I inquire for some technical assistance from the hardware guru meditators.
Background (if you want to skip to the questions, they are towards the bottom):
Someone brought me an A500 rev.6a to fix. It had the infamous green screen problem. I checked all socketed chips on a functioning A500, and they all passed. I also cleaned the Agnus socket and pins. No improvement. So I assumed it was the soldered ChipRAM. Went ahead and did the piggy-back trick, but no improvement. Figured the chip on top didn't make good contact with all the pins, so I decided to start desoldering. After removing each chip, I tested it on a functioning A500. Turned out a couple were dead. Went ahead and cleaned up the holes on the motherboard, traced with a DMM/VMM the data/address bus wires to make sure I didn't clip any by accident while unsoldering, and after all was good went ahead and added 4 sockets. Plugged in working chips and fired it up. Bingo! Worked the first time. Unfortunately I didn't test it for too long, because I wanted to go back and add 4 more sockets for the 1MB upgrade. Went ahead and did that. Plugged in chips, fired it up, went into WB, only 512KB showing. Doh! Forgot the jumper hack. Tried plugging in a keyboard to use WB, but got a reset (typical) and then a guru error. No problem, turn off, wait, turn on... but then problems started appearing: graphics going all bonkers (corruption) when I moved or slight bent the motherboard, or touched parts of it (no specific part) etc. Eventually green screen again!
Now, I'm not sure if the keyboard business is related or not, although the problems started appearing at about the same time I plugged it in.
Here's where I stand and need to ask about R201 and R202:
If I leave the motherboard off for 10-15 minutes and turn it on, it gets to the white Kickstart screen and within 20-30 seconds I get graphical corruption and screen whites-out very quickly (2-4 seconds). The machine reboots within a few more seconds, and not too long after I get the dreaded green screen again. Turn off, wait, turn on again, and still green screen (unless I leave it off for a while).
I measured the voltage going to R201 & R202, and I get 4.94V (PSU is a BigFoot, and known good PSU). BUT, when I measure the voltage drop across the resistors, on the functioning A500 I get around 4.7V (~200mV drop), while on the crippled one I get 0.3V or 1.2-2.0V (depends, at different times I got different values, and different values on each of the two resistors). Also interesting is that if I unsolder the resistors, they both measure 4.7KOhm, but if I plug them in, and measure them (machine turned off), I get some ridiculous value starting at 28-30Ohm and climbing (as if there's a capacitor in-line). This happens with or without the ChipRAM in the sockets. On the other hand on the functioning A500 they give a steady 4.7KOhm measurement while in-circuit.
I also replaced both 4.7KOhm resistors, and I get the exact same results, leading me to believe that R201 & R202 are fine, and the problem is "downstream".
So we come to my questions:
1)
What part of the circuit "downstream" could be causing these resistors to be acting up?
2)
Any specific capacitors I should be looking at?
3)
Would any of the chips (U34 & U35, U10, U11, U12, U13) on the datapath cause such a strange behaviour if they're shot?
4)
Would anything near the keyboard connector cause such a weird problem? What can happen if one plugs/unplugs the keyboard while the machine is running? Is it like external keyboards, or is it possible to fry something like plugging and unplugging expansion cards?
Remember that the machine DOES get to the Kickstart screen, but only after it's been off for a while. So most likely the traces are fine, nothing is broken, or so I believe (I was pretty careful and did a good job, compared to other times, because I figured a new trick for myself: use lots of flux while using the desoldering braid, it works MUCH better than without!)
Thanks in advance!
PS. Anyone wanting to suggest to replace the motherboard, please don't bother. I know the easy solution or the cop-out solution. I'm doing this because I'm learning to fix these problems for the time when there are no more motherboards.
