Avometer Model 7 Testing

[Uncle Bulgaria]

Ahoy,

I have a Model 7 and wondered if there were any accepted methods for testing/setup. I have done some searches but nothing specific has come up.

The voltage readings seem to be spot on, at least for DC. Are there any parts that 'go' or any normal repairs that are recommended as a matter of course?

I have cleaned it all out, replaced the battery with a C-cell in a clip and got new ovoid torch cells in the newly polished compartment.

One thing I noticed was the idea in another thread to shunt the movement for postage by shorting the contacts and setting it to a low DC range. On this model nothing happens to the needle when this is done (it can still swing and isn't braked), so does this mean a movement problem if the volt/ohm readings are spot on (I haven't tried current), or just an idiosyncrasy of the Model 7's design?

[ex 2 Base]

What low DC range are you using? Can't exactly remember, but I think we used to set ours AVOs to low DC amps with a piece of wire across the bottom terminals and that dampened the movement. Stand correction as it was many years ago.

[Techman]

There's not a lot you can do with these old Avo 7 meters, as the shunts and multipliers are all 'wound' components and you would need to be quite skilful to alter the length of the windings to correct ranges.

As said, you need to damp a low DC current range for transportation.

I have several old Avo meters of this type and although VERY useful when working with vintage equipment, you have to remember that they will quite heavily 'load' a high impedance circuit and give you false readings as well as possibly stopping the circuit from working altogether while you are taking the readings.

[Uncle Bulgaria]

The lowest DC Amps setting is 0.002. With the terminals shorted, and this setting selected, when the case is wobbled the needle moves as much as when the connections are not shorted.

I have just tried some tests with a 12V bench supply, and AC is read up to 10, but there is very little deflection if I select the 100V setting which is the next one up. The same with DC. Mains AC causes very little deflection at the 400V setting.

Various batteries read spot on 1.5V DC, and the resistors I've tried up to a few kOhm also agree with my DMM.

Any ideas?

Also the instructions on the back of the case (what a great idea!) don't have initial setup guidelines. I'm not sure of the utility of the knobs at the top, and wonder if zero has been properly set (which I imagine is the set screw where "Model 7" is written). Any written procedures to make sure I'm starting from the right place?

Yours confusedly.

[peter_sol]

I think you set both switches to off in other words AC and DC to brake the movement
Simply set Zero of the needle with the screwdriver slot at the top.

[pmmunro]

Dear Uncle,

The attachment is the Spares List for the Model 7, from about the age of yours until 1968. It is more than just what the title would suggest; there is considerably more information which should be of interest.

It is very likely that one of the multiplier resistors in your meter is open-circuit, possibly R8 or R9. They are likely to be wound with resistance wire in your meter. They may also have lower value carbon resistors in series to trim the value to the required resistance. If these wire wound resistors are open circuit and if your are not too concerned to keep your meter completely original, they can be by-passed by metal-oxide resistor combinations selected to give the correct values. Identification of the components is greatly aided by the service manual.

There is no specific provision in the Model 7 for movement damping. As previously mentioned, the correct procedure is to set the switches for the most sensitive current range (DC) and connect the terminals together with heavy guage wire. The Model 7 is the only conventional Avometer which needs a 1.5V cell for ranges other than resistance. The 1.5V cell is used to bias the rectifier at the low end of the alternating voltage ranges.

The only "routine" maintenance I would suggest is to check that the switch contacts are clean and to clean the cut-out table and the stem of the cut-out button. Do not use a spray switch cleaner as a spray; it is very likely to reach the hairsprings and deposit droplets of lubricant which will stick between the coils of the spring.

Work on the cut-out is quite delicate and there are some very small parts which try to escape. Skills somewhere between watch and clock repairing are about right.

With good information, care and skill, these meters are far from impossible to maintain and repair, but there a number of points to watch to avoid trouble. You should find a great deal of information on Avometer maintenance in previous posts and threads.

Send me a private message if you need further detailed technical information.

PMM.

[Uncle Bulgaria]

Thanks all, and particularly PMM.

That document is downloaded, and will probably be perused on the train tomorrow. Luckily I have my complement of watchmaking tools, as one of my other hobbies is repairing old watches!

I will report back when I have investigated. I'm renovating various houses as part of the day job, so don't know when that'll be. As I hope the Avometer will be useful in my valve work it shouldn't be too long though!

[Phil G4SPZ]

The Avometer Model 7 is often referred to as the 'standard' instrument in Trader and other service sheets. Yes, the meter does load the circuit under test, but it's by a predictable amount. If you use the same model or sensitivity of instrument when taking measurements, you can get an immediate comparison.

As PMM has said, it is quite feasible to replace open-circuit wire-wound resistances with modern components, and still achieve very good accuracy. I have a large stock of resistors to hand and can supply you with most values for the cost of postage; feel free to PM me with your requirements.

[Uncle Bulgaria]

Thank you Phil.

I didn't fail to reply out of lack of interest, rather work and a defunct computer have kept me away from the workshop! I hope to have a serious look at the Avometer this weekend...

[Uncle Bulgaria]

So I've had the meter apart and perused the document that PMM kindly uploaded. Here's a picture of what I take to be Rs 8 & 9. When metering across the marked places I get a rapidly changing reading, which increases and steadies, implying they're not open circuit and C1 is being charged. I'm loath to disconnect them in order to get an accurate reading, but I'm unaware of another way.

Readings:
A-C (through carbon comp I take to be R8): 779R.
A-D (148k pink resistor, R7 149.2k on diagram): 149.6k
C-B (bobbin I take to be R9): ~30M

So R9, if it is so, is very high...

Am I entirely off the wall here, or on the right track?

[Phil G4SPZ]

Yes, I think you're on the right track. The bobbin you have identified as R9 should read 44,200 ohms, and it seems to be open-circuit. This is not an uncommon fault. To replace it you could fit conventional 39k + 5.1k resistors in series; I can supply you with some selected resistors to make up the right value FOC for the price of postage. Drop me a PM if interested.

One useful trick you can try is to connect your DMM, on the ohms range, across the Avometer's terminals and measure the resistance presented by the DC voltage ranges as you work upwards. The basic sensitivity, without pressing the /2 button, is 500 ohms per volt. So on the 1 volt range you should expect close to 500 ohms, on the 10 volt range 5,000 ohms, on the 100 volt range 50,000 ohms and so on. If the resistance shoots skywards on a particular range, you've pinpointed the open-circuit section of the multiplier chain.

[Uncle Bulgaria]

That's a good tip Phil, thank you.

I followed it and got:
1V 0.499K
10V 5.01K
100V 16M (fluctuating upwards)
400V ditto
1000V ditto

The 100, 400 and 1000V settings had the same fluctuating resistance that continued past 20M, as I suppose it should do if the chain is broken.

Switching between them on the fly had no effect on the resistance, while switching to the 10V setting immediately reset the meter which then returned the same 5.01K value as before.

I have sent you a PM for the resistors, so many thanks for the kind offer.

[Phil G4SPZ]

PM received and reply sent, sorry for the delay. The resistors will go in the post tomorrow. Good luck, and let us know how you get on!

[Uncle Bulgaria]

Well that seems to have done the trick!

I've just put the resistors instead to test of the bobbin, and now get spot-on resistance measurements for all voltage settings. AC reads as it should up to mains.

I'll solder the resistors in a rather more permanent fashion, and just have one more query. I've run some measurements from a bench supply I've got hanging around and while AC is all right, I get different DC readings between my cheapo DMM and the AVOmeter. The AVO is consistently a volt below the DMM. Both are high, but that's the supply's fault. Is it something to do with the AVO's impedance?

[Silicon]

If you connect both the AVO and the DMM simultaneously to the power supply you will eliminate the issue of the low input resistance.

The AVO will still 'load' the power supply but the voltage readings should be the same.