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Old 8th Apr 2020, 1:03 pm   #21
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Default Re: AVO VCM MKII restoration starts

Martin - I checked and the anode link indeed does connect as you say between the wipers of SHba and SGab - I've changed the redrawn circuit, in doing so I found a couple of minor issues, I'll go over the whole thing few more times before I repost it. Haven't checked out the diode connections yet but will do so before I repost the circuit.

Fitting the roller switch cover has not been plain sailing I've had to do a lot of hand fitting, I feel as if the silver solder should not be harder than brass but it sure feels like it when filling, the worst is that a few of the holes are off centre, I did take care in marking their positions but they were still off, I did not notice at first but I changed drill bits part way through drilling the holes and the new drill bit I used (which came from a bulk lot of cheap drill bits from China) had been ground off centre causing the drill to wander - a good example of you get what you pay for.
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Old 8th Apr 2020, 3:00 pm   #22
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Default Re: AVO VCM MKII restoration starts

Thanks for checking the circuit! Good that your tester had the same connection on the Anode link.

I've included the circuit that I redrew from the original circuit with the corrections.

I once bought twenty M3 threaded taps from China and not one of them was M3! I tried them all in a few M3 nuts but not one fit nicely and the nuts could only be turned on three of them and those were very loose fits, measuring these with a caliper they where more like M2.6, all others were M3.5 up to M3.8!
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Old 9th Apr 2020, 2:04 am   #23
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Default Re: AVO VCM MKII restoration starts

Your redrawn circuit at first glance looks identical to mine - pity I didn't know about it's existence before, still it filled in some evenings for me while my wife was watching a "really good show" (that I don't care for) on TV called married at first sight.

Yes the Chinese stuff is very variable, they do make good stuff but also make some not so good as well and it can be pot luck for you when ordering from them.
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Old 9th Apr 2020, 9:58 am   #24
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Default Re: AVO VCM MKII restoration starts

The circuit was published in Radio Bygones No.134 Christmas 2011 with an article named "Servicing And Calibrating AVO Valve Characteristic Meters Mkl And Mkll".
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Old 9th Apr 2020, 11:03 am   #25
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Default Re: AVO VCM MKII restoration starts

I don't have the article, and as mine is all working now I probably don't need it - as you have done such a good job with the circuit redraw it is not much point in me doing the same thing, so as I have the MKI 3 transformer version I'll change my redrawn circuit to reflect that version and post it once I'm done, besides the 3 transformers there are other minor differences such as selenium rectifier rather than a miniature vacuum diode and no capacitors on the cutout relay contacts.
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Old 9th Apr 2020, 3:11 pm   #26
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Default Re: AVO VCM MKII restoration starts

It would add valuable information to the AVO community if you publish your diagram as I don't know of any diagram with the three transformers and other differences. If you also could add some detailed photos of the differences in this thread it would be excellent.
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Old 10th Apr 2020, 10:43 am   #27
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Default Re: AVO VCM MKII restoration starts

With the issue of low filament volts still hanging over my head, thought that I would have a go at cleaning the filament volt switches with the ultrasonic cleaner, it looked as if the wires were long enough to dangle the switches into the cleaner tank - if I could perch the VCM up high enough to clear the tank, it was do-able. I had to remove the front chassis bar to clear the tank and also unsolder one wire that was too short to allow the switches to drop right down, in unsoldering the wire I found a faulty solder joint, so once the switches come out of the tank and are dry I'll check all of the joints. The tank had to be filled almost to the top, I used hot water from the tap this, is around 55 deg C and a few drops of detergent, and set the timer for 30min.

Once done a quick rinse with cold water, a good blast with compressed air and then a good blast of hot air finished the job. I'll leave it overnight for further drying before I turn it back on - having cleaned the switches and located a dry joint I am a little optimistic. While the tank was still full of hot water I threw all the knobs in, it will save me going over them all with a tooth brush later. The ultrasonic cleaner was salvaged from a skip located behind a dental surgery, the mechanical timer was faulty so I fitted a similar timer scrounged from an old microwave oven
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Old 10th Apr 2020, 11:35 am   #28
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Default Re: AVO VCM MKII restoration starts

That looks nice!

I've only seen something similar done once before, have a look at this old thread, made by another Swede: https://antiqueradios.com/forums/viewtopic.php?t=93437

I really like your renovations, I would never have the focus to do such an extensive job.

I once had an idea of making a new finger guard for the roller switches by laser cutting a piece of black Perspex and then make a mold that mimicked the rounded part which I could then wrap the Perspex around after having heated it so that it would it get the correct form. Maybe someone can try that? 3D-printing is another option.
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Old 12th Apr 2020, 4:41 pm   #29
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Default Re: AVO VCM MKII restoration starts

With the filament voltage switches back in I soldered a 1.5ohm resistor directly to the H+ and H- terminals on the rear tag board to simulate a valve heater drawing current, I was dissappointed when I measure the voltage at the resistor and found I still had just under 2 volts, I was running out of ideas.
Having found two bad solder connections so far I decided to check solder connections at each of the low voltage taps on the transformer, I found another bad solder joint - I redid the one bad joint and reflowed all of the other joints - there was little or no change in the voltage, I checked the voltage between the transformer terminal and switch and found 15mV - surely this should be just about zero after all I was checking the voltage at each end of a piece of wire which should be close enough to 0 ohms, while I was cleaning the wires and terminal prior to resoldering I noticed that the transformer filament winding wires soldered to the terminals were quite heavy and looked to be around 1.2mm or so in diam. but the wire connecting the transformer terminals to the filament switch looked only half that diameter, at the time I thought nothing of it - looking further and tracing the path of the wiring the diam of the wiring changed to a smaller diam. once it reached the switch. Checking with a digital caliper the transformer filament winding was 1.3mm diam, wiring to the filament volts switch was 0.8mm and from there on changed to 0.6mm.
I jumpered the connection between the transformer and switch with a length of heavier gauge hookup wire and again measured the voltage across it - now it was only 5mV. The original length of connecting wire is just tinned solid copper wire within a sleeve and I just managed to slide another length of tinned wire through the sleeve and reconnect. Tracing the wiring through from the switch I did the same to all of the filament wiring - sliding another length of solid tinned wire through the sleeve and reconnecting.
Measuring again the voltage across the resistor I now had 2.38 volts and I had only dealt with one side of the filament wiring, there are 2, I still had the other side to do.

Probably when new with good switch contacts the standard hookup wire would be fine but 50 or so years down the track and the switch contacts get a little dirty then it is easy to lost half a volt at 2 amps it only needs 0.2 ohms resistance in the circuit to cause a 0.4v drop, with a 2.5v filament this is nearly 20% loss.
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Old 13th Apr 2020, 10:24 am   #30
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Default Re: AVO VCM MKII restoration starts

Really nice that you found this problem but what a job to fix it!

Do you have any serial number or any thing else identifying the date on your three transformer tester, it seems strange that AVO didn't take the wire diameter into account (or perhaps not if you count all other errors that you can find in an AVO VCM).

Sometimes it is a good idea to check the contact resistance even in the mains circuit with the mains switch and of course checkking the cut out relay contacts for high contact resistance - briding them with wires like you did and comparing the result is a good way of finding these faults.
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Old 13th Apr 2020, 11:28 am   #31
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Default Re: AVO VCM MKII restoration starts

Quote:
Originally Posted by Dekatron View Post
I once bought twenty M3 threaded taps from China and not one of them was M3! I tried them all in a few M3 nuts but not one fit nicely and the nuts could only be turned on three of them and those were very loose fits, measuring these with a caliper they where more like M2.6, all others were M3.5 up to M3.8!
By far the best place I've found for precision taps and dies (and tapping drills) is this place https://www.tap-die.com/ . They are not the cheapest as compared with eBay, but you can absolutely rely on the quality. Any obscure thread you need. Cycle threads, BSP, left handed - they have pretty much everything.

I have a box of BA taps and dies I bought from them a few years ago - that has come to my rescue on a number of occasions!

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Old 14th Apr 2020, 3:03 pm   #32
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Default Re: AVO VCM MKII restoration starts

I looked and can't find an Avo serial number, there is one in top but is a PMG (Post Master General) serial number that has nothing to do with Avo.
The two diodes on left of one of the pics both test ok and both have quite a low forward voltage drop around 230mV so they may possibly be germanium, the cathode end is painted with red.

The roller selector cover turned out ok, while we are not in total Covid19 lockdown and I could have walked to the hardware shop to buy a can spray paint I thought I better do the right thing and avoid unnecessary travel so I initially painted it with ordinary gloss black enamel, after 2 coats with a small brush it was looking lumpy and sad - I then saw my air brush that I had purchased 20yrs or so ago and never used, a bit of prep with wet and dry and the air brush worked like a charm I was surprised how easy it was, ordinary turps based enamel, I was able to leave the slightly thinned out paint in the jar between the daily coats, 3 in total.

With the roller selector back in (I stripped it down a second time to retension the contacts) I ran a couple of tests - a 2A3 that started me looking for lost filament volts, the filament volts are still on the low side at 2.35V but much better than the 1.9-2V that I had before I also tried another 2.5V filament valve a balloon shaped type 56 triode, the 56 filament only draws 1 amp so the filament volts were up to 2.45V

I found 2 handles that were a very close fit with respect to hole spacing so these have gone onto the front panel, the rear panel is missing altogether so I'll need to make something to fit and I still have to make a pair handles for the sides.
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Old 19th Apr 2020, 4:42 pm   #33
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In a previous post I mentioned that the meter was an issue, and after some 'work' from me became open circuit.
As this is a 'restoration' project it would have been nice to have an original looking meter on the front so now that tester is nearing being finished I thought I might have a go at rewinding the coil of the defunct original meter - if I could get it working then I could make an opamp driver for it and so have an original looking meter.
I unsoldered the hair springs and stripped the bare movement down to remove the coil/needle assembly - I gently prised the top needle bearing and pointer assembly away and could see the end of the coil was still soldered in place, must be a break somewhere else I thought so I cut the wire, then I had another thought what if the solder joint was faulty, too late to now resolder it, I tinned it anyway and as best I could and checked the coil for continuity - sure enough it was complete, my hunch was right it was the solder joint, and must have been a suspect joint that was disturbed when removed the top bearing assembly the first time, it was though too late to reflow the joint I had already cut the wire. Just unwind one turn and resolder, not possible - I got less than 1/3 the way and the wire disappeared into the coil, must be the beginning of the coil not the end.
I found a small spool of wire the same diameter and managed to join it, no easy feat, the wire is quite small I measured it at 2 thou (.002"), then I made one complete turn, reglued the top needle bearing and attempted to solder the new bit of wire to the hair spring boss, after 5 or so mins I admitted that the hair spring was in the way and I would have to remove it completely, once it was removed I did manage to solder the wire, it was however a tricky process and I thought I had it a few times only to discover it wasn't so. The plan of attack was to try to wind the wire around the 'terminal' with fine tweezers and then solder but, the pointer, and needle bearing are glued together and I was concerned that the heat from the solder iron would cause the glue to let go, so I decided to tin the wire first. Solderable insulation is good as it means you don't have to strip the insulation varnish away, but it is not a 1/2 sec process like it is with normal wire and a hot clean iron, it takes a few seconds for the insulating varnish to 'melt' and the wire to tin. I found that once it was tinned it became stiff and hard to wind around the 'terminal', perseverance did pay off and it was finally soldered, under a jewelers loup I could see the joint was good but a bit untidy. The end of the wire I joined on can just be seen in the last pic.
The hair spring was soldered on next, it is quite fragile so I cut some thick card to make a support so I could just lay the hair spring down flat in the correct orientation, I was concerned that if I held it with tweezers I would probably distort it.

Reassembling the movement was still tricky but still easier than the joining the wire and reattaching the hair spring, with bearings adjusted the pointer moved easily and always returned to same zero mark, I gave it a test, 459uA fsd and 227Ohm resistance, next step is an opamp driver.
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Old 19th Apr 2020, 6:10 pm   #34
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Default Re: AVO VCM MKII restoration starts

I take my hat off to you. Makes my AVO valve tester meter fix look like childs play. I don't think I would even attempt it.
Well done!

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Old 19th Apr 2020, 10:45 pm   #35
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Default Re: AVO VCM MKII restoration starts

I agree, that is some serious repair skills!

Now I just hope for you to say that you made some new pivot bearings too....

That's what I am trying to be able to do by buying a lathe, but they are so small that it only seems like a watch makers lathe will be able to turn them, unless I can find som very thin threaded brass pins so I "just" have to drill the pivot hole....

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Old 22nd Apr 2020, 2:16 pm   #36
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My initial meter success has been a bit soured by my discovery that the movement was no longer balanced, most probably due to the top needle pivot being positioned in a slightly differnt location, I added some weight in the form of solder to the balance bobs at the rear and to the right of the needle, this restored the balance somewhat - not perfect but certainly as good as some of the meters I have in my collection.

The next blow came when I was testing my meter amplifier, I did not take a great deal of notice initially but eventually decided to check the linearity and found it was not great, varying by up to 5 or 6%, meter position made no difference, I could not see why as the pointer always returned to the same zero position from any place on the scale and always returned to the same position when powered down and then powered up again, it is almost as if the scale is not linear but I know that is not the case as I checked that, so I can only assume it is the meter, 2% would have been nice.

After all that I decided to check if I had any movements I could adapt to the Avo meter case and found an identical physical movement - the Avo movement I have not the original as I can see that one of the internal mounting brackets has been hand made. While checking it out I noticed that the pointer on the 'donor' meter looked a bit on the short side, sure enough it was around 7mm too short, having come across damaged meter pointers before I was ready with a pack of micro Al tube put out by Albion metal hobby supplies. The pack consists of 1each x300mm lengths of 0.5,0.6 and 0.7mm diam sliding fit tube, the sliding fit means each tube will slide into the next size up. The pointer on the 'donor' meter measured 0.65mm in diam. so my 0.6mm tube should fit. I applied a little heat from a soldering iron to soften the glue holding the pointer in place and in 5 seconds or so it slid off.
I cut off a length of the 0.6mm tube squashed and about 85% of its length between 2 pieces of flat steel in the vice and I had a new longer pointer, I knew the the balance would be out but hopefully it would be only in one plane as all I had done was lengthen the pointer by 7mm. I was able to slide the balance weight to the rear which restored the balance to an acceptable level.
Checking the linearity it was within 2% or so from 0 to full scale, just now need to wait until the adhesive holding the new pointer dries and I should be able to assemble the meter.
Pictured are the movement minus the original pointer and the movement with the new pointer in place to check for length and still to be flattened out, the old pointer is lying to the right, bit hard to see I think the forum software resizes pics smaller.

Martin - I haven't tried to make pivot bearings, the ones I have seen look to be made from steel probably hardened and probably more clock sized than watch sized, They are small I agree but I feel they could turned up even on my lathe which is a 12x24, I have turned things down to 0.75mm and smaller in diam. The brass pivots could be drilled with a D bit drill.
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Old 22nd Apr 2020, 4:27 pm   #37
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Default Re: AVO VCM MKII restoration starts

Two notes on AVO meters and scales:

AVO drew scales by hand so that they would fit the meter linearity, now that you have refurbished/repaired the meter the linearity could have changed so that the scale doesn't fit the meter any longer. I've read this in AVO documentation as well as read about it here on the forum and also seen the difference between different original AVO VCM scales on different meters.

The shorter needle might mean that the meter has come from an AVO Transistor Tester or from an AVO Electronic Test Meter as those have the same case but shorter needles compared to AVO VCM meters. I've checked this with AVO TT's and AVO ETM's that I own.

It's a fantastic job that you are doing in fixing these!

Oh my, I wish I had a lathe and your knowledge so I could start to make some new pivot bearings.
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Old 23rd Apr 2020, 3:05 pm   #38
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The Avo meter scales may have been hand marked and I can see that almost certainly the leakage ohms scale would have been done this way with the aid of known value resistances, the mA/Gm scale though should be linear as a the moving coil movement is also linear, it did have me wondering what I had done to cause the movement to act in a non linear way, I thought that I must still have a lot to learn, however the story does not end there.

As I was preparing to remove the offending movement and transplant the donor I noticed that the meter scale did not look to be central to actual movement proper, with the movement out of the case I separated the components of the mounting brackets. As I mentioned in a previous post the movement that came with the tester is not an Avo supplied movement it looks different to ones I have seen in photos and one of the mounting brackets is hand made to accommodate the movement, the hand that made the bracket did not take a great deal of care see pic.

I laid the hand made bracket on a sheet of paper and marked the hole positions with a pencil, drew some lines with a set square and measured the hole spacing, the movement was mounted at least 1.5mm to the left of the scale. An hour or so at the work bench and I had a new bracket still hand made but with a bit more care. Meter reassembled and tested at 1/4, 1/2 and 3/4 of fsd, all were within 2% or better. The meter scale being offset to one side would produce the strange results I was getting. Looks like I lengthened the donor meter pointer for nothing, but I guess I did learn something in the process.
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Old 23rd Apr 2020, 3:29 pm   #39
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Default Re: AVO VCM MKII restoration starts

Nice that you found the reason. I have the same experience with an AVO meter I got with a VCM Mk III, someone had tried to repair it and slide the movement to the side, took me some time to find that. Then I accidentally made the same error when I was making a new meter, but on this one I had accidentally slid the scale to the side.

I also came across a meter in an AVO VCM163 which had had the bracket replaced with an iron bracket which affected the movement as it was slightly magnetic, the original bracket is made from brass.
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Old 24th Apr 2020, 2:35 pm   #40
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Default Re: AVO VCM MKII restoration starts

Just when you think you have all issues solved another crops up, the peg on the meter zeroing screw was too short to reach the metal adjuster that moves the hair spring, the old bracket had the movement around 2 mm too high and there was a lot of gap between pointer and scale, I made the new bracket so it would lower the movement by 2mm just enough to prevent the meter being zeroed, had to cut a new adjuster from black acrylic looks like it will work ok once the glue holding the peg in place sets.
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