Sequential Circuits Prophet 600 -1982    Weight = 22 Lbs. Number mfg.~7000 MRP=$1995

Circuit Overview:
     Z80 controlled synth using 6 pairs of Curtiss CEM3340 vco synth with 6 CEM3374 filters. With the T8, the first MIDI synth. The DAC stepping sounds a bit course on some of the controls as typical with resonance cranked up or with the vco coarse tunes of course. The Prophet 5 is smoother also because it has real EG's and the digital stepping in the Envelope control of the 600 is very evident in some instances. The sudden drop at the end of envelopes is particularly bothersome to me. But it's a wonderful synth for some of the sounds it can do and some of it's special capability like 12vco stack by cranking mix knob all the way left in unison. Also to smooth things out a bit the pedal input is a direct analog control and a 10V voltage control pedal is a marvelous addition.

Service Tips:
     You'll need new EPROM (see above) if MIDI is onmi always. Capable of stacking all 12VCO's for ultra thick leads by cranking the mix knob all the way left before storing the patch as I recall. (Helpful while troubleshooting also). The one I just worked on had two failed 4067 chips on the front panel boards leading to just noise coming in from there. The DAC also had a bad pin in the socket, the msb...which made for knobs acting like two position switches of course :-).

NOTE: Regarding CEM3372 VCF's. We had a recent bit of experience trying to get these to work in Doug Terrebone's Prophet 600 and in one I had here. None of the stock I had would sound the same (not to mention that two I got from chipforbrains some time back were totally doa... bummer. Thought those were NOS!) They lacked the whispy overtones with Resonance all the way up in particular. THey were all "C" revision. The "B" rev's were in the P600's but Doug said he finally got in an AX80 that had Rev "C's" and those worked fine! Go figure! Anyway just beware that there is some variation in these chips.

Very early 600's (like mine) might have a .22 instead of 1uf capacitor for the power-good filter. I'd change that. Mine might have also just had a leaky 74C02 though. It measured ok on my chip checker but I put a new one in and it worked after that. The signal was crumpled to under threshold on pin 3.

Also some of the electrolytics used might be failing. Check esr's. Dead voices randomly scattered about would most likely be that issue I think.

Also this same dandy machine wound up having an LFO that would distort on the bottom half of the waveform and be clean on top. I mean gnarly thwappy noise..my friend said the sounds I was making would be a hit in the glitch music realm. I saved the bad cpu that was heating up (though not hot...just ..I would put a little ethanol on it and the lfo would start working..ethanol evaporates and a minute later it starts doing it again.) and I have no expanation at the moment for why it would manifest in this way. Intermittently the machine STILL boots with voices out of tune though! I have to retune..then it's fine usually. The caps are all good in this one. hmm. ALSO this machine (5679) had version 7 eprom which had some kind of issue according to the summary I got on Noteit's site years ago. Let me just post the update I made to that file though right here

Prophet 600 Revision History


  Prophet 600 Revision History 
  This page details the history of the Prophet 600 synthesizer revisions. 
(my notes)

    Six_0_1 was in my #512.  Omni mode only. Not sure of other problems.  But this was apparently the real original.

    Six_0_2 was recommended in the manual I have as a replacement for it.  I have one of these too in my drawer. Smith?
.

(back to Notelt's notes)

    Six_0_4 
      This was the (one of..obviously) first revision sent out with the Prophet 600. Omni Mode only. 


    Six_0_5 
      Added Mode and Channel select features. 

    Six_0_6 
      Corrected a problem with the envelope adjustments while playing and fixed 
      a recognition of running status Note On events with zero velocity. 

    Six_0_7
      A pretty rare revision. Very few Prophet 600s went out with this version. 
      There was an error in the OS which was quickly corrected in the Six_0_8 
      release. [Note: A special thanks to M. Schorer for providing this 
      information].  (this was in my #5679 machine)

    Six_0_8 
      Added the following: 1. MOD wheel range was expanded (for better 
      compatibility); 2. Fixed incompatibility (droning notes) with Roland 
      sequencers (MIDI timing clock was interfering with Note Off events) and 3. 
      Fixed a bug where a note may stick if UNISON is switched off while the key 
      is held. 

    * Record + 1 = program change on/off
    * Record + 2 = current program dump
    * Record + 3 = center pitch wheel
    * Record + 4 = wheel send/receive on/off
    * Record + 5 = sequence dump
    * Record + 6 = Omni mode on
    * Record + 7 = voice defeat (hold key down first)
    * Record + 8 = Poly mode on
    * Record + 9 = channel +/-