exxos's Atari, Amiga & retro forum

HCRX27 wrote: 16 Sep 2025 13:33 @rubber_jonnie
Indeed, thank you. I was looking for, but somehow missed attachment option earlier on. Edited :)

Good stuff, thanks.

Hello @exxos and Gang.

I need your help, I'm afraid, on my 520STM (C070243 REV.H) I've replaced all 9 transistors around shifter, and only to get no video output signal at all. That is both on ST LOW/MED and HIGH modes.

As mentioned earlier, I did it inspired by the lecture of:
https://www.exxosforum.co.uk/atari/last/VIDEODAC/

The shifter itself seems to be the cause of the problems. I tried simply buffering the RGB outputs with a Schmitt buffer to clean up the lines before they go into the transistor drivers. But it did not help much, if at all. I also have swapped out the 2N3904's with a generic BC548 transistor and this helped reduced the problem.

What can be a problem, please?
1) did I replace too many 2n904 transistors in Shifter's area (all 9), while only some could be replaced?
2) can it be a problem that I replaced them with BC548C, instead of BC548, as mentioned in the article?

Any help would be greatly appreciated. I'm pretty sure the soldering is OK, used desoldering station to get rid of 2N3904's. There's still +5V on Shifter and Atari motherboard in general, no smoke, no hot IC's, just no picture.
The photo of Shifter's area I attached is just an example from someone else's photo, but mine is the same revision.

I'm gutted, to be honest :-/

You should not replace all... Some are special HF transistors for oscillator..

Plus replacing the RGB transistors was only an experiment.. Not any official upgrade..

Also one of the issues I've run into is that the collector, base and emitter positions can vary depending on make, I've run into it where they are reversed and of course they won't work like that.

It's usually the collector and emitter I've found to be that way.

You should not replace all... Some are special HF transistors for oscillator..
Plus replacing the RGB transistors was only an experiment.. Not any official upgrade..

Understood and of course, I know it was my own risk, still I thought it was worth trying. I'll replace them back with 2N904's, just leaving RGB transistors with BC548C. I can't find schematic of my exact board model and revision, based on another ones, it looks like R,G,B are Q3, Q4 and Q5.
How about Q2 (mono), can it stay BC48C as RGB's?

News:
Good: I regained the video output with replacing back to original all transistors except RGB and mono.
Bad: BC548C's smoothed everything to the level of unacceprable blurriness (attached).
Best: nothing got damaged permanently, I came back to all stock 2N9304's and have it like it was before (jail bars) :)
At least I tried, thanks @Exxos and Gang!

You might want to install a RGBtoHDMI in it? I use one in my 520ST and it's great.

HCRX27 wrote: 28 Sep 2025 18:19 News:
Good: I regained the video output with replacing back to original all transistors except RGB and mono.
Bad: BC548C's smoothed everything to the level of unacceprable blurriness (attached).
Best: nothing got damaged permanently, I came back to all stock 2N9304's and have it like it was before (jail bars) :)
At least I tried, thanks @Exxos and Gang!

Well that at least is a result. I might add RGB2HDMI im my own machine as @Steve suggested as mine has awful jailbars too.

The jailbars are just a feature of this design, and part of its "charm".

No machines from this era have flawless video output. If you want that go for an emulator. :-)

The video "faults" are far less noticeable on a CRT. LCD displays just make them stand out.

stephen_usher wrote: 28 Sep 2025 19:39
The video "faults" are far less noticeable on a CRT. LCD displays just make them stand out.

Very true, just annoying I only have one CRT and it's for my One Per Desk, so not suitable. Actually, not 100% true, I have a 5 inch B&W tv with composite in, but I do think I need to try and find a decent CRT at some point soon.