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Old 14th Nov 2019, 8:42 pm   #1
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Default Avo VCM Mk4 sticking relay

Hi all,
After success with my efforts to construct a Heath Robinson 9 pin compactron adapter I set to measure my first EL509 and it is a hefty powerful tube. Yay and thanks to all who replied to that thread!

However on switching from 'test' to 'gas' and delightedly finding none of that I then switched back to 'test' on my way through to 'check' and at this point the relay tripped. I don't imagine that this is this a big worry as it does this with some of the heftier tubes and some showing gas, I presume related to grid currents or somesuch. However in my VCM if the relay does switch off I have to unplug and remove the back and physically reset the relay if this happens. It doesn't appear to be gunk on the relay and I've attempted to demagnetise it, but I can't get the relay to stay 'off' by fiddling with it, only if it trips. The primary issue is that this is a PITA, especially if I'm going to have to do it after measuring each of my sweep tubes. I can't work out why this is happening? Has anyone else had a problem with this?

The unit appears unmolested, all diodes, trafos, etc seem original and nothing seems to have ben undone on the stand-offs. It measures to within about 5% of an Amplitrex so accurate enough for my needs. Any help much appreciated, and thanks in advance
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Old 14th Nov 2019, 10:26 pm   #2
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Default Re: Avo VCM Mk4 sticking relay

I’ve seen this on a few occasions. You need to make sure that there is a slight gap between the u-formed piece and the moving/tilting piece. AVO put a ”hinge” made from thin sheet metal screwed with two screws to the u-shaped piece, in some cases you can loosen the screws and push it upwards to increase the gap. If that doesn’t fix it you can file down the arm of the u-shaped piece where the hinge is screwed so you get a gap that way. You only need a few tenths if a millimeter gap, but you will have to try it out until it works.

I think I have photos of this somewhere here on the forum so do a search and see if you can find them, otherwise I’ll try to post them when I am back home next week.

/Martin
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Old 15th Nov 2019, 3:26 pm   #3
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Default Re: Avo VCM Mk4 sticking relay

Here's the thread with the photo in post #4.

/Martin
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Old 15th Nov 2019, 5:16 pm   #4
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Default Re: Avo VCM Mk4 sticking relay

I'm sorry, where is that thread? Also thanks for the reply. Do you know why it sticks like this? It seems rather odd to me!
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Old 16th Nov 2019, 12:35 am   #5
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Default Re: Avo VCM Mk4 sticking relay

Sorry, the link didn’t get through:

https://www.vintage-radio.net/forum/...d.php?t=123200

I’m not sure why some relays develop this behaviour and other’s don’t.

/Martin
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Old 16th Nov 2019, 6:47 pm   #6
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Default Re: Avo VCM Mk4 sticking relay

Hi Martin,

Thanks for all your kindhelp. What I (think I) understand that you are suggesting is that there is some weird temporary magnetisation that is difficult to eradicate between the two poles of the relay mechanism? Once the relay is re-set it is not possible to get the armature to attach to the pole piece, it just springs freely back to the contactor as you would expect.

Unlike your picture but exactly as per your text my pivot piece was set waaay too low so one of the poles is acting as the pivot, and there is not enough movement to change that without making a new piece or doing the re-drill and solder as you also suggested.. The unit is currently in apparently unmolested condition and I am slightly unwilling to change that although I have no intention of selling it, maybe because I use it frequently and know that if I start to pull it to pieces I will go down a rabbit hole and spend the next five years fiddling with its guts instead of using it as a tool, and I am not anywhere near as experienced as most on here so potential for error is significant.

I have come to a conclusion that I will just turn the negative grid volts up a notch to drop the emmissions which prevents the relay from tripping unless its a duff valve that causes this (as it should) when the back will just have to come off as it has done presumably since it was made! At least when I retire I will know what to do to improve the unit. Thanks again for your valuable help.
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Old 16th Nov 2019, 9:08 pm   #7
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Default Re: Avo VCM Mk4 sticking relay

There are two more things you could try:

Either using an extra coil driven by some AC-voltage which you put over the U-shaped piece, and also the pivoting piece, to de-magnetize them.

Or you can make a rectangular piece of copper wire that you slide on top of the thin metal piece which acts as the current pivot, but underneath the pivoting piece, that way you will get a small distance as the copper wire will act as the new pivot point on top of the U-shaped piece, and it will stay in place.

I haven't tried any of these two methods so you are welcome to test them and report your findings.

/Martin
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Old 21st Nov 2019, 5:59 pm   #8
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Default Re: Avo VCM Mk4 sticking relay

Quote:
Originally Posted by Dekatron View Post
There are two more things you could try:

Either using an extra coil driven by some AC-voltage which you put over the U-shaped piece, and also the pivoting piece, to de-magnetize them.

Or you can make a rectangular piece of copper wire that you slide on top of the thin metal piece which acts as the current pivot, but underneath the pivoting piece, that way you will get a small distance as the copper wire will act as the new pivot point on top of the U-shaped piece, and it will stay in place.

I haven't tried any of these two methods so you are welcome to test them and report your findings.

/Martin
Hi Martin,
Wel, I decided to be brave and lose a lot of time I can ill afford
I had a go at the copper wire insert into the relay, it goes in easily enough, and I roughly adjusted the two grub screws. The relay now doesn't trip, I will come back to this later I guess as I haven't checked voltages in there yet because I decided to have a look at the calibration instead. It is mostly in surprisingly good order. With minuscule tweaks I have got the voltages where they ought to be (following your clear instructions) and most is in order as follows with exceptions after that:

Anode volts - within 2% across the range
Grid volts - within 1% across the range excepting <-0.5V where it rises a bit (+ 0.03V) but still way inside calibration.
Vg and RV6 pots all adjusted and good.

But...

Screen volts - consistently 11-12% high on all ranges.
mA/V adjustment - I can't set it to 104mV when Vg is at 0V. There is not enough play after undoing the mA/V pot screw and nut. It sits at 104mV on about 4.2mA/V or around 86mA on 5mA/V. Is the potentiometer shank round or half round? If it's half round and there's a grub screw holding it then the only adjustment is in the screw holding the pot to the chassis - this allows about 10mV range. If it's fully round then I think the quadrant dial is stuck to the pot shaft so I'll need the clear plastic screen off the mA/V dial, and for that and I'll have to figure a way to get the split pin out of the mA/V dial shank to get hte knob off first, and then how to un-seize the dial from the shank?

Can you offer any insights and thoughts into how I should approach this? I can't help but feel the screen volts must be straightforward - is it just that RMS = 1.1107 x average reading or am I grasping at straws? But the transconductance dial is a bit of a worry as it is well off value... Are there resistors or other components that I should be checking over? My circuit tracing skills are limited and I don't want to damage anything!
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Old 21st Nov 2019, 7:29 pm   #9
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Default Re: Avo VCM Mk4 sticking relay

Do you have a scope that you can connect to an isolation transformer, for ground loop protection, so you can look at the waveforms on the voltages with to check them?

The potentiometer can be worn out in some places, or it might just be a little bit non-linear. AVO says somewhere that you should split the error between the different points if possible.

On some of the potentiometers that AVO used you can pry off the metal back and look at the wound track and in some cases you can see if it is worn flat on all of the track or just in some places, and in some instances you can see if it has moved (separated some) across the track in some places, that will then explain some of the non-linearity.

Some photos of the relay and insides of your tester might help in spotting some faults.

/Martin
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Old 21st Nov 2019, 7:36 pm   #10
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Default Re: Avo VCM Mk4 sticking relay

Hi Martin,
Yes I do have a scope and a isolation transformer, and currently the VCM is connected to a variac (with continuous earth connection). Wouldn't I connect the VCM to the transformer rather than the scope? Either way I can do it, what should I be looking for?
If I can work out how I will post pics of the innards, the panels are off, it all looks pretty tidy in there.
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Old 21st Nov 2019, 7:49 pm   #11
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Default Re: Avo VCM Mk4 sticking relay

It's better to hook the scope to the transformer as the isolation transformer might affect the waveform of the mains voltage which will affect the AVO.

Check all of the voltages that are out of tolerance and where possible compare them to the ones that are within tolerance, for instance Anode and Screen voltages (where they are identical on the switches, not all positions are) and if possible hook up the scope to different places in the Vg circuit to see if you have some suspect voltage drops anywhere.

Depending on what version of the AVO MkIV you have there can be different rectifiers used, older or more modern types for instance and in the older models two diodes in series with 2.2Meg resistors across each diode (older versions used flat metal disc diodes with usually yellow glue on one side, more modern ones used black cylindrical diodes similar to 1N4007 types). Both the diodes and the resistors can be bad after all these years, resistances changing and the diodes can become leaky.

I usually replace the old rectifiers and all capacitors with modern types if the AVO is going to be used regularly and you want nice repeatable measurements.

/Martin
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Old 21st Nov 2019, 7:55 pm   #12
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Default Re: Avo VCM Mk4 sticking relay

Pics!
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Old 21st Nov 2019, 8:04 pm   #13
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Default Re: Avo VCM Mk4 sticking relay

Thanks, I will hook the kit up but it will take a while. The Mk4 is a later model than your one I think - s/no 4966, and the diodes mostly look like little black components, there are a few silver ones as well like the fellow below on the component board.
Edit: To be fair the measurements have been very consistent when re-measuring valves, it hasn't put a foot wrong which is why I am slightly concerned about going down this rabbit hole!.
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Old 21st Nov 2019, 8:37 pm   #14
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Default Re: Avo VCM Mk4 sticking relay

Well, they say that if it isn't broken don't fix it.

If it measures repeatably and reliably I would keep it as it is.

If you have the possibility you could do some measurements on a few spots of the curve of some valves with DC-voltages and compare to your AVO, then you'll know how well it performs.

If you remove the relay again you might clean it to get somewhat lower contact resistance, there are a few places where the voltage has to pass from the wire t the wiper, like through the small spring. I took some extra flexible silicone instrument wire and soldered from the stud where the wire was soldered that goes to the spring and then I soldered the other end to the pivoting piece, that way I bypassed the spring and small contact and with just a quick cleaning of the contacts I git much lower contact resistance through the relay.

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Old 21st Nov 2019, 9:11 pm   #15
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Default Re: Avo VCM Mk4 sticking relay

Thank you, I will get some static measurements on a few valves done to compare with the AVO before going ahead, the transconductance does concern me but maybe its the old OCD kicking in...I re-measured the scren against anode voltages and they are essentially the same, it must have been an artifact of my bad technique before I suspect (I probably measured from the wrong place) - they are reliably within 1.3% of each other on all equal scales with the anode always being the larger voltage, so panic over there!
Should I disconnect the relay, clean it up, and try it with known dc voltages to see if it is behaving, or measure in circuit to see using the DMM on DCV? I could also remove the inserted wire to check that this has not caused the situation in some manner, not that I can see how it could?
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Old 23rd Nov 2019, 5:58 pm   #16
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Default Re: Avo VCM Mk4 sticking relay

Update:

First of all a big thank you Martin for your patience and generosity with your hard won knowledge. We are all fortunate to benefit from your kindness in keeping these venerable machines alive.

Being me I was not happy letting sleeping dogs lie, and since the AVO was in pieces on my bench (= office desk thus no work being done) I pulled the 6 resistors out of the mA/V circuit and measured them. The worst (R3) was 0.6% off nominal at 2013 vs 2000 Ohms, the rest were between 0.1-0.2% off nominal, pretty darn good for 50+ year old components, even the pot was only 0.2% off, and its resistance changed smoothly with rotation with no 'flats' evident. Encouraged I re-soldered everything, repeated the calibration steps as before which was fast as esentially nothing had changed and, for who knows what reason, the required reading of 104mV for 5 mA/V now sat at 4.95 on the scale. I have adjusted this to spot on via the set screw and voila - I believe my machine is now calibrated! Please advise if I am getting ahead of myself here...

Anyway next up is to try to reset the relay to operational, then get some valves static measurements and compare then, assuming that all works out ok, re-assemble and get back to doing some work
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Old 23rd Nov 2019, 7:05 pm   #17
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Default Re: Avo VCM Mk4 sticking relay

And a final update: relay has now peculiarly STOPPED sticking. I have adjusted it so that it trips as specified for the shorts and with a 5U4G installed. Yay, and thanks!
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Old 23rd Nov 2019, 7:21 pm   #18
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Default Re: Avo VCM Mk4 sticking relay

Congratulations on having repaired it and calibrated it!

I've never seen one with that good tolerance of the resistors! I once spoke to a gentleman who had worked with manufacturing of these resistors and he said that they all more or less degraded faster than designed due to poor coating paint, you must have gotten resistors from the good batch then.

/Martin
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