[sdiy] Jupiter 4 overall tuning drifts after it gets warm

[Gil W.]

Thanks,

Well the 726 ICs are on each of the four voice boards. Basically I assume the problem is not on any of the voice boards themselves as the total tune changes, not the tune of an individual voice... Also, if I'm not wrong, the 726 are not responsible for the tunning itself but for tone generation in the JP4 ??

I haven't used it extensively enough since I bought it to be able to tell if this is a "new" behavior. Also, it gets stable quite fast, actually almost as soon as you power it on. But after an hour the overall tuning start to get up and down (a very slight change) slowly.... I believe that some specific part changes its value due to heating ? I checked the +15/-15/+5 voltage line as it happens, and they are very stable (meaning the tuning will rotate while the voltages will keep on being stable).





--- On Thu, 1/6/11, lanterma at ece.gatech.edu <lanterma at ece.gatech.edu> wrote:

> From: lanterma at ece.gatech.edu <lanterma at ece.gatech.edu>
> Subject: Re: [sdiy] Jupiter 4 overall tuning drifts after it gets warm
> To: gil_we at yahoo.com
> Date: Thursday, January 6, 2011, 4:05 AM
> On Jan 5, 2011, at 8:57 PM, Gil W.
> wrote:
> 
> > My JP4 overall tunning (the scale and the individual
> tuning of each card) is generally ok. How ever, after about
> an hour of work the overall tuning starts to move up and
> down a few semitones...
> > The tuning of all voices "moves" as one piece, so I
> guess the problem doesn't lay at any of the voice cards but
> somewhere else in the circuit ??  From my experience
> this can happen when a precise capacitor (usually a
> polypropelene) goes bad, but I couldn't find any capacitor
> of that type other than on the voice boards. Any suggestions
> please ?
> 
> I think the Jupiter-4 uses Fairchild 726s if I recall
> correctly, and that's a "heater" type of design. Has your
> Jupiter-4 always done this or is this a new behavior? If
> it's the former, maybe the heater mechanisms are just taking
> a long time to stabilize, and Jupiter-4s need to "warm up?"
> 
> - Aaron