This is a very simple project indeed: add a line-in input feeding the amplifier in my sister’s CD/Tape/Radio, which has much better sound than the ‘device’ she mainly plays music from. How hard can it be to find the amplifier IC and feed in directly?
Taking the thing apart was none too hard, and it’s cheap ’90s hardware so all through-hole and single-sided PCB. I found the mode switch, traced it back to the amplifier, paid careful attention to the pinouts and soldered a small cable onto one input, the other end going to a 555 ‘signal generator’ kit I had lying around. Power up: the signal generator exploded and the lcd backlight died.
Apparently I’m not so good at tracing PCBs as I thought. The thing now was to repair the backlight: it showed no continuity in either direction so I wondered if it were blown. Getting at it to find out required taking the whole front off, in the process of which I lost a tiny screw and had to find a replacement, and an annoying half hour of trying not to break everything. And then the thing was fine… So now I needed some way to turn it on when in CD mode. Fortunately I’d taken everything apart, so it was quite easy to trace the connection to the CD board and carefully measure the voltage at the requisite pin. I found two: 12v and 5.2v, switched by the mode switch. Trying to trace back the led driving circuit showed a lot of very complicated things everywhere; I’ve no idea why. A 220R (I think) and the backlight lights up fine.
On the way I brushed the probe against the next pin, and the left speaker crackled. So I got interested, guessed, and touched another pin: the right speaker. Attaching a 3.5mm socket in place of my dead signal generator confirmed that I got good sound out with about 50% volume in. I should probably fit an attenuator, but meh. What happens if both CD and external input run? I expected a simple mixing, but instead one dominates. It’s probably not a good idea to risk backfeeding the A/D converter by doing this: really I should add a switch, and have it illuminate the LCD when the internal CD is selected. But after hunting I couldn’t find a 3-pole double-throw switch anywhere and gave up. So here is the new unit:
[gallery ids=“164,165,166” type=“rectangular”] The flexible lamp in the first photo is jolly handy for this kind of thing, even if over-using it gives me a headache. It also uses the worlds silliest regulator. Maybe at some point I’ll write that up as well…
If you try this with a bit of old audio equipment don’t rush in with a signal: just brush the suspected input pin with a probe or a bit of wire: if you’ve got the right pin you’ll hear crackling, and if you haven’t it won’t blow anything up ;)