exxos's Atari, Amiga & retro forum

Hi. I'm trying to convert an atari stfm motherboard c103414-001 to "f", because it have some corrosion in the modulator circuit, many components seems to be in bad shape and I dont need RF output. The image from the begining was broken:


Once removed the RF output box, I noticed that there is not circuit to make the composite sync needed to get the right image. Searching on the net, I found an image of a F motherboard C103175 that seems simillar to my board, and as mine, there is nothing under the output box:


I can't find any schematic of this board, and I dont know if Q12 in C103175 motherboard is inside the shifter box, in my board, there isn't, and it makes me think that may be not neccessary on my board (my shifter is labeled C07013-002) or it MUST be inside the RF output box


So, before to have to buy a new RF output box, where I presume that the composite sync circuit have to be fitted, may I be wrong? may be the RF output box is not necessary for this board? I measured the glue syncs V and H and seems to be ok, also both
quartz oscilators, and ok too.


what else may I have to check?

I feel silly now that I've noticed when checking the board again, that the circuit to generate the composite sync signal was right next to the GLUE chip :oops:

The modulator is required to generate composite on these machines. It is possible to add just the necessary components in place of the modulator (I think a transistor and some resistors, possibly a diode too) but without them you won't get a composite signal IIRC.

Sorry, I can't recall what components are needed in place of the modulator and where they go.

Yes, following the schema of another board revision:


This give me the gue of the components to generate the sync signal for the image RGB components, and once I found the place (near the GLUE chip), I get a clean image with the follow components where the numeratión on this board version changes (c103414-001):


R131 is R217 -> 100 ohm resistor
R129 is R125 -> 10K ohm resistor
R130 is R126 -> 150 ohm resistor
CR9 and CR10 are CR12 and CR13 -> the 1N914 diodes
Q10 is Q12 -> the 2N3904 transistor

It does not generate a composite image as modulator does, AVs cables don't work, and may does not work on modern tvs, but now I get a clear image by using a SCART cable:


I hope this information may be useful for somebody :-)

thanks!

What I was talking about was some components installed in the modulator section to enable composite, not where you've installed them.

As long as it's working for you it doesn't really matter though :)