PDP-8/I at the RICM

I am not sure I understnd the point of the monitor circuit. If the capacitor fails dead-short, then the PSU fuses will blow (You do hae fuses in series with the transfomer windings, I hope). No monitor circuit is going to do much of use in that situation. If the capacitor fails open, or high ESR, or loses capacitance, the ripple on that supply line will increase. A monitor could detect that, but equally if that happens, the machine stops working correctly. And the first thing I'd do to any piece of vintage hardware that wasn't working -- whatever the fault -- is to check all the supply lines for voltage and ripple. At which point you home in on the faulty capacitor quite quickly. A monitor would perhaps save you 2 or 3 'scope tests. Is that worth it? -tony