Den Sun, 25 Feb 2018 05:40:10 -0700 (MST) skrev Sherry Haibara <v.castelli92@gmail.com>: > Took a couple of measurements on the monitor PCB. The AC input is ok > (16.5Vac), but the voltage across the capacitor and on the LM7812 > voltage regulator seems to be very off. > The voltage on the capacitor oscillates between 0 and 8.7 volts in a > rhythmic fashion (it goes like: 0-1-8.7-1-0). On the LM7812 (I > assumed it's the one mounted on the big heatsink right next to the > capacitor) I measure 22V DC on two pairs of pins and 0V on the other. > The 4 diodes on the bridge test fine with the diode test of the > multimeter, showing 0L on one side and a 0.55V drop on the other. Unless there is some mistake while taking the measurements, I can only see theese explanations: * The main rectifier capacitor is more or less totally worn out causing a too big ripple for anything to work. You can try connecting another capacitor with similar specs in parallell with the existing one. * Bad diodes in the rectifier, measuring correctly with the low voltage your measuring instrument uses but conducts in the wrong direction whit enough voltage across them. Not that likely but also not totally unlikely. * Severe overload on the 12V regulator. That would also cause a big ripple across the main capacitor but then some stuff would also run very hot. * Extremely unlikely: Short circuit between the secondary windings on the transformer. Easy to measure, just disconnect the transformer from the monitor and ohm measure between this winding and the other windings. You could try disconnecting the output from the 12V regulator (and disconnect the transformer) and feed the monitor from some other 12V supply like a PC PSU or similar. -- (\_/) Copy the bunny to your mails to help (O.o) him achieve world domination. (> <) Come join the dark side. /_|_\ We have cookies. Message was sent through the cbm-hackers mailing listReceived on 2018-02-25 21:00:03
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