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Compaq Portable 486c Screen Brightness Issue - Part Deux

Shadow Lord

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Quick recap: I picked up a nice looking but problematic Compaq Portable 486c - most of the issues have been resolved. My last remaining issue is that I can not adjust brightness on the screen at all. It is on full max blast all the time. Initially I had thought the pot controlling the brightness had gone bad. However, after taking the pot out of circuit and testing it it turned out to be fine.

I worked (fumbled around really) on the circuit some more and looking at the four transistors on it one of them appears shorted open.

You can see pictures of the board here (with the offending transistor already removed):

BrightnessBoard.jpg


and the transistor here:

Transistor1.jpg
.

Checking the two outer legs of the transistor shows a short. I took the transistor out of the circuit and it continues to show a short along the two outer legs. As I understand it that should not happen. Would that be a correct assessment? If it is shorted open would it explain the light being on at full force? I found a replacement for the transistor but it will take a month to get here from China. Anything else that a trained eye sees that I should check on? The cap looks OK visually but I am going to check it with an ESR meter to be sure. TIA.
 
that's not the same part. note that the center lead is cut

i'm not sure this is the data sheet, but it has the two outer leads shorted
https://www.datasheetarchive.com/pd...3663ec865f7df046a&type=P&term=motorola%20b330

you need to trace those three leads back and see if it is hooked up as two rectifier diodes

I had chucked that up to different packaging of the same part I.e. a TO-252 vs. To-251

COOLMOS-7N65-12071932143995.png

I think that is the correct datasheet. Thank you for that. Based on the sheet it is working as designed (two outer legs shorted). So back to more fumbling... And learning ;)
 
Hi, I recently found a 486c which has the exact same problem. Brightness is maxed out with no way of adjusting it through the pots.
Did you ever find out how to fix this?
 
  • Is J501 the connector for the backlight? If so, you could trace backwards - I see some caps (10uF) that you could use an ESR meter to test (or even replace them).
  • Have you tried running some continuity tests to ensure the circuit traces are still intact?
  • Is there any voltage/current making its way to the pot? Or is it shorted (eg max brightness) in the circuit, negating the pot completely?
  • You can also use an ESR/multi tester like mine below that identifies and tests those 3 legged things - as others have already said, some are trannies and some are diodes.

1643023068221.png
 
I will get one of those testers (will come in handy anyway) and give it a try - thank you! I will report back here...
 
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