AVO CT160 Valve Tester. Protective relay.

[Station X]

Once the protective relay in a CT160 has fully operated (ie not just buzzing) how long should it take to release once power is removed?

Mine seems to be held in forever by residual magnetism and has to be released manually. The return spring cannot be tensioned enough to release the relay and in any case I would be reluctant to do this as it would also affect the operate current. A piece of thin card inserted between the coil face and the armature cures the problem, which makes me wonder whether there should be a piece of tape or something in this position.

Is there a document showing how to adjust the relay? The manual gives operating currents, which I guess you'd have to set by using resistors between anode/screen and cathode. Do you then adjust spring tension or contact gap though? Both would affect the operating current.

[Dekatron]

In this thread: https://www.vintage-radio.net/forum/...d.php?t=123200 you can read on my findings for sticking relays in post #4.

The EMER documents and the service manual doesn't give any direct adjustment instructions for the two screws, they only mention the currents that should trip the relay and to make sure that the pygmy lamp is working.

On all sticking relays that I have come across the solution has been to file down the U-formed piece where the pivot point is - this is how a working relay is looking, with a small distance between the moving armature and the U-formed piece at the pivot point.

[Station X]

Thanks Martin.

There seems to be a design issue here. Filling down the coil cores seems a bit drastic to me, so I'll try using tape to break the magnetic circuit. It's not that often that my trip operates in any case.

I note that your contact screw is screwed in much further than mine, which is going to reduce the gap between coil core and armature, resulting in the relay operating on a lower fault current.

[Dekatron]

The photo was just taken to show the distance between the moving armature and the U-shaped piece, the screws were in the position the previous owner had left them when he tried to repair the relay. They had to be turned a lot for the relay to work properly so your relay probably has them in the right position.

I have also moved the hinge piece upwards on some relays as it is just screwed by two screws but it didn't want to stay put there so filing down the U-formed piece was easier than placing something on top of it that would stay put there. I tried making a small "hat" of non magnetic metal like copper that could be placed on top of the U-formed bracket and secured by the hinge piece and its screws but it was easier to just file down the U-formed bracket.

[Station X]

For the moment I've reassembled the CT160 without any modifications. As I've said it doesn't trip very often. Next time it locks up I'll look at implementing a solution. What I've done in the past when I've had similar problems is to drill a hole in the armature and rivet a copper or brass rivet into it to form a residual stud.

Thanks once again.

[David Simpson]

Graham, back in the 60's when I used to service RAF CT160's, the relays were sometimes temperamental. In most cases the problem was mechanical. Cleaning the contact & the pivot with ctc, then a touch of grease. Folk should never clean the contact with a file - just a wee strip of crocus paper. But of course, we had an inexhaustible supply of AVO spares. So many a time we just fitted a new relay assembly. No fannying about. Back then, CT160's were only ten years old, at the most. Nowadays, they are well over 50 years old, and spares are like rocking-horse manure. I wonder if anyone has thought of building a solid state relay unit ?
Of course, the relay unit functions, to some extent, as an audio alarm, should one be approaching fault current levels. So a solid state device would need to incorporate a modern wee buzzer circuit, or something.

Regards, David

[Station X]

The armature is very strongly attracted to the coil, even with the power off, so I don't think anything other than breaking the magnetic path is going to cure this.

I wonder if it's an age related problem due to the iron used in the coil core and armature becoming less soft?

[Dekatron]

I've soldered new relay contacts, cut off from modern relays, onto the moving armature with great success when the original contact was worn down too much to be refurbished. Most had a large black hole in the center going all the way down to the moving armature itself, where the pin was supposed to make contact. Some contacts had even come loose from the moving armature and disappeared at some time, and on some the pin had struck the hole left from the contact and been badly burned, almost welded in place.

The Polish designers used a current transformer and a schmitt trigger circuit in the ELPO P-508 (lot of nice photos here: http://www.fonar.com.pl/audio/fotki/..._p508/p508.htm) - which also uses a copy of the roller selector found in the AVO Valve testers. You can see the cut-out design in the PCB near the end of the photos. The ELPO works in the same way as the AVO Valve testers do and relies on the same maths and patents.

I have one such original cut-out assembly and have tested with a modern circuit which could be used in the AVO Valve testers but I haven't finished the design yet. I use a modern current transformer like the ones used in modern electricity meters and copied the old Polish design. It works well but I haven't had time to finalize the circuit nor make a PCB for it. The original design works unmodified in an AVO valve tester, you just have to adjust the point that you want it to trigger at. In the P-508 they used a large heavy duty relay to cut the power to the transformers. The ELPO P-508 is probably a somewhat later design than the AVOs. I've only used closed core current transformers so you'll have to cut (or unsolder) a wire in the return path to add it to the AVO, I haven't tried with any split core current transformer mostly because I needed some 10-20 turns of wire through the current transformer to get proper output signal levels. I don't know when I will have time to finalize the design, I am always short of time for all my projects.....