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Old 11-11-2021, 11:08 PM   #19
Join Date: Feb 2010
City & State: Ames Iowa USA
Line Voltage: 120VAC 60Hz
Posts: 69
Default Re: Motor control board repair

Originally Posted by R_J View Post
107vac is likely that they have 3 phase power supplying the building and they use one phase to supply the board, this is normal.
Locate the source for the -15v, trace the -15volt point back, the ac for this circuit is likely supplied from the transformer and then through a diode or bridge to give a negative voltage.
It looks like the -15V goes back to SR3 and the voltage regulator that doesn't get warm, AVR2. From looking at the trace lines and from the meter, it seems like the AVR2 is in parallel. SR3 has a + and - and two middle pins that go to the AC input, so it seems like a rectifier bridge, but I don't know what the SR3 designation could mean. Any ideas?
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