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Author Topic: Broken A500 Yellow screen - diagrom 1.3  (Read 6822 times)

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Offline Castellen

Re: Broken A500 Yellow screen - diagrom 1.3
« on: January 28, 2025, 06:44:37 PM »
Can anyone "translate" this error to me?

It's showing that the data bus is likely working OK, but test data written to the DRAMs was found at unexpected addresses.  That means that you're either got an address bus fault, or one or more DRAMs is bad.  Given the previous issues you've seen with one of the address bus lines, I'd suspect the former.

To check the address bus, use the scope to look at the column/row data which should appear all of the time on all 9 address lines, A0 - A8, on each DRAM.  See page 4 of the schematic:
http://amiga.serveftp.net/Schematics/A500_schematics/A500_R6_schematic.pdf
Expect to see continuous groups of 4 x low pulses on each of the address lines.  If they're missing on one of the address lines, that's the problem.  Work backwards from buffer U34 towards the address bus generator (U2) the until you find where the fault is.

Also check that you're seeing column and row address strobes at each DRAM pins 17 & 4.

If that all looks OK, then you've probably got one or more bad DRAMs.  It's fairly complicated to work out which one it is in this case, so the simpler option would be to replace all 4 of them.

 

Offline Castellen

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #1 on: January 29, 2025, 08:00:15 AM »
The green waveform is exactly what I'd expect to see.  The yellow waveform looks unusual, the half levels suggests there might be a short between A0 and another address line.  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.  In which case you're probably going to be looking for a short between A0 and something else.

I just had a quick look at an A500 here, some of the other DRAM address lines have two or three strobes.  A0 should have 4 strobes, A1 has 2 strobes.

Also, it would help to perform this measurement with no ROM installed.  If the CPU is stuck there doing nothing, all you should expect to see is the constant DRAM refresh cycles.  i.e. Constant activity on all of the address lines + column/row address strobe.
 

Offline Castellen

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #2 on: 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 Castellen

Re: Broken A500 Yellow screen - diagrom 1.3
« Reply #3 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).