On Thursday, January 28, 2016 at 5:24:00 AM UTC-8, N_Cook wrote:
> I replaced the triac with similar rating and same symptom.
> Runs perfectly smoothly, bypassing the control pcb
Ordinarily, stuttering means the brushes are sticky or dirty,
and not making consistent commutator contact. It could be,
though, that your triac is false-triggering (there ought to be
a gate-to-cathode resistor or resistor+capacitor that prevents
false triggering).
Are you sure that component is a triac? There are similar devices
called 'quadracs' that have an inbuilt trigger.