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Author Topic: High brightness causes video shutdown  (Read 14234 times)

JaDaveIII

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High brightness causes video shutdown
« on: February 16, 2025, 06:19:24 pm »
The last time my Lisa 2/10 was powered up was over 10 years ago so now is the time to get it going. I replaced the refa caps (yeah, the smoke was coming out of the 1.8A P/S) and also the 2 x 470 uf 250v caps as one of then didn't have a totally flat top. All other caps looked good so I let them be. I am using a BMOW floppy emulator and a ESProfile HD emulator.

Everything was proceeding as planned. I loaded up Lisa Office System 3 and was in the process of trying to figure out how to actually transfer / install Write and Calc when all of a sudden the screen just faded to black, in about 1/2 second. No smoke or noise. I pushed the Power button and an orderly shutdown happened.

After a while, I powered up. All is good. But after maybe 20 minutes it happened again. After Shutdown, I wait and power up.  I started playing with the Brightness control on the back. Ya! As soon as I turned it up the video when off.

My best guess is the U1 -  78M24CT, 28V regulator. I don't have the widget hooked up so I don't have its current draw on the +12v line so I'm putting that test aside for now. Any other guess?
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compu_85

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Re: High brightness causes video shutdown
« Reply #1 on: February 16, 2025, 09:00:36 pm »
I had this problem with mine and had to replace the regulator.

I don't know if some component is drawing too much power, the replacement regulator gets quite hot.

-J
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sigma7

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Re: High brightness causes video shutdown
« Reply #2 on: February 17, 2025, 03:15:03 pm »
Quote from: JaDaveIII on February 16, 2025, 06:19:24 pm
U1 -  78M24CT, 28V regulator

In later revs, the higher current 7824 was used instead of the 78M24. (It is actually a 24V regulator, the surrounding circuit is designed with an output offset to generate the 28V rail)

I haven't encountered this specific symptom myself, but your guess seems reasonable to me and compu_85's experience is good confirmation.

Good luck!
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JaDaveIII

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Re: High brightness causes video shutdown
« Reply #3 on: February 19, 2025, 01:56:53 pm »
Quote from: compu_85 on February 16, 2025, 09:00:36 pm
I had this problem with mine and had to replace the regulator.

I don't know if some component is drawing too much power, the replacement regulator gets quite hot.
When I get time I'll change it out to the non-M version of the regulator. In the mean while if I keep the brightness low it seems to work fine.
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Lisa2

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Re: High brightness causes video shutdown
« Reply #4 on: February 19, 2025, 02:42:08 pm »
While you are replacing the regulator, check on R1 and R3.   When I had a bad 7824, it let out the magic smoke from R1.
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TorZidan

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Re: High brightness causes video shutdown
« Reply #5 on: February 24, 2025, 06:35:03 pm »
I would start by applying DeoxIT on the potentiometers on the video board and in the power supply, and turning each a few times, to clear any dust and gunk. There are eternal discussions whether it's better to use DeoxIT D5 or F5 on potentiometers. I use F5.
 
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