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

[poroxius]

Hi, here is an update of the situation:

I got a new MOS 8364R7 that was made in the 22nd week of 87' off of eBay.
I pulled out my old Paula, inserted the new one and booted my A500.

I had my motherboard connected to PuTTY with my null modem cable setup and realized that I'm not getting DiagROM's green flashes from keyboard's Power light. I also was not getting anything from PuTTY as I did while I was testing with my old Paula.

I took my multimeter and tested the voltage between Paula pin 27 (VCC) and GND. I'm getting only 3.62V. Measuring the resistance between the VCC pin and GND, I'm getting 101Ω.
I'm getting the same 3.6V on all VCC pins, as well as a 101Ω resistance.

I have a Meanwell RT50B in my PSU box which gives me a solid 5V output on my A500 power connector, but when I was measuring voltages across my motherboard earlier, with my old Paula connected, I was getting max 4.82V. Something on the motherboard was causing a 0.18V drop.
I was addressing this issues on r/amiga earlier this year and they've suggested that I should search for the root cause of that drop. Unfortunately, I forgot to mention it to you as it is a big clue for my issues.

My motherboard was getting 4.8V until I inserted my new Paula.
After I pulled out my old Paula, I sprayed her socket and it's surrounding area with WD-40 Specialist Fast Drying Contact Cleaner, which is meant for electric circuits. However, I didn't do it properly.
WD suggests that the user should spray 15-20cm away from the circuit and that the user should tilt the circuit so any extra residue fully dries before powering it on again. I was spraying really close to the motherboard and was using a wider mist mode that was maybe to harsh for it. I didn't wait for too long before inserting my new Paula, so maybe it didn't dry thoroughly.

Basically, I think that the spray could have caused a connection which made the voltage drop even worse. As I was using the WD Contact Cleaner earlier, maybe it's residue are causing shorts which result in a voltage drop across the whole mobo.

I'm now going to clean my whole motherboard with 99.9% isopropyl alcohol in order to remove any contact cleaner residue that could cause shorts.

If I'm still going to have low voltage, I will:

• remove all socketed ICs that Amiga doesn't need for booting and will measure the voltage without them connected
• insert removed ICs one by one while measuring the resistance between VCC and GND - if the resistance drops, I found the main cause of the voltage drop

• if that won't solve the problem, then I'll check for faulty sockets, pads, traces, capacitors and resistors

Q: Should I assume the harsh WD Contact Cleaner as the main cause of prior voltage drops as my A500 was working totally normal after I bought it, but problems started to appear after I used the Contact Cleaner on my motherboard? If so, I will give my motherboard a clean with 99.9% IPA.
If not, I will do the steps I mentioned above.
Also, is it safe to boot the Amiga now that I have such a low voltage? I’m afraid that maybe the ICs could draw more current in order to compensate the low voltage, making the booting unsafe for the mobo.

I don't think that the new Paula is bad as I managed to boot to the DiagROM's System Info and it's still showing BADPAULA, which suggest that Paula wasn't working due to low voltage it was getting, not a fault within the chip itself.
The voltage drop is definitely a big factor in the issues I was describing to you earlier, A500's ICs can't operate normally with such low voltages (4.8V and now 3.6V). Something is definitely wrong with my motherboard as my PSU gives me 5.01V.

-poroxius, October 5th, 2025

[Castellen]

Q: Should I assume the harsh WD Contact Cleaner as the main cause of prior voltage drops as my A500 was working totally normal after I bought it, but problems started to appear after I used the Contact Cleaner on my motherboard? If so, I will give my motherboard a clean with 99.9% IPA.

I'm not familiar with WD Contact Cleaner, so I don't know what's in it or its compatibility.  Some of these products are intended for electromechanical contact cleaning (switches, relays, etc) and can contain a lubricating oil.  The oil can sometimes create problems with very low current electronics contacts that aren't intended to move, such as IC sockets.  Some cleaners are also intended as degreasers (removing oil, carbon, etc) and can attack some types of plastics, which can create other problems.

I'd only recommend isopropyl alcohol for electronics cleaning.

Also, is it safe to boot the Amiga now that I have such a low voltage? I’m afraid that maybe the ICs could draw more current in order to compensate the low voltage, making the booting unsafe for the mobo.

It doesn't work like that, but in summary you won't do any damage when trying to operate at a lower voltage.  Below a certain operating voltage, things simply won't work.

The power supply to U3/Paula is 5V via a 1 Ohm series resistor (R309) for AC noise decoupling.  If U3 is drawing excess current, then you'll have excessive voltage across R309.  I just measured a test board here and the normal voltage across R309 is around 170mV.  So U3 supply current is  0.17V/1Ohm = 170mA and power dissipation as heat will be (5V-0.17V) x 170mA = 0.82W meaning it will feel warm to touch, but not hot.



After you've sorted out whatever contamination problems you might have with the IC socket contacts, compare the supply current of the old and new 8364 (measure voltage across R309) and see if one is significantly different or not, which will give you a clue if one of them is obviously defective.

[poroxius]

Hi, sorry for the delay,

A couple of days passed since I mentioned the problem which occurred after I cleaned the area around Paula with the WD contact cleaner. After it happened, I quickly went to clean the area with 99.9% IPA and left it to dry for a few days.

When I turn on my Amiga now, I'm once again measuring the 4.66-4.67V on my Paula's VCC pin and 4.82V on other VCC vias on my motherboard (voltages on my mobo were like that before I encountered the error I described in my last reply).

It doesn't work like that, but in summary you won't do any damage when trying to operate at a lower voltage.  Below a certain operating voltage, things simply won't work.

The power supply to U3/Paula is 5V via a 1 Ohm series resistor (R309) for AC noise decoupling.  If U3 is drawing excess current, then you'll have excessive voltage across R309.  I just measured a test board here and the normal voltage across R309 is around 170mV.  So U3 supply current is  0.17V/1Ohm = 170mA and power dissipation as heat will be (5V-0.17V) x 170mA = 0.82W meaning it will feel warm to touch, but not hot.

When I'm measuring the voltage across R309 with my new Paula connected to the socket, I'm getting 150mV and 4.67V on her VCC pin (4.82V across the mobo).
When I'm measuring with my old Paula connected, I'm getting 160mV and 4.67V on her VCC (4.82V across the mobo).

When I power on my Amiga, I’m getting the same DiagROM screen with bad graphics, and system info still shows “BADPAULA”.

I suppose that Paula isn't the main cause of the 0.18V voltage drop (which I suppose is the cause of my Amiga not working properly as changing the Paula with a new one didn’t help much) across the mobo after all, so I will check for other possible causes of that issue (other ICs, resistors, capacitors, maybe even the power connector?).

Although, I'm curious about how maybe the IPA solved the low voltage issue I described in my last reply. Leaving my Amiga to dry after cleaning for a couple of days seemed helpful. I definitely won't be using the WD contact cleaner for my A500 again.

-poroxius, October 11th, 2025

[Castellen]

When I power on my Amiga, I’m getting the same DiagROM screen with bad graphics, and system info still shows “BADPAULA”.

Did you clean possible contamination from other IC sockets as well?  It's not clear what state the system is currently in, can you post the entire output from the DiagROM serial output from power on?

I suppose that Paula isn't the main cause of the 0.18V voltage drop

180mV across R309 sounds normal, that indicates the 8364 (Paula) is drawing the correct supply current.

[poroxius]

Did you clean possible contamination from other IC sockets as well?  It's not clear what state the system is currently in, can you post the entire output from the DiagROM serial output from power on?

That’s what I was thinking about doing. I will definitely try to pull out every socketed IC, clean it’s pins and socket with IPA and also give the rest of the board a clean.
I will also check the DiagROM for new information while using the new Paula.

Thank you,
-poroxius, October 12th, 2025