Trio CS-1554 Blew the PIO Intensity Capacitor, why?

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I was doing some measurements last night of my TDA1543 dac's I/V stage and I was using a Trio CS-1554 Dual Trace Oscilloscope when I heard a loud fizz and it was coming from the Trio.

I was using it for approximatley 15 minutes with the input voltage range set correctly.

Sure enough this afternoon I took off the covers to the Trio and noticed that the oil that had covered the inside of the case was coming from a 1500 WV PIO 0.5uF capacitor up at the front of the unit.

This unit is a fully transistorised one and works perfectly fine aside from being the age that it is and this PIO capacitor blowing.

The PIO cap in question is attached to the Intensity trimpot, The second one from the bottom.
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What caused this? was it caused by old age or some other faulty component like the main filtering capacitor losing capacitance?, ie that one going to the trimpot from the pio cap?

I was hoping to get this cleared up as I have a sister Trio back at home and it will have an identical pair of capacitors in it that I can use for spares
 
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According to my experience, PIO caps are not very trustwhorthy, and fails by no appearant cause, maybe of old age sometimes. In my years as a TV repair technician in the ages of tube equipment I have changed thousands of PIO caps due to leakage or shorts. And they can emit lots of smoke, to scare the owners.
 
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