Forget tin whiskers, now there's .. plastic whiskers?

[Agent24]

I'm troubleshooting one of my oscilloscopes which has lost its trace. Since the PSU seems OK and the front panel LEDs still light, the buttons still switch the relays etc, I'm guessing it's in the CRT section.

I have begun performing visual inspections, and while looking around the neck board I noticed a trimpot which seems to have started growing white crystals - out of the plastic!

I also noticed these to a lesser extent, on the main power switch body and the case of what seems to be some part (as yet unidentified) in the mains filtering circuit.

Has anyone seen anything like this before? I assume it's some breakdown of the plastic due to contamination or incorrect manufacture. I don't think it's environmental, only those three parts seem affected. Another trimpot on the neck-board looks totally normal.


Interestingly enough, this crystallised trimpot is part of a circuit which seems(?) associated with the cathode voltage (the schematic is labelled in French) - so maybe it has failed and is the cause of the fault?

[stj]

Re: Forget tin whiskers, now there's .. plastic whiskers?

someone posted this issue on eevblog just a few days ago.
nobody knows if it's from the plastic, or just attracted to it.

scoop a bit up and put the meter probes in it - check resistance and capacitance.
be interesting to see if it can actually conduct!!

[R_J]

Re: Forget tin whiskers, now there's .. plastic whiskers?

I suspect there is a fairly high voltage on that control, and it is caused from the enviroment it was in. Just like dust on high voltage controls etc. but this is white, maybe from a salty humid enviroment. Have you always had this scope or was it bought used from somewhere?

[Agent24]

Re: Forget tin whiskers, now there's .. plastic whiskers?

Salt crystal build-up is probably a good explanation, actually.

The oscilloscope was inherited, I don't know its history before I ended up with it. But I can say it probably spent most of its life in Australia, and if so, it also most likely resided in a coastal area for some time. Some of the shielding metal is also rather rusty.

The trimpot sits between the 110v and 5v rail, with the wiper going off to the cathode (KAT) through a diode and capacitor. Through another diode it connects to G1 which they have called "WHE".

The other trimpot is between a 58v rail and ground, with the wiper feeding the base of a translator which seems to adjust G4 and G5.


I'll see if I can poke around with the crystals a bit...

[R_J]

Re: Forget tin whiskers, now there's .. plastic whiskers?

With high voltage in the area any moisture evaprorates and leaves behind the mineral. Does the heater light up in the neck of the tube?

[Agent24]

Re: Forget tin whiskers, now there's .. plastic whiskers?

Yes, the heater lights, although it is quite bright for a second at power on. They are powering it from an unregulated winding in the main SMPS, so maybe it's 'normal' before the SMPS stabilises. Seems like a bad idea though.

Ignore the unplugged wire in the photo. I had unplugged it due to reading a rectifier diode as shorted, but it was just due to the heater resistance in parallel through the transformer winding.

[Agent24]

Re: Forget tin whiskers, now there's .. plastic whiskers?

http://www.eevblog.com/forum/chat/wh...783/#msg163783

Same thing (maybe) but probably no high voltage there to cause electrostatic attraction?

[R_J]

Re: Forget tin whiskers, now there's .. plastic whiskers?

Similar to this I think https://www.wired.com/2012/04/snowfl...owing-machine/

[petehall347]

Re: Forget tin whiskers, now there's .. plastic whiskers?

looks like its growing from inside .have seen similar to this before .