This is something you wouldnt think of I dont think. But if you got an M1a, with no sound, its not the sound on the proggy card thats nessecarily at fault.
Basically I have this monopoly clubber here and it had no sound. Fair enough I thought, bad sound card, I will sort it another time. But when testing a couple of boards that were sent to me I was surprised to hear a whump out of the speakers (the boards didnt boot but the amps started up) and it got me thinking. What can be causing a no sound issue on a board where the sound is directed straight out to an external audio amp?
Out came the tools and the mission started. First port of call was to investigate the sound synth chip . There were no working meters on this board either and as they use one of the two output ports on this very versatile chip I put that down as a suspect. Logic probe out , and after a bit of fiddling, powered up and checked out the lines. D0-D7 were pusling away fine and so were the select , BDIR and clk lines but I was getting nothing at the port outputs, so I pulled and socketed both it and the 74HC373 running into it and checked again- still nothing!
changed C14 (couples the external audio out to the program card) and still nothing. Then I got wondering and it dawned on me. Theres no whump out the speaker. Even if the audio synth on the M1 was dead I would still get noise out the amp. So I metered the amp, 2volts at the +12v pin. A bit of a problem there! traced it out to the program card edge conn (pins 25a-26a) and out under a ULN2803a which happened to be the meters. And there lay the fault. The +12v power rail runs along and under that IC (U41) and theres a via there that the battery likes to eat. It had done just that on this board. Continuity checking between pin 25 program card and pin 10 of U41 (which is the common pin) I found high resistance, not open but it was certainly not a complete circuit, and it explained why 12v was appearing as 2v at the amp. It also explained why no meters and some HI-Z floating lines at the AY8930. Jumped the track with a piece of medium duty hookup wire, and fired up and WHUMP! and then as an added bonus the satisfying noise of ram clear alarm, job done.
To cut a long story short paul, check contiuity between U41 pin 10 (on an M1a board anyway, the numbers vary but its the ULN2803 next to the meters plug) and program card pin 25-26. It should beep, if it doesnt, jump the track between the via at pin 10 and its junction down to pin 26, itll be straight across from it on most boards.
next!
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