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...
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