Help with Roland Juno 6 Issue

hobgob_inc · Joined: Jun 09, 2009 Posts: 46 Location: Australia

Hey guys,

I am trying to repair a Roland juno 6 which had a couple of faults. I cleaned some sliders etc and fixed a few small faults but the main problem has me stumped at this point.

The main problem is that the sawtooth and PWM waveforms both are extremely thin in all voices, ie. the sawtooth does not ramp back up (or down) as it should, it looks like a thin spike not the normal sawtooth shape. It has the correct amplitude however, 12Vpp.

The sub oscillator is fine. The square wave which comes into the waveshapers from the programmable counter ships is also perfect.

I first thought that it was the integrator caps in each voice, as they were way outta spec but new ones made no difference. I have also replaced several of the 4000 series chips. The fact that the fault is present over 6 voices would indicate something futher back in the circuit, however my understanding of the digital circuitry is not great. I do have a scope.

If anyone knows anything about juno 6 circuits it would be great to hear from you.
Thanks a lot

Luke

hobgob_inc · Joined: Jun 09, 2009 Posts: 46 Location: Australia

Oh I have the full service notes too, which are quite helpful

Uncle Krunkus · Posted: Wed Aug 31, 2011 9:40 pm Post subject:

Is the PWM programable?
ie. Can you set the Pulse Width for each patch and save it?
If so, there a possibility that the PWM inputs are recieving a permanent low PW amount from the program chips.

PS I don't know Junos that well, (I'd love to work on one) I'm just thinking out loud with some ideas.

ian-s · Posted: Thu Sep 01, 2011 12:13 am Post subject:

The PWM comes from the saw and it sounds like the saw is not working. From memory, the saw DCO is a regular integrator that gets reset by the digital section. My guess is the CV that controls the ramp rate, that should come from a dac and probably a demultiplexor, is not getting through.

Check the DAC output and multiplexor (the polysix used a 4051)

If the filter still tracks the keyboard then the DAC is most likely ok.

Good luck.

hobgob_inc · Joined: Jun 09, 2009 Posts: 46 Location: Australia

Thanks for the replies guys. Krunkus- Yeah i did actually think that the PWM section was a definite possibility, as you said with something sending an incorrect voltage into the PWM inputs. But as Ian said the PWM comes from the sawtooth, so it would indicate something else.

After reading both posts I went back and took a step back... I had already replaced the critical 4051 demultiplexors, and realised that there was a voltage sitting near the rails where there should have been Keyboard CV straight after the DAC. through isolating different sections of the circuit I was then able to trace this down to a bad TL082 pulling the KCV up.

So at this point its looking all good!

Thanks again
Luke

Uncle Krunkus · Posted: Fri Sep 02, 2011 12:41 am Post subject:

Great news,
I don't have the dosh to lash out on an old synth which needs a bit of TLC, but it's a dream I love to share. Following your work can be both educational and inspiring.