Severe Graphics Issues - A500 Rev6A, DiagROM 1.2.1

[poroxius]

Hi!

A couple of months passed since I last wrote under this post.

In the last reply, I said that I was going to check the DiagROM output with a new Paula. I connected the null modem cable to my A500's serial port and opened PuTTY. I got the DiagROM booting text and could easily use the menu.

This is what DiagROM gave me in System Info:

                          Information of this machine:

               Dump of all readable Custom Chipset HW Registers:
BLTDDAT ($dff000): $8098   DMACONR  ($dff002): $6380   VPOSR   ($dff004): $A000
VHPOSR  ($dff006): $9A61   DSKDATR  ($dff008): $FFFF   JOY0DAT ($dff00a): $0000
POT0DAT ($dff012): $0505   POT1DAT  ($dff014): $0000   POTINP  ($dff016): $0606
SERDATR ($dff018): $4B2D   DSKBYTR  ($dff01a): $9000   INTENAR ($dff01c): $4B2D
INTREQR ($dff01e): $1000   DENISEID ($dff07c): $0000   HHPOSR  ($dff1dc): $0090

Workmem: $0006C478 - $0007FFFD Size: 78kB   ROM size: 156kB  Order: End
Chipmem workarea: $00000400 - $0007FFFF Fastmem workarea: $00000000 - $00000000

Doing ROM Checksumtest: (64K blocks, Green OK, Red Failed)
$CED3BA58 $98C6080B $3980DA0F $003EC000 $003F0000 $003F4000 $003F8000 $00DFBCEA

CPU: 68000  FPU: NONE  MMU: NOT CHECKED
Is $1114 readable at addr $0 (ROM still at $0): NO
Is $1114 readable at addr $f80000 (Real ROM addr): YES
Is $1111 readable at addr $f00000 (expansion ROM addr): NO
Stuck buttons & keys etc at boot: BADPAULA

Serial: 9600  BPS - CPU: 68000  - Chip: 512kB  - kBFast: 0      Base: $0006C47A

I'm still getting the P1RMB/LMB stuck at boot during the DiagROM's booting process. Everything else seems to be normal as DiagROM's menu functions normally on the serial output.

However, when I try to connect my A500 to my 1084S-P1 or SCART TV, I'm not getting any video output.
Maybe the contacts on the A500's video connector should be cleaned as I didn't do it for a couple of months now, but it's really weird as I'm not getting anything on both the 1084S and SCART output of my Sony TV.

Upon researching, black screen with DiagROM inserted suggests a faulty CPU, but as I'm getting normal and healthy serial output and strong 10 short + 1 long flash from my keyboard, I think that something might be wrong with my A500's video connector, which I will firstly check.



After I read some of my older posts on r/amiga about this issue, I remembered that I used to fix this yellow screen and boot normally by pulling out Fat Agnus with a PLCC extractor, cleaning her with IPA and reinserting her into her socket. When I reinserted her, along with pressing on other ICs, the Amiga would start working again. But, after a leg broke on that original Agnus, I replaced her with a new one, tried the same trick again, but this time it didn't work. I forgot to mention this to you in earlier replies as I totally forgot about that.

Q: Maybe it's the faulty Agnus socket that's causing this yellow screen? The pins on the socket look ok, but maybe because one of the legs broke off of Agnus, I should replace the original socket with a new one as it may also got damaged when I was pulling Agnus out?

Also, I think that it's the right time to get an oscilloscope involved and look into the data/address busses.

I did some research about what kind of an oscilloscope I should buy, but I just got confused really quickly as I never used an oscilloscope before or did any research about it, so all of the specifications were kind of advanced for me.

As the 68000 operates on 7.09MHz (for my PAL A500), I found out that I should minimally use 20MHz, but ideally should go for >50MHz just to be safe.
I also found out that I should look for an oscilloscope that has a sampling rate of >200MSa/s, DC coupling, 1MΩ input impedance, X10 + short ground probe etc.
I was thinking about that maybe I should go for an oscilloscope that YouTubers like Adrian's Digital Basement use, but they cost a lot.

Q: If you maybe know, what are the important specs I should look into when buying an oscilloscope that I will use for my A500's diagnosing? Are there any minimal values of the specs I should look out for? Would you recommend a specific oscilloscope that maybe you are using?

The oscilloscope is pretty new to me, as I have never used it before, so I'm not sure if I'm doing my research right. I wouldn't like to spend a large amount of money on a oscilloscope that would be overkill and maybe could be replaced by a cheaper one that would work the same. Or maybe I shouldn't cheap out on this.

-poroxius, February 1, 2026

[poroxius]

This would suggest:
1. The original 8364 is still defective, sounds as though that part is still fitted?
2. The replacement 8364 is also defective, as ~3.6V measured at the supply pin suggests it's drawing excessive current, and there's no serial data output
3. The CPU, ROM, most of the data/address bus, etc, is likely OK as the serial data out looks sane

1. The original 8364 isn't in the socket anymore as it gave me low voltage on these pins pin 32 (P0X): 0.05V, pin 33 (P0Y): 0.05V, pin 35 (P1X): 0.06V, pin 36 (P1Y): 0.06V. The newly bought Paula is currently in the socket.
2. I cleaned Paula's area with IPA and am getting the 4.67V on the new Paula. Will check voltage on the new Paula's P0-1X and P0-1Y though.
3. You are right, apart from the BADPAULA and weird graphics issue that is probably related to the bus between Denise and Paula, everything else seems to be working fine as I'm getting valid serial output.

I think that the best approach from this point on is that I have to dig in the world of the oscilloscope and learn how to use it properly or get a working A500 Rev6A board and test the ICs from my original board with it. I will definitely look into the oscilloscope and learn as much as I can about it, as it is a powerful tool that will definitely be useful if I encounter more issues further on.

These are definitely the best two approaches I could do now, so I will reach out when I successfully boot into the 1.3's Floppy Screen once again.

Thank you for your help Castellen,

-poroxius, February 2, 2026