There are a very limited number of ways the choke can fail. It can be:
- Open: use an ohmmeter to measure the series resistance. If it's under 200 ohms or
so, it's probably not open. Over 1K is certainly a problem.
- Internally shorted: This will mean that it just does not filter very well, and the ripple
on the B+ supply will be high, so the amp may hum. There's no good way for the beginner
to detect this other than the difference between a good choke and a bad choke in reducing
- Shorted/leaking to the iron core, and therefore to the chassis. Measure the resistance
from the wires going into the choke and the chassis. This should be over 50K at least. If
you suspect that the choke is leaking, measure the resistance to the chassis and then unbolt
the choke and measure again. If the resistance goes up, the choke is leaky. If it's same, it's
not. Sometimes there will be a short that happens only when the choke gets hot or vibrated,
but that is rare.