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Old 23rd Dec 2023, 7:41 pm   #1
Christoffrad
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Default AVO model8 mk3 - component layout + advice plse

The last time I used my AVO 8 mk3 it worked fine but is now completely dead on every range although the movement deflects as resistance switch is moved between ohms and ohms x 100 even without probes attached.
The cut-out continuity is OK. The service documentation doesn't give much guidance and there is no diagram showing component locations.
Does a more thorough document exist. Can anyone give step by step guidance here please.
Chris
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Old 23rd Dec 2023, 8:59 pm   #2
The Philpott
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Default Re: AVO model8 mk3 - component layout + advice plse

Sorry for covering ground twice Chris, but can you confirm that ACV, DCV, ACA and DCA ranges are definitely dead?

Suggest first using a different set of leads, or just a couple of bits of wire for a low voltage test. The leads are quite robust but fatigue isn't unknown where the moulded section starts at either end.

Dave
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Old 23rd Dec 2023, 9:33 pm   #3
Christoffrad
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Default Re: AVO model8 mk3 - component layout + advice plse

Thanks for your reply Dave
I can hardly believe it myself .... So in response to your request I have connected simple new probes and I can verify that there is no response on AC volts ranges or DC volts ranges to appropriate input voltages.
Also nothing on resistance ranges except for the flick when switching (on Resistance) between R and R x 100 (which shows the movement is seemingly OK)

Chris
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Old 23rd Dec 2023, 10:23 pm   #4
Christoffrad
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Default Re: AVO model8 mk3 - component layout + advice plse

And DC A also dead
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Old 23rd Dec 2023, 10:46 pm   #5
The Philpott
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Default Re: AVO model8 mk3 - component layout + advice plse

Hmm. I'm familiar with the twitch of the pointer when switching between ohms ranges (it lets me know i've put batteries in!)

Perhaps a dry joint or cracked joint somewhere- i've been caught out before by joints that look fine until you poke at them with a screwdriver.. so the cutout is OK..but could be a loose screw, terminal or bad joint that leads to or from the cutout?

I don't have the circuit diagram in front of me at the mo, that might inspire me tomorrow.

Dave
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Old 24th Dec 2023, 9:48 pm   #6
Chrispy57
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Default Re: AVO model8 mk3 - component layout + advice plse

Hi - sorry your Avo is giving you some grief at the moment. I'm not an expert, but have been learning my way around an Avo 8 Mk3 that my wife picked up for a fiver over the summer. The circuit diagram is a bit overwhelming at first sight, but I found that highlighting the circuit path for a specific range was very helpful. An example is attached for the 100V DC range. (Just a JPG and Paint.Net in my case)

Then try methodically checking the continuity along that path. You say that the cut-out continuity is OK, but perhaps start at the black common terminal and check through the cut-out, then on to the next component, 10k resistor, then the meter etc to look for a break?

If you've suddenly lost everything, then something pretty fundamental has gone open circuit - broken wire, dirty contact, misaligned switch contact etc so hopefully you should soon identify and rectify the fault.

Good luck
Chris
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Old 24th Dec 2023, 10:31 pm   #7
Christoffrad
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Default Re: AVO model8 mk3 - component layout + advice plse

Thank you Chris. I'll take another look based on your note and diagram.
BTW the cut-out on my Mk3 seems to have a double (ie ganged) set of contacts that are isolated from each other.

Happy Christmas

Chris
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Old 24th Dec 2023, 11:27 pm   #8
pmmunro
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Default Re: AVO model8 mk3 - component layout + advice plse

Chris,

Send me a private message with an email address and I will send you servicing information to help you with this fault.

The cut has two sets of contacts. The large heavy contacts are to break the current when a fault occurs and have a corresponding rating: the smaller conical contacts are to provide a parallel, low resistance, path to ensure accuracy. The spring leaves ensure that the light duty contacts break first and make last and so are relieved of the switching duty.

PMM
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