Keithley 179A, V and A dead

[khs]

Last days I ordered from evilbay a tested(!) Keithley 179A 20A.

I have two, but some of my boards I make have three power supply voltages so it's fine to see all three currents during the first power up.

The "new' 179A looks like new, but during the first test volt and ampere readings failed. Only Ohm readings were fine.

All fuses were OK and no sign of damage was visible.

Checking the power supply voltages using the LO input as reference I noticed the +-15V were too low and not stable.

Checking the connection of the LO input to ground I noticed the LO input was not connected to ground. But is should be connected via R154 (a 0.01 ohm resistor) if not in ohm mode.

The ohm switch was OK. This is not bad, because I assume removing the mode switch without damaging the whole board may be quite difficult.

If the ohm switch is OK and there was no sign of a damage of the wires on the pcb visible it should work. Or I have overlooked something.

Following the wire from the ohm switch to R154 the wire disappeared under CR111.

And a second view showed a tiny piece or blank chopper more or less under CR111. This was the end of the wire.

After removing CR111 it was visible under CR111 this piece of the wire was blown. Before and after CR111 the wire on the pcb was fine so the repair was simple.

 
P.S. The picture of the CR111 could be better, but the piece of blank wire should be visible.

[lowimpedance]

P.S. The picture of the CR111 could be better, but the piece of blank wire should be visible.

Sorry too blurry, I cannot see what your talking about. Are you saying there was a thin wire, (fuse like !?) under CR111 that had gone open cct ?.

[khs]

P.S. The picture of the CR111 could be better, but the piece of blank wire should be visible.

Sorry too blurry, I cannot see what your talking about. Are you saying there was a thin wire, (fuse like !?) under CR111 that had gone open cct ?.

Here a better picture. I've removed CR111 to show the damaged wire better.

Yes the width of this part of the wire was not as wide as the rest of the wire, so it was more or less a fuse.

I've learned the fuse must be fast to protect the wire. Otherwise the wire protects the fuse...