Jim Wilkins wrote:

OK , I don't know a lot about SCR's , but I have some experience with
electronics . How do I test it ? I was planning on starting by checking
the resistance of both sides of the control pot ... but I have no idea
how to test an SCR .

In principle, it's easy 8-). First, test it like a diode,
however if it conducts either way it's faulty.

Then test it forward biased and momentarily connect gate to anode.
It should start conducting and stay that way.

There are two traps in this: One is that an SCR generally needs at
least two forward diode drops, 1.4 volts, to work at all. The other
is that "latching" the SCR on requires some minimum current, maybe
1% or less of running current. Far more than a normal diode tester
provides. A car battery and taillight bulb would probably work.

HTH,

bob prohaska