Broken A500 Yellow screen - diagrom 1.3

[PetterM]

Hi,

I'm trying to fix a really nice looking Rev.6a motherboard (512k), no corrosion damage. First power on, only give me yellow screen.
Taken out all chips, and give them a round with ipa and deoxide, all looks like new. Still yellow screen.
Measured all voltage, cpu got good clock, reset, and halt stays high.
Don't know the history of this board, but caps will be replaced later, un-soldered a few, and they measured good.

Inserted Diagrom 1.3, but it only give me a strange color output (attaching a short video) (don't have a serial cable yet)
I hope some of you know what this it, I cant find any documentation explaining this.

So now i'm stuck.     Please help me bring back this lovely board to life.

My guessing is that the first bank of chipram is bad, so kickstart/diagrom can't execute code, but that just a hunch.
(Chips and sockets are in the mail)

(Sorry for any bad english, norwegian is my preferred language) 

Petter

[PetterM]

Thanks, that's what I'm hoping for, but it should give me a Green screen.
Yellow is more like "I really don't know whats wrong" message, so it can actually be anything.

I was hoping that anyone knows what the flashing screen output from diagrom 1.3 means. But i cant find a documentations, other than:
"No Chipmem, Fastmem detected:"
Flashing bars on screen when chars is printed

From https://github.com/ChuckyGang/DiagROM/blob/master/DiagRom%20Codes.txt

Could it be that easy?

But then, it could be the Fat Agnus, or some of the memory logic chips, i don't know.
I have to wait for the mailman, bringing me memory chips and sockets.

[PetterM]

Hi, sorry for the delay. I have made "some" progress. After i got the serial cable, it was clear the no chip mem was detected ok, and that values written to one address was found on another address. So playing around with the scope, i could see that address line 4 (A4) on the chip ram was pulled low, and if i measured resistance between ground and A4, it was 68ohm (same as resistor pack 201), so its almost shorted to ground.

After snipping out U34 (74F244), i no longer measure 68ohm on address line 4.
My hope is that U34, somehow got a internal short, that pulled down that address line.
So now i waiting for mailman to come with some 74F244.

Does this sound even remotely possible, or are my hopes to high? 

[PetterM]

Hi, new update.
Found a broken trace from resistor pack 202 to u34, that give low activity on A0 (adressline).
Fixed that, and now available chipram is 128kb. Huge improvement from 0.

So now diagrom shows this: (se attachment)
Can anyone "translate" this error to me?

[PetterM]

Ok, thanks. This info was very accurate, and to the point. I will try to probe and look for this, and report my findings. I hope I'm find something wrong.
Thanks again.

[PetterM]

Hi, just did a quick test, was not sure my scope was up for the task, but this is what we are looking for, right?
Will do some more testing later this week.

First attachment was from C16, A4, not A0 as named.
When i probed A0, its only 2 good pulses, and 2 short. So this can be the whole problem?

[PetterM]

Hi, is there å place online i can see what pulses is expected from a chip? Can't remember seeing that in any documentation.
I'm quite new with the oscilloscope, so my biggest problem is to know what to look for, what is ok and not.
I will do some more probing during the weekend.

[PetterM]

Hi, sorry for the long delay, but pictures will come. Just so much other tings to do.

I have 2 issues i want to "check out" before i proceed, and that is, should there a connection from U34-Pin12 to RP202-Pin2?
As picture from Amiga PCB Explorer marks with a dotted line her.  Attachment u34.png

And second, should the via at U34-Pin 10 be soldered to anything. Previous this was bridged to Pin10, ground. Attachment Via.jpg

[PetterM]

Thanks, did you notice that "via" connection at U34. It was a solder bridge to Pin 10 her, that i cleared up. You think it should be like it is on picture?
They are really close together, so no wonder the previous owner has been a little unlucky with the soldering here.

[PetterM]

Just had a quick look at a rev 6 board here, and it correctly matches the schematic, not your PCB reference.
http://amiga.serveftp.net/Schematics/A500_schematics/A500_R6_schematic.pdf

i.e. U34 pin 12 connects to RP202 pin 6.  You should measure approx. 68 Ohm from U34 pin 12 to pin 7 of any of the DRAMs.  If you measure between all combinations of the address lines (at the DRAMs or on the U34 output side), they should all be a very high DC resistance, i.e. over 100k Ohms.  If you see a short circuit or fairly low DC resistance between any two address lines, that's your problem.

Hi, i'm just had a "light bulb" moment her, studying Schematic, I'm not used to read this type, most experience with old audio amps.
I haven't seen that little number beside the actual pin number, now it's much easier to follow the trace. Felling so stupid here now.
Thanks again. Now its time to hunt down broken traces again. And this this time with schematic.

[PetterM]

Hi, that did it. I found a missing connection between U35-Pin 9 and RP202-Pin 2. That's adressline 8.
I have been so close all the time, but now its all good. 512kb chipram, and diagrom is all green, all the way.

Thanks to you all.

And now, let the mods begin.