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Compa Portable 486c - can not adjust screen brightness

S

Shadow Lord

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So the brightness on the screen seems to be maxed out. I took the machine apart and it appears to be a bad pot. At least I think it is. Measuring the resistance with the dial set to either max produces no change (i.e. at both positions the resistance read 4.8k ohms). Unfortunately I can't seem to find a replacement for it.

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I guess Clarostat is now Honeywell? But I can't find this model or equivalent anywhere. If anyone knows where I can order a replacement or if you have one in your junk box that would be great. TIA!
 
Does the screen look washed-out? It’s an extremely common issue on the 486c and some other old active matrix color displays. It is caused by failing SMD capacitors on the LCD display itself. Replacing the capacitors usually fixes the issue.
 
Not sure if I would call it washed-out. The backlight seems to be at full intensity. Even with the screen blanked the screen glows grayish white and I can not dim it to black. Plus turning the brightness knob doesn't change anything.
 
Last edited:
Shadow Lord said:
Anybody have a line on this pot?
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I'm not sure how specific you want, but you might be able to get away with a regular 10K Ohm pot with Linear sweep with the shaft reshaped to fit the Compaq Knob. I used to have one of these so I feel your pain on finding parts for them. I spent 3.5 years looking for a screen for mine before It went to someone on this forum.
 
Mad-Mike said:
I'm not sure how specific you want, but you might be able to get away with a regular 10K Ohm pot with Linear sweep with the shaft reshaped to fit the Compaq Knob. I used to have one of these so I feel your pain on finding parts for them. I spent 3.5 years looking for a screen for mine before It went to someone on this forum.
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Tell me about it. I am also missing the converter that allows the weird power connector to be used on a regular molex connector. If I can't find the right pot I may try repairing it. It looks like the pot is capped from the top and it maybe possible to pull it apart that way. Or I guess I could leave well enough alone but I still have hope someone somewhere has this pot....
 
Brightness failures are really common on these machines.

Next time I can get into my lab I'll see if I still have any front panel parts
 
Al Kossow said:
Brightness failures are really common on these machines.

Next time I can get into my lab I'll see if I still have any front panel parts
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Thank Al. I appreciate it. The pot is located on the PCB that also has the speaker on it: Compaq PN: 128900-001.
 
So I have managed to get most of the upgrades/fixes done on this system:

1. RAM upgraded to 32MB (system won't take any more then this - installing 4 x 16MB RAM sticks still gives you 32MB)
2. Installed a DOM to replace the missing HDD
3. Upgraded the CPU - my machine already had an aftermarket 80486DX2-66 in it. I initially tried to upgrade to a 133Mhz Power Stacker chip. However, the system would not recognize the new CPU no matter what DIP settings I used. I know someone on the board got an Evergreen upgrade running on theirs so a non0intel upgrade is possible. I took the easy route and installed an Intel Dx4 OverDrive Processor. It identifies itself to the BIOS as a 80486-33 so no issues there. However, the speedup is there (even the ram count is significantly faster) and the Compaq Dx Suite correctly IDs the chip as a DX4.

The only remaining issue is the burnt pot for the screen brightness. If anyone has parts from a nonworking unit or just that particular model pot (Clarostat CM46895-3) please drop me a line!

TIA!
 
So further testing has determined the pot to be good. I desoldered the pot from the board and on its own it provided a smooth resistance from 0 to 10K ohm. That is the good news. The bad news is that I have no idea what could be causing the problem now. Any one with ideas?
 
Al Kossow said:
The circuit for modulating the power to the CCFL has failed to maxium.

Like I said, this is a really common problem on these machines.
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Al,

Were you able to find any parts for the front panel? Failing that any particular failure points in the circuit I should look at? Remeber you got to dumb it down for us non EE pleebs. :D TIA!
 
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