Only half of screen showing on PowerBook 100 (red solid color on rest)

[vetz]

I bought a PowerBook 100 and it didn't even power on the first time I tested it. I've done the following:

- Recapped caps in LCD panel, replaced with tantalums
- Recapped all SMD capacitors, replaced with tantalums and polymers
- Opened up the Conner drive and unstuck the head.

Generally there were very little capacitor leakage across the machine. I've rechecked my re-cap work multiple times.

System now chimes and boots up into MacOS as normal. Keyboard + mouse works fine. Brightness + contrast control works.

But, only half the screen is showing. The rest has a solid red color. I suspect a signal issue to row drivers 3 and 4. The only other topic I've found that closely resembles is this one: https://68kmla.org/bb/index.php?threads/powerbook-100-half-screen-dead.46200/

If I unplug the brightness+contrast control cable, then the screen is dark across the whole panel, no more red color.

What more should I check. I've done continuitity check on the flex cable + wiring harness, those were fine.

Could it be related to the remaining through-hole capacitors (which normally holds up a bit better than the SMD ones) and/or inductors?

 

[GRudolf94]

Ha, I had a recollection of already having explained how that works - and it's the thread you linked.
Most likely cause is a missing bias voltage to the lower row drivers. Check continuity, and compare voltages on each pin at the top row drivers with the bottom ones.

Which voltage rating did you use for the panel caps? Tantalum caps might not have been the best choice there.

 

[vetz]

Forgot to mention, but I already checked continuity as it was a tip in that other thread and that was fine.

I used 35v 3.3uf tantalums caps. I've used the same ones in a PowerBook 150 with no issues. Mouser link: https://no.mouser.com/ProductDetail/581-TAJB335K035SNJ

Will check voltage on the lower and compare!

 

[luRaichu]

I had basically the same problem with the panel in my PowerBook 160. Hope you figure it out

 

[vetz]

I checked the voltages and they were the same to both set of drivers at just above 5 volts.

Anyone have some more ideas on what to check?

 

[3lectr1cPPC]

Which voltage rating did you use for the panel caps? Tantalum caps might not have been the best choice there.

I don't know whether or not the voltage load on those caps is enough that same-value tantalum replacement is "not advisable", but as far as I know, that should at worst cause a shortened lifespan for the caps., rather than causing an immediate failure. FWIW, I used tantalums on my PB100 to no ill effects (so far at least).

 

[GRudolf94]

I checked the voltages and they were the same to both set of drivers at just above 5 volts.

Anyone have some more ideas on what to check?

The only STN machine I have with me atm is a PB520, and that's a slightly different panel IIRC. I can have a peek later and see if I come up with anything else.

Check the lower data bits just in case, but it'd be weird if they happened to be undriven.

I also think there should be more voltages (or am I misremembering and that goes to the column drivers only?)

 

[Byrd]

Can you elicit any change in the LCD with pressure applied to the ribbons, controller etc?

 

[vetz]

The only STN machine I have with me atm is a PB520, and that's a slightly different panel IIRC. I can have a peek later and see if I come up with anything else.

Check the lower data bits just in case, but it'd be weird if they happened to be undriven.

I also think there should be more voltages (or am I misremembering and that goes to the column drivers only?)

There is a -22v for the backlight (measured to -21v)

I checked the databits from the J12 connector and I couldn't see anything special difference between upper and lower. I do not have a oscilloscope available right now so I had to use the Hz setting on my multimeter for checking.

Can you elicit any change in the LCD with pressure applied to the ribbons, controller etc?

No, its static. The only change I can trigger is if I unplug the small ribbon cable to the brightness/contrast board. Then the screen goes fully dark blue.