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Author Topic: Broken A500 Yellow screen - diagrom 1.3  (Read 6547 times)

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Offline Castellen

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #14 from previous page: January 29, 2025, 07:38:45 PM »
The short answer is no.  Datasheets for logic such as the 74F244 buffer will explain electrical characteristics of the IC such as voltage limits, switching thresholds, drive levels, slew rate, propagation times, etc.  In the case of the buffer, the output function is the same as the input function, so that's easy - the output will be the same as the input.  The buffer input signal is generated directly by the DRAM controller (part of U2/Agnus).  Meaning you'd need to have a good understanding of the details around DRAM access.  I've been an electronics design engineer for 30 years and often find that confusing, so not a good topic for someone new.

Or to explain another way: the easiest way to know what to look for is to measure on a working A500 for comparison, not by reverse engineering the DRAM control logic.  Remove the ROMs from both working and defective boards so they're in the same state.

But you're on the right track.  As I mentioned above: "See what A0 looks like at the input of the buffer (U34 pin 11).  It should look the same on the output (U34 pin 9).  If the address data looks OK at the buffer input, but it's bad on the output, that's the problem."
 

Offline PetterMTopic starter

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #15 on: February 05, 2025, 04:09:15 PM »
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
 

Offline Castellen

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #16 on: February 05, 2025, 10:51:46 PM »
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?

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


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.

U34 pin 10 connects to ground.  As with most TTL logic, the 'bottom left' pin is ground (0V), and the 'top right' pin is VCC (+5V).
 

Offline PetterMTopic starter

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #17 on: February 07, 2025, 09:50:11 AM »
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.
 

Offline PetterMTopic starter

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #18 on: February 07, 2025, 10:11:16 AM »
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.
 

Offline PetterMTopic starter

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #19 on: February 07, 2025, 01:27:28 PM »
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. :)