Scored this little guy today. Rejected , can't be fixed. A neighbour of mine (who knows i like to tinker with electronics) fished it out of the skip where he works and gave it to me. It was pretty greasy and grimey.
Basically on poweron the meter was constantly complaining about the leads. Set it to voltage, ohms or any non-ampere setting and it continuously beeps and shows 'LEAD' on the display. Set it to amps and on certain ranges it says 'LEADs or Fuse' without beep , and on others it says 'Leads' with beep.
No matter what you try it is stuck in that mode. i took it apart and figured there must be something wrong with the lead sensing system. Looking at the connector contact block it turns out there are two pins per banana jack. when you plug in a banana jack you short the two terminals and that is how they detect where the leads are. the sensing mechanism is only present for the 2 ampere contacts even though the GND and volt contact also have this split terminal.
the sensing is done via two fat carbon compiste safety resistors. disconnecting those from the sense wire made the machine operational. hmmm so there is something in the contact block... i desoldered the contact block from the PCB and foudn a kind of oily substance between the pcb and contact block. i cleaned it of using alcohol prep swabs ( prep swabs are what a doctor uses before you get a shot. they are made from woven cotton and impregnated with alcohol. the nice thing is that they are virtually lint free , as opposed to ear-cleaners that can leave lint behind )
i measured the resistance between the pins in the contact block. some were 30 megaohm , some were 300k some were 5 meg . wtf ? there should be no resistance ... i cleaned the inside of the contactblock using alcohol , terpentine and a universal degreaser ... that's better but some are still in the order for 20 meg ... that can't be. looking inside the block i found the same weird oily goop in the ampere contacts... looks like somebody spilled something in there...
so i threw the entire connector block in my little ultrasonic cleaner , put in a mixture of soap , degreaser, alcohol and water and let it sit at full power for half an hour. blow dry witht he hot air gun and lo and behold: infinite between all pins.
Soldered the block back on using a low solids flux , cleaned the board off with alcohol , powered up the meter and ... tadaa it works perfectly fine. after a thorough cleaning of the case using terpentine and alcohol it looks like new.
I'll have to invite my friend next time i go for a slice of pizza.
First picture : the main board top view. notice the guard traces aroun the main divider hybrid. input stage is well protected with MOVs , PTC's and big power resistors. the necessary cutouts in the PCB are rpesent to guard high voltages
Second picture : closeuo of the guarding aroudn the divider hybrid as well as the contacts of the rotary switch. the little sot23 transistors on the left side are the sensing circuit for the testleads
Third picture : the brown resistor left bottom is the lead sense resistor for the 10 a range. there is another one lower for the 400mA range.
fourth picture : backside witht he big fluke asic that does the measuring and true rms. notice the big high quality filmcaps (104M and 223) for the sampler and true rms convertor. also notice the guarding aroudn the hybrid pins. the resistoras around the rotary switch hole (somewhat arranged in a circle) are the range selector. fluke uses a single pin to select function. this is essentially a potentiometer. every click of the rotaary switch increases with 1 kilohm. the other contact are for physical divider switching and sense switching,
fifth picture; the removed contact block you see the greasy stuff on it ...
sixth picture: alcohol prep swapbs. these are dirt cheap. get them at costco .1000 for like 5$
seventh picture : contact block after ultrasonic cleaning. grease is gone.
i have no idea what that grease was but it was conductive enough to mess up the lead sensing..
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