Severe Graphics Issues - A500 Rev6A, DiagROM 1.2.1

[Castellen]

Q: How can I fix the issue with the stuck mouse buttons during boot-up?

The stuck mouse buttons are a valid clue and something solid to work with.  The report shows that both middle and both right mouse button inputs are active, which is definitely not normal.  The common factor is that these are all analogue to digital inputs on U3 (Paula).  Check the DC voltage on all of these lines to start with, U3 pins 32, 33, 35, 36.  They should normally be around 4 to 5V when in an idle state.  What you find will determine where to look next.

Q: Should I try another terminal, such as RealTerm, as PuTTY obviously isn't working right, or is something still wrong with my cable setup, even though that the loopback tests shows that TX and RX work? It obviously was working as I got to the main menu, so the connections on the DB9 F - DB25 F are right and the RS232-USB is fine.

The output you've copied above shows that the serial data and terminal is working, and the A500 is fundamentally alive.  I've no idea why PuTTY was working and now it isn't.  There is no hardware or software flow control with DiagROM, so configure the terminal to Flow Control = None.  Maybe something related to the USB serial port, I've seen instances where they randomly stop working, or they do something unexpected such as transmit data working but not receive data; then it starts working again when you disconnect/reconnect the USB cable.  I mainly use Term 4.8 on the Amiga, which works reliably.

Q: Maybe there is something wrong with Paula's busses after all?

It's not completely broken because you're getting valid serial data out.  The UART (serial port communication) is a function of U3/Paula, so if the data or address bus had a problem there, the serial output will usually not work, or be corrupted.

You could sanity check that the data looks valid on each of the 16 data lines and 8 address lines at U3 if you wanted to, that's quick and easy to do with an oscilloscope.

You could always try exchanging U3 with a known good 8364.  It's the same part between A500, A1000, A2000, A3000.

[poroxius]

Check the DC voltage on all of these lines to start with, U3 pins 32, 33, 35, 36.  They should normally be around 4 to 5V when in an idle state.  What you find will determine where to look next.

I first measured voltages on mentioned pins without the mouse connected: pin 32 (P0X): 0.05V, pin 33 (P0Y): 0.05V, pin 35 (P1X): 0.06V, pin 36 (P1Y): 0.06V. Next, I measured voltages on mentioned pins with the mouse connected: pin 32 (P0X): 0.05V, pin 33 (P0Y): 0.05V, pin 35 (P1X): 0.06V, pin 36 (P1Y): 0.06V.
These pins are really pulled low, I tried looking for continuity between them and the ground as maybe they are shorted with it, but they are not.

Maybe something related to the USB serial port, I've seen instances where they randomly stop working, or they do something unexpected such as transmit data working but not receive data; then it starts working again when you disconnect/reconnect the USB cable.

I will try doing that in the future if I won't get anything on PuTTY. Maybe reconnecting the USB will help.

You could sanity check that the data looks valid on each of the 16 data lines and 8 address lines at U3 if you wanted to, that's quick and easy to do with an oscilloscope.

You could always try exchanging U3 with a known good 8364.  It's the same part between A500, A1000, A2000, A3000.

I also did the following:

• visually inspected top and bottom side of my A500 near Paula and joystick connectors - nothing obvious appears broken, corroded, lifted
• looked for continuity between U3 pins 32, 33, 35, 36 and E417 (P0X), E416 (P0Y), E427 (P1X), E426 (P1Y) - they are connected

• checked if RP401 is healthy - getting 9.2kΩ between pins 1-2, 1-3, ... 1-9 and 4.6kΩ between 1-10. I'm getting 4.83V on all of them. I'm not sure if pin 10 is GND (it is according to amigapcb.org's Rev8A), but if it is maybe that explains the 4.6kΩ reading. However, I'm getting 4.83V from pin 10 too, which I suppose suggests that it isn't GND because if it was it would read 0V.

I think that the smartest thing I could currently do is to first check the data and address lines with an oscilloscope and totally replace Paula if needed according to the test. As I'm getting really low voltages from P0X, P0Y, P1X and P1Y, I suppose that something definitely is wrong with Paula, as DiagROM suggested.

I don’t know if I should try checking Paula with an oscilloscope due to bad voltage readings I got for P0XY and P1XY, maybe I should just replace it? If I should, is there a chart online that shows good and bad reading examples for specific pins on A500’s ICs?

-poroxius, September 17th, 2025[/list]

[Castellen]

I first measured voltages on mentioned pins without the mouse connected: pin 32 (P0X): 0.05V, pin 33 (P0Y): 0.05V, pin 35 (P1X): 0.06V, pin 36 (P1Y): 0.06V.

That suggests U3/Paula is defective.  The 5V source/pullup for those four lines is inside this IC, not externally.  RP401 is not related to the middle/right mouse buttons.

Quickly check that U3 pin 27 (power supply) has around 4 to 5V DC on it.  Which I expect will be the case, else you wouldn't have seen any serial data out.

As I'm getting really low voltages from P0X, P0Y, P1X and P1Y, I suppose that something definitely is wrong with Paula, as DiagROM suggested.

I don’t know if I should try checking Paula with an oscilloscope due to bad voltage readings I got for P0XY and P1XY, maybe I should just replace it? If I should, is there a chart online that shows good and bad reading examples for specific pins on A500’s ICs?

I doubt if there's any such chart, but when you understand how it works, it's fairly obvious what many of the signals should be.  i.e. The system is in reset state when /RST is low, the 7MHz clock should always have 7MHz on it, the data and address lines should always have data activity, etc.  You can also compare signal conditions against a good working system.  My printed service manuals are covered in 25 years of handwritten notes about various signals and system conditions, but they're unlikely to make much sense to anyone else.

Anyway, you probably don't need to go to this level of detail now.  There's an obvious issue with U3, so replace that with a known good 8364, re-test and see if the system is working correctly now or still has other issues.  If there's further issues, then the symptoms you find will determine where to look next.

[poroxius]

Anyway, you probably don't need to go to this level of detail now.  There's an obvious issue with U3, so replace that with a known good 8364, re-test and see if the system is working correctly now or still has other issues.  If there's further issues, then the symptoms you find will determine where to look next.

Such low voltages are definitely not normal and suggest that something is pretty wrong with Paula, so I was thinking that maybe I don't have to check it with an oscilloscope as the voltages suggest that there is something wrong.

I will order a new 8364 as I unfortunately don't have any spare parts and test with a new Paula.
Thank you so much for your help, I'll post some info after I replace my faulty Paula.

-poroxius, September 17th, 2025