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Jupiter 4 Tuning

PostPosted: Sun Jun 22, 2014 11:30 pm
by harry
Hey guys,

i know a few of us have Jupiter 4's here..
was wondering how yours all hold up tuning wise?

Mine has always been a bit out of tune.

Today i decided to try and tune it - I warmed it up and tuned it nicely (using the 4 trimpots) - the scaling is almost perfect so i didn't want to mess with that (internal voice card trimmers). My psu voltages are all spot on (within tolerances set out in the service manual)

Thing is, when it's turned off, the next time the voices are all out either way by say 20 cents..

I could just tune it before every session (only took a couple minutes), but i would like to make it more stable.
Curiously - wikipedia says this about tuning - i can't find the article that this references:

[i]The VCOs can garner unstable tuning if aging or low-quality electrolytic capacitors are used (two for each voice card). Synthesizer repair shops can replace these tuning capacitors with stable polystyrene capacitors for an instant perfect tune. Individual oscillator card VCO tuning is accessed by four capped holes in the middle of the back of the case, but the unit should be allowed to warm up before adjusting for at least 20 to 40 minutes prior to adjustment.[/i]

Has anyone done this? i.e.: changed the 'tuning' caps for polystyrenes? and which ones are the tuning caps?

I'll eventually go through and re-cap all my synths, but i'd rather not pull it apart to do so unless i have to.

I'd be very curious to know how your Jupiters hold tune? and if you have recapped and noticed a difference etc. I've a desoldering station on the way, so it wouldn't be too hard.

Re: Jupiter 4 Tuning

PostPosted: Mon Jun 23, 2014 1:01 pm
by Chinagraf
Hi Harry, mine holds up fine. I've never touched those pots in the back, I did mine with the pots on the actual voice boards. I must have got the procedure from the service manual. Sorry don't know which are the 'tuning capacitors'.
From memory there aren't too many EL caps on the voice boards, I'd just replace them all.

Re: Jupiter 4 Tuning

PostPosted: Mon Jun 23, 2014 8:16 pm
by harry
Hey Andy, thats great that yours stays in perfect tune.

Mine is really good - but i notice it when 1 note is 10cents (1/10th semitone) out.

I didn't realize that the voice boards control overall tune - i thought they only contained the 'scaling' trimpots.

I'll have to re-read the service docs again and have another go.

Re: Jupiter 4 Tuning

PostPosted: Tue Jun 24, 2014 9:49 am
by Chinagraf
From memory I think they will control the scaling but also the pitch center. I vaguely remember something in the service manual about centering those pots in the back before started the calibration process, so I may have done that possibly.

Re: Jupiter 4 Tuning

PostPosted: Tue Jun 24, 2014 7:03 pm
by Thirteen
There aren't electro caps in the oscillator core or the antilog amp in the JP-4 from memory, certainly the integrator cap is not an electro. If the VCO's are scaled correctly but the tuning is out it may be an issue with the digital-analog converter... Correct scaling would indicate that the VCO's are not faulty.