Quote:
Originally Posted by zeno
Sticking relays were kinda common. Also a good spot for cold joints.
You can double check it by measuring the DC where the relay coil connects
to the relay driver transistor. Just follow the trace to find it.
When on relay coil will be source, usually 12 VDC on one pin & very low
on the other.
When off both will be the same.
Relay driver transistor will be when on
C = very low VDC
B = .7 > VDC
E = 0 VDC
When off
C - source, usually 12 VDC
B = 0 VDC
E = 0 VDC
Last the diode across the coil everyone wants to know about.
Without the diode when you turn off the set the coil field collapses
& the relay will chatter as it re energizes itself. The diode will short it out
& prevent chatter.
73 Zeno 
LFOD ! |
IIRC those diodes also absorb inductive voltage spikes caused by the field collapsing...ISTR that those spikes can fry lower voltage switching transistors if not suppressed by a diode.