AVO VCM MK II overload relay too sensitive

[ianbatty311]

Hi, folks

I have been given a VCM MKII to repair.

It looks to be in 'fair-but-neglected" condition. Complete externally and internally, reported as not working, some damage to paintwork, needs a good clean.

It seems to be out of calibration, but I reckon that's an 'easy' fix.

The main problem is that the overload relay keeps popping out.

Even just stepping the mains voltage selector switch will cause it to trip, sometimes.

I'm assuming that the inrush current when I change settings is the problem.

The VCM only draws maybe 10~12 watts from the 240 V mains, so I'm assuming that is about a normal current draw and should not trip the relay.

It does reset manually, and so long as I don't change the mains setting switch, or overload the VCM with incorrect settings/switch selections, it seems to work OK. I've tested a 6AQ5 (6V6 equivalent) and the VCM is behaving about as I recall a MK IV did.

Any ideas? I *did* find this forum to be an absolute goldmine of info for the MK IV I did up a while ago, so any and all advice will be really, really welcome.

Thanks from Down Under (Australia, that is),

Ian.

[Dekatron]

Have you read the excellent article by Euan MacKenzie in Radio Bygones #134 Cristmas 2011 or the excellent article by Gary Tempest in Radiophile Summer 2000?

Article by Euan: https://www.vintage-radio.net/forum/...05&postcount=5

They don't discuss the relay in detail but many other aspects of refurbishing.

You should check C2 & C3 and also check if any of the windings on the relay have shorts to cathode, anode or the common ground. I have seen relays where the plastic material in the bobbin,where the wire has been wound on, that has been gnawed through and where the wire has made contact to the metal armature shorting the winding.

Also try to set all of the top switches to the zero (0) position and see if the relay still trips.

[Dekatron]

I forgot to mention that if the relay has tripped a lot the armature can become slightly magnetic which means that it will trip easier, if that is the case you will have to de-magnetize it.

[ianbatty311]

Hello, Martin,

Thank you for the tip and for the link to the magazine.

I'll let you know how I go.

Cheers,

Ian.

[retailer]

The overload relay on my MKII had been removed by the previous owner by clipping the leads with side cutters, took me a while to figure out all the connections, but I did in the mean time find out how it works. When it trips it has AC going through it and this has a tendency to demagnetise it, the reset button pushes the contacts together and at the same time energises the coil with DC via a diode to remagnetise the central core so it holds with residual magnetism once the reset is let go, possibly your switch contacts are bad or the diode is toast and the core it isn't being remagetised.

[ianbatty311]

Hi, thanks for the reply.

Fascinating to find out just how complex a superficially-simple piece of kit can be.

I'll have a look at the components you indicated.

It would have been nice if AVO had given a fully-detailed description of the MKII -maybe they did and it's just not clear.

Cheers,

Ian.

[ianbatty311]

Hi, folks.

I think the overload relay should have enough magnetism to stay closed, even when the VCM is turned off.

It did not when I looked at it.

I could push it closed and feel a weak retaining force, but not enough to hold it closed.

With the VCM disconnected from the mains, I used my bench supply to put 'a bit' of DC current through the coil, noticing which polarity *increased* the hold-in force.

I put about 200 mA through, and it would stay closed when I removed the 200 mA.

But, if I now open it (using an insulated tip), and push it closed again, it will not stay closed.

So it seems that I can magentise it temporarily, but I'm not putting in enough current to make it permanent.

(and I think that having lain unused for a long time is the cause of the reduced residual magnetism)

Am I right so far?

I think I could get more effective residual magnetisation with a higher DC current, but I *don't* want to burn out the coil (!).

Any ideas on a safe "zap" current to bring up the residual magnetism?

Cheers,

Ian.

[Dekatron]

Unfortunately I've never had to repair these relays, nor do I own a Mk II, I've only read about the relay in the service and instruction manuals and there is no mention of any holding force there.

Is it possible to attach a permanent magnet to the relay to increase its holding force to see if that is the problem here?

[Dekatron]

I just remembered one thing and that is that a friend of mine who repaired one of these Mk II relays found that one of the relay windings had been turned the wrong way so that it counter acted against the other winding. Check your relay to see if it has been fiddled with and if either the windings have been replaced and inserted the wrong way or if the wiring has been soldered to the wrong connections.

I have seen the same fault on one of the AVO Mk IV relays that I repaired, someone had probably tried to fix the relay and removed the windings and accidentally put them in the wrong way. I accidentally made the same mistake myself but since I had seen the fault earlier I could correct my own mistake immediately when checking the cut-out function.

[ianbatty311]

Hello, Martin,

Thanks again for your reply.

The wiring is all original, so it's not been hacked, happily.

The relay is a nice bit of engineering - if the valve anode/screen are working correctly, they are supplied with AC from their respective transformer tappings and they act as *diodes* for this circuit.

This means they only pass pulsating DC, which causes the relay to pull in even more positively than when the set is turned off.

In practical terms, normal (or even excess) anode/screen currents will not throw the relay out, as higher pulsating DC currents simply pull the relay in harder. So it does not protect the AVO against a normal valve drawing excessive electron current.

A *short*, however, will allow AC to pass through either or both windings.

That is, the expected diode action of the valve has been overcome, and the short will allow both half-cycles to pass through the relay winding/s.

Now, on the 'formerly-not-rectified' half-cycle, an (effective) throw-out current will flow during the formerly-not-rectified cycle, will oppose the remanent magnetism (which holds the relay closed even with the power off) and trip the relay.

That all makes sense to me.

On my original 'trips when changing the mains setting switch' problem, it happens if I switch quickly, but not if I do it *slowly*.

Ummm....

Beats me - any ideas?

Ian.

[Dekatron]

That reminds me of another problem I had when my RCD tripped when someone rang the doorbell - the transformer for the doorbell was right next to the RCD which induced a current in the RCD so it tripped - took me quite some time to find that fault as I was looking for a ground fault and not the magnetic field inducing a current in the RCD.

Could it either be that the bulky transformers induce a large enough magnetic field to trip it or is it just because of the AC voltage changing phase when you change the mains voltage switch, tripping the relay for exactly the thing it is designed to trip for - when it sees the other half wave period?

I only see two possibilities to test this, one being a mu-shield around the relay and the other being that you feed the relay from some other transformers that don't switch on and of via the power switch in the AVO VCM, that way they won't change phase if that is the problem.

[retailer]

Are you able to work out the magnetic polarity of the central pole pieces using the RH rule or even with a small toy compass and then try to remagnetise the central core with a stronger rare earth magnet ?.
It's been a while since I delved into the innards of my MKII but I recall a spring on the cutout relay and indeed there must be one that pulls the contacts apart when the relay activates - has this has been adjusted for too much tension ?