Memotech Limited MTX 500, Service Manual

Page: 42 / 78

38
4 If pulse is present on both 4E (374) and 5E
(SN76489AN) check for pulse on pin 1 of J13 on
pal board and pin 3 of J11 on pal board. If
either are absent check for open circuit or
replace pal board.
5 If pulse is present on J11 and J13 check the
tuning of modulator. See ref Page 42.
b)
Cause of Trouble
Faulty SN76489AN or 74LS374N
Check Point
As above
Repair Method
1 Often the sound channels become distorted once
the components have reached their optimum
temperature. You can heat the SN76489AN and
74LS374N systematically with a soldering iron
(whilst checking pulses on SN76489AN) to
recreate several hours of use.
NB: For further information on modulator and
video board fine tuning see page 42.
SYMPTOM 8
Faulty keyboard response
a)
Cause of Trouble
Key bounce
Check Point
Upper keyboard chassis
MTX Service Manual