PC Engine LT - Schematic Nov 17, 2021 11:23:20 GMT
Post by mmora on Nov 17, 2021 11:23:20 GMT
I'm new to the forum but I have been drawn here due to this thread:
The thing is, I have a PCE LT that had been losing brightness for a while until it almost gave it last breath a few months ago. I knew it was capacitor related, so I went to a friend of mine that has been a long time technician and is the best one I know. Long story short, we basically replaced all capacitors and, thus, repaired the LT and the screen is now perfectly bright and usable again. However, an audio issue appeared - a buzzing sound. This is a sound that only happens in game mode and in portable mode - so, if connected via EXT, sound works perfectly. If something is connected via AV IN, sound is also perfect, both through the speaker and the audio jack. We traced all the rails - quite hard in the PCE - and we found a strange signal that basically was present in a rail that connects one of the PCE card pins and the B pin - middle one in the far side - in the EXT port (A and C are used for sound, but B should be quiet). For some reason, this signal is also present in one of the connections to the audio amplifier and all goes wrong. We guess there must be some issue with the rail - maybe some kind of short - as we were able to isolate the problem by removing a single resistor, but we would still like to find the source of the issue. This signal must be - probably by mistake or failure - generated in some component. The thing is that, without the schematic, it is an exhausting task to follow the traces. Ah, the processor seems to work fine, both L and R audio work well; it is an "extra" signal which is added to the mono signal at some undetermined point. We could just kill the signal - grounding it - but we have so far avoided doing so as it might have some reason to exist.
Apologies if that explanation is a bit confusing, but I didn't take any pictures of the oscilloscope graphs
So, any hints? Does anybody have the LT schematic or service manual? There was supposed to be one in the now disappeared PCE forums but I can't find more than mentions to it.
Thank you in any case and kind regards,