simonfr Posted August 20, 2012 Report Share Posted August 20, 2012 Hello everyone, I hope you guys can help and i have posted this in the right place!! I dont have a MIDIbox, but i do have a Commodore 128 computer with a 6581 SID chip, stamped 2385 (manufactured wk 23, 1985). Anyway, the sound output is very faint and i have to turn the volume control up on the computer monitor to hear the sound. The sound works fine but is very faint. I have replaced the inline electrolytic capacitor in the circuit, and the 1kohm load resistor seems fine, and i have also replaced the 2SC1815 transistor in the circuit. All other capacitors and resistors like for the low pass filter seems ok (removed most of the components from the board and tested them). The SID is getting +12V and +5VDC, and the earth continuity seems fine. But after all this, i still only get faint sound....... From reading output on a multimeter (no oscilloscope available). I programmed the SID in 128 mode (basic) to cycle the volume from 1 to 15 playing all notes in a sequence ABCDEFG repeat. When the SID is commanded to change the volume setting in one step, there is no difference in the output volume, but the step causes the output to 'crunch' loudly. Then when it goes from volume 15 down to 1, it makes a really loud statically 'crunch' (if you know what i mean). At volume 0, the output on pin 27 is 3.7V DC. As the SID volume is commanded to go louder, the output DC voltage goes down from 4.7V DC to about 3VDC (15 down to 1) in uniform steps. There is no apparent AC output on the multimeter. So i dont know if its the inputs/output circuit or the chip itself.... Does this sound normal? Has anyone had any similar issues and ideas what could be wrong or what to look/test for? Do you think the SID chip is dud?? Hope you can help. Cheers, Simon Quote Link to comment Share on other sites More sharing options...
Hawkeye Posted August 21, 2012 Report Share Posted August 21, 2012 Probably the best way to test would be with another C64 or another 6581 SID chip and swapping chips... Most failed SID chips have broken filters, i´ve not yet heard that that the master output volume control breaks, but of course, it could be possible, especially if you are sure that the output circuit components work fine... From the 6581 Datasheet: Audio Out (Pin 27) -This open-source buffer is the final audio output of SID, comprised of the three SID voices, the Filter and any external input. The output level is set by the output Volume control and reaches a maximum of approximately 3 volts p-p at a 6 volt DC level. A source resistor from AUDIO OUT to ground is required for proper operation. The recommended resistance is 1 KOhm for a standard output impedance. As the output of SID rides at a 6 volt DC level, it should be AC-coupled to any audio amplifier with an electrolytic capacitor in the 1-10uF range. Best of luck! Peter Quote Link to comment Share on other sites More sharing options...
simonfr Posted August 22, 2012 Author Report Share Posted August 22, 2012 Thanks for your reply Hawkeye. It is increasingly looking like the SID is probably dud. The IC isnt even responding to the VOLUME 0 (zero) command which should turn the sound off. Otherwise its the even more unlikely that the input A0-A3 and D0-D7 lines to register the SID arent working properly, considereing it is actually generating 'perfect' sound albeit without volume!! Anyway, i have another supposedly genuine 100% tested guaranteed 6581 SID winging its way to me soonest, so soon find out :-/ Cheers, Simon Quote Link to comment Share on other sites More sharing options...
Hawkeye Posted August 22, 2012 Report Share Posted August 22, 2012 Hi Simon, I do agree and would also guess, that the SID has a problem! Would be glad to hear about your findings after you´ve installed the new one! Many greets, Peter Quote Link to comment Share on other sites More sharing options...
simonfr Posted August 29, 2012 Author Report Share Posted August 29, 2012 Hi Simon, I do agree and would also guess, that the SID has a problem! Would be glad to hear about your findings after you´ve installed the new one! Many greets, Peter Peter I have some good and bad news to report.... The good news is, after installing a 1983 flavour 6581 SID chip into my Commodore 128, normal service is resumed.... Banging tunes with volume control, from such great games such as Zoids and Spellbound (music author Ron Hubbard). The output is as it should, ~6VDC signal with overlaid AC coming out of pin 27 on the chip (as it should). The bad news is that there is another dead 6581 SID chip in the world...... If anyone does want this chip for whatever evil experimentation they may have in mind, then please do not hesitate to contact me. As mentioned before, from what i can tell, perfect sound output, but no volume control as described above.... Cheers, Simon Quote Link to comment Share on other sites More sharing options...
Hawkeye Posted August 29, 2012 Report Share Posted August 29, 2012 Hi Simon, bad to hear that, but good that you worked it out :) Many greets, Peter Quote Link to comment Share on other sites More sharing options...
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