[sdiy] Jp-8 head scratcher, solved or....need solvent

Jack Jackson jackdamery at hotmail.co.uk
Tue Jan 21 17:05:45 CET 2014


I take it you mean the NF510 when you say the JFET? What did you replace it with?

> From: diy at artefacts.nl
> To: synth-diy at dropmix.xs4all.nl
> Date: Tue, 21 Jan 2014 16:13:41 +0100
> Subject: [sdiy] Jp-8 head scratcher, solved or....need solvent
> 
> Hello all,
> 
> a little update for the archives and for everyone who helped me on the 
> tuning issues of the Jp-8.
> 
> I repaired the jp-8 some time ago. The problems in the 2 oscillators where 
> not related to each other, altough.......
> When I looked more precisely at the waveform I found some strange 
> oscillating effect just after when the cap was reset by the JFET. I have 
> seen problems with FETs in oscillators of a JP-8 but usually this was low 
> drain source current extending the reset pulse or leakage of the gate. 
> Replaced the FET and retuned the oscillator and the auto tune also tuned it 
> spot on.
> The other oscillator had a leaking JFET input of the integrator opamp. 
> Replaced it, retuned it and voila, worked out fine.
> My conclusion was: The error of the oscillation has more effect on high 
> frequencies and the lower input impedance has more effect on the lower 
> frequencies , therefore the autotune miscalculates the values for fine tune 
> and scaling. Tuning by hand you can sort of make it work because you also 
> have the high frequency compensation.
> 
> Now I have an MKS-80 in for repair that makes me think again about the 
> problems I had with the JP-8. The MKS-80 had large dc feedthrough of the 
> envelopes of the filters in 3 voices. I replaced one filter chip, worked 
> fine, replaced the two others with no result. Tried serveral chips but no 
> change. Replaced caps on both filters. One solved, the other still 
> unchanged. Tried all kind of things with no result. Then I cleaned both 
> sides of the board with flux cleaner. Problem gone.
> 
> Are the more people having troubles with the Roland flux? There boards are 
> always covered with a thick layer of flux and I have never had problems with 
> it before or not that I know at least.
> 
> Cheers
> Maarten.
> 
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