RichCini wrote:
EDIT: I now get the start-up tone. If you look at the C64 schematic (attached), the audio output is amplified by a 2SC1815 or, in some versions, a PN2222. At the output of the amp there is a polarized electrolytic capacitor. If I connect my bench-top amplifier (simple LM386 amp from Radio Shack) to the "audio out", the tone is inaudible. If I connect right to the emitter, I hear the tone but have to turn the volume up kind of high. Similarly, if I connect right to the chip output, I get the tone with.
I'm not well-versed enough on the MEGA88, but I wonder why the stock amplifier circuit from a C64 doesn't work considering this is supposed to be a drop-in replacement. Maybe the PWM output level?
If you get no signal at the output, and only a little at the emitter, it sounds like the output is shorted, and the small impedance of the capacitor in between is providing a slight load, and the output signal current times that impedance is developing a small signal voltage. That assumes the transistor is installed correctly. 25+ years ago at work, a circuit I designed came back from the board house and nothing at all worked. The new engineering manager (whom
I had hired!) implied I was incompetent and didn't know what I was doing, and that it was a waste to have had boards made from my design. Then we figured out that the CAD operator who had laid out the board used the wrong transistor pinout, and all the transistors were connected wrong. After the tech removed them and re-soldering new ones with their leads twisted around to meet the errant board layout, everything worked right.
Back on track, take C13 out and see if you get a lot more signal at the emitter. If you do, the output is shorted. If the transistor is able to work in an analog range, ie, the PWM carrier is sufficiently filtered out, the DC voltage at the emitter should be about 2/3 of a volt below that of the base. FB15, R7, and C37 are apparently to filter out the PWM carrier. Its -3dB point is near the top of the hearing range. The 0.9x voltage gain (in spite of a lot of current gain) comes from the voltage divider at the base, formed by R7 and the AC impedance of the base, which is roughly the load (1K ideally, that of R38) times the current gain of the transistor which we can very roughly guess is 100. (Actually it's probably more.) 100K/(100K+10K) is about .9. (There's a little more to it, but the resistors have probably a 5% or 10% tolerance anyway; so getting into more detail with the transistor's curves would be more of interest to distortion than to getting a more accurate gain number.
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