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Old 1st Mar 2021, 8:58 am   #1
MelJon66
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Default Avo model 40 - saved by the solder

Greetings All,

This is an Avo 40 query.


When I put the Avo into circuit the meter needle went full deflection and the cut-out popped up. I then spotted that I had the Avo in the 12 volts DC position in error. Those stylised A and V markings are not helpful if you just glance at them from odd angles. I disconnected, set the Avo into the 12 amps DC position, reset the cut-out, reconnected and got.......nothing! Not a flicker on the Avo.

With the back off the Avo case there were obvious signs of excessive heating on the first section (12 amp range) of the series copper/brass shunts as the varnish had bubbled up. Photo attached. The input from the positive meter terminal goes into the cut-out then out to these shunts via the grey insulated wire coming in from the left. When I touched the grey wire the soldered end was not connected to the end of the shunt. It seems that by design the insulated wire is acting as a spring and when the shunt gets hot it melts the solder and the spring releases, breaking the soldered connection to the shunt. The area between the solder pads is varnished so the solder cannot reconnect in that area. It's a very simple overload device and it obviously did its job in this case. When I re-soldered the joint the meter is up and running again so hopefully no long term harm done.

My questions are:

1) Did Avo use a solder with a specific melting point in this connection? I have used standard 60/40 so I wonder if the solder overload device will now work as intended.

2) Is the cut-out a current or voltage device? Why did it pop up when inadvertently set to 12 volts as I always thought the voltage setting was the safe setting to be in? The battery was fully charged so may have been at around 14 volts but the Avo has internal resistance and was in series with the rest of the system so I would not expect it to see all 14 volts and the current should have been low. Is there something I am failing to understand?

3) Why did the cut-out not pop up when on the 12 amps range? It may have gone over current for just a few seconds initially, as the DVM may have done, and should have operated then. The cut-out now seems to set and release mechanically and the contacts are not damaged but how can I test it electrically without taking the Avo to it's limits again?

Hopefully there are some Avo experts in the membership who can enlighten me. Any help and advice always appreciated.

Cheers
Mel
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Old 1st Mar 2021, 12:07 pm   #2
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Default Re: Avo model 40 - saved by the solder

I don't know about the Avo, but if the joint melted then low temperature solder is a real possibility.

This could be the bismuth solder, 140C.

I have a Nightstor boiler which uses two thermal fuses at the top just in case, and uses this bismuth solder. Replacement fuses are available, £40 each, so I just bought some Bismuth solder wire and made my own.

But, DO NOT contaminate bismuth solder with lead/tin solder, either way round! It affects the melting point, just like contaminating tin only solder with lead. Use a new tip on the iron and use it only for that material.
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Old 1st Mar 2021, 12:33 pm   #3
The Philpott
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Default Re: Avo model 40 - saved by the solder

The main cut-out is an inertia device which (as i understand it) operates when the pointer slams into the high end of the scale- if it's properly adjusted, that is. Some Avo's of this type may be fitted with an acceleration cutout as well-(Model 40? Model 7?) this detects unwise acceleration and can actually disengage the pointer before it reaches 1/2 scale (If it's working properly that is!)

Critically, the meter is connected in series when measuring current (so it becomes part of the circuit) and voltage is measured ACROSS a component, or battery, or power supply, or whatever. Deviating from this is not an option. (Sorry if i'm teaching you to suck eggs there.) Note that an Avo 40 on it's 12vDC range only presents 2000 ohms of resistance- it's the least sensitive Avo, but as compensation it's rather robust.

The font on the selector switches of these meters has caught me out as well; particularly in poor light or if the white infill has worn away. Glad it seems to be working again.

Dave
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Old 1st Mar 2021, 2:38 pm   #4
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Default Re: Avo model 40 - saved by the solder

Another reason for using LMP solder could be the huge amount of heat that gets wicked away from the iron by the current shunt. I have a couple of scrap shunt boards that i could take a sample from and compare it with 60/40.
Dave
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Old 1st Mar 2021, 4:41 pm   #5
MelJon66
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Default Re: Avo model 40 - saved by the solder

Thanks,

The 60/40 solder flowed very easily so the wicking away issue didn't seem to cause any problem.

Connecting the meter in series when on a voltage setting was entirely unintentional. An Avo on the 12V DC volts range connected directly across the terminals of a fully charged battery would have every reason to object but in series with the car circuits I would expect the voltage across it to be lower. If the car circuits are low resistance relative to the 2000 ohms of the Avo most of the voltage would be dropped across the Avo so that might explain it, particularly if there is a fault somewhere in the vehicle as I suspect there may be. I don't know if the Model 40 senses acceleration but if so that might also explain the cut-out popping. The Avo is old and despite being in great condition it may have had some abuse over the decades and the cut-out may just be a bit sensitive. Anyway, the lesson is to check the Avo settings twice before connecting. I use the Model 40 for heavy duty work so it doesn't get used much and I am not as familiar with it as I am with the Avo 8, which I keep for bench use only.

The shunts aren't in circuit on voltage ranges so I am sure the overheating occurred when it was correctly set on the 12A range and maybe it took a few extra amps and the cut-out failed to operate. Maybe the cut-out was sticky after tripping on the volts setting a few seconds earlier. I am always wary of car batteries because of the frightening amounts of current they can produce so I won't rely on an ammeter in circuit from now on as 10 or 12A doesn't seem to be enough. I plan to buy a 50A shunt and use a DVM across it.

Any further insight on whether Avo used LMP solder would be appreciated. Also, is there a good method of testing the cut-out/overload without damaging the meter further?

Cheers
Mel
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Old 1st Mar 2021, 5:09 pm   #6
The Philpott
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Default Re: Avo model 40 - saved by the solder

There's a method for checking the cutout setting on the forum here somewhere, but it does involve overloading the meter to a degree. (Can't remember the details)
I think the later Model 40 has the die-cast cutout, with two tiny coil springs attached..maybe yours is one of these..

You could invest in a UT210D or UT210E clamp multimeter, (maybe £40 on Amazon..?) they have DC current capability, i'm quite pleased with mine.

I have very tatty old Avo 40 from around 1970, i must admit i got fed up with the DC voltage ranges of 12/6/1.2/0.6 and altered it to have 24/12/4.8 and 2.4 instead- it lives in the garage now!

Dave
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Old 1st Mar 2021, 8:47 pm   #7
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Default Re: Avo model 40 - saved by the solder

Vehicles electrics other than car radios and clocks not permitted on this forum

Cheers

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Old 1st Mar 2021, 9:33 pm   #8
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Default Re: Avo model 40 - saved by the solder

For measuring high and possibly erratic currents, I insert a shunt into the circuit of low enough resistance and high enough current carrying capacity for all the systems being powered to work normally, with little voltage drop.

Using a lesser shunt will result in damage.

So the problem becomes one of measuring the very low shunt voltage, when the systems have stopped acting and settled down to their background consumption.

This means using a very high resolution voltmeter across the shunt in order to get the resolution to measure the small current with instrumentation scaled for the maximum surges which may be demanded.

This is a good application for 4 or 5 digit DVMs. You may not need such instruments every day, but they can be the solution to unusual problems.

With one you can also do tricks like measuring the voltage drop along PCB tracks, across ground planes, and even up the leg of an SMT IC. You can be a wiz at tracing shorts.

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Old 1st Mar 2021, 9:36 pm   #9
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Default Re: Avo model 40 - saved by the solder

The Model 40 Avometer has both an inertia cut-out and end stop tripping, both of which act through the pop-up cut-out mechanism.

The acceleration cut-out should operate before the pointer reaches two-thirds of the scale. The principle of operation is shown in the two attached photographs. The pointer is not rigidly attached to the moving coil but is free to swivel a small amount on the pivot. On a heavy overload, due to inertia, the pointer will lag behind the moving coil causing the pointer stem to bear against the outer end of the pawl. As the pawl swivels on the stud on the pointer guide bracket, the inner end of the pawl (bent downwards) will catch the teeth on the ratchet disc. As the ratchet disc turns, it puts pressure on the bell crank which pushes the cut-out leaf spring downwards. This causes the cut-out to trip and should happen before the moving coil hits the end stop. On lesser overloads, insufficient to invoke the inertia tripping, the moving coil frame will come into contact with the bell crank, also causing the cut-out to trip.

There is nothing in the service instructions about a thermal fuse arrangement, so I would be surprised if this provision had been made. However, the Model 40 service manual is really just a supplement to the Model 8 service manual and does not give much servicing detail specific to the Model 40, it is more of a parts list with circuit diagram.

At different times in the production (from 1939 to c. mid 1970s) the cut-out setting instructions varied, but the best source of information is, paradoxically, the "Recommended Spares List" issued late in the production run. This states that the cut-out must trip on a 12 times overload but not trip on 4 times overload.

I wouldn't expect anyone to set the cut-out on the 12A range, but if they did, the test current would be 144 A. Obviously, the Model 40 is quite a robust meter!

On the 12V (DC) range, connecting the meter across the car battery terminals should have resulted in nothing more than the pointer reaching full-scale, or a little beyond if the terminal voltage was higher than 12V. Except on the 12(AC) range, the meter will draw 6mA with the divide-by-two button unpressed; when it is pressed the full scale current is 3mA.

Had the meter been on the 12A (DC) range when it was connected across the battery terminals, or between battery positive and the body of the car, the battery voltage would be applied across the 12A shunt, which has a resistance of 0.02 ohms. The internal resistance of the car battery is of course very low, so roughly 600A would flow in the shunt and 300 mA in the moving coil until the cut-out operated.

PMM
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Old 2nd Mar 2021, 9:03 am   #10
MelJon66
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Default Re: Avo model 40 - saved by the solder

Thanks PMM,

The solder fuse does appear to be an original feature and it seems typically "Avo" in the way that it has been done.

I have attached a photo giving a wider view so you can see how it works. The insulated grey wire coming in from the cut-out on the left is stiff springy wire and has been shaped to give a specific amount of spring tension and amount of movement at the end of the wire. The resting position of the end of the wire spring is between the solder pads at the end of the flat 12A shunt on the right. The end of the wire spring is soldered away from its resting position so that when the solder melts due to overheating the wire springs back to its resting position between the pads, which is varnished so contact is broken. On its way across the tops of the shunts the wire spring passes through a hoop that has been soldered to one of the shunts. The hoop acts as an end stop so that when the solder breaks the end of the wire spring cannot overshoot and remake contact with the solder pad close to it.

It all seems very deliberate and it seems to work as intended. This is an ex SWEB Avo and I wonder if this was a special extra specified by SWEB or possibly SWEB carried out these mods themselves.

Cheers
Mel
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Old 2nd Mar 2021, 10:03 am   #11
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Default Re: Avo model 40 - saved by the solder

Thanks to all for your excellent contributions to the thread. If I join all the dots together I think all of my questions have been answered.

PMMunro has verified that the Model 40 has inertia and end stop tripping so when I set it on the 12V DC range by mistake and hastily inserted it in series into circuit the cut-out tripped, as designed, in one or other of these modes. With the battery even at its maximum of 14V any potential over voltage would have been minimal so no damage was done and the meter movement still works normally. The tripping occurred on a voltage range, when the current shunts were not in circuit so I conclude that my range setting error could not have contributed to the overheated shunts and the solder fuse blowing. But, I confess to bad practice and from now on will follow some of the very good advice that I did manage to read before the mods took the posts down. Plus I will double check the settings on this meter before I connect it up from now on. It seems I am not the only one to have misread the range markings on these old meters.

When the cut-out was reset and the correct 12A DC range selected and the meter was inserted into circuit I remember there was an immediate click from the Avo and no meter reading. I expected to see that the cut-out had popped up again but it had not. There was either no pointer deflection at all or it was just a flick and I missed it while I was still focussing on connecting the croc clips. PMMunro confirms that the cut-out is designed to trip at no less than 4 x overload which would be 48A in this case. It is unlikely that this level was reached as I had previously used my DMM on its maximum 10A range and that survived in circuit for up to 1 minute without blowing a fuse. The Avo became unresponsive immediately so I suspect the click indicated that the solder fuse had melted, possibly at a much lower current level than it was designed for. If the solder fuse was still intact there would have been visible meter deflection for some time and/or the cut-out would have operated. None of this happened so there was very little time for any heating of the shunts to occur before the solder melted. It is quite likely that the heat damage to the shunts was already there as a result of prolonged overloading at some other time.

I bought the Model 40 a few years ago from an ex. SWEB engineer who lives near me. He was very keen to tell me that he only ever used it for “synching generators”. It’s in very good condition but judging by the list of names written in and then crossed out inside the lid of the leather case it had changed hands a few times so he can’t have known it’s full history. As the solder fuse appears to be an unusual addition it could be a mod that SWEB specified from Avo or carried out themselves. When Avo's were the ubiquitous meter of choice damage on high current ranges must have been a common problem within electricity boards and SWEB may have asked Avo to provide additional protection. They may well have done as requested as the volumes were high and SWEB would have been an important customer. It does make me think that the solder fuse may have already taken a bashing before it came in to my possession and it may have been hanging on, literally by a solder thread. My bad practice of slamming the Avo into circuit has caused a transient taking it over the edge and the solder fuse gave up without much of a fight. Whatever the case I will not be fussing about re-soldering the possible LMP solder joint with the correct solder as standard 60/40 just removes the solder fuse feature which seems to be uncommon and nice to have only.

So, I am now happy that I understand what happened and the failure is just as likely to be the result of damage sustained in the Avo’s industrial past as it is with what I did. In the end it still works and I know much more about it than I did before. I also know that an Avo or DMM are not the right tools for the job I was trying to do. I like the idea of a DC clamp meter but I already have an excellent clamp meter but it does not measure DC current. Another one that will get little use seems extravagant but perhaps it will make it onto my birthday or Christmas lists. My wife and daughters are very happy to by me gear for special occasions because they can never think of anything else. That’s an outstanding win win result in my book.

Thanks again for everyone’s contributions.

Cheers
Mel
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Old 2nd Mar 2021, 11:31 am   #12
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Default Re: Avo model 40 - saved by the solder

I don't think what your photograph shows is any kind of thermal fuse. It's a tapping along the length of the flat shunt ribbon to give the correct calibration and was common practice in instruments of this kind. It is extremely difficult to cut the ribbon to the exact length needed to give the very low and precise 0.02 ohm value for this section of the shunt. This problem was overcome by making the shunt slightly too long giving a higher than required resistance. The grey wire would then be soldered to the ribbon at a point which gave the correct shunt value. The springyness is just a side effect of how the wire has been formed.

I'm sorry to be controversial, but I think your meter must have been on the 12A range when the severe overload occurred. I know, from experience, that it can be very easy to confuse the setting of the multimeter and the Model 40 marking are not a model of clarity. Anyone who hasn't done such a thing has either been very lucky to notice the mistake in time, had to reset their meter or change a fuse hoping no-one else would notice, or hasn't used multimeters much. (Excluding perfection of course).

On the 12V range the meter has a terminal resistance of 2000 ohms. No matter what is in series, whether it is the car's system or a short circuit to earth or the opposite terminal, no more than 6mA can flow from a 12V source, which of course will not do any damage to the meter.

If the shunt ribbon was hot enough to burn the SRBP support then it was well above the melting piint of tin/lead solder anyway. Even disconnecting the solder link would do little to reduce the current in the shunt since the wire is simply by-passing a short length of the shunt.

The critical factor here is the energy let-through (I^2 t) before the cut-out operated. At a fault current of 600A, and at trip time guessed to be around 100mS thats 36 kJoules - a lot of energy!

PMM
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Old 2nd Mar 2021, 11:47 am   #13
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Default Re: Avo model 40 - saved by the solder

--One more Achilles Heel that occurs JUST often enough to warrant a mention- the selector cam rotors under the rotary switches are not pegged in alignment, so if the screw (and crinkle washer underneath) become loose, range selection is lost. If the screw is tightened with the cam rotor in the wrong place, range selection is incorrect! On most models these screws are accessible without further dismantling. They don't need to be really tight, and they only normally loosen when there's been arcing across the fixed radial contacts in the meter proper.

Dave
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Old 2nd Mar 2021, 3:14 pm   #14
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Default Re: Avo model 40 - saved by the solder

Thanks PMM,

I had not considered the possible fine calibration function of the sliding wire but it makes perfect sense now that you have explained it. I thought the wire passing through the hoop halfway along the wire to limit lateral travel at the end supported my theory but of course it could equally be there to constrain movement of the wire in readiness for soldering during final calibration. Your suggestion sounds much more plausible and I am very happy to accept your greater knowledge of these devices. I have enjoyed learning a little bit about the unit but that leaves a lot I have yet to learn.

On your second point you are not being controversial at all, the meter must have been in the 12A range when the overload occurred because that is the only time when the 12A shunt is in circuit, barring potential selector misalignment as put forward by The Philpot. I did venture the idea that the overheating may have happened before I purchased it, leaving the Avo with a potentially high resistance soldered joint (not solder fuse) that failed prematurely when I used it. This may still be correct as I saw no pointer deflection at all so the soldered joint failed very quickly. When the meter needle didn't move I knew there was something wrong and I removed the croc clips pretty quickly but in all fairness I don't react particularly quickly these days. As I explained I initially used my DMM on the 10A range and this settled down to show 6.85 amps after a few seconds so if there was an overcurrent situation it wasn't taking excess current for long at all and the DMM coped with it. The DMM probably does have good quality modern fuses but I'll bet the PCB tracks are not as chunky as the 12A shunt in the Avo and it was in circuit for much longer. It's a Brymen BM235 but I have never had any reason to peer inside. I also noted that the Avo cut-out did not operate but you have already confirmed that this would probably not operate below 48 amps so this is not a surprise really and still leaves plenty of excess amps to heat the shunt while staying well below the cut-out threshold.

Anyway, we are agreed that the shunt got overheated at some point while on the 12A range and the solder joint melted and protected the meter from destruction. It may have been me that did it but the Avo survived and I will try not to repeat the error.

This leaves me with one outstanding challenge. How do you learn from mistakes without having to go through the stress of making them? Answer, you don't but you post your mishaps on the forum so that others learn from your mistakes and you learn from theirs.

Happy days!
Mel

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