Solved: Noisy JX-8P Outputs

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Solved: Noisy JX-8P Outputs

Postby synthRodriguez » Fri Aug 05, 2016 6:26 pm

For the archives...

Bought a nice JX-8P with noisy, garbled, crackling outputs. All six voices were working, but lots of snaps and pops especially on certain patches. None of the patches sounded right, everything sounded "in the background" with white noise on top of it; some voices were faint, others sort of ok.

It wasn't a dirty H-M-L level switch. Traced the signal back to TP5 which is the voice outputs right before they go to the Chorus, Limiter, and HP filters. Sounded the same. Found some info online where the HD14052 multiplexer right there tends to go bad in Juno 106 synths. Swapped it out, no change. Ok, time for some better sleuthing.

Hooking up a PG-800 I found when using the Mixer sliders, DCO1 would always sound all six voices (although fuzzy and inconsistent) but DCO2's six voices would round-robin <Loud-Loud-Soft-Nothing-Nothing-Soft>.

Probed the 8-channel HD14051 Mixer multiplexer, IC45. All outputs were operating properly when sliding the mixer controls. The six Mix A-F outputs respond to the DCO2 slider, the one Balance A-F output responds to the DCO1 mix slider. Ok, the multiplexer's doing its job.

Moving downstream, the multiplexer feeds its outputs into two TL064CN quad opamps which condition the signal into something the voice circuits can use; sliding a PG-800 mix control from full down to full up should cause a 0 to 5 volt excursion on the output legs of the opamps.

Everything looked fine on IC53, but on IC54 all four outputs would stay buried at -10 VDC no matter what position the slider was in.

Summary: Bad TL064CN opamp in the IC54 location. Changed it out, all's well again.
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Re: Solved: Noisy JX-8P Outputs

Postby johnstimley » Sun Aug 14, 2016 1:19 pm

Its great to hear when someone manages this kind of detective work and fix.
I would buy an old synth myself but have neither the know or experience to do such repairs.
Is there a dummies guide to fixing synths?

ATB JS.
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