Tandy 1000 SX motherboard mod?? (not working) \ VOGONS

Tandy 1000 SX motherboard mod?? (not working)

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First post, by Dusko

Posted on 2022-10-30, 19:45

Dusko Offline

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Hello All!

I recently bought a Tandy 1000 SX secondhand that doesn't turn on and when I opened the case I found that the motherboard has several weird mods.
I wanted to think that it was a last minute factory revision/fix, but I'm starting to think that that's not the case. I compared it with a few pictures or youtube videos I found on the web and none of them have this changes.

I wonder if I should remove all those mods and reconnect the traces that were cut off, but I really don't know what was the purpose of all that to begin with.

Does anyone has any idea what's this all about?

1. Motherboard overview:

The attachment Front.jpg is no longer available

2.Motherboard back:

The attachment Back.jpg is no longer available

3. Two caps in series with the original trace filed off:

The attachment 2 caps and trace cut off.jpg is no longer available

4. Audio/Video: Inductor with 2 wires, funny looking connections and a trace filed off:

The attachment Audio Video 1.jpg is no longer available

The attachment Audio Video 2.jpg is no longer available

I'll post more pictures below...

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Reply 1 of 8, by Dusko

Posted on 2022-10-30, 20:00

Dusko Offline

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A few more pics...

5. IC CD4529BE with two pins shorted:

The attachment CD4529BE two shorted pins.jpg is no longer available

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

The attachment U54 with cap.jpg is no longer available

7. A ceramic cap in the back:

The attachment Cap in the back.jpg is no longer available

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

The attachment Back cables and trace filed off.jpg is no longer available

9. A bunch of ICs with caps and resistors:

The attachment ICs with caps and resistors.jpg is no longer available

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.

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Reply 2 of 8, by Dusko

Posted on 2022-10-30, 20:16

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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....

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Reply 3 of 8, by Shponglefan

Posted on 2022-10-30, 20:18

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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.

Last edited by Shponglefan on 2022-10-30, 20:22. Edited 1 time in total.

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Reply 4 of 8, by weedeewee

Posted on 2022-10-30, 20:20

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

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Reply 5 of 8, by Dusko

Posted on 2022-10-30, 20:25

Dusko Offline

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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!

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 6 of 8, by Dusko

Posted on 2022-10-30, 20:26

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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.

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 7 of 8, by BloodyCactus

Posted on 2022-10-31, 14:33

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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

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Reply 8 of 8, by Dusko

Posted on 2022-11-01, 03:13

Dusko Offline

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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?

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

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