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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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22nd Nov 2011, 10:00 pm | #1 |
Triode
Join Date: Jul 2011
Location: Cambridgeshire
Posts: 43
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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 |
22nd Nov 2011, 10:26 pm | #2 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,935
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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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23rd Nov 2011, 1:26 am | #3 |
Dekatron
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
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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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23rd Nov 2011, 9:42 am | #4 |
Octode
Join Date: May 2011
Location: Lancashire, UK.
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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. |
23rd Nov 2011, 11:12 am | #5 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,677
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Re: AVO equivalent
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23rd Nov 2011, 1:46 pm | #6 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: AVO equivalent
Except cheapo DVMs are often only 1MΩ input resistance......
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23rd Nov 2011, 2:07 pm | #7 |
Triode
Join Date: Jul 2011
Location: Cambridgeshire
Posts: 43
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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 |
23rd Nov 2011, 2:48 pm | #8 |
Dekatron
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Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
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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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23rd Nov 2011, 4:56 pm | #9 |
Nonode
Join Date: Jan 2010
Location: Edinburgh, UK.
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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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23rd Nov 2011, 8:32 pm | #10 |
Triode
Join Date: Jul 2011
Location: Cambridgeshire
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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 |
23rd Nov 2011, 9:14 pm | #11 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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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? |
23rd Nov 2011, 9:24 pm | #12 | |
Moderator
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Re: AVO equivalent
Quote:
None of this matters if you use a meter similar to that used by whoever compiled the service sheet.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
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24th Nov 2011, 2:51 am | #13 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
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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.. |
25th Nov 2011, 1:46 am | #14 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: AVO equivalent
Quote:
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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25th Nov 2011, 11:51 am | #15 | |
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Re: AVO equivalent
Quote:
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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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
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