Post by Vyper68 on Oct 4, 2018 6:34:44 GMT
So last time I talked about the imbalance in the audio from the +2 Grey with the Beeper audio being substantially louder than the AY audio and the resistor change to make them equal. I also talked about the noise on the audio which manifests itself as humming and white noise and can be very distracting. This problem seems to affect +2's made in Taiwan more than +2's TIMEX made in the UK. The problem I thought at the beginning was the grounding on the Datacorder the consensus on the Spectrum websites seemed to back this up as the problem. It seems to be caused because the PCB for the Datacorder uses the metal chassis of the Datacorder to connect two ground lines before it goes to the Spectrum PCB. The solution is to connect the two ground points with a wire link and bypass the chassis and reduce/eliminate the noise. I did this and to my disappointment it did not work.
This was a curveball as I was convinced it was the problem, I then started to diagnose the problem with some help from the guys on the Spectum forums. I disconnected the Datacorder from the main PCB and the problem was still there, so in my case it was never the Datacorder in the first place. So the problem was on the main PCB, because it was humming and white noise it seemed like it would have something to do with the power or ground rails on the PCB. I was hoping it would be the ground rail as the 5v rail would be more problematic. So first off I replaced the 7805 voltage regulator, cleaned off the old heatsink adhesive and applied new and set aside while I worked on the PCB. The Spectrum was fitted with Ceramic de-coupling capacitors, the normal 104's ( 100nf ) it was unlikely they were bad but they were 30 Years old and you never know how good they were when fitted, so they were all replaced with new MLCC capacitors. I then turned to the more likely candidates, the Electrolytic capacitors. The Electrolytic capacitors were all replaced as well.
So now we have replaced a fair few things it was time to test my handiwork, the Spectrum was reassembled and powered on and the results were very good. The noise/interference had been reduced by around 75% to my ears and in my eyes that was a win. I was hoping to get rid of it entirely but I am not sure this is possible now, thinking back to my +2B and other Spectrum's there has always been a small amount of noise and I think it is the design of these computers. My Amstrad CPC's have a small amount as well.
So now I have a +2 Grey which is much quieter than when it arrived and ready for another 30 Years....