Amprobe AM83 faulty

[andypandyplod]

Hi,
I got this at a boot Fair.It was fully working then.After about a few years it started giving odd readings?. When I looked inside,an IC marked U4 (see photo),a 4011 device had blue crystals around its legs?.I thought it was IC adhesive,or old flux gone wrong? It had corroded some of the IC pins,and PCB pads. I removed the original IC carefully as was possible,and replaced it with a new one. The meter now works,but gives random results and has about + or - 25% difference,when alternating the probes on a 3v battery. This fault is also reflected when measuring mains AC. I have also lost some of the button functions. I have looked for other possible faults ,like the rotary switch, battery,and fuse connections. I had trawled the Internet for a complete circuit without success. I know its an outdated meter , and should throw it away, but I like a challenge. Any Help Please? thank you

 

[Nigel Goodwin]

It sounds like something has been spilled in it, causing the corrosion - and the problems most likely stem from that - either components corroded, or the PCB itself.

And of course it's most likely to be UNDER components, where you can't see it.

 

[large_ghostman]

I had this from cheap chinky solder and the flux in it, I replaced the one that looked corroded but it didnt work (different board to yours). In the end it turned out that although only one component had the bad corrosion some other had developed bad solder joints, maybe they would have eventually also gone green? I ended up resoldering with decent solder and it was ok again

 

[andypandyplod]

Hi,
thanks for your advice,I always use Multicore 40/60. I will have yet another go around the area,even though I have looked many times before.If only I could get a circuit it would be a lot easier,I could trace the tracks.

 

[large_ghostman]

Like Nigel says its always where you cant see it or almost like a dry joint, funny how a single tiny whisker touching something can short a board out and yet I have had fat wires in a screw block that needed to be screwed down tight to make a connection! If you havnt spilled anything then I would get one those fibre pen things and clean all the soldered areas and resolder any that even looked at me the wrong way. Failing that and a scope its going to be hard, also check the switch contacts!! Thats caught me so many times along with ribbon cable connectors!

 

[andypandyplod]

Thanks again I will keep trying! In the past I have always fixed everything,unless I cannot get the parts,parts are too expensive,or it is too far gone

 

[Nigel Goodwin]

Thanks again I will keep trying! In the past I have always fixed everything,unless I cannot get the parts,parts are too expensive,or it is too far gone

When something has been contaminated it's usually non-repairable, even if you manage to get it working it's likely that something else will fail in the near future.

If contamination is limited to a small area, and it's fairly large components, it's sometimes possible to do a decent repair - but if there's corrosion under SM IC's you're fighting a losing battle.

On an even nastier note, one of the earliest digital satellite services was in South Africa, and Pace made the receivers for them. A few months in they had massive identical failures of the boxes, and it turned out that the PCB manufacturer hadn't cleaned the multilayer PCB's properly before joining the individual PCB's together. This left traces of etchant in the middle layers of the PCB, which over time etched some of the centre layers away

Following extensive fault finding, they managed to identify the most likely track failures, and were able to add external wires to the board to restore operation. Obviously it would be cost prohibitive to spent huge amounts of time of every board, so a production line was set up to fit the extra wires - and any which didn't pass testing afterwards were scrapped.

 

[large_ghostman]

Man you wouldnt of wanted to be at that meeting!

 

[Nigel Goodwin]

Man you wouldnt of wanted to be at that meeting!

No, or to be from the PCB manufacturer

 

[andypandyplod]

thank you

 

[andypandyplod]

meter now fully working! I removed the IC,I had previously soldered in. Carefully checking tracks and vias,I found the via from pin 1 was open circuit,and repaired it with a single strand wire put through the hole,and soldered to the offending track either side of the open circuit. This got the meter working,but readings were dancing around the correct reading.By grounding 4 inputs on the unused inputs(not originally done when manufactured),and recalibration,meter fully worked!