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Old 21st May 2019, 9:47 pm   #1
audiomm
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Default AVO VCM MkIII repair

I;ve got a MkIII AVO valve tester on the bench for repair and have discovered one of the 3 coils on the protection relay is open circuit. Before I contemplate the thought of removing the offending coil for rewind , does anyone know where I can get a complete assembly or even a replacement coil?
I have attached a photo of the relay assembly - and the resistances measured are noted below............
A - B O/C
C - D 5 ohms
E - F 228 Ohms
E - G 0.5 ohms

I would expect the O/C coil between A & B to be the same as E - F i.e. 228 ohms or thereabouts.
Any thoughts?
Thanks Mike
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Old 22nd May 2019, 12:02 am   #2
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Default Re: AVO VCM MkIII repair

Have you checked so that it isn't just bad soldering on the solder posts, has happened a few times on relays that I have repaired, when the cloth around the wire hadn't been removed properly when soldered so it made poor contact.

I've also repaired a few relays where the black bobbin had been gnawed through by the armature it sits on and that has sometimes shorted the wire to the armature as well as severing it. I repaired the bobbins with a combination of superglue and PTFE (Teflon) tape.

It isn't really that hard to repair these relay coils. Just be careful when unwinding the wire, both so that you don't damage the wire and so that you don't crush the black bobbin.

I've just unwound the wire and soldered it back together and then used a thin PTFE (Teflon) shrink tubing on the repaired spot and then wound it back on to the bobbin - just remember winding direction and how it was placed on the armature so the magnetic fields add up - take a lot of photos when you dismantle it.

The resistance value is more or less correct, some coils I've measured have had up to 260 Ohms resistance.
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Old 22nd May 2019, 4:42 am   #3
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Default Re: AVO VCM MkIII repair

Thanks Martin for your reply. I have now dismantled the assembly after taking lots of photos and removed the offending lower coil. It wasn't hard to find out what had happened....and it would have to be the inner/start of the winding.
See photo. Fortunately I have been able to measure the winding = 230 ohms so at least there aren't any other breaks. I am going to see if I can 'lasso' that end and solder on another wire before I have to resort to unwind the coil and redo it. Nothing ventured, nothing gained eh?
However I think on reflection that a rewind would be best and definitely the more reliable option. I'll report back once I've got it sorted.
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Old 22nd May 2019, 8:10 am   #4
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Default Re: AVO VCM MkIII repair

Iím glad you found the problem and that it will be possible to repair it. I always unwind them carefully so that I can put the wire back with the same bends/kinks in each corner. Shortening it by one turn so you will get a new wire stickibg out the hole will be fine in your case.
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Old 27th May 2019, 1:14 am   #5
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Default Re: AVO VCM MkIII repair

An update on the repair. I rewound the coil and refitted the relay. On power up the red meter light came on and the relay was chattering away. I figured that maybe the windings were out of phase wih the others so swapped over the yellow leads but it wasn't that. Note, I haven't done any adjustments to the relay yet as I saw a few other issues. Someone has been in here before me - I noticed the meter connections were loose and the wires to the mains on/off switch appeared to have been messed with. One side of on/off switch had the white wires connected together bypassing the switch. Also noticed a bit of sleeeving rolling about -obviously left behind by whoever worked on it previously. Anyway I have checked the meter - its OK, and I've retightened the connections.

Also disconnected the top panel to give easier access to the innards - the relay still tripped . I've removed the DH77's to no avail so maybe the relay might need adjusting after all. I was loath to tinker with it initially but in view of the previous 'repair' it is possible it was fiddled with in an endeavour to fix whatever was wrong?
But before I do, I've tried to isolate the fault condition by disconnecting the HT transformer from the LT transformer but that didn't change anything. So I will disconnect the LT transformer from the heater volts switches to see if that highlights anything. Hopefully the LT transformer is OK and all the needs is for the relay to be adjusted - I'll give it a rest for a day and have another look tomorrow.
Mike
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Old 27th May 2019, 8:18 am   #6
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Default Re: AVO VCM MkIII repair

It's all go around here. I got my second wind and got back to the VCM late this afternoon. I pulled the relay out again as I had virtually convinced myself that it was the main offender. And it was. Before re-installing it I hadn't adjusted anything which in retrospect was a mistake. As you can see from this photo I took before I rewound the coil shows that lower arm which tensions the spring was bent as a result, no doubt, of a previous repairer winding down the adjustment screw to try & stop the relay chattering. Sigh!
Anyway I straightened it up and unwound the adjustment so that there was a good gap between pole piece & rocker arm. Then I adjusted the points for good contact pressure (resistance measured .6 ohm) and refitted the relay.
On power up it behaved, phew!..... and I waited....and waited for the valve diodes to warm up and for the meter to respond . Nothing! Damn...what now? So I swapped V1 and V2 around and hey presto the meter moved to the set mark . That means I need a new DH77 for V2 and hopefully the machine will then work . Obviously I'll have to check calibration and reset the relay later but it looks like I might yet have a long-winded win - assuming there aren't any more hidden bodges I have yet to discover.
I'll wrap it up in a day or two. Mike
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Old 27th May 2019, 9:05 am   #7
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Default Re: AVO VCM MkIII repair

You can replace those D77 diodes with 1N4007. The MKIV used silicon diodes.

Craig
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Old 31st May 2019, 3:52 am   #8
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Default Re: AVO VCM MkIII repair

This repair is turning into a mission. I put the brand new valve in - it (V2)provides the voltage for Screen - switched the Circuit selector to Test and checked voltages via the top panel. All screen voltages vary as they should when the Screen selector is switched thru the steps from zero up to 300v BUT only approx half the voltage appears.So dial up 100v and get 49v. Similarly dialled up 300v and got 148v. Is this correct or am I missing something obvious?
The Anode volts are approx 10% higher than Anode switch settings which seems about right.
Just out of curiosity I swapped the valves around and reset circuit selector to C - expecting to see needle move up to ~set mark, BUT the meter moved gently backwards? Swap the valves back and meter behaves and settles on the ~ set mark OK . That is weird.
Also set up the relay to trip when a short is detected BUT, if on top panel I short out C to A1 it trips when Screen is selected instead of A1 or A2. Conversley, if I short out C to S the relay trips only when A1 or A2 selected!
The original issue was an o/c relay coil and I am now wondering if a fudge had been done somewhere - maybe power supply board? I'll have a close look to see if I can spot anything obvious but would appreciate some pointers/clues as to where to head next. Thanks Mike
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Old 31st May 2019, 9:29 am   #9
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Default Re: AVO VCM MkIII repair

Have you read the service manual for the AVO Mk III? If not, download it from the forum or elsewhere on the Internet, and read the chapter on how measurements should be performed and what measurements to expect.

In the Mk III you have full sine wave signal on the Anode voltage and half wave rectified voltage on the Screen and Grid.

If I remember correctly now the service manual says that you should short the rectifier valve in the Screen circuit when performing those measurements.

Since the relay was a problem and you had it removed you might have to check the wiring as it might be incorrect.
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Old 31st May 2019, 11:00 am   #10
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Default Re: AVO VCM MkIII repair

Mike, follow Martin's advice & study the Manual. AVO VCM's & just about all valve testers, use transformer secondary derived AC voltage's peak waveforms for testing Ia & mA/V. Yes, switch selections indicate RMS voltages, but they are NOT DC.
I've attached a very simplified diagram of how an AVO CT160 (A MK3 in a clam-shell case, more or less) works.
Have a good shufti through "Search" - masses of info on MK3's & all the other AVO testers, & much banging-on by some of us VCM enthusiasts over many years.
Just to help you understand the AC waveforms & 20mS half-wave pulses - use an oscilloscope on the anode, screen & grid. Also remember - full AC peak to peak voltage is on the meter terminals & the relay contacts, so please take care.

Regards, David
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Old 31st May 2019, 8:58 pm   #11
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Default Re: AVO VCM MkIII repair

Firstly a thanks to Martin & David. Yes I have a copy of the service manual and in between other jobs am reading up studiously
I am also well aware of the voltages racing around inside this tester and am very careful.
Anyway I have, not surprisingly, discovered another problem - the resistor R36 has an overwind of wire to make up 200 ohms and one end of this was dry jointed. My copy of the manual shows R36 as 200 ohms (and no tolerance stated) but I have found that other versions of the service manual show it as 200 ohms +/- 0.5%. This one measures 198.1 ohms inc. overwind so I'll replace the overwind with some suitable guage enamel wire sometime today.
I've also found the missive written jointly by Martin & Euan Mckenzie on the VCM's and this is way better than any of the Avo manuals so am confident that a fix is getting close. Another day-another challenge! Mike
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Old 31st May 2019, 9:39 pm   #12
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Default Re: AVO VCM MkIII repair

Sometimes it is enough to spread the turns on the overwound wire to separate them so they donít make contact to restore the original resistance and then cement it with some araldite or similar to keep them separated.
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Old 31st May 2019, 10:47 pm   #13
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Default Re: AVO VCM MkIII repair

Thanks Martin. I've got the resistor out on the bench so will see if spreading the wire along the body of the resistor makes any difference. I does appear to have been messed with before as the soldering on the metal strap that the the overwind attaches to was .......how can I put this politely.......somewhat less than satisfactory . I am fast coming to conclusion that the previous repairs were done by that well known firm Bodge-it & Leggit.
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Old 1st Jun 2019, 1:42 am   #14
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Default Re: AVO VCM MkIII repair

Success!! I spread the overwind out on R36 as suggested by Martin and that got me 199.9 Ohms so used araldite to fix the winding in place, refitted the resistor and everything is working fine. I have yet to go through the calibration process, but a quick test with a new ECC88 I had lying around showed the results were very close to the data book figures.
I think it might be prudent if I also fit the 1N5711 diodes across the meter as protection as recommended in other threads.
Thank you to everyone for your help - it is greatly appreciated. Mike
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