Essentially at this point, you're just looking if the serial data out is generally working or not. If it isn't, then there's probably a common register data/address bus issue that's also affecting your video output.
After I set up the null modem cable and the serial adapter like this (
https://youtu.be/oGBFVtYDbM0?si=b5e1wQQPVQCGb6WZ&t=738 - starting at 12:18) to PuTTY on my PC, if there is a faulty data/address bus, I won't get anything or any info on the PuTTY terminal as there would be no serial output?
0L on most Ohmmeters means over-range, or open circuit in this case. Even if the resistor was open circuit, which is extremely unlikely unless it was physically damaged, you'd still measure parallel resistances in the same circuit. Meaning you'd not know if it was defective or not unless you took it out of circuit (un-soldered one end). I'd suggest not wasting time with this as a resistor fault is very unlikely.
Oh, got it! I didn't keep in mind that most of the resistors are connected in a parallel circuit, it makes sense now! I won't consider them as a possible fault any further.
I gather the same fault was still present before you removed ICs and re-inserted them, etc? i.e. Removing/re-inserting them made no change to the video issues? That would make a socket contact issue less likely. Though it's possible you may have introduced a secondary fault if there are bent/damaged IC leads that aren't making reliable contact with the socket.
Exactly, upon removing and re-inserting ICs nothing changed at all. I did that as it is one of the easiest tests I could do in order to determine what's wrong and potentially fix it that way. But, it wasn't the case for my A500
.
Using a flat screwdriver to remove DIP (dual in-line package) devices is OK if you're careful. You need to carefully work at each end to gradually lift both sides up. I've seen cases where people have 'crowbarred' the entire IC up from one end, which bends all of the leads, and sometimes damage top side tracks on the PCB where the screwdriver gouges it.
I think that I was careful as I was working each end carefully and equally, and thankfully didn't bend a lead. Crowbarring them is definitely the worst thing I could have done to the ICs so I kept that in mind.
It can be difficult to remove PLCC devices from sockets even with the correct tool. That's why there are often holes in the base of the socket and in the PCB to push the IC out from the bottom side, sometimes the extractor tool can't grip enough in the IC corners, especially if they've been worn from previous extraction attempts.
Yes, especially if the PLCC extractor isn't a high quality one. The holes on PLCC's corners really make extracting Agnus easier.
It's very difficult to check that the socket is reliably contacting all IC leads in this way. Not only is it physically difficult and time consuming, just the act of a multimeter probe pressing on the IC lead can sometimes be enough to temporarily "fix" a poor/intermittent contact. So it measures OK, but then goes open circuit after you remove the pressure from the probe.
It indeed was very time consuming lol. Could I do something better in order to test the sockets and is it even necessary or should I focus more on testing data and address buses with a serial port diagnostic tool?
That's a reasonable approach. A null modem cable is quick and easy to make if you happen to have the right D-range connectors, and will give you a few more clues before you proceed.
After that, it's quick and easy to check the address/data lines at Denise, which will determine where to look next. Presumably you have an oscilloscope? Else this is quickly going to get a lot more difficult.
I will definitely make a null modem cable and test my A500 with the serial output.
I was thinking about making it like this:
- get a FT232 USB to DB9 M cable
- connect the USB to my PC and DB9 M end to the DB9 F end of a regular DB9 F to DB25 F cable that I will turn into a null modem one
- connect the DB25 F end to the A500's serial port
There is no point in testing just with DiagROM inserted to my motherboard as the graphics are so bad that I'm not getting much info from it apart that something is wrong with the graphics.
I will have to get my hands on an oscilloscope because it would be much easier then testing the data/address bus connections with just a multimeter. I presume that I wouldn't get much info about them with a multimeter?
By the way, I didn't mention much about my RAM expansion.
It wasn't inserted into the RAM expansion slot but when I connected it now - I got the same green screened frozen menu, but DiagROM read the additional 512kB and now
I'm reading 1024kB of memory, which probably means that there is nothing wrong with RAM on my mobo and RAM on the expansion.
I went and checked if maybe the connections of jumpers JP2 and JP7A are causing issues - JP2 pins 1 and 2 are connected together and JP7A is left floating, their pins aren't connected. According to
https://www.retrosix.wiki/jumpers-amiga-500, my Amiga should boot normally with those connections, which it does so
none of the RAM expansion jumpers are connected wrong.
However, I have a question about JP4. Something wrong happened to it while it was still owned by it's previous owner or during my testing.
For some reason, there is a hole in the mobo between JP4's two pins (Picture -
https://imgur.com/a/AaE4yqK). I'm worried that it shouldn't be in that state and that there is something wrong with it.
I read that JP4 should be bridged for a NTSC output, but as my Amiga is PAL, I think that I should leave those pins unconnected and as they are - with a hole between them.
I'm just trying to be sure that I could leave it that way and that it isn't causing these issues.
Again,
thanks so much for your reply! I will definitely get a null modem cable and test the serial output. Also, I will get an oscilloscope so I could test the data/address buses.
