen meters with close to 2600 Ohm moving coils from untouched AVO VCMs. So there certainly seems to be a difference from what batch or revision the meters are from. So yours is in fact low in resistance compared to his figures. When I discussed this with him many years ago we decided to go on his figures in the document I wrote, even though my figures differed, but that was because his were from meters that included the UK too and mine were only from early serial numbers from Sweden.

[David Simpson]

Another thought Graham - just recently another Forum chap(Sven in Germany ?) said he'd sent his AVO 30uA meter to the UK for repair & it went missing ! Might be worth your while & find out which company he sent it to for repair, beforehand, should you consider posting off yours.
Hopefully you can sort the problem yourself. Otherwise - get cast iron insurance cover from Royal Mail, &/or recompense assurance from the repair company - considering the £200 + that these meters cost to replace with a genuine AVO one.

Regards, David

[Dekatron]

All of the AVO Mk III, IV and CT160 that I checked from sales from SRA had serial number below 437, so they were quite early ones, whereas the others all has serial numbers above 2000 and most in the 4000 range. So it is quite possible that AVO made changes to the specifications in later serial number ranges.

Sven's meter finally turned up he told me a short time ago and was serviced by Herts Meters, it was DHL that finally found the package.

[John_BS]

Graham: #


By far the simplest solution is to trim the series & shunt resistors to bring the meter back to spec (current and total resistance)


New Swamp resistance 1041 New Shunt Resistance 7647

[Karsten]

Graham,

You might consider a meter amplifier like my suggestion
https://www.vintage-radio.net/forum/...t=55782&page=4 .
This will not only resolve any problems with internal resistance but also enable you to employ the best parallel resistor for needle dampening regardless of external circuit load. It protects the movement against overload and makes it virtually undestroyable. The "dip" function is also nice as it greatly assists in nulling anode current.

Good luck!
Karsten

[M3VUV51]

is it the same as a bradley ct-471?

[Station X]

Thanks for all your replies.

I should have a replacement magnetic shunt in my hands soon:-

https://www.vintage-radio.net/forum/...d.php?t=160352

I'll post the results of fitting it and recalibration here.