RIFA Safety Capacitor Replacement in HP Gear
In a previous post, I had documented the condition of several RIFA Y Safety capacitors, showing clearly the cracked condition of the outer case. This condition is a precursor to the well documented failure of this style of RIFA capacitor. Reportedly, these problems have been solved since Kemet squired RIFA in 2007. Of course the equipment which contains these potentially failing capacitors is typically older than 2007.
For example, I recently replaced six (two from each instrument) of these capacitors from the following equipment:
- HP 3245A (1992)
- HP3245A (1992)
- HP 3458A (1989)
Each and every RIFA Safety Capacitor I removed from this equipment showed cracks in the case.
These capacitors are connected from line to ground and designed to fail “open”. Unfortunately these RIFA capacitors failed short and can result in smoke and fireworks. Why is this?
Examining the partial schematic shown below (HP 3245A AC Line Input) we can see that the two line to ground safety capacitors are always connected to power, even when the main power switch (S3) is in the off position. If the safety capacitors fail open as they are designed to, then there is no danger and no fireworks. The capacitors no longer filter line noise but the instrument will still function. However if they fail short (as do the RIFA capacitors we are discussing) then full line voltage and current is applied to ground through the capacitor. This results in noise, smoke and fireworks.
I feel much better having replaced the suspect RIFA capacitors with modern X1/Y2 safety capacitors.
Thanks for reading.