Am 04.10.2011 17:32, schrieb Thomas Harte:
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?
MC1377P mixes RGB to composite video, so if it is dead, you wont see any
picture on composite output, but all is looking normal on RGB output. If
you have no bright, but composite video is otherwise okay, then MC1377P
is okay.
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.
I think, the supply voltages are maybe on different pins, and different
levels (Modern FCGA chips use often 3.3 Volt or less, but SAM has 5
Volt). If so, there will be no drop in replacement
Using SMD technology it would still be possible to create a drop-in
board with powerful chip that can act as relpacement for the ASIC.
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.
Check how stable the voltage from your transformator is, maybe one of
the capacitors is dried out.
I just ordered a SAM drive board without the WD1772 controller (I still
have some working controllers desoldered from old Ataris ST, these are
very cheap on eBay) and drive from samcoupe.com I think, your problem is
the drive too, not the controller, as mine is.
LCD
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
