I hadn't considered that bright might be variable by channel — I'll have to check that out.
If it were MC1377P affecting all channels then presumably I'd see the effect only through the aerial or a single-line video output? If I were to find a suitable RGB scart cable then I'd still get the expected output? Re: the ASIC and primarily because I'm curious, how would making a replacement for that work? Are there readily available FPGAs that could drop straight into the same slot, if not then how expensive is it to build some sort of bridge? As I said, I'm quite an electrical dunce so don't hesitate to be patronising. Given that at least some of my bright comes and goes, my floppy drive seems to be dead (it makes a spinning noise and lights up but fails to report that a disk is inserted) and there's a real chance I may emigrate soon it might be time for me to put my real SAM away for good. That'd be a little sad. On Tue, Oct 4, 2011 at 10:11 AM, Leszek Chmielewski <retr...@gmail.com> wrote: > > > 2011/10/4 nev young <pasiphae1...@yahoo.co.uk> >> >> On 03/10/11 23:40, Andrew Collier wrote: >>> >>> On 3 Oct 2011, at 16:22, Thomas Harte wrote: >>> >>>> It looks like a previous owner of my current SAM has had occasion >>>> to replace resistor R55, or at least, to solder an additional copy >>>> of R55 on top of the existing one. See >>>> http://postimage.org/image/1g4kbz490/ >>>> >>>> Immediate follow-on questions, mostly resulting from me being an >>>> electrical dunce, are: what does R55 do, what would be the likely >>>> effect if it was a bit dodgy and is it really okay just to solder >>>> an extra resistor on top of an existing one? >>> >>> According to the schematics in the tech manual, R55 is doing >>> something to do with the MIC tape interface, and should be a 100kΩ >>> resistor - which if I'm reading the photo correctly (the colour bands >>> look {brown, black, yellow, gold}) is exactly what it is. >>> >>> Two of them wired in parallel are equivalent to a single resistor of >>> 50kΩ (assuming they both work) though I'm not certain what the >>> implication of that is for the rest of the circuit. >>> >> R55 and C28 form a feedback circuit that should "square up" the audio >> signal coming from the tape cassette. Reducing R55 from 100K to 50K, by >> putting two in parallel, will increase the amount of feedback. >> >> The Bright signal is generated by the ASIC and appears on pin 18 (If I >> read my diagram correctly). It then goes to R65, R69 and R73 (all 36K >> [orange, blue, orange stripes]) to drive each of the colour driver >> transistors M3(green), M4(red) and M5(blue) (3x BC547). >> >> If you have lost bright on one colour look at the corresponding resistor >> and PCB connections. If the transistor has blown you would lose that >> colour completely. If you have no bright on any colour then check the >> output of the ASIC and the PCB connections from there to the 3 resistors >> for cracks, dry joints, broken through plating etc. >> >> If there is no signal coming out of the ASIC then get used to a dull >> life. :-( >> >> Nev >> > My SAM has no BRIGHT too. There was a shortcut between Composite and +12V, > so the MC1377P was burned out (Just got a replacement by desoldering a Atari > Mega STE), I lost BRIGHT too as the ASIC was toasted a little too. After > replacing it with ASIC from my spare SAM the BRIGHT is back again. > It is time to design a replacement ASIC. Velesoft is working on one since > years, but he is too ambitious: 4096 Colours, Hardware sprites and > scrolling... > LCD
