On 24/03/2014 21:23, N_Cook wrote:
Repairing a digital audio procesor. Symptom it makes an horrendous noise
like swarm of hornets and bagpipes with chorus and reverb.
There is a reset line that connects reset pin of the system micro to a
reset pin of the DSP 144 pinner and reset pin of an associated memory.
Long trace and ribbon around and between a couple of boards. Nothing
found in the way of power-on reset IC to this line.
But touching this line with a probe tip , resets the system to full
operation. Just touching a finger to this line does not reset.
Touching the metal of the probe when touching the trace does reset.
No equipment available to check for any circa nanoseconds reset pulse,
seems to be H always.
A 15nf cap to ground and 15 inches or so of test lead , one end to
ground does reset, with a 680pF cap there insted, no reset.
main ICs are
ATMEGA88, p1
DSPB56720ag, p111
S29AL032D70TF103 , p12, pinning assumed to be same as S29AL016, data
found .
Incidently no ferrite torroid or cylinder on the maker's SMPS wall wart.
How to regularise / repair this situation?

Faulty cap on the reset line ? Open cct pullup on the reset line on the
mega 88 ? Mega88s can run with an internal reset timer, no need for a
cap or resistor but may be fitted for 'belt and braces'.Also check the
clock to the 88 and its Vcc.