Old ex BT DVM.

[Oldcodger]

I can't remember the Fluke number from an earlier discussion on this model. Problem I've got is that it now over reads on ( all I've checked) DC volts by 30% .
e.g. 29v nominal mobility scooter battery reads circa 37volt. I've tried it on some of the other DC ranges and it seems to show the same result. I've noticed that playing with the function switches ( AC/DC & mA/ V ) changes the makes a difference for a short time. Ideally a circuit diagram would be nice, but failing that the Fluke model would also be helpful to try and localise the problem on paper before doing any more.
BTW- I know it's not in mint condition, but i got it second hand and it was a working meter in my lifetime.

[Andrewausfa]

When clean the accuracy will improve by 30% It's an 8022A I believe.

Andrew

[Oldcodger]

The dirt is only on the face. Inside it's quite clean, but I'm suspecting one of the function switches as I see variations on the readings if I take readings before and after moving them .

[The Philpott]

Here's hoping it's just a filament of copper wire that's managed to get inside and float around.

[MrBungle]

Originally Posted by Andrewausfa

When clean the accuracy will improve by 30% It's an 8022A I believe.

Andrew

That’s not actually wrong. Dirty PCB will load the front end down.

It is almost an 8022. Has different holes apparently to BT spec.

[Oldcodger]

Originally Posted by The Philpott

Here's hoping it's just a filament of copper wire that's managed to get inside and float around.

Highly unlikely as the case would need to be opened for that. I'm temped to suspect one of the function switches having contact problems as the readings do change when either of the function switches are moved .

[MrBungle]

The likely cause of this is one of the electrolytic capacitors in it leaking all over the board. They are quite destructive. Get them out and replace, them scrub the board with isopropyl alcohol around the switches and the main IC. Probably be fine after that.

I got this one working as new which was reading non zero.



Also replaced the display with generic one but that was painful.

[Oldcodger]

Looking a photo- it's a different setup. My meter only has one board it's an 802xa-3001 with the date ( possibly the copyright date) as 1979. There's only two caps visible .Both look in mint condition, no leaking /bulging which I'd expect would lower the reading, not inflate it.
I've looked at your post on the Maplin DVM and checked the impedance into the 8022 using my good Fluke and it's a weird reading. +VE TO +VE on the 200mv range I get circa 2Mohm. + to -ve gives either a very low reading or the same. Possibly a cap charging somewhere. ??That's why I'd hoped there might be a circuit diagram somewhere to let me work out the problem logically. The face of the doubl side PCB looks reasonably clean .
ETA- I've found useful info on this meter on line now. I'll add my links in tips etc .

[MrBungle]

You tend not to be able to see the leak until the cap is removed unfortunately. Usually indicated by an oxidised leg. I had a picture of one somewhere from the above but from above it looked perfect.