Fluke 77 Repair

[TheOEL]

I had an incident the other day, one of those days when the world would have been a better off place if I had stayed in bed all day.
A friend is out of the country for a while so I was checking the battery in her car. Down to 12.2 volts so I put my quick charger on to it and the volts came up. Decided to check the charging current, so put my trust 30 odd year old Fluke 77 on to amps, just over 5. Neighbour arrived to see what I was up to, we chatted for a while and, after he had gone, checked the voltage. You guessed it, still on amps.

Result. Nice display but no change in readings any more. The 10 amp socket connects to what I presume is a shunt, shown in the photo.

Any thought on repairability?

[Mr.B]

Welcome to the forum.

Yip, we have all done it at one time or another.
I would be inclined to use it as an excuse to buy myself a new meter... 

[lowimpedance]

Depends on how badly toasted the PCB around whats left of the pads !. ie not too much carbon or pad damage.
And how keen you are. ( You can download the schematic)
Take a look at this Bourns current sensing resistor from EL14.
http://au.element14.com/bourns/pwr4412-2scr005f/current-sense-resistor-0-005-ohm/dp/1561181
Will need to create a 4 terminal connection like the original at the PCB which is simple.
Should be able to bodge it in.
All this is assuming the meter is still good on all modes, you have checked that the DC volts and ohms etc are still good ? (i think they will be).
So a new fuse and shunt, and a check to see if it is roughly in spec for the10 A current range.
Is it worth it, or as MrB says perhaps its time for a meter update ??

If it was me I couldnt resist the challenge 

[retiredcaps]

And how keen you are. ( You can download the schematic)

Same here.  I would expend a ton of energy and time trying to fix it only because it would be a good learning experience for me.  For others, it may not be worth the time and effort and would welcome an upgrade.

If you decide to fix it, I can also offer suggestions to the ones already presented after we know that your 77 is mostly functional.

For inspiration, I direct you to this repair regarding the current range.

http://mrmodemhead.com/blog/yokogawa-7534-03-multimeter/

PS. I know Modemhead would be all over this repair if the meter were in his hands. 

[Mr.B]

...If it was me I couldnt resist the challenge  ...

Yes, me too... However I would tell my wife I needed a new meter... 

There is no such thing as too much test equipment!

[ModemHead]

Oh my, it looks like you have the earliest 77 model with the unfused 10A input.  Chances are the shunt has changed value after its brief stint as a cigarette lighter.  Probably a vaporized PCB track or two as well, although that should be limited to the current measurement paths.

I'm also curious as to whether any other function still works.  The shunt voltage sense line goes straight into U1 with only a series resistor to protect it.  No voltage clamps, diode bridges, etc. like on a more modern unit.  So I'd be a bit concerned about U1.