In only one circumstance to date have I had to replace a chip, and I've debugged a lot of model Ts! In many cases, I have found trouble with PCB tracks. I can't say for certain about your signal issue. BTW nice scope ! One of my favorites.
What I have done is gone trace by trace and really looked for 2 things 1) clean edges 2) correct and repeatable levels the tricky thing is what happens when the signal goes tri-state. Also, for a bus of identical signal types, I look for "one of these things is not like the other". look at each of the data signals on AD0-7 and make sure the "look the same". Eventually I find the problematic signal. Also, I would confirm that your socketed chips are pulled and reseated. To me, the traces on the peripheral side of 81C55 look like normal signals and some tristate superimposed. How about playing with the trigger to see if you can isolate the noisy part of the signal. I know that is not a storage scope but it might reveal some info. Also, you could trigger the scope off of the chip select line to the 81C55 to get better insight, as it is called at 256 hz. Another word of advice - go to the "end of the line" on the memory and data lines, and confirm you have signal all the way to the end. Often a mid trace break means memory is good to some ram and not to others. ..Steve On Fri, Mar 27, 2015 at 8:14 PM, Jovan Trujillo <[email protected]> wrote: > Hi Guys, > > I'm trying to fix a dead Model 100 which gives a blank screen when > powered on. The machine is not responsive to keyboard input as well. I > can't get it to beep. While poking around with the oscilloscope it > seems the power is ok. The clock input to the CPU is also probably > good enough. The only chip that gives me a messy signal is the 81C55, > which routes all of the I/O from the keyboard and LCD screen. I've > tried reflowing all the connections to the chip. There was a moment > when the thing beeped as I pressed a bunch of keys and the screen was > giving me garbled output, but upon hitting the reset button everything > died again. > > I'm thinking I need to replace the 81C55 but wanted a second opinion. > What do you guys suggest is a good replacement if I do need to buy > one? > > Here is a lonk to my oscilloscope output: > http://jovan.freeshell.org/weblog/ > > Thanks, > Jovan >
