Voice broken... please help

The Roland Jupiter-8 Analogue Synthesizer
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Joined: Thu Apr 08, 2010 8:28 pm

Voice broken... please help

Postby roflcopta » Sun May 02, 2010 7:55 pm

Its a 14bit model with groove midi interface.
When the Problem started there was only VCO1 dead, VCO2 was still working from time to time but now both are dead.
The test Program says its Voice A on the second MOD, i also swapped the boards and it turned out that the fault must be on this Board and not on the INT or MOD CON.
I did the Calibration Procedure till point 5 (VCO MOD) and those Point before were all in the tolerance and i just did some finetunning, so Power etc should be ok.
On the Voice the Noise is still Working and so do also the VCF and VCA.
When i turn the Volume really loud i fund out that i can hear the VCO1 & VCO2 as well on a very low level and they seem to work normally…
so i guess its somewhere between the VCOs and the Filter, maybe an OP?
I also did some measurements with the scope and compared them with the other working Voice (B) on the Board (and some other Voices as well), so here are the differences ive found:
TP1B: 13.764V; Squarewave
TP1A: 9.302V DC
TP2B: 13.765V; Squarewave
TP2A: 9.131V DC
i guess i could have to do with an OP(s) or maybe a CV for one (them)?

It would be glad i someone could give some advice, since i do have only basic electronic knowledge. I originally learned electrician, so i know how the electric laws, how to measure and what the parts in general do.
Thanks in advance

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Location: Gold Coast, Australia

Postby adam8it » Mon Jun 07, 2010 2:02 am

Sorry Kris I don't have much to add here, but would like to know how you get on. It sounds like you are localising the problem though, and it's encouraging that you can hear the voices behaving normally although at low volume.

If you look at the "module" (voice) board schematic, there is a TL082 op amp that mixes VCO 1 and 2 (the overlay graphic says "Source Mix"). If you don't have a scope, maybe you just check with some kind of audio probe as to whether you are getting signals either side of this?

(As you're an electrician we don't need to remind you of the mains voltage warnings etc etc!!)
Adam Inglis
A Disco Ate My BABY!

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Joined: Thu Apr 08, 2010 8:28 pm

Postby roflcopta » Wed Mar 23, 2011 8:53 pm

its been now quite a while since my post i just would like to let you know how it went with my jp8:
brought the jp to my favorite synth tech (martin from synthorama) and he was able to fix it :-) (so no jp-7 anymore)
here in short:
both ICs CA3046 were broken, one RC4558 and a capacitor as well, he was able to replace them and now everything works fine again!
he couldnt tell me why they were broken, since its quite strange that both chips on different voices gone dead.
since then it works like a charm :-)
and btw, if your ever in switzerland: you have to check out the synthorama, one of the worlds biggest collection of analogue synths!
stay tuned

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Joined: Tue Nov 06, 2007 1:41 pm

Postby noidear » Thu Mar 24, 2011 11:56 pm

Hi Kris,

Glad to hear you got your JP-8 fixed, and thanks for letting us know what the issue was. Very useful to know these infos as it gives a good starting point for others when they have similar probs.

Re the synthorama museum, yes I heard about it recently when a freind of mine went there last month. He said it was unbelievable. I believe it is only open to the public the first saturday of every month but my friend was lucky to be there at the right time. Anyway hopefully one day I will be able to visit it as it sounds like synth paradise. :)

All the best and enjoy using your fixed JP-8

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