AVO 8 mk 2 not working for ohms - UK Vintage Radio Repair and Restoration Discussion Forum

Spencervs · 23rd Oct 2022, 9:14 am

Hi all, I have an AVO meter which is working for Volts and Amos but not ohms. When I got it a few years ago the ohms were fine. I checked the batteries and they still read at 15v and 1.5v on my dmm. Contacts also are not corroded and the leads have continuity. Any ideas?

I hear there is a fuse but I can't see it anywhere and am unsure if that's for later models.

pmmunro · 23rd Oct 2022, 10:34 am

There's no fuse in a Model 8 MK II; that was added in the MK III and the Model 9.

Do you have a circuit diagram?

PMM

Spencervs · 23rd Oct 2022, 9:27 pm

Hi there, I have found a schematic online here:

https://irationaudio.files.wordpress...2schematic.jpg

Is that any help? I'm not sure what could have gone. Is it possible that the 15v reads a voltage but is actually kaput?

Chrispy57 · 24th Oct 2022, 11:07 am

You could test this theory by connecting say a 10k resistor across the 15V battery and re-measuring the terminal voltage with your DVM. If it holds up at 1.5mA the battery should be OK for the AVO high Ohms range measurements.

Spencervs · 24th Oct 2022, 11:34 pm

So I just tested the batteries with the resistor and it works fine. I'm wondering what possibly could have gone wrong here. Any likely components to suffer a surge or disconnect?

Spencervs · 26th Oct 2022, 8:49 pm

Or failing this is there any technique to fault finding with these? It looks really daunting to dismantle without knowing the best methods

Chrispy57 · 28th Oct 2022, 11:26 am

Hi Spencer, good news that you managed to eliminate the batteries as the source of your problem - but what next? As I look at the circuit diagram that you have provided what caughty my eye was the note that the DC Switch Cam has contacts k & l closed for the 15V battery powered resistance range measurement, and m & n closed for the 1.5V battery powered resistance ranges. Are you able to test for continuity, or at least identify and prospectively clean these contacts?

Spencervs · 25th Nov 2022, 12:12 pm

Hi all! Thank you so much for the replies! When I was observing the unit, I realised that the leaf switches on the DC side were not engaging at all. I gently moved aside the resistor board which lay on top of the switch mechanism and realised that the screw securing the cam had come loose! No idea why at all. Anyway upon tightening it the unit came back to life. I used some switch cleaner on the relevant parts and pots and I think it's all fine and dandy. Seems relatively accurate compared to my dmm.

I'm wondering how these were calibrated now? And if there is still a service that does it professionally and for a reasonable price.

Thanks again all!

Chrispy57 · 26th Nov 2022, 11:56 am

Hi Spencer,
well done for bringing an old workhorse back to life!

Cheers
Chris

barrymagrec · 26th Nov 2022, 2:49 pm

Quote:

Originally Posted by Spencervs

Hi all! Thank you so much for the replies! When I was observing the unit, I realised that the leaf switches on the DC side were not engaging at all. I gently moved aside the resistor board which lay on top of the switch mechanism and realised that the screw securing the cam had come loose! No idea why at all. Anyway upon tightening it the unit came back to life. I used some switch cleaner on the relevant parts and pots and I think it's all fine and dandy. Seems relatively accurate compared to my dmm.

I'm wondering how these were calibrated now? And if there is still a service that does it professionally and for a reasonable price.

Thanks again all!

Well done. This was the same fault as my Dad`s 8 MkII had about 40 years ago, I guess AVO didn`t always do their screws up properly. No trouble since, mine now.

The Philpott · 26th Nov 2022, 3:07 pm

The (cheesehead) screws have a crinkle washer underneath, which does the job...most of the time. Unfortunately one of the screws on an early type Model 8 is inaccessible under the transformer.

Spencer, basic calibration of the magnetic flux can be done using a precision voltage source (usually on the 10v DC range) If all the resistances are in spec. then tweaking the magnet shunt (with non ferrous tools) to ensure accuracy on this one voltage range should transfer accuracy to the other ranges- provided all switches are clean and operating correctly. The resistance ranges are inherently calibrated by the operator when he manually zeroes the pointer @ FSD. (Always provided the movement is healthy, of course)

Component accuracy on a Model 8 MkII has stood the test of time well, it's unusual to find a shunt that has gone outside spec. (though you might find a winding here and there that has rotted and gone open circuit) AC readings can drop out somewhat if the rectifier has been abused or has corroded.

Dave

23rd Oct 2022, 9:14 am	#1
Spencervs Hexode Join Date: Apr 2011 Location: Grove Park, SE London Posts: 371	AVO 8 mk 2 not working for ohms Hi all, I have an AVO meter which is working for Volts and Amos but not ohms. When I got it a few years ago the ohms were fine. I checked the batteries and they still read at 15v and 1.5v on my dmm. Contacts also are not corroded and the leads have continuity. Any ideas? I hear there is a fuse but I can't see it anywhere and am unsure if that's for later models.

23rd Oct 2022, 10:34 am	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,709	Re: AVO 8 mk 2 not working for ohms There's no fuse in a Model 8 MK II; that was added in the MK III and the Model 9. Do you have a circuit diagram? PMM

23rd Oct 2022, 9:27 pm	#3
Spencervs Hexode Join Date: Apr 2011 Location: Grove Park, SE London Posts: 371	Re: AVO 8 mk 2 not working for ohms Hi there, I have found a schematic online here: https://irationaudio.files.wordpress...2schematic.jpg Is that any help? I'm not sure what could have gone. Is it possible that the 15v reads a voltage but is actually kaput?

24th Oct 2022, 11:07 am	#4
Chrispy57 Pentode Join Date: Mar 2020 Location: Rochdale, Greater Manchester, UK. Posts: 205	Re: AVO 8 mk 2 not working for ohms You could test this theory by connecting say a 10k resistor across the 15V battery and re-measuring the terminal voltage with your DVM. If it holds up at 1.5mA the battery should be OK for the AVO high Ohms range measurements.

24th Oct 2022, 11:34 pm	#5
Spencervs Hexode Join Date: Apr 2011 Location: Grove Park, SE London Posts: 371	Re: AVO 8 mk 2 not working for ohms So I just tested the batteries with the resistor and it works fine. I'm wondering what possibly could have gone wrong here. Any likely components to suffer a surge or disconnect? Last edited by Spencervs; 24th Oct 2022 at 11:49 pm.

26th Oct 2022, 8:49 pm	#6
Spencervs Hexode Join Date: Apr 2011 Location: Grove Park, SE London Posts: 371	Re: AVO 8 mk 2 not working for ohms Or failing this is there any technique to fault finding with these? It looks really daunting to dismantle without knowing the best methods

28th Oct 2022, 11:26 am	#7
Chrispy57 Pentode Join Date: Mar 2020 Location: Rochdale, Greater Manchester, UK. Posts: 205	Re: AVO 8 mk 2 not working for ohms Hi Spencer, good news that you managed to eliminate the batteries as the source of your problem - but what next? As I look at the circuit diagram that you have provided what caughty my eye was the note that the DC Switch Cam has contacts k & l closed for the 15V battery powered resistance range measurement, and m & n closed for the 1.5V battery powered resistance ranges. Are you able to test for continuity, or at least identify and prospectively clean these contacts?

25th Nov 2022, 12:12 pm	#8
Spencervs Hexode Join Date: Apr 2011 Location: Grove Park, SE London Posts: 371	Re: AVO 8 mk 2 not working for ohms Hi all! Thank you so much for the replies! When I was observing the unit, I realised that the leaf switches on the DC side were not engaging at all. I gently moved aside the resistor board which lay on top of the switch mechanism and realised that the screw securing the cam had come loose! No idea why at all. Anyway upon tightening it the unit came back to life. I used some switch cleaner on the relevant parts and pots and I think it's all fine and dandy. Seems relatively accurate compared to my dmm. I'm wondering how these were calibrated now? And if there is still a service that does it professionally and for a reasonable price. Thanks again all!

26th Nov 2022, 11:56 am	#9
Chrispy57 Pentode Join Date: Mar 2020 Location: Rochdale, Greater Manchester, UK. Posts: 205	Re: AVO 8 mk 2 not working for ohms Hi Spencer, well done for bringing an old workhorse back to life! Cheers Chris

26th Nov 2022, 3:07 pm	#11
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 3,681	Re: AVO 8 mk 2 not working for ohms The (cheesehead) screws have a crinkle washer underneath, which does the job...most of the time. Unfortunately one of the screws on an early type Model 8 is inaccessible under the transformer. Spencer, basic calibration of the magnetic flux can be done using a precision voltage source (usually on the 10v DC range) If all the resistances are in spec. then tweaking the magnet shunt (with non ferrous tools) to ensure accuracy on this one voltage range should transfer accuracy to the other ranges- provided all switches are clean and operating correctly. The resistance ranges are inherently calibrated by the operator when he manually zeroes the pointer @ FSD. (Always provided the movement is healthy, of course) Component accuracy on a Model 8 MkII has stood the test of time well, it's unusual to find a shunt that has gone outside spec. (though you might find a winding here and there that has rotted and gone open circuit) AC readings can drop out somewhat if the rectifier has been abused or has corroded. Dave