Yes, good point. I spent so much time on this, that doing this first would
make sense. I actually enjoy reading those schematics and trying to make sense
of it, and how everything works, that I feel it has not been a waste of time. My
undergraduate electrical engineering 101 class is now starting to come back to
me.
The various input to the PS (5V sense, On/Off, line sense, current sense,
power fial, shutdown etc....) and how they all interact with the PS function is
still something I am wrestling with, and I have not yet find a way to test the
full functionality on the bench. I will try to find a way to load the 5v output
(a 1 ohm, 5W power resistor is what I have in mind), feedback the voltage back
to the PS on pin N.
Sent: Wednesday, October 01, 2014 2:27 PM
Subject: Re: I blew it - literally
Totally random thought, but to me it seems odd that just opening
the door while the machine is on would cause permanent damage. It's
possible you shocked the machine with static electricity and damaged something,
but one thought that occurred to me: Try checking the safety switch itself
to make sure that it is OK - IIRC the rear safety switch is built into the power
supply and soldered to the board, make sure that when you press the switch you
can see the circuit make using an ohmmeter, and also make sure that when you put
the rear door back on and tighten the thumb screws that you actually hear the
safety switch click on.
On Saturday, September 20, 2014 1:38:04 PM UTC-4,
pintoguy wrote:
So, I had a perfectly good Lisa 2/10 which I had fixed earlier (I
had a short in the widget fan). Then, as I was demo-ing it to a friend, I made
the mistake and opened the back cover while it was turned on. As expected, the
safety switch tripped. The problem is that, since then, the Lisa won't power
up. I checked that the 5V standby is there, and I also checked that I get the
18V and 170V DC in the power supply. When the PS is connected in the unloaded
state, I noticed that resistors R2 and R33 and hot, but this might be normal,,
as the schematics tell us that they will be dissipating about 0.5 W at
idle (170V in 51kohms dissipates 170x170/51,000=0.5W). However, resistors R4,
R5, R7 and R8 are also very hot to the touch. They are, from what the
schematics say, part of the push-pull switching regulator.
Would anyone
be able to help me ?
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