STFM and OSSC Woes

[Heywood]

Hi,

I have a 1040STFM that I have a cool novelties ST to SCART cable that I use to connect to my OSSC, however some strange things seem to happen.

I can get into GEM without problems (TOS 1.02/1.04 Switcher that I bought through here) and have a Gotek Floppy installed running Flash Floppy (my STFM came this way)

What seems to happen is when I try to do things that appear to be a different mode of operation (say like launching a game), the OSSC goes haywire and says there is no sync lock. Since my ST is a STFM variety was able to connect to the modulator output to a TV and tune it in and I can see things are still working so I'm not sure if this is an issue due to the OSSC or the cable (or both), my OSSC (1.6 hardware, 0.88 firmware) seems to place nice with other systems but perhaps there is an Atari ST specific profile I should be using?

Any advice would be welcome

[exxos]

I think a lot of people have trouble with it. You could try this to see if it helps https://www.exxosforum.co.uk/atari/last/ ... /index.htm

[rubber_jonnie]

The OSSC may need different options, have a look at this: https://videogameperfection.com/support ... kstart.pdf

I have a BBC B that drops sync via my OSSC and I find I have to use linemult mode 2 to make it work.

[Heywood]

Thanks for the ideas

Cheers Rubber_Jonnie - I've looked at this before and have attempted to get things to work - I'm suspecting that it could be as exxos is suggesting.

exxos - thanks for pointing me to the article. My local electronics store (Jaycar) doesn't offer all of the electrolytics listed as you recommend, specifically:

• 16V 10uF - they don't have the low ESR variant but the one they have states a 45mA ripple current

• 50V 2.2uF - they don't have this at all but they have a 63V version (not low ESR either) with a 22mA ripple current

• 16V 47uF isn't available but it is in 35V (is low ESR though)

The others they do have as low ESR. I'm guessing in general a higher voltage is okay but usually this means a physically larger cap so the footprint might be a little larger and lower ESR is preferred especially from a longevity and filter ripple perspective.

Do you think it's worth giving it a shot with what I can do locally first?

[exxos]

Worth a try but I would try the resistor mod first.

[Heywood]

I installed the 2K7 resistor and it seems to have sorted things out! Thanks for pointing me in the right direction!

Just as a question - is the RF shielding necessary (including that around the PSU)? I kind of bent the metal work getting access to the IC and if it's more for old school EMI compliance than anything safety related (doesn't appear to have negatively impacted the video signal) maybe I can leave it out?

[rubber_jonnie]

I installed the 2K7 resistor and it seems to have sorted things out! Thanks for pointing me in the right direction!

Just as a question - is the RF shielding necessary (including that around the PSU)? I kind of bent the metal work getting access to the IC and if it's more for old school EMI compliance than anything safety related (doesn't appear to have negatively impacted the video signal) maybe I can leave it out?

I have modded machines where the upper shield won't fit, so I run some of them (Megas, STEs and STFMs) without it with no ill effects. As you say it was mostly to stop the computers of that era causing interference with other equipment, and these days it has lost its relevance.

For the ST the lower shield is also a carrier for the mainboard, and though it will fit without, it isn't the best, as the ports are not properly centered in the case holes, and in some revisions the lower shield provides support for the cartridge socket via a bent metal lip, but others have this support as part of the lower plastic case.

[Heywood]

Sweet - thanks for that, I have the bottom shielding still in place, so I can screw the case back together.
Now to start experiencing the ST