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Old 22nd Nov 2011, 10:00 pm   #1
2E1CIH Mike
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Default AVO equivalent

Hi,
I am soon to start restoring a Marconi CR300. I have a manual which gives expected resistance and voltage measurements at various points in the circuit - using an AVO.
Could I use a modern, digital meter and expect the same readings please? Also, can anyone suggest what is considered to be a good make of multimeter please?
Thanks.....Mike
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Old 22nd Nov 2011, 10:26 pm   #2
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Default Re: AVO equivalent

I'd guess you can use a modern DMM. I think the main significance of them specifying an AVO was that AVO's probably had higher input impedance than other cheaper models, so AVO's gave a truer reading than the others which loaded the circuit and dragged the volts down. I think the input impedance on a modern DMM will be higher than an old AVO, so you may find the voltages you read just a little higher than quoted.
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Old 23rd Nov 2011, 1:26 am   #3
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Default Re: AVO equivalent

Mike,

An AVO meter is good to start with - you cant easily kill one!

To be honest, you dont really need the immediate ease of reading that a DMM gives - a rough idea of voltage is close enough. The AVO will do this, and wont slide off the bench...

If you want a DMM you will be hard pressed to beat a Fluke.

I will have several different meters with me on Sunday - have a go with them and see how you get on.

Sean
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Old 23rd Nov 2011, 9:42 am   #4
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Default Re: AVO equivalent

The 20KΩ/volt sensitivity of an AVO (Such as AVO 8mk5) will influence readings compared to the 10MΩ/volt of a typical DVM when dealing with "high impedance or resistance" circuitry.

The readings on a DVM may be higher than expected. To understand this take a 1MΩ and 10MΩ resistor and measure the voltage of a PP3 or similar with the resistors in series with the positive lead. The voltage is of course the same for either but what does your meter/s show. The AVO will give very different readings depending on range, less noticeable on the DVM.
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Old 23rd Nov 2011, 11:12 am   #5
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Default Re: AVO equivalent

Quote:
Originally Posted by Mooly01 View Post
10MΩ/volt of a typical DVM
DVM's are not 10MΩ/volt. They're just 10MΩ on all ranges. That's the same resistance as a 20kΩ/volt AVO on its 500V range. On lower voltage ranges, the AVO's resistance will be lower but the DVM's will be the same.
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Old 23rd Nov 2011, 1:46 pm   #6
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Default Re: AVO equivalent

Except cheapo DVMs are often only 1MΩ input resistance......
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Old 23rd Nov 2011, 2:07 pm   #7
2E1CIH Mike
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Default Re: AVO equivalent

Many thanks for the replies.
Sean, I'm afraid I can't make Cottered this Sunday now. My elderly mother has a problem, and it is the only time I can go to help her.
Are there other regular meetings in the area at all?
I really appreciate you offer though.
Mike
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Old 23rd Nov 2011, 2:48 pm   #8
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Default Re: AVO equivalent

No, no other regluar meetings.

Stick with an AVO for doing the CR300 - its what the manual specifies, so the readings you get will be relevant to the manual.
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Old 23rd Nov 2011, 4:56 pm   #9
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Default Re: AVO equivalent

Does the manual specifically mention using an Avo8. The set is old enough that it could be that the measurements were made using an AVO7. The '7 is even less sensitive than an 8 so the readings would differ even further when using a DMM.

Al
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Old 23rd Nov 2011, 8:32 pm   #10
2E1CIH Mike
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Default Re: AVO equivalent

It just says "AVO."
It's a bit strange how readings can vary from meter to meter. When compared to physical measurements, it seems even stranger. What I mean is; a centimeter is just that irrespective of the instrument used to measure it. However, I can see there would be differences in accuracy between an engineers steel-rule, and a cheap plastic ruler. Maybe that's it!

My next question; does anyone have an AVO for sale?

I believe buying these on ebay can be risky.

Mike
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Old 23rd Nov 2011, 9:14 pm   #11
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Default Re: AVO equivalent

Mike,

Assuming all the different types of meters are working correctly, to within their specified accuracy of measurement if all are presented with, say 100 volts at their terminals they will all read 100 volts. However, depending on the input impedance (for simplicity lets assume we are measuring DC volts so we can say input resistance) of each meter, it will load the circuit under test to a greater or lesser extent and disturb the voltage present at the point being measured. so the meters may read differently because the voltages being measured are actually different because of the presence of the meter in the circuit.
So when a manual lists a table of voltages, to reproduce these you need to use a type of meter which loads the circuit under test in the same way. The easiest way to ensure this is to use the same model of meter if possible.
Does this make sense?
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Old 23rd Nov 2011, 9:24 pm   #12
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Default Re: AVO equivalent

Quote:
It's a bit strange how readings can vary from meter to meter.
AVO meters and other meters using a moving coil movement are basically current meters, even though they may be measuring Volts or Ohms. When you make a measurement you draw current from the circuit which changes its operation. The voltage you read is actually correct, but it is different to the voltage present when the meter is disconnected. DMMs draw less current, so they disturb the circuit less resulting in a reading closer to the voltage present with no meter connected.

None of this matters if you use a meter similar to that used by whoever compiled the service sheet.
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Old 24th Nov 2011, 2:51 am   #13
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Default Re: AVO equivalent

I think you may be over-concerned about this, and if the service sheet doesn't specify which AVO, then who knows what was used.

Although I very rarely have anything good to say about Maplin , they do seem to have some nice DMM's; I have two of theirs, neither costing more than £10 and both are excellent (when checked against my two AVO's, which I now rarely use, but are not for sale ). Fluke are wonderful, but much too expensive.

I think that with a DMM on your CR300, you might soon recognise it is consistently reading voltages a tad high, but where there there's a fault, it will show up as more than a tad off spec..
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Old 25th Nov 2011, 1:46 am   #14
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Default Re: AVO equivalent

Quote:
Originally Posted by M3MJV Mike View Post
Hi,
I am soon to start restoring a Marconi CR300. I have a manual which gives expected resistance and voltage measurements at various points in the circuit - using an AVO.
Could I use a modern, digital meter and expect the same readings please? Also, can anyone suggest what is considered to be a good make of multimeter please?
Thanks.....Mike
Err...doubtful, especially with any in circuit resistance measurements that have capacitors in the that same circuit.
Unless you are lucky you might find that doing resistance checks on the anodes and screens can give you a false sense of security as some times capacitor leakages will not show up on resistance tests with a meter due to its low internal test voltage.
By coincidence I started the electronics refurbishment on a CR300 this morning and I have already run out of replacement capacitors !

If you are about to power up the receiver for the first time there is a filter capacitor directly across the HT line (C104) I would replace that before you do anything, then do a resistance check across the HT if it's over 20k then it is probably ok to power up, assuming that all the heaters lightup check G1 on SOP valve, any +Ve would indicate leakage in the grid coupling capacitor, after that the most common points are any anode and screen decouplers, if the voltage is down at their respective points then they are probably leaking, if they are they also tend to increase the value of their respective feed resistors.
If you are up to spec. with your sevicing skills then disregard the above, having said that the one I am working on at the moment will be on the bench for a few more days yet so if you need to compare notes or throw any questions please feel free.
If you have the correct power power supply (Type 889) I might be asking you a few questions as mine differs slightly from the description given in the manual.

When it comes to meter selection for receiver servicing my own preference is an Avo 8 plus the cheapest DVM I can find!

Lawrence.
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Old 25th Nov 2011, 11:51 am   #15
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Default Re: AVO equivalent

Quote:
Originally Posted by pwdrive
Unless you are lucky you might find that doing resistance checks on the anodes and screens can give you a false sense of security as some times capacitor leakages will not show up on resistance tests with a meter due to its low internal test voltage.
This is certainly true, but unless they're very leaky neither an AVO or a DMM will detect leaky capacitors on a resistance range. However leaky capacitors will generally show up as incorrect voltages on valve pins ie low voltage on screens, anodes or cathodes and postive voltages on control grids.

As for comparing readings between AVOs and DMMs, just expect a higher reading on a DMM. Where a voltage is wrong it will generally be so far wrong as to leap out at you.
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