Mac Plus Clicking — #4

Post #4 by imagineer

Source Forum	68kMLA
Category	Hardware
Post Date	Sat, 25 Sep 2010 - 03:24
Original URL	https://68kmla.org/bb/threads/mac-plus-clicking.21337/

Post

Harrison, I see you meant a different voltage adjustment, on the foil side of the power board, towards the screen.

Tried that, full - then full +, same clicking sound ( It clicks about 4.5 times a second and is more a flick, flick sort of sound.)

In his pdf "plus_analog" Lee says:-

The computer is protected against gross regulation failures that would lead to excessive supply voltages. Twelve ampere crowbar SCR Q8 turns on if there is a dangerous over-

voltage (on the order of 15%) on the +12V supply,indicating a regulator failure (or misad-

justment of R56). The resulting short circuit shuts everything down. Q8 is a 2N6394 in

some versions; the only difference is that it is rated at 50V instead of 100V. Both SCRs

will work ﬁne.

After the crowbar has been activated, the power supply periodically tries to recover, emit-

ting a characteristic “whup-whup-whup” sound as long as the overvoltage problem per-

sists. To make sure that the problem is not caused by an improper setting of voltage

control R56, turn it to the most clockwise position (when viewed from the foil side of the

PCB) to minimize the output voltage setpoint. If the “whup-whup” sound is still heard, it

indicates a major problem elsewhere.

Assuming the "whup" sound Lee describes is what we have here, I have been unable to locate the Q8 on this Plus board, however, it seems to be doing it's job judging by this text. As I said, adjusting the R56 produced no new result. It appears the "elsewhere" is where I now have to look.

Any ideas where this "elsewhere" might be would be much appreciated!