Fluke 8375 tin whisker

[RolandK]

Because of a hint in the TEA thread i got a 5 1/2 digits (119999) fluke 8375, which of course only showed zeros in the display.

For the 8375 i did not find any manual, but the 8300 manual works fine. The 8375 seems to be a 8300 with 10 ohm, 100mV and true rms voltage options (numbers are ranges, resolutions are eg. 1 µV, 100 µOhm) and a better reference (0.002 % vs 0.005 %).

The reason for the zeros display was a short between two strip lines on the main pcb which where parallel through the whole pcb about 40 cm / 16 inch. Of course there was at first sight nothing visible. So i used a 4-wire milliohm capable meter, connected the 2 source wires each on oposite sides of the two strip lines and measured the resistance between 2 parallel points with the 2 sense lines. Think of a H. In the middle is the lowest resistance. So i found the position with the lowest resistance.

And on further inspection i found a needle forest of ultra thin tin whiskers of about 2mm length (1/10 inch). After removing them with a rubber the instrument went to life.

Reason for tin whiskers are many. In this case it seems copper lines with tin plating and no other coating between.

[Gyro]

That looks unpleasant, especially for a Fluke. I wonder if it has been stored in cold damp conditions.

You could either try a decent conformal coating, not guaranteed to prevent regrowth indefinitely, or, more tedious unless localised, tin the affected tracks using Tin/Lead solder. The Lead should suppress further dendrite growth.

[RolandK]

I just used a rubber and blew them away. All the tin wiskers were in one location, where no parts were mounted, nowhere else. Indeed the area may have been the coolest location in the instrument. But i could not see under the switches and the 50 years history are unknown. Hope it will last the next 50 years.