SID failure modes

[Davo]

I'm checking out a C64 using the "Music Machine" cartridge.  I figure that ought to be a halfway decent and quick exercise of the SID.  Anyhow, this C64 has a dodgy keyboard, but I'm able to get one key to make a sound.  Repeated presses of the key causes the same note to appear onscreen, but each time, it gradually rises in pitch by maybe an eighth of a step.  After maybe 20 presses, the note is constant.  Is this a good SID or a bad SID?  What would be a better SID test?

[TK.]

I don't think that this is a bad SID, it seems to be a feature of the software (e.g. an envelope on the pitch?)

If you hear any sound, you can be sure, that the SID is ok

Best Regards, Thorsten.

[Davo]

I also determined that the SID is good, but was left puzzled.  I put the SID in question into a known good machine and tested with the same Music Machine cartridge.  The SID sounded great.  I then tried the good machine using the bad keyboard.  Problem reappeared.  Put the SID back in the first machine and plugged in a known-good keyboard.  The SID sounded great.  So, the keyboard was somehow able to cause this odd problem.

Regarding hearing any sound indicated a good SID, I've heard that some SIDs can have good oscillators but bad filters.  I wish I could find my Frogger2 cart.  That was a good sound test.

[TK.]

Yes, especially the newer (shrinked) 6581 versions which were produced 1984 and later have bad filters - if good filter sound and low noisefloor is desired, consider the use of a 8580

Best Regards, Thorsten.

[Davo]

I've been trying to get 8580s and just not realized that only certain C64Cs have the 8580 chip.  So I now have six 6581 SIDs of varying vintages and one 8580 still in the computer.  How do I determine a shrinked 6581 from the markings on top?

[TK.]

How do I determine a shrinked 6581 from the markings on top?

there isn't that much documentation available about this, but the lower line shows the production date which gives a hint

Best Regards, Thorsten.