General Category > Lisa Troubleshooting and Repair
Lisa 2 PSU Woes
sigma7:
While reviewing the May 1983 Hardware Reference Manual for another PSU thread, I see it refers to U3 as an "opto-isolator" so I guess YMMV and tomayto / potaahto.
warmech:
Well, update for y'all. I took the PSU to a friend's place who is infinitely better at this than I am (this is his post-retirement day job, so he knows what the heck he's doing) and let him take a look at it. We went over as much of the PSU as he had the equipment to test and basically ruled out every component but the transformer. Nothing shorted, nothing blown - everything checked out. However, while testing, it appears that CR10 failed, as the crowbar has reverted to a failed state and our subsequent testing confirmed as much. All that having been said, I think this PSU is a lost cause at this point. I have another lead on a replacement (working) PSU, so that may be the path forward from here.
Adjacent to this, however, I've been trying to test the CRT to make sure no damage occurred when the system took a hit during the brief overvoltage it took when the PSU was fired up the first time. I have a bench supply putting out +33v and +12v and have wired up a fingerboard running both to the video board. Additionally, I've wired the fingerboard to the old PSU's focus/contrast controls, so the CRT is wired up with all it should need. All that considered, I get no life out of the CRT - no neck glow, no high-pitched whine you normally get out of CRTs, nothing. I tested a couple of points on the video board while it was mounted, and it's getting just under +12v/+5 (11.8-ish/4.9-ish) and also very slightly under +28v out of the regulator, so I know it has power and that the regulator wasn't fried from overvoltage. I'll pull the board to start testing components tomorrow, but does the CRT require anything else to power up? I have the faceplate on the front to engage the interlock, but does the motherboard/CPU board have to be connected for it to power up, or am I missing something else? The hardware manual doesn't mention anything specific in regards to requiring the CPU to turn the dang thing on, but I do know a good part of the video control is on the CPU board.
Edit: After craning my neck a little further over, there is in fact neck glow and I just wasn't looking well enough. Still no high voltage, though. I'll test transistors in the morning and see what things look like.
sigma7:
--- Quote from: warmech on June 18, 2022, 04:46:17 am ---... test the CRT ... I have a bench supply putting out +33v and +12v and have wired up a fingerboard running both to the video board. Additionally, I've wired the fingerboard to the old PSU's focus/contrast controls, so the CRT is wired up with all it should need. All that considered, I get no life out of the CRT - no neck glow, no high-pitched whine you normally get out of CRTs, nothing. I tested a couple of points on the video board while it was mounted, and it's getting just under +12v/+5 (11.8-ish/4.9-ish) and also very slightly under +28v out of the regulator, so I know it has power and that the regulator wasn't fried from overvoltage. I'll pull the board to start testing components tomorrow, but does the CRT require anything else to power up? I have the faceplate on the front to engage the interlock, but does the motherboard/CPU board have to be connected for it to power up, or am I missing something else?
...
Edit: After craning my neck a little further over, there is in fact neck glow
--- End quote ---
Without any video signals, I think all you'd get is the dim glow of the filament.
The horizontal sync is what drives the flyback transformer and without that, there will be no whine or HV.
All of the video signals come from the CPU board (the video data come from the slot RAM, then through a shift register on the CPU board to make the video bit stream).
The front panel interlock switch disables the PSU like the back panel interlock (so not a concern if you are supplying power otherwise).
Since the PSU was generating all the voltages (albeit too high due to apparently no feedback), I don't think the transformer is a likely suspect, and not surprising that CR10 didn't like sinking all the energy. I've been pondering how to troubleshoot this particular design and will let you know if I come up with a potential procedure.
patrick:
--- Quote from: warmech on June 18, 2022, 04:46:17 am ---(...) and basically ruled out every component but the transformer.
--- End quote ---
In your first posting you write that you get voltages on the secondary side. Your voltages are 50% too high, but the ratio of the 5V, +/-12 V and 33 V is correct. -5V does not count, this is regulated by a separate 7905. That means primary drive circuit is working, transformer is working, secondary rectifiers are working. But your regulation is not working.
At first: when operating the PSU outside the Lisa, make sure pin N (+5 V sense) is connected to pin 11, 12, M (+5 V output)! Otherwise your controller will measure 0V and continue outputting power until the crowbar triggers.
You already replaced R29, so I assume it is working. This is a common failure, the wiper can have intermittend contact. The Q3 will never conduct, the controller circuit thinks the output voltage is much too low, same as above. Rotate R29's wiper to the Q4 side, then the controller gets the biggest actual value available, which means it should set the lowest output voltage. Target value is 0,7 V at the base of Q3 when +5 V and -5 V have equal magnitude.
If the thing still bangs into the crowbar: 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.
sigma7:
--- Quote from: patrick on June 18, 2022, 03:11:27 pm ---At first: ...
--- End quote ---
That's a most excellent & helpful troubleshooting process, thanks Patrick!
Perhaps you can tell us half-voltage folks... when this PSU was configured for the European market (ie. 230V), how was the compatibility of the standby transformer managed:
The schematic shows a jumper, but no notes for its use... was a different transformer part number used, or something (resistor, diode, inductor, capacitor?) used in place of the wire jumper that we have?
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