Hi.

I am trying to help a friend troubleshot a System 11B board that does not boot, board is a bit of a hack job, had some acid damage, and a bunch of parts replaced.

Here is the problem, board seems to do its thing with the Leon test rom, Pia's are pulsing, reset circuit is good, irq signal, vma, clock also good, relay clicking, with game roms the trouble starts, diagnostic led flashes around 60 times, then blanking led 10 times, and it starts over, no difference if ram or U26 is installed, same behavior.

All Pia (6821) has been replaced, and are socketed, U25 ram has been replaced, U26/U27 has been replaced, sockets for U25/26/U27 replaced.

Never worked on a System11 board before, so i am a bit stuck, any help will be appreciated.

Edit: Got it to boot one time, but no luck since, fault was the same before parts got replaced.

Okay this is a bit odd, when touching the legs on IC31 with my finger the board will boot as it should.

Another odd thing is pin 23 on the 6821's, it should be around 3 volt, but its around 6.5.-7v.

Board is on the workbench, only 5 volt connected, and the jumper from 5 volt to zr1.

Really no advice ? I did find a few posts with the same symptoms, but never a solution to what's causing it.

It does seem like its not reading U26 and U25, even though Leon's test rom does confirm U25 is good.

Pins on U31 was a bit dark, so it got replaced, traces are good, and yes the reset circuit looks bad, not my work.

20201126_131338 (resized).jpg

Here is a few scope pics from the R/W signal, checked on pin 21 of the PIA's not sure if it has anything to do with the problem, this time it booted by touching the pins on U38.

First two is when the board is working, and last two when it just flashes in diagnostics.

20201126_160611 (resized).jpg

20201126_160715 (resized).jpg

20201126_160807 (resized).jpg

20201126_160746 (resized).jpg

Finally caught it on video, first try freezes the board, second try makes it boot, dont mind the music.

The "magic" happens when touching around pin 21 to pin 25, so R/W, CS0, CS2, CS1 and E

I will take the music over the power supply fan noise any day.

There is not flexing, so it has to be capacitance from my finger that triggers the boot, it can also be done on U31, and U33, on the pins facing the battery location.

U31 and U33 both has something to do with the A13/14 lines, U38 pin 24 is connected to U31 pin 4, CS1 on the 6821, and it looks like pin 24 on the 6821 is connected to A13.

It seems to react mostly on pin 24, 25 on the 6821, CS1, E. CS1 is Chip select ? That would make sense why it is not reading U26 maybe?

Screenshot 2020-11-27 at 08.10.46 (resized).png

I will check it out tonight.

What about the R/W signal ? When tracing it back to U15 pin 34 there is only a high signal, not pulsing, not sure what creates the R/W signal ?

I tried the finger trick, and it works on every single PIA in the game section of the board, i am a bit lost to be honest.

Checked the traces, no faults found, i wonder if it could be an IC not doing its thing ?

Looks like ive been chasing my own tail for a bit, took a better look at the power supply i've been using, turns out the 5 volt is very noisy, after switching to another powersupply the board no longer boots when dragging a finger across the PIA's, i am suspecting the noisy 5 volt had something to do with that.

So back to square one, board flashes 60 times in diagnostic, and 10 and blanking and repeats.

Faulty U12, board is fixed.

And board is dead again, this time only 5v led on..

Not even Leon's test rom will boot atm.

It does.

Everything also seems to be pulsing, but no relay, and no pulse on U10 pin 23.

This did,

but now its stone dead, 5v led on, clock, reset, vma, irq works, leons not booting anymore.

And now back to working, no idea what went wrong, left board yesterday, and today board started running the test rom no problem.

Go figure.

Nope, it was 4 AM when i gave up, and went to bed, came back next day in the afternoon, and it fired right up, not sure what it could have been, a solder whisker maybe from previous work ?

It seems to be working perfectly now, had it turned on, off a bunch of times, moved around, turned upside down, boots every time, board is going back it its owner, and will be installed in the game again, will update when it does.

Thank for all the help to all, great forum.

PCB is back in the game, and working perfectly, so the main problem was a faulty U12.