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Old 23rd Mar 2020, 7:11 am   #1
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Default AVO VCM MKII restoration starts

I have had for some years now what I believe is an early No 3 MKI Avo VCM with 3 transformers, T1 is for the filaments, T2 is the HT (anode,screen), and T3 has the 25V, 145v and 2v supplies. I was going to part it out as it came partially dismantled, at the time I needed the bakelite roller switch surround to complete a CT160 project, the seller promising to send on the missing bits when he located them, needless to say this didn't happen I wasn't surprised or upset as we had negotiated a price consistent with the condition of the tester as I received it, the most notable missing part was the meter bezel, however I recently did mange to pick one up via this forum so I started to think it might be worth saving the tester.

The VG trim pot was missing, simply removed by cutting wires - I manged to locate one in my stash of salvaged pots, took a while to work out which wires went where (some of the other wires had been cut as well). Once that was done I turned my attention to the cutout relay, this had also been removed the easy way by cutting the wires with side cutters, it took me quite a bit longer to wire up as I decided to clean and adjust it while it was out - the circuit diagrams that I have show the anode volts selector switch connects straight to the relay, however I could not reconcile this with what was in front me, so I had to resort to cutting the cotton lacing on the loom to separate the wires and trace them out. On my tester the anode switch connects to the circuit selector first and then to the relay, so the circuit selector needs to be switched to the correct position before the wire can be traced with a continuity tester, eventually perseverance won the day and the relay was successfully wired in.

The backing off pot had also been removed - with only 5 wires this did not take long to install, initially I had 2 of the wires in the wrong place - the overload relay is reset by applying a voltage from the 25v backing off supply through a diode, this remagnetises the relay after an over load so it will hold - the reset voltage is taken from the backing off pot and then to the reset switch, end result of the wrong connections was that the cutout relay would not reset correctly.

This brings me to the meter, my meter has a sticker on the back from an instrument repair company indicating it has been worked on in the past, the rest of the stickers reads, 440uA 110ohms, I believe it is meant to be 440uA and 100ohms but I thought 10 ohms difference should not make make for a huge error, checking this with a series resistor and 1.5v battery I found the meter was actually fsd at 470uA, the voltage across it around 87mV according to ohms law the resistance calculates to around 185 ohms so something was not right, at the correct fsd it should have 44mV across it.

I installed the meter anyway just to see if all of the functions were working. The set mains voltage was close but slightly under so I set up a 6V6 and checked the actual anode current against the meter indicated current, the info I have is that the meter should display double the current measured at the anode link on the back panel - I don't recall how much but it was out. I removed the meter and opened it up, it has an internal shunt 1198ohms, I removed this to check the fsd of the movement itself - 397uA at 87mV, from this the resistance calculates to approx 220ohms, clearly with a shunt of 1198 ohms the total resistance is not 110ohms but around 185 ohms, seems like someone at the instrument repair place was stretching the truth,I did not actually measure the meter coil resistance as I don't like seeing the needle slam itself against the stop.

So it seems as it is, my meter may be unusable, to have a resistance of 100ohms and a fsd of 440uA doing a few calculations I figure the basic movement needs to be 200uA fsd at 44mVwith a coil resistance of 220ohms - to bring it 440uA a shunt calculates to 183 ohms this paralleled with the meter resistance of 220 ohms gives 99 ohms total. I think my calculations are correct. I don't think I can bring the meter into correct specs with a new shunt resistor, to get a fsd of 440uA with 87mV across coil the shunt needs to be just under 2000 ohms this paralleled with the 220ohms meter coil gives a total meter resistance of around 190 ohms. The Avo internal anode current shunts have been calculated for a meter resistance of 100ohms so I would then have to recalculate and replace all of the shunts, as far as I'm concerned this is not an option. What could have gone wrong with the meter - I'm guessing that as the coil resistance can't change, it has lost magnetism somehow possibly from mis-handling.

My options are either a meter amplifier as others have done or have a go at remagnetising the movement, a remagnetiser is something I've wanted so I may give this a go, with the meter as it is I don't have a great deal to lose.
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Old 23rd Mar 2020, 6:52 pm   #2
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Default Re: AVO VCM MKII restoration starts

I have had some moderate success remagnetising the magnet on a 2 panel AVO valve tester.
I had managed to ber the meter by applying external magnets to increase the sensitivity, didn't work for me. The meter is supposed to be 600uA fsd and was 2mA fsd after I had finished.
Nothing to lose I wound about 20 turns of PVC insulated wire round the magnet.
In order to decide which way to pass current through the coil I applied an external supply via a 10K resistor to the meter and increased the voltage to give about half scale reading. I then passed a small amount of current through the coil I had wound round the magnet and observed if the reading increased or decreased, I was lucky it increased so I knew which way to pass the current to attempt remagetisation.
I paralleled 4 off 15,000uF capacitors and charged them (eventually) to 45V (I know the caps are only rated at 35V but what the hell).
I discharged the caps into the coil I wound round the magnet with the expected flash and crack. I did not take any special precautions to protect the meter coil, just left it open circuit.
The result was that I managed to remagnetise the meter to a sensitivity of 700uA, not the 600uA I needed.

Ideally I should have been able to magnetise it to more sensitive than I needed and then "aged" it with a controlled dc current to reduce the sensitivity to what I needed.

I suspect I would need yet more volts and thicker wire but I have put it aside for now.

Peter
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Old 24th Mar 2020, 12:27 am   #3
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Default Re: AVO VCM MKII restoration starts

Well that is a very interesting process Peter - certainly something I might try with some old AVO8 movements which are all low sensitivity to a greater or lesser extent.
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Old 24th Mar 2020, 3:06 pm   #4
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Default Re: AVO VCM MKII restoration starts

I found a Western Sangamo meter in my collection of old meters that tested at 42.5mV and 98uA fsd, a small value resistor in series with the meter bought it up to 44mV for full scale and then a shunt across the meter/resistor bought it to 440uA fsd with 100ohms resistance, the movement is very well damped maybe even too much so as it takes the needle a good few seconds to come up to its reading, an evening on the computer with Corel Draw and I had a meter scale ready to glue over the existing scale.
Mounting the meter is a bit of a headache I really don't want to go drilling holes all over the front of the tester as I plan to try and get the original meter operational so for now I've made a setup that requires me to reach in through the back, to fix the meter into the tester, just got to make sure I unplug it first. The replacement meter is smaller than the original so I cut a surround from a sheet of black perspex.
Ran through the setup and calibration procedure and found that my scale is a few percent out at the lower end thinking about it now I realise I should have set it up with a variable resistor and battery and ticked off the major marks across the scale before I scanned it. Tested a used 6V6 and it came very close to new specs, so for now I'll leave this part.
In my original post I mentioned that I used the bakelite roller switch surround on another project so this is next - I doubt I'll be able to find one so I'm going to have a go at fabricating one from sheet of 6mm thick black perspex that I have.

Electronpusher0 your results from such a quickly lashed up setup is impressive and must be worth taking further.
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Old 24th Mar 2020, 10:42 pm   #5
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Default Re: AVO VCM MKII restoration starts

I made my lash up after reading this thread:
https://www.vintage-radio.net/forum/...ad.php?t=76514
Discharging a capacitor into a coil wound round the magnet is the method used in the RFL-440 remagnetiser, the manual and schematic for which is included in the above thread.
I have in mind to make a modern version of this, I have a box of meaty 440V capacitors and several 300 ish volts transformers. The power rating of the transformer is not an issue as I can just use a series resistor to limit the charge current, it will just take longer to charge.
Care will be needed re safety as it could be lethal. The RFL-440 has a table and hood assembly with interlock as an accessory.

Peter
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Old 25th Mar 2020, 8:50 am   #6
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Default Re: AVO VCM MKII restoration starts

Thanks for the link to the RFL-440, quite a beast.

In the early '90s I was technical director of Wharfedale (long before Stan Curtis sold them to the Chinese), and on the production line was something that put the RFL-440 to shame. We used to make high hundreds of thousands of drive units a year. They were assembled with the magnet unmagnetised. There were two magnetising stations. The assembled speaker magnet was put in a basket and a foot switch operated. Thuuungggg.

These were operated 8 hours a day, with one operation every ten to 20 seconds or so.

Because of that, these were rugged units, with the magnetising coil water cooled, and some very serious capacitors, relays and switch gear.

No protective box on that! It was built back in the day when health and safety was not what it is today.

Craig
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Old 25th Mar 2020, 9:24 am   #7
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Default Re: AVO VCM MKII restoration starts

Retailer, forgot to add, ref post 4, the meter looks the part and rescues an AVO tester from being broken so has to be good.
Very impressed.

Peter
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Old 27th Mar 2020, 8:49 pm   #8
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Default Re: AVO VCM MKII restoration starts

I have decided to make my home confinement project a full magnetiser.
I have started a new thread to report progress.

Peter
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Old 28th Mar 2020, 1:39 am   #9
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Default Re: AVO VCM MKII restoration starts

My original Avo MKII meter reads under and seems to have lost magnetism I called a local instrument repair business, a one man show run by the tech I spoke to he said they do remagnetise movements, most can but some can't be, it is done with the coil in place and the movement is calibrated after by gently demagnetising (slowly decreasing ac field) until the required calibration point is reached, cost wise would be around $50 if the bare movement is supplied which is quite reasonable.

Following this conversation I contemplated making a magnetiser and as the movement is a centre pole type I wondered how to get to the centre pole so I unsoldered the top hair spring and removed the top pivot bearing, while examining it I found why the movement was sticking, the adhesive that fixes the top assembly to the coil had let go and I could move it around on top of the coil, the top assembly is the needle pivot, hair spring and pointer.
To cut a long story short I reglued it and once assembled the movement was no longer sticking but somehow had become OC, bit of a shame as I would have liked to experiment with it. I did notice that the centre pole looked to have a ceramic magnet set into the middle of it, don't know if these can be remagnetised.
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Old 29th Mar 2020, 5:37 am   #10
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Default Re: AVO VCM MKII restoration starts

Mine is one of the MKII's that did not have a B9A socket, not sure why it wasn't fitted I don't think it is that old that B9A sockets weren't around when it was made, the fitting went smoothly I was surprised at how soft the Al top valve panel was, looking at the wiring I can see long signal paths - it is wired from the central roller switch to the sockets on one side and then a harness of shielded cables runs across to the other side not does not look very well laid out, the screened wiring looks like an attempt to prevent valves from breaking into oscillation. With the top panel back on I tried a couple of valves all good with 12AX7 but EL84 was very touchy - adjusting the grid bias had to done GENTLY any sudden movement and the anode current would soar from 45 odd mA straight to 100mA, a sure sign it was breaking into oscillation, increasing the bias bought it back. While I was at it I tried a couple of EL34's, knowing these are also high slope valves I wondered how the tester would handle them - the results were much the same as the EL84, very touchy on the grid bias control, looks like I have some work to do on the top valve socket panel.
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Old 29th Mar 2020, 6:49 am   #11
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Default Re: AVO VCM MKII restoration starts

Sounds like a job for ferrite beads.

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Old 29th Mar 2020, 8:56 am   #12
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Default Re: AVO VCM MKII restoration starts

I rewired the valve base panel on my 2 panel AVO tester and fitted ferrite beads to every pin on every valve base and also on each pin of the roller switch. It has shown no signs of oscillation with EL84s.
I also left out the two capacitors normally fitted on this panel, relying on the ferrite beads alone.

Peter
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Old 29th Mar 2020, 2:46 pm   #13
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Default Re: AVO VCM MKII restoration starts

Ferrite beads were just what I was thinking I had some left over from another project so I removed the top panel and installed ferrite beads on every connection to the B9A socket the area circled in pic 1, the harness that can be seen across the front carries all of the connections from the left side sockets across to the right side sockets.

I rewired directly from the roller switch to the left hand side with sleeved solid tinned wire, while making the connections to the roller switch I found a dry joint, I just touched it with the soldering iron and it came loose possibly it was just the flux holding it, circled in pic 2 as each connection was made I cut one of the harness wires and when done I was able to lift it completely away pic 4.

I had enough ferrite beads to also place one on each pin of the IO8 octal socket, I thought may as well do it, removing the screened harness may not be enough to overcome the issues I had testing EL34's, the beads can be seen in pic 4. Only 3 beads left so I hope I don't come across any more valves with oscillation issues.

Once reassembled I ran it through a few tests with some El84's and El34's - much better behaved, with no obvious signs of oscillation. I also tried a few 2A3's and noticed that the filament did not appear to have much glow, I checked the filament voltage on one of the other sockets, 2.59v with no valve but it dropped back to 2.1v with a 2A3 plugged in. I know some of the cheaper emission testers have wimpy transformers and it is quite common for the filament voltage to sag once 2.5 amps are called for but I would not have expected this from Avo, so checking at the 2.5v tap on the filament transformer I found 2.58v under load at the transformer but only 2.1v at the sockets, at the top panel tag strip I measured just over 2.1v so it looks as if most of the fault is before this point in either a dry joint or the filament volts switch, I'm hoping it is a dry joint which would be easier to tackle than cleaning the filament switch contacts.
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Old 3rd Apr 2020, 2:42 pm   #14
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Default Re: AVO VCM MKII restoration starts

I haven't had much success chasing down the cause of the missing filament volts - including the valve panel roller selector the filament supply is routed through 4 switches, I did remove the roller selector and gave it a clean before reassembling but it made minimal difference, I did not re-tension (bend) the moving contact has anyone cleaned these before and if so did you re-tension the moving contact. I'm losing around 0.45 volts with a 2 amp load (2A3 filament), that works out to around 0.05 ohms resistance per switch. I checked all of the rotary switches and the wafers and contacts are all in good physical condition, I was able to clean the moving part of the rotary switches but did not want to mess with the fixed contact, I have regular contact cleaner but not Deoxit, has anyone used this, does it work. This is something I'm going to have to go back to once some of the other things are done.

As mentioned in a previous post the roller switch bakelite cover is missing, while it is not essential it does help stop dirt etc from getting into the tester and also acts as a stop when adjusting the roller switch. I had a look at fabricating one from a black perspex sheet that I have but it is just a bit too thick so I will have a go at fabricating one from brass, once it is silver soldered together and given a coat of black paint I hope it will look the part.
I made a start by turning a length of timber the required diam to use as a former to bend the brass into the right shape, once bent I set it up in the mill and cut the grooves for the "ears" that are used to rotate the rollers, it all seems to fit well - the rollers turn smoothly, next step will be to cut the brass sections for the base and sides and silver solder it together.
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Old 3rd Apr 2020, 6:38 pm   #15
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Default Re: AVO VCM MKII restoration starts

Quote:
Originally Posted by retailer View Post
I did not re-tension (bend) the moving contact has anyone cleaned these before and if so did you re-tension the moving contact.
I have not worked on the MKII but did strip, clean and reassemble the roller switch assembly on a 2 panel AVO valve tester.
I did retension the moving contacts.
I reassembled with a smear of vaseline over the contacts and moving parts, I did add a couple of very thin spacing washers as the assembly was very stiff.
While it was apart I took the opportunity to fill the engraved numbers with chinagraph wax pencil.
I also removed the zero rod so that selecting 0 meant the pin was open rather than short to cathode (I think this was standard on later testers)

Peter
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Old 3rd Apr 2020, 6:42 pm   #16
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Default Re: AVO VCM MKII restoration starts

And a picture of the finished switch.

Peter
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Old 3rd Apr 2020, 8:05 pm   #17
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Default Re: AVO VCM MKII restoration starts

Peter that is an fantastic job well done.
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Old Yesterday, 2:50 pm   #18
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Default Re: AVO VCM MKII restoration starts

Managed to spend some time on the roller switch cover, silver soldered the brass frame together it is starting to come together and is quite sturdy, I'll need to do a fair amount of work with a file and the Dremel to clean it up.

I have a a few different copies of the MKI/MKII circuit and they all seem to be copies of copies, the one in the EMER documents is about the best but it is split in two, so I took the time to redraw it over 3 or 4 evenings - I may as well share it, - it is A3 and a PDF so it can be blown up with no loss of clarity.
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Old Yesterday, 4:37 pm   #19
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Default Re: AVO VCM MKII restoration starts

Could you please check two things in the circuit diagram, I've redrawn the circuit diagram some years ago (in 2011) for a friend who wrote an article about it. He and a friend of his traced the circuit diagram in their two AVO VCM Mk IIs and found the the Anode link was positioned between thw wipers of SHba and SGab and also that the diode connected to R3 had its anode connected to R3 and its cathode to SGab.

I've never owned an AVO VCM Mk II so I don't have anything to compare with so it would be very kind of you if you could check these so that we can either see if there are different versions of the design (but the diode must be connected in teh same way I think).

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Old Yesterday, 9:35 pm   #20
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Default Re: AVO VCM MKII restoration starts

That is a brilliant roller cover you have made. Cracking job.

Peter
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