General Category > Lisa Troubleshooting and Repair

Edge Connector Replacement

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Lisa2:
If you want to keep your system original then John Woodall has used OEM cables for sale, eBay item number: 144765847489

fri0701:
(I know this thread's pretty old, but I wanted to post an update after I finally got around to taking a look)
I ordered a couple new-old-stock MPU131s from here: https://smallbear-electronics.mybigcommerce.com/transistor-mpu131-programmable-unijunction/
I soldered it in to my PSU, put it in the Lisa, put the back cover on (engaging the interlock switches), and connected the power cord. Before I even pressed the power button, I could hear a crackling noise and saw some magic smoke come out the side.
I took the PSU back apart, and sure enough, the new MPU131 I'd put in at CR21 had a big crack and burn hole in the side. Clearly something else is wrong somewhere "up the chain". I have a couple more MPU131s at least, but clearly I need to do some more investigating around the schematic.

sigma7:

--- Quote from: fri0701 on September 24, 2023, 11:09:01 pm --- CR21 had a big crack and burn hole in the side. Clearly something else is wrong somewhere "up the chain". I have a couple more MPU131s at least, but clearly I need to do some more investigating around the schematic.

--- End quote ---

Patrick has an excellent strategy for troubleshooting this PSU in another topic:


--- Quote from: patrick on June 18, 2022, 03:11:27 pm ---disconnect mains, apply +5 V from a lab PSU to the +5 V rail and -12 V to the-12 V rail. Do not apply -5 V to the 7905 output, these don't like to be reverse-powered. Put an 1k resistor in series with the +12 V rail and apply 12 V. The resistor is necessary to limit the current through CR21 when it fires.

Now start with R29 away the Q4 side. Rotate it slowly and see what happens with Q3 and Q2. The voltage at the base of Q3 should rise until it reaches 0.7 V. Then Q3 and Q2 should conduct and the PUT triggers. That means the voltage at pin 3 goes from 12 V to almost zero.

If this works, connect another 1k resistor from pin 3 of CR17 to 12 V line. It might be necessary to disconnect pin 3 from the PCB, so that only the bias voltage is connected. Now repeat the procedure above and check if CR17 triggers, too. Your fault is likely to be somwhere in this area.

The only PUT you can buy today is the 2N6027 from OnSemi. This works as a replacement for the obsolete MPU-131.

--- End quote ---

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