A few more pics...

5. IC CD4529BE with two pins shorted:

6. Another IC (U54) with a cap across two pins:

7. A ceramic cap in the back:

8. The white cable is not connected, I wonder at which point it was. Another two traces filed off:

9. A bunch of ICs with caps and resistors:

As I said, I don't know if this is one of the very early SX motherboards or someone was trying to do something with it...

Thanks in advance for your comments.

BTW... Power supply voltages are good.

In case someone has an idea of what should I do with it: I'm not an electronics engineer but I have some knowledge and the tools (soldering station, de-soldering gun, logic probe, oscilloscope, etc.).
I have fixed several mobos over time, including a Tandy 1000 (not A), so my success rate is decent, BUT, I have no idea where to start with this one....

I've opened my own Tandy 1000 SX to compare. While I can't see the entire motherboard (I haven't fully disassembled it), the visible parts show the same modifications you are seeing on your board (i.e. the caps soldered to the ICs).

My first instinct is these are probably factor modifications. Even though it's clearly a lot of factory modifications.

The caps diagonally soldered onto the ic's are for extra decoupling, so shouldn't give any problem.
All the other mods... no idea.

Shponglefan wrote on 2022-10-30, 20:18:

I've opened my own Tandy 1000 SX to compare. While I can't see the entire motherboard (I haven't fully disassembled it), the visible parts show the same modifications you are seeing on your board (i.e. the caps soldered to the ICs).

My first instinct is this is probably factor modifications. Even though it's certainly a *lot* of factory modifications.

Thanks for the time you took to compare it with yours!!
So, that might be a good thing I guess. I was thinking someone started to "play around" with it.
I think is time for me to have another look at it and make some more measurements...

Thanks again!

weedeewee wrote on 2022-10-30, 20:20:

The caps diagonally soldered onto the ic's are for extra decoupling, so shouldn't give any problem.
All the other mods... no idea.

Thanks for the info! I didn't know that.

my SX does not have any of those mods. (mine is a REV F mobo, silkscreened between the cpu + big plcc). Yours has the MV1-4586, mine is MV1-1187, thats probably the fab date of week-year. So our boards are not too far apart, 3 months-ish. I guess enough to have converted over to a new revision.

most of your mods are as stated filter caps.

the av mod looks like an EMI filter choke, maybe to reduce some noise on the output

BloodyCactus wrote on 2022-10-31, 14:33:

my SX does not have any of those mods. (mine is a REV F mobo, silkscreened between the cpu + big plcc). Yours has the MV1-4586, mine is MV1-1187, thats probably the fab date of week-year. So our boards are not too far apart, 3 months-ish. I guess enough to have converted over to a new revision.

most of your mods are as stated filter caps.

the av mod looks like an EMI filter choke, maybe to reduce some noise on the output

Thanks for the info and thanks for checking!

I might have some bad ram but I'm not sure yet. I need to check the datasheet to know what I'm looking for when probing.

The computer doesn't make a sound or display anything on the screen.
Do these Tandys make beeps for errors? like "newer" PCs?