AVO Two Part Tester

[Chrispy57]

Hi Wavey - I have attached some photos showing my meter sensitivity tests and the access to the movement that I had (without having to remove it) and a couple of improvements made while I had it apart.
Unless there was a change of meter to a 1mA movement at some stage during production, or a previous owner transplanted an old AvoMeter movement at some point, it does sound like yours is now low sensitivity, probably due to weaker magnetisation? Hopefully those other threads will point you in the right direction.

Cheers
Chris

[WaveyDipole]

After reading some of the threads I found I decided a close visual inspection of the movement was in order. It is pretty difficult to see the spring, so I decided to take some photos from various angles and blow them up on the display. I thought it would be helpful to post the photo up on here so its clear what we are dealing with.

It seems that the spring is not flat, but bowl shaped. I thought that the outer two leaves were touching, but photos 2 and 3 seem to suggest that they are perhaps not. The spring is clearly not centred but tighter on the closed side to the right and looser towards the open side to the left. The photo also makes it seem that there are a lot more dust particles around than there seems to be to the naked eye.

What is the vertical bar in front of the movement and connected to the block in which the rotator sits? It seems to move quite easily and see that in your photo is is positioned vertically? Another difference is that the notch in the adjuster on your meter seems to be closest the movement, whereas in mine it is at the other end of the slot.

I am using a PL320 and can't get a voltage as low as 0.15VC on mine!

[WaveyDipole]

Furhter to my last post, I found a description of the metal bar here:

https://www.jacmusic.com/Tube-tester...ers-Part1.html

Quote as follows:

Originally Posted by www.jacmusic.com/Tube-testers/Analog-Testers-Part1.html

As you can see, the panel meter itself is rust free. Look at that old fashioned horse shoe magnet. Such are not small size, but very stabile, and have a very uniform field, because the ends are flat. Note, at the front, is the magnetic shunt. This is just an adjustable piece of iron, with brass screw, and the iron shunts the magnetic gap. So when put nicely in the middle, the gap is maximum shunted. When you shift it too one side, or even totally remove it, the magnetism inside the gap it at it's highest. As you can see, it is in the middle, so in the (unlikely) case the magnet would loose some magnetism, it could conveniently be compensated.

In the photo he shows the piece of metal is different and it is no clear to me from the description in what position is maximum shunt (lowest sensitivity). The one in mine, like the one in your photo, does not have a slot, so it just swivels. I am guessing here that horizontal is maximum shunt and vertical is minimum? If so, then the movement in mine is already at minimum shunt and therefore maximum sensitivity....

I presume that the PCB in yours is some kind of sensitivity amplifier?

[Superscope]

If the Movement is low on Flux, I have had some success with adding
Neodymium Magnets to boost the Flux on these Avo Horseshoe Magnets.

https://www.vintage-radio.net/forum/...=4343&page=159

Check post # 3179

With the right type of Neo, the repair can be almost invisible when the Movement
is back in place.
I was able to restore the sensitivity of an Avo Air Ministry Type D Movement (2.5mA FSD) that was about 20% low.
So it can be done.
Although the Scale Shape can suffer.


Ian

[Chrispy57]

Hi Wavey - I can see that you have been busy researching the movement, and those photos are quite enlightening.
As you have noted we don't have the centre slot, unlike the guy in your article, so could only swivel rather than swivel and slide our bars, but our two bars are aligned with a 90 degree difference to each other.
Also, I noticed that there is an obvious brass screw holding my bar and the other guy's bar, whereas your screw doesn't look like brass - is that just a lighting issue because a steel screw would screw things up!
My little PCB is just an overvoltage protection circuit, again described in some threads on here, to protect these sensitive and unobtainable meters.
I guess you could use an external potentiometer with yout PSU to wind it down to lower voltages than currently achievable if you wanted to try a similar check? Mine was done on the 100mA selector switch setting, so the 6 indicated 60mA as per the DVM 60.05mA as pictured, but satisfactory over the whole scale from my rough notes. Incidently, they also reminded me that the anode voltages were on the high side until I changed some of the capacitors and used a variac to standardise at 240V - my isolation transformer was giving slightly high output and Rochdale raw mains was 245V on the day! Screen voltages were pretty close though.
Nice to see Ian's advice on the magnetisation issue too.

Cheers
Chris

[Electronpusher0]

There has been some speculation as to whether an AVO 7 movement is the same as the one in the 2 panel valve tester.

The answer is a qualified yes.

Its not obvious as the fsd of the 2 panel is 600uA and the AVO 7 is 1mA.
BUT the ANGLE of deflection is completely different. The 2 Panel tester only uses a restricted part of the deflection range corresponding to the 600uA marker on the AVO 7.

The pointer is longer on the 2 panel to compensate for the restricted deflection range.

So, if you are desperate, an AVO 7 movement CAN be used in a 2 panel but the pointer will need extending and the movement re-balancing by adding an additional counter weight.

A word of warning, these movements are extremely sensitive to being touched by anything magnetic, if you use a steel screwdriver to undo the mounting screws you will almost certainly reduce its sensitivity, you must use a non magnetic screwdriver. I bought a bronze one specially.

Peter

[Superscope]

Originally Posted by Superscope

I was able to restore the sensitivity of an Avo Air Ministry Type D Movement (2.5mA FSD) that was about 20% low.
So it can be done.
Although the Scale Shape can suffer.


Ian

Small correction to my post above.

The Movement I worked on in my Link is from a Rare version of the AM Type D Avo meter with the Model 7 Movement.
i.e. 1 mA FSD and not the 2.5mA I stated above.

Which is good, as it makes it all the more relevant here, but it should apply equally to any Avo Horseshoe Movements within reason.


Ian

[WaveyDipole]

Originally Posted by Superscope

If the Movement is low on Flux, I have had some success with adding
Neodymium Magnets to boost the Flux on these Avo Horseshoe Magnets.

https://www.vintage-radio.net/forum/...=4343&page=159

Check post # 3179

Thanks. That was helpful. Being able to place them out of view like that helps to preserve originality. I was curious how the N/S poles are oriented on these magnets and it seems that this depends on which way they are magnetised - axially (N/S on opposite flat sides) or diametrically(N/S on opposite curved edges). Most eBay listings do not state this, although some do. I expected that they would need to be placed in such a way so as to be enhance rather than suppress the exiting flow of magnetic flux generated by the horseshoe, so was very interested to read your comment about placing them so as to repel their neighbour.

Originally Posted by Chrispy57

Also, I noticed that there is an obvious brass screw holding my bar and the other guy's bar, whereas your screw doesn't look like brass - is that just a lighting issue because a steel screw would screw things up!

No, its not the lighting. It does appear to be a steel screw.

Originally Posted by Chrispy57

My little PCB is just an overvoltage protection circuit, again described in some threads on here, to protect these sensitive and unobtainable meters.

I had wondered about that since some actions seem to swing the pointer hard left, but have also read about instances of it being swung hard right, so that does sound like a sensible precaution.

Originally Posted by Electronpusher0

There has been some speculation as to whether an AVO 7 movement is the same as the one in the 2 panel valve tester.

The answer is a qualified yes.

Had wondered about that as they do look very similar. Perhaps not surprising really that AVO would use the same factory parts and make some adjustments to arrive at the movement characteristics they wanted.

Originally Posted by Electronpusher0

A word of warning, these movements are extremely sensitive to being touched by anything magnetic, if you use a steel screwdriver to undo the mounting screws you will almost certainly reduce its sensitivity, you must use a non magnetic screwdriver. I bought a bronze one specially.

Thank you for that word of warning! I can see that there are a number of "non-sparking" screwdrivers made from various non-ferrous alloys such as Berryllium-coper or aluminium-bronze. I don't know whether B&Q carries anything like that but will have a look on Thursday. I imagine the screwdriver itself must be considerably softer than the usual ferrous steel variety. They do seem a little expensive compared to those made from standard materials.

[Chrispy57]

Hi Wavey - just a passing thought as you await your trip to B&Q for a non-ferrous screwdriver on Thursday. If, as you say the magnetisation is strongest with the bar in your position, yet this is inadequate, combined with a steel rather than brass screw causing a further reduction, then why not just remove both screw and bar to maximise the magnetisation and see if that improves your readings?

Cheers
Chris

[WaveyDipole]

An interesting thought. I didn't know for sure that magnetisation was strongest with the bar in the vertical position but surmised it based on the information I had read. To confirm it, I have just tested the FSD with the bar in both positions. When it is positioned horizontally, the reading drops by around 2 minor ticks when compared to the vertical position. Therefore, it seems to provide only a relatively minor adjustment.

I then proceeded to test with the bar removed. Fortunately, the retaining screw was not tight so with the help of a fingernail initially, so I was able to remove it using only my fingers, avoiding the use of any ferrous implement. As soon as it was slackened slightly, the magnetic force from the magnet caused the bar to flip into a horizontal position. The upshot is, unfortunately, that even with the bar completely removed, the reading doesn't go higher than 7. There might have been only a hairs width of difference.

[Chrispy57]

Hi Wavey - thanks for the update - worth a try but disappointing result.

Cheers
Chris

[WaveyDipole]

I couldn't find a suitable screwdriver at B&Q or locally so ordered a couple of Aluminium-Brass screwdrivers online. They arrived today, but having tested one of them with a magnet, I find that it is actually slightly magnetic? Hopefully it will be safe to use on the two big mounting screws?

[WaveyDipole]

I haven't updated this thread in a while because I had been waiting for magnets to arrive. The movement had been removed and stored using one of the previously mentioned aluminium-brass screwdrivers. The magnets arrived over a week ago but I had been unwell so waited until I was feeling better and for the heatwave to pass, both of which would have affected my concentration.

Today, I felt that I could tackle it and firstly, checked for debris using a piece of thin card between movement and magnet. A handful of small bits were extracted. Only one of them, I think, might have been large enough to cause a problem. Next, I investigated and tweaked the adjuster and retaining screw to get the tension just right. It seemed a little tight and slackening it somewhat allowed the pointer to fall back to zero properly. Some progress made!

At this point I discovered that the outer end of one of the springs had come adrift from its soldering point so a bit of delicate soldering was required to re-attach it. Fortunately it was easily reached and accomplished without and further damage. Phew!

Next, I then tried Superscope's idea of using Neodymium magnets. Without additional magnets, the meter reads just over 6 at 600µA of current being passed through it. With the Neodymium magnets the pointer could be pushed up to about 7, but no further. One thing of note is that the meter coil measures approximately 41Ω but I came across a circuit showing it at 30Ω so that's a considerable discrepancy. There are also signs on the dial screw slots indicating that it has been removed at some point. I did wonder whether the movement might have been replaced at some point and the dial attached to a 1mA movement, e.g. from an AVO 7? Given the discussion about different pointer lengths between ATPT and AVO7 movements, I checked and the pointer has been extended. It also does not have the small circle shown on some photos that aids visibility, however the extension does look neat enough to have been done at the factory.

I am not sure where to go with it next? Build an amplifier circuit perhaps? Adjust the shunt resistor?

[Freya]

The meter on my first two panel had a similar problem, in that a hairspring had come away from its mount and had been resoldered, not by me. the FSD was also wrong. At the time i decided to send it to Herts meter company, someone called Robert repaired it.
The results of its repair was the FSD was out because the hairspring had been altered by the soldering creating more tension. A complete new hairspring was fitted.

[WaveyDipole]

I can see how that could easily happen. The hairspring balance is very delicate. If it were broken and a segment removed so that the end could be soldered to the mount point again, or else if consecutive windings of the coil accidentally got soldered together that would create additional tension which would result in an imbalance.

Fortunately all I had to do was hold the end of the hairspring with a pair of pointed tweezers against the solder point and dab the solder point with the iron to reflow the solder. Took all of about 5 seconds. The meter read exactly the same after the repair as it did before the repair so I was very fortunate.

I re-tried the test I did earlier with a 10k resistor in series and varying the voltage between 1Vdc and 10Vdc for a current of between 100µA and 1mA. Previously the movement was sticking at around the 2 mark and below and not landing on zero. It
was re-assuring to see it now move smoothly all the way across the scale without sticking, and landing on zero each time the power was removed. However, is it not a curious co-incidence that the FSD of the meter is almost exactly 1mA. What are the chances it would be almost spot on in the event of a fault?

[Chrispy57]

Hi Wavey,
good to hear that you are feeling a bit better and rising to the challenge again! Well done for sorting out those little mechanical problems, and getting some experience of the extra magnetisation possible with your new neodymium magnets. The evidence seems to be pointing in the direction of having a 1mA meter movement doesn't it - (extended pointer and near 1mA readings)?
If the modification was carried out by the factory/Admiralty workshop/competent technician then presumably the tester was recalibrated and working correctly at that point?
I wonder if after resolving all your earlier, fairly minor but irritating, issues it may be sensible to step back and reassess the situation? What is actually "wrong" with the tester now that prevents you using it to test your valves? Do you have a known good valve to test and compare those results with your own unit? Good luck!

Cheers
Chris

[WaveyDipole]

Chris, thanks.

Yes, the evidence does appears to suggest a 1mA movement. According to the "Second to fourth echelon work" document, the meter resistance should be "approximately 32Ω" and the snubber approximately "approximately 40Ω", totalling up to 72Ω. I measured the resistance of this movement at approximately 41Ω and the snubber resistor at approximately 36Ω, giving a total of 77Ω which is a little high. I agree that if its a factory fit, the circuit should have been adjusted accordingly and the instrument calibrated for the alternative part. I am not sure whether the 36Ω represents an adjustment or an approximation....

Since the reading is exactly on 6 for 600µA I was wondering whether it might be worth leaving a couple of Neo magnets on to provide some leeway so that some adjustment via the magnetic shunt is made possible. I agree with your suggestion to "step back and re-assess". While the movement is still out of the case, I was going to carefully clean the interior to remove of any loose debris and then replace the movement. There is nothing much further that I can do with it now in any case. Previously, the movement was sticking and it was tricky to get a proper zero adjustment. Because of this readings were often low and rather erratic. It would be useful to re-test and compare the performance with the previous results.

Other than that, the document does mention that "adjustment can be effected on the swamp resistance. I thought it might be an interesting exercise to do some calculations to work out the effect of the shunt with the nominal values (40Ω shunt, 32Ω movement, 600µA FSD) and then try to work out what the value of the shunt would need to be for a 1mA meter. In any case, para 10 of "Second to fourth echelon work" gives the procedure for calibrating and adjusting the circuit, which can be done, if required, once the movement has been replaced and re-tested.

[WaveyDipole]

The movement is back in the case and I am glad to say, the pointer is operating smoothly and no longer sticking.

The tester seems to function as one might expect, but I am still getting rather inconsistent readings. I tested a couple of EF39's both tested bad and well into the 'replace' category, yet one of them is an unused NOS part. Both work properly in a radio.

I also tested a couple of PCL series valves. Am I correct in thinking that where there are two sets of figures in the valve manual, that one applies to pentode/triode 1 and the other to pentode/triode 2 so one has to switch between Normal and A2 and adjust anode and screen volts accordingly? Both seem to test bad on Normal but Ok on A2.

I did note that the heater setting is important. One of the PCL valves I tested required a 9V heater. I cautiously set it to 7.5V and it tested bad. When I set it to 10V (there is no 9V setting), the valve tested good, but the readings were on the low side.

I intend to go through the calibration procedure, but the fact that some valves or parts of them test good seems to suggest that the tester works. It could be that I have some faulty valves, but I wouldn't expect unused NOS to test bad yet actually work in the radio? Not quite sure what to make of it yet, but at least the meter movement now seems to behave as expected.

[Chrispy57]

Hi Wavey - your progress to date sounds very promising, I'm sure that the sticky meter was the biggest issue and that's sorted. If I may address the points that you have raised, with the preface that I am not an expert on valves or valve testing, but have a little recent experience of refurbing and using this tester, then I would first point out the often used remark that the best test is how the valve performs in the radio.

Your EF39s do work in your radio, and that circuit will be well designed and quite forgiving of valve characteristics. I happen to have tested 3 such valves in 2020 and while the quoted mA/V figure is 2.2, my samples tested as 1.9, 2.3 and 2.8mA/V. Let's remember also that those NOS valves are probably 70 years old now with an unknown storage history.

You don't specify which PCL type you tested but looking at the first in the manual, PCL82, the data shown for use with this tester are 414 237 516, then 100, 50, 3 and below that 100, 100, 6, finally B9A TP.

I interpret these as meaning that it is a B9A base Triode Pentode, the roller switch settings should be, from left to right, 414 237 516. In the Normal position of the Select Anode switch for the Triode Section - Anode switch = 100V, Screen switch = 50V, expected result = 3mA/V, and in the A2 position of the Select Anode switch for the Pentode Section - Anode switch = 100V, Screen switch = 100V, expected result = 6mA/V.

As you observe, the heater setting is critical to the results obtained, and also over-voltage will be detrimental to the integrity and longevity of the valve.

I've read that in the past other Members have used standardised valves to compare their own tester results, and if reading this thread may chime in more authoritatively. If it would help, I'd be happy to send you one of these EF39s to compare your result with my (non-standardised) results for some peace of mind?

Cheers
Chris

[WaveyDipole]

Chris, firstly my apologies. The NOS that I compared with the working valve was the EBC33, not the EF39. I see from the datasheet that the EBC33 is a "dual diode triode" which is rather different to the EF39 which is a single pentode. I do also have two EF39's but both are used and neither is NOS. I also realise that I have been rather vague, so I repeated some of the measurements this morning, making notes of the results which I have presented here.

Since the EF39 has the simplest configuration, I started with the spare and compared it to the one already in the radio. The settings used were:

Dial - 026 510 310

vf = 6
top = G1
va = 100
vs = 100
ma/v = 2.2

The actual readings for the spare valve were:

ma/v = 0.5
stat = replace (2.0)
c.ins > 10

The readings for the vavle in the radio were:

ma/v = 1.3
stat = white mid area (5.4)
c.ins > 10

Anode select was in the 'Normal' position. From this it would seem that the spare valve is bad and the one in the radio is weak but at least the measuring process seemed to make sense.

The second valve I measured was a PCL82. One thing of note with this one was that on power up, the heater briefly flared but settled within a couple of seconds to a normal glow. Is that indicative of a fault? The valve otherwise seems to look quite normal.

The settings were:

Dial - 414 237 516
vf = 16

Triode (Normal setting)
va = 100
vs = 60
ma/v = 3

Pentode (A2 setting)
va = 100
vs= 100
ma/v = 6

The results for the triode (Normal setting) were:

ma/v = 0.5
stat = 'replace' (1.5)

The results for the pentode (A2 setting and screen adjusted to 100v) were:

ma/v = 5.2
stat = 'good' (9.5)

C.ins was approximately 5 megohms.

Faulty triode, good pentode?
Would the 'good' reading be expected to be as high as that?

Going back to the EBC33 double diode triode, what position should the anode and screen switches be in? The valve manual shows one set of settings:

Dial = 026 890 310
vf = 6
va = 100
vs = unspecified
ma/v = 2

The diagram of the valve on the datasheet shows an anode, two diode anodes, a single common cathode and what appears to be a single common grid. Should there be readings on D1 and D2 as well as Normal? As mentioned previously, this is the vale for which I do have a spare NOS unit and a working unit in the radio, but neither give any ma/v or status reading on the Normal position on the tester.

As a sidenote, given your last comment, I am also considering what to do about valves that have a 9v heater, since there is no such setting on this tester. It jumps from 7.5V to 10V. Evidently when run on 7.5v the PCF80 I tested last night runs to cool to give the correct reading. At 10V the readings started to look sensible but you also caution against over-voltage. Are there many valves that run with heaters at 10V? If not, would it make sense to add a series resistor perhaps? In any case, that can wait for now. My main focus at the moment is on understanding the measurement process and getting that right for various valves.