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
