IIgs ROM-0 Mainboard Differences

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IIgs ROM-0 Mainboard Differences

I own a ROM-0 that was part of a //e Upgrade Kit. Today, I saw photos of another ROM-0 board, and it has two slight differences that mine lacks.

 

There is an extra resistor on it, plus a little bodge board.

 

Do any of you know the purpose of these?

 

 

 

 

I cannot tell where this bodge board is wired other than to that 3340, so IDK what it is intended to remedy. 

 

I don't recall ever seeing a board with that resistor in place, so again, IDK what purpose it serves, nor why it was removed in later revisions.

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Just guessing, but being

Just guessing, but being 75ohm I would say that R121 is part of the video out circuit. Maybe RS4 in the schematic which looks optional (feeds the modulator connector?)

 

Other board might be part of the audio out as well.

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jeffmazur wrote:Just guessing
jeffmazur wrote:

Just guessing, but being 75ohm I would say that R121 is part of the video out circuit. Maybe RS4 in the schematic which looks optional (feeds the modulator connector?)

 

Other board might be part of the audio out as well.

That is an interesting guess. I will look at the circuit paths and see if i can validate it. 75Ohms would indeed tend to be video, bt bear in mind that tose photos are the first time that I have seen this resistor wired, and I have had several Rom 0 boards. I was curious on the purpose of these mods/bodges because I am curious what they are intended to correct. That little sub-board is something that I have absolutely never seen before, although I have seen bodges on the back side of the PCB, so I might want to compare if any of those are to the same points. 

 

To date, I hven't documented them, or tried to do a logic check on what they are doing. I wish that I knew where the other side of that sub-board was wired. (I can't logistically justify spending the money required to buy that specific eBay board,  just to determine this.)

 

I suppose when I have anything to add, I will post it, in the event that it benefits anyone else. 

 

P.S. Would it be typical for the component IDs not to align on these schematics? I would expect R121 to be identical on the schematic as it is marked on the PCB, as otherwise, the schematics are somewhat borqed. 

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