UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 23rd Oct 2022, 10:14 am   #1
Spencervs
Hexode
 
Spencervs's Avatar
 
Join Date: Apr 2011
Location: Grove Park, SE London
Posts: 382
Default 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.
Spencervs is offline  
Old 23rd Oct 2022, 11:34 am   #2
pmmunro
Octode
 
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
Default 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
pmmunro is offline  
Old 23rd Oct 2022, 10:27 pm   #3
Spencervs
Hexode
 
Spencervs's Avatar
 
Join Date: Apr 2011
Location: Grove Park, SE London
Posts: 382
Default 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?
Spencervs is offline  
Old 24th Oct 2022, 12:07 pm   #4
Chrispy57
Hexode
 
Join Date: Mar 2020
Location: Rochdale, Greater Manchester, UK.
Posts: 439
Default 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.
Chrispy57 is offline  
Old 25th Oct 2022, 12:34 am   #5
Spencervs
Hexode
 
Spencervs's Avatar
 
Join Date: Apr 2011
Location: Grove Park, SE London
Posts: 382
Default 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; 25th Oct 2022 at 12:49 am.
Spencervs is offline  
Old 26th Oct 2022, 9:49 pm   #6
Spencervs
Hexode
 
Spencervs's Avatar
 
Join Date: Apr 2011
Location: Grove Park, SE London
Posts: 382
Default 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
Spencervs is offline  
Old 28th Oct 2022, 12:26 pm   #7
Chrispy57
Hexode
 
Join Date: Mar 2020
Location: Rochdale, Greater Manchester, UK.
Posts: 439
Default 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?
Chrispy57 is offline  
Old 25th Nov 2022, 1:12 pm   #8
Spencervs
Hexode
 
Spencervs's Avatar
 
Join Date: Apr 2011
Location: Grove Park, SE London
Posts: 382
Default 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!
Spencervs is offline  
Old 26th Nov 2022, 12:56 pm   #9
Chrispy57
Hexode
 
Join Date: Mar 2020
Location: Rochdale, Greater Manchester, UK.
Posts: 439
Default Re: AVO 8 mk 2 not working for ohms

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

Cheers
Chris
Chrispy57 is offline  
Old 26th Nov 2022, 3:49 pm   #10
barrymagrec
Octode
 
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 1,552
Default Re: AVO 8 mk 2 not working for ohms

Quote:
Originally Posted by Spencervs View Post
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.
barrymagrec is offline  
Old 26th Nov 2022, 4:07 pm   #11
The Philpott
Dekatron
 
The Philpott's Avatar
 
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
Default 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
The Philpott is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:45 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.