Avo mk1 and calibration help required - UK Vintage Radio Repair and Restoration Discussion Forum

oceanobsession1 · 26th Oct 2015, 8:44 pm

Can anyone help with checking the calibration of my machines please , I have
read the manual but I'm finding it confusing , I have a fluke multimeter if I were to use the b9a valve holder to take readings and set circuit selector to
test I would need to know the correct pin numbers to test for voltages , and I think I would need to buy an amp meter to set the meter scale . thanks phil.

trh01uk · 26th Oct 2015, 9:33 pm

Phil,

I take it we are talking about an AVO Mk.I valve tester?

Richard

oceanobsession1 · 26th Oct 2015, 9:43 pm

Hi Richard, I have one mk1 and two mk2 that need checking, recently purchased and the mk1 seems to read about the best and the other two seem to read low. cheers phil.

trh01uk · 27th Oct 2015, 9:05 am

Phil,

right, well the AVO Mk.2 manual can be downloaded from here.

That manual includes a page on calibration (see page 28). It also has the schematics of the Mk.2 and Mk.1

I can't find a manual for the Mk.1 - though someone on here may be able to supply one. I suspect the calibration for the Mk.2 will work equally well for the Mk.1, though that needs checking.

I am no expert on these VCMs, and there are people on this list better placed to advise than I am. I will however point out that these units do not use a simple smoothed DC supply system. They use "rectified AC", effectively half sine waves. This means that you generally need a "true RMS" voltmeter to get an accurate RMS measurement.

Reading the calibration method for the Mk.2, I note the instruments specified are:

AC voltmeter
"A moving coil DC ammeter" - AVO 8 for instance
valve voltmeter

I don't have time right now to study this calibration process to see exactly what they mean by a "valve voltmeter". Chances are that you can substitute a modern DVM, but that needs confirmation.

Richard

oceanobsession1 · 27th Oct 2015, 11:44 am

Hi, thanks Richard for your thoughts. I have the manual but its hard to understand and make sense of.
I've only got a Fluke multimeter so I will need to borrow some instruments to check the calibration. thanks phil.

trh01uk · 28th Oct 2015, 7:52 am

Phil,

just to add a bit more on manuals, you may have noticed my recent thread on the "Testers, Valve, AVO, No.3". If you read that you will see that the AVO Mk.I and Mk.II only differ in minor ways, mostly cosmetic. That means that the EMER for the military version is also relevant - and its available on the net.

In EMER Y814, which describes repairs, setup, etc, paras 21 to 28 cover setup and performance testing. Reading that, it seems you need an Avometer 50 range Mk.2 and it seems straightforward to me if you go through it step by step. As far as I can tell, the AVO required is the AVO 7 Mk.2, which has 50 ranges.

The AVO7 was particularly insensitive, with a loading of 500 ohms/volt. That is crude by modern standards, but given that the calibration is given for that instrument, a modern meter with much lighter circuit loading is likely to give different results.

Which Fluke multimeter do you have?

Richard

oceanobsession1 · 28th Oct 2015, 9:29 pm

Hi again Richard and thanks for taking the time to help with this matter, my
fluke is a 110 true rms multimeter, I think you may be right due to the tester
being rectified ac you need an analogue meter to get a smooth reading ,
where the digital meter goes up and down so its hard to determine what its
reading , so I think I need to purchase another meter , I will take a look at the
manual cheers for your help phil.

Radio Wrangler · 28th Oct 2015, 11:37 pm

Sorry, but a true RMS meter is definitely NOT right for this job. the whole thing hinges on the response of a rectifier/moving coil meter to halfwave sinusoids. A true RMS meter will tell you the equivalent heating voltage of all sorts of waveforms. I'm pretty certain AVO never needed true RMS instruments in making VCMs.

Calibrating the most important measurement of these instruments was handled by the availability of calibrated valves.

David

trh01uk · 29th Oct 2015, 10:02 am

David,

I am sure you are right - the Fluke 110 isn't suitable. That's why the EMER at least is quite specific about the use of the AVO 7 which will take average readings, though it may well be calibrated for RMS voltage on a sine-wave.

I haven't studied the detail of the calibration process, and it may be that the low impedance of the AVO 7 is also a factor - at least if the published procedure is to be used.

I think some members of this list have offered calibrated valves in the past.

Richard

Radio Wrangler · 29th Oct 2015, 2:05 pm

Ironically, true DC testers like the Sussex and Dave Simpson's unpronounceable special are ideal machines for calibrating valves, and they themselves canbe calibrated by normal meters.

David

trh01uk · 29th Oct 2015, 11:44 pm

And presumably if a DC test is what is required to produce a calibrated valve, then it should be fairly easy to take a particular valve, connect up the necessary supplies - and calibrate it?

Presumably the calibration involves setting precise anode and screen voltages, and monitoring anode and screen currents for a particular value of grid bias. And then getting the gm?

Richard

oceanobsession1 · 30th Oct 2015, 12:40 am

Just looking at some of these ampmeters on ebay for setting the mu meter
on the avo mk2 , which would be a good buy if any me phil.

26th Oct 2015, 8:44 pm	#1
oceanobsession1 Retired Dormant Member Join Date: Oct 2015 Location: Birmingham, West Midlands, UK. Posts: 49	Avo mk1 and calibration help required Can anyone help with checking the calibration of my machines please , I have read the manual but I'm finding it confusing , I have a fluke multimeter if I were to use the b9a valve holder to take readings and set circuit selector to test I would need to know the correct pin numbers to test for voltages , and I think I would need to buy an amp meter to set the meter scale . thanks phil.

26th Oct 2015, 9:33 pm	#2
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,652	Re: Avo mk1 and calibration help required Phil, I take it we are talking about an AVO Mk.I valve tester? Richard

26th Oct 2015, 9:43 pm	#3
oceanobsession1 Retired Dormant Member Join Date: Oct 2015 Location: Birmingham, West Midlands, UK. Posts: 49	Re: Avo mk1 and calibration help required Hi Richard, I have one mk1 and two mk2 that need checking, recently purchased and the mk1 seems to read about the best and the other two seem to read low. cheers phil.

27th Oct 2015, 9:05 am	#4
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,652	Re: Avo mk1 and calibration help required Phil, right, well the AVO Mk.2 manual can be downloaded from here. That manual includes a page on calibration (see page 28). It also has the schematics of the Mk.2 and Mk.1 I can't find a manual for the Mk.1 - though someone on here may be able to supply one. I suspect the calibration for the Mk.2 will work equally well for the Mk.1, though that needs checking. I am no expert on these VCMs, and there are people on this list better placed to advise than I am. I will however point out that these units do not use a simple smoothed DC supply system. They use "rectified AC", effectively half sine waves. This means that you generally need a "true RMS" voltmeter to get an accurate RMS measurement. Reading the calibration method for the Mk.2, I note the instruments specified are: AC voltmeter "A moving coil DC ammeter" - AVO 8 for instance valve voltmeter I don't have time right now to study this calibration process to see exactly what they mean by a "valve voltmeter". Chances are that you can substitute a modern DVM, but that needs confirmation. Richard

27th Oct 2015, 11:44 am	#5
oceanobsession1 Retired Dormant Member Join Date: Oct 2015 Location: Birmingham, West Midlands, UK. Posts: 49	Re: Avo mk1 and calibration help required Hi, thanks Richard for your thoughts. I have the manual but its hard to understand and make sense of. I've only got a Fluke multimeter so I will need to borrow some instruments to check the calibration. thanks phil.

28th Oct 2015, 7:52 am	#6
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,652	Re: Avo mk1 and calibration help required Phil, just to add a bit more on manuals, you may have noticed my recent thread on the "Testers, Valve, AVO, No.3". If you read that you will see that the AVO Mk.I and Mk.II only differ in minor ways, mostly cosmetic. That means that the EMER for the military version is also relevant - and its available on the net. In EMER Y814, which describes repairs, setup, etc, paras 21 to 28 cover setup and performance testing. Reading that, it seems you need an Avometer 50 range Mk.2 and it seems straightforward to me if you go through it step by step. As far as I can tell, the AVO required is the AVO 7 Mk.2, which has 50 ranges. The AVO7 was particularly insensitive, with a loading of 500 ohms/volt. That is crude by modern standards, but given that the calibration is given for that instrument, a modern meter with much lighter circuit loading is likely to give different results. Which Fluke multimeter do you have? Richard

28th Oct 2015, 9:29 pm	#7
oceanobsession1 Retired Dormant Member Join Date: Oct 2015 Location: Birmingham, West Midlands, UK. Posts: 49	Re: Avo mk1 and calibration help required Hi again Richard and thanks for taking the time to help with this matter, my fluke is a 110 true rms multimeter, I think you may be right due to the tester being rectified ac you need an analogue meter to get a smooth reading , where the digital meter goes up and down so its hard to determine what its reading , so I think I need to purchase another meter , I will take a look at the manual cheers for your help phil.

28th Oct 2015, 11:37 pm	#8
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: Avo mk1 and calibration help required Sorry, but a true RMS meter is definitely NOT right for this job. the whole thing hinges on the response of a rectifier/moving coil meter to halfwave sinusoids. A true RMS meter will tell you the equivalent heating voltage of all sorts of waveforms. I'm pretty certain AVO never needed true RMS instruments in making VCMs. Calibrating the most important measurement of these instruments was handled by the availability of calibrated valves. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

29th Oct 2015, 10:02 am	#9
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,652	Re: Avo mk1 and calibration help required David, I am sure you are right - the Fluke 110 isn't suitable. That's why the EMER at least is quite specific about the use of the AVO 7 which will take average readings, though it may well be calibrated for RMS voltage on a sine-wave. I haven't studied the detail of the calibration process, and it may be that the low impedance of the AVO 7 is also a factor - at least if the published procedure is to be used. I think some members of this list have offered calibrated valves in the past. Richard

29th Oct 2015, 2:05 pm	#10
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: Avo mk1 and calibration help required Ironically, true DC testers like the Sussex and Dave Simpson's unpronounceable special are ideal machines for calibrating valves, and they themselves canbe calibrated by normal meters. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

29th Oct 2015, 11:44 pm	#11
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,652	Re: Avo mk1 and calibration help required And presumably if a DC test is what is required to produce a calibrated valve, then it should be fairly easy to take a particular valve, connect up the necessary supplies - and calibrate it? Presumably the calibration involves setting precise anode and screen voltages, and monitoring anode and screen currents for a particular value of grid bias. And then getting the gm? Richard

30th Oct 2015, 12:40 am	#12
oceanobsession1 Retired Dormant Member Join Date: Oct 2015 Location: Birmingham, West Midlands, UK. Posts: 49	Re: Avo mk1 and calibration help required Just looking at some of these ampmeters on ebay for setting the mu meter on the avo mk2 , which would be a good buy if any me phil.