AVO VCM MK1? & meter FSD

[vintage_8bit]

Hi all.

I have just got around to looking at this AVO VCM, it's been on the, to do shelf for a while now.

I have been reading previous posts, but I would still like to confirm 2 points.

1. What model do I have? I think it's an early Mk1. Please see attached. You will see it has 3 transformers.

2. I believe from previous measurements that the meter is suffering from loss of magnetism, but what should the D.C. resistance be, & what is the FSD current on this meter? (440uA 100 Ohm?)

I use a mk3 tester, so I have some knowledge of these excellent instruments & how delicate the movement is, but I would just like to confirm my thoughts before proceeding. Colin

[Dekatron]

The meter itself should have an FSD current of some 440uA and an internal resistance just a little above 100 Ohm, usually around 110-120 Ohm. Combined with the S-potentiometer of 2500 Ohm used to shunt the meter it should be possible to adjust the meter and shunt to 500uA at 100 Ohm

[vintage_8bit]

Thanks, Martin. I will concentrate on the meter fix 1st, before I proceed further.

So is this a Mk1? I haven't found any details on the 3 transformer model, apart from a comment I read, saying "some early MK1's had 3 transformers" This VCM has been previously had a B9A base poorly fitted. Colin

[David Simpson]

Hello Colin,
Yes, there were a 3 T/F version. I acquired one I think about 10 or 11 years ago( check "SEARCH"). It had been well "got-at" & the cut-out was absolutely phooked. The meter was OK as was the thumbwheel switch & the valve holder panel. All three transformers were OK.
No (3 T/F) circuit diagrams could be found. Even Dekatron couldn't find one. He reckoned they were a prototype MK1 or MK2.
So - stripped it down. The v/h panel with the T/Wh. switch, and the top quarter of the cabinet with the lid were incorporated into a DC Valve Tester I built. The HT transformer was incorporated into a 500V DC PSU I built. The two LV transformers went to a Forum chap. I used to own two CT160's, but now just use a MK3 for general valve testing.
Your VCM is certainly worth repairing/restoring/calibrating.

Regards, David

[vintage_8bit]

Thanks for the reply, David. If I get the meter going ill spend some time identifying the difference between the 2 & 3 transformer versions & label up an image of it all.

The meter is now on the bench, see attached. It was sticking in several places. After cleaning the pole pieces it seems ok, for now but I have been here before so wont be surprised if it happens again. It takes about 460uA to reach FSD. So, I'm just considering extra magnets or an op amp circuit. Colin

[vintage_8bit]

I have just read Martins post again. 440 uA is the target FSD so my meter isn't as bad as I thought. I've yet to measure the resitance properly but it looks like its around 115 Ohm. Colin

[John_BS]

Meter should be fine as there's the parallel trim-pot

[vintage_8bit]

Thank you John.
I notice there appears to be no swamp resister in this meter. My Mk3 has a bobbin within the meter housing. Colin

[retailer]

I've a 3 transformer Avo VCM, it was very close to a basket case, but I managed to bring it back to life, part of the process was to redraw the circuit, not sure if I uploaded the redrawn circuit but in case I didn't here it is - I'm 99% sure it is correct.

[vintage_8bit]

Thanks Retailer. That is most helpfull, much better than I could ever have done. Colin

[David Simpson]

Colin, I wouldn't worry about a slight imperfection in your meter's fsd.MK1's & 2's are about 70 years old now. All they need now is loads of general TLC.
Testing nowadays with modern DMM's is bound to throw up a few discrepancies, compared to the old AVO7's & early 8's in use back then. Retailer has done a great job with his circuit diagram.
I'll be doing some DC Standardised Valves very soon. If you PM me your address, I'll send you one & its Gm(mA/V) Graph, backsheesh.

Regards, David

[vintage_8bit]

Thanks David for the advice & kind offer. Its easy to forget how old this equipment is. I will be going right through this, Selector switch stripdown, checking all the R's & C's. I see its recomended to change out the rectifiers as well & fit meter protection. Colin

[vintage_8bit]

I have now checked out the 3 transformers, & its general condition. It all seems good, so I will be busy for a while going through the other components. Thanks for all the comments above. Colin

[vintage_8bit]

Caliration update.

Unfortunately, I was unable to reach FSD when adjusting the shunt RV7 - the "S" pot. I experimented with some rare earth magnets but settled for re-magnetising the horse shoe magnet. I got this idea from another forum post somewhere.

I wound 8 turns on each pole piece connected in series, cw on one pole & acw the other. I then discharged an 8200uF cap into the coils. It took a bit of experimenting with the charge, around 150V & some repeat flashing. The meter FSD was now spot on at 440uA. I know in manufacturing it would be over magnetised & then brought back to spec, but I will monitor it to see how long it lasts. Another observation before the re-magnetisation was that the linearity was not that good. It read 3 - 4% high mid range. This is now spot on. The "S" pot position is now mid range & the anode current calibration is good on all ranges.

The valve bases top panel has yet to be fitted & the rest of the calibration completed. The valve tester, prior to the above meter problem, also had 4 wiring errors. All joints concerned, still had the original inspection paint on the terminals & could never have worked properly. Very strange!

Colin

[retailer]

Excellent result ! This may the first time I've read in this forum that a member has successfully bought a meter back to spec by remagnetising the magnet.

[vintage_8bit]

Thanks Retailer. You circuit diagram was invaluable to finding the wiring faults. Colin

[David Simpson]

Great work Colin. I'm glad that you've acquired a circuit diagram for your 3 T/F MK1. Years back, when I acquired a 3 T/F one, I was unable to find a C/D. Just the standard 2 T/F version. Martin & other VCM guru's were of the opinion that it was a prototype of limited production.
When you refit the top V/B panel, you might consider fitting 9 Ferrite beads over the valve base connection wiring, whilst you are at it.

Regards, David

[vintage_8bit]

Thanks David.

The link for the circuit is in post No 9 above, if anyone needs it.

My valve base panel had a poorly fitted B9A socket. I put in a new one and fitted beads on all the 9 pins. So I hope that will be OK. They used a few screened leads in the panel.

Colin.

[ianbatty311]

Hi, you might like to check out my three-part presentation on the Historical Radio Society of Australia's website:

https://hrsa.org.au/

and look for our Youtube Channel.

There's a complete description, plus two Q&A sessions.

It's good that your meter is in calibration, as that's the vital starting point.

You may see references to a "calibration valve". (i) these are usually costly, and (ii) I consider them unecessary.

If all the voltages are correct in your AVO, then it will give the correct readings, *provided* that the current readings are correct.

To check this, plug in a valve and set it up for (say) 40 mA plate current. Break the plate current connection on the front panel and insert a DC milliammeter on the 100 or 50 mA range. The milliammeter should read exactly *half* of the indicated value on the AVO's own meter. If not, stop here and fix the problem.

If this basic calibration is correct, then you need to check that the selected electrode voltages are correct, *allowing for* the conversion factor between what the AVO says are applied, versus the *measurable* voltages on your DC-reading voltmeter. See either Martin's or my descriptions (thanks to Martin for guiding me to my level of expertise!)

I also have published several articles in our Society's Radio Waves magazine, and had several publicly published in Silicon Chip magazine. I can send you PDFs of any/all if you wish.

Be aware that, as Martin, I, and others have found, the original AVO drawings are incorrect/incomplete, especially regarding the Mains Calibration circuitry. I recommend that you check out the outstanding work Martin has done in redrawing as-built instruments.

Good luck with the AVO, "they are mysterious beasts whose resurrection will shower gold on the restorer".

Oh, did I really say that?

(you get the idea, I hope)

Ian.

[David Simpson]

Ian, maybe CV451/491's(ECC81/82's), duly standardised, which AVO recommend in their manuals - are expensive in Oz. However, using a St'd valve is necessary for calibrating AVO VCM's(or any valve tester for that matter).
Me - I now prefer a decent Ia drawing valve. Therefore, in years past, I've given away to Forum folk - well over 20 6AQ5's & 6AU5's, all DC standardised & accompanied with an A4 tabulation & graph of the Gm(mA/V) slope(curve).
Surely, some VR valve testing enthusiast in Australia could build a DC testing rig. Its not rocket science. Thus providing backsheesh St'd valves for VR Cobbers. Prid pro quo - - guys could donate surplus decent/NOS valves to the project in return. Its worked in the UK, & even "Dekatron(Martin) in Sweden kindly donated a couple of 6AU5's a few years back.
Note - AVO VCM's & just about all other vintage valve testers - all operate on AC mains derived sinewaves & half-wave pulses for Va,Vs & Vg. So, with calibration procedures & a St'd valve, getting one's VCM to achieve a slope following & close to a DC slope is what to aim for.
Of course, a modern "Sussex", or a super dooper "RoeTest", or even a "rocking horse manure" TEK 570 could be a great standard to check our (mostly 60 or 70 year old) vintage VCM's & Testers against.

Regards, David