Re: Issue 3 Spectrum 48K dodgy output
Posted: Sat Nov 06, 2021 7:08 pm
Check crystal X2 and the nearby components for dry joints, solder splashes, track damage or any physical damage.
Further to what hellraiseruk said, ULA pins 15 and 16 are the colour difference outputs. As he says, set the border and paper to the same colour, then record the voltage on each of these. Do this for all eight colours. If one pin always has a constant voltage, the the ULA is suspect.
Do the same with pins 2 and 4 on IC14 (LM1889). The details of the voltages are in the service manual. This chip only does the colour encoding. Test the supply voltage to it on pins 14, 15 and 16 (they should all be the same voltage).
The output is on pin 13. Test the voltage here.
Capacitor C65 (22µF on later boards, but I’m not sure what value is on your board) is the coupling capacitor that feeds the colour signal to join the monochrome signal at the base of transistor TR2. So check it’s connections (did you renew this one?). And check / test the PCB tracks between pin 13 of IC14 and it. And from the other leg of C65 to the base of TR2.
If you want, you can carefully lift one leg of C65. Then the machine should produce a rock solid and consistent monochrome image only.
If you have suitable test gear, you may want to measure the frequency that the oscillator in IC14 is running at. See the service manual for further details.
If you don’t have any suitable test gear, try replacing crystal X2.
Note that failure of TR8 or TR9 is rare. In terms of semiconductor failures, the normal causes are IC14 or the ULA.
Mark
Further to what hellraiseruk said, ULA pins 15 and 16 are the colour difference outputs. As he says, set the border and paper to the same colour, then record the voltage on each of these. Do this for all eight colours. If one pin always has a constant voltage, the the ULA is suspect.
Do the same with pins 2 and 4 on IC14 (LM1889). The details of the voltages are in the service manual. This chip only does the colour encoding. Test the supply voltage to it on pins 14, 15 and 16 (they should all be the same voltage).
The output is on pin 13. Test the voltage here.
Capacitor C65 (22µF on later boards, but I’m not sure what value is on your board) is the coupling capacitor that feeds the colour signal to join the monochrome signal at the base of transistor TR2. So check it’s connections (did you renew this one?). And check / test the PCB tracks between pin 13 of IC14 and it. And from the other leg of C65 to the base of TR2.
If you want, you can carefully lift one leg of C65. Then the machine should produce a rock solid and consistent monochrome image only.
If you have suitable test gear, you may want to measure the frequency that the oscillator in IC14 is running at. See the service manual for further details.
If you don’t have any suitable test gear, try replacing crystal X2.
Note that failure of TR8 or TR9 is rare. In terms of semiconductor failures, the normal causes are IC14 or the ULA.
Mark