On Sun, Jan 3, 2021 at 11:35 AM, walter shawlee wrote:
barely visible input resistor (62 ohm carbon film) was BURNED
Hi Walter:
I'm pretty sure that 62 ohm is a damping resistor for the relay switches, to prevent oscillations when they switch.
If someone had the channel one set to ground that leaves that input floating. (The center pin of the BNC is connected through the 62 ohm resistor.... and through a NC relay... and then to a NO relay... so the center pin is not grounded... it's floating.)
My guess is that someone had channel one hooked up through a 1X probe to SMPS... either the input caps... or a transformer... and got a high voltage/high energy pulse they didn't expect.
Either there is a spark gap on the PCB, or it arced through to the relay coil.
A further guess is that it arced through the first relay. (Those potted Panasonic relays probably don't have enough isolation.) So the relay might be damaged.
When the scope is set to ground, all the vulnerable goodies (like the attenuators) are disconnected from the input when channel one is set to ground. But the 62 ohm carbon film resistor is still there. Carbon films are not good at handling high energy pulses (they blow up)... carbon composition is best (the entire body of the resistor can absorb the impulse, instead of just the carbon ribbon), but carbon comps don't have the stability of carbon film, or metal film resistors.
