[SOLVED] Juno 106 issue: no saw wave

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[SOLVED] Juno 106 issue: no saw wave

Postby polysixer » Fri Dec 16, 2016 3:14 pm

Hi, I'm troubleshooting a Juno 106 and have almost everything sorted, including filter chips. Still remaining there is one issue present from the moment I picked it up: there is no saw wave produced. Mind you, if I turn the volume all the way up only with saw selected I CAN hear a very faint note (which doesn't seem like it's square leaking, as it doesn't respond to PWM) which should be a very silent saw. By the looks of it it's not the saw wave tact switch as it seems to enable the LED perfectly. It doesn't seem like the waveform generators are at fault either, as the are perfectly fine with square and sub, so it should be something common to the three after that. Besides a saw failing in three chips would be very rare, and even then AFAIK the square share is derived from the saw in the Juno 106, which makes it even more puzzling.

I've already replaced IC24 which muxes all DCO CVs, but no dice. IC17 seems to only manage square and sub signals but maybe I'm missing something. Should I be looking more towards Panel Board IC and diode failures instead of Module Board? Anyone with experience/knowledge on how the saw wave is handled? Thanks in advance.
Last edited by polysixer on Tue Dec 20, 2016 11:08 am, edited 1 time in total.
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Re: Juno 106 issue: no saw wave

Postby cornutt » Fri Dec 16, 2016 4:43 pm

I'll take a look at the schematics this weekend. But yes, the DCOs are saw core, so if the pulse wave is working, the saw has to be working. (The pulse waves are working, right? Not just the sub-oscillator? The reason I ask is because the sub-oscillator isn't derived from the saw; it's derived directly from the DCO counter.) I'm trying to remember how the voice mixer circuit works... IIRC there are separate VCAs for the saw and pulse waveforms on each voice, and then everything gets mixed prior to reaching the final VCA. That's where I'd be looking. I think all of that is on the module board.
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Re: Juno 106 issue: no saw wave

Postby polysixer » Fri Dec 16, 2016 5:25 pm

cornutt wrote:I'll take a look at the schematics this weekend. But yes, the DCOs are saw core, so if the pulse wave is working, the saw has to be working. (The pulse waves are working, right? Not just the sub-oscillator? The reason I ask is because the sub-oscillator isn't derived from the saw; it's derived directly from the DCO counter.) I'm trying to remember how the voice mixer circuit works... IIRC there are separate VCAs for the saw and pulse waveforms on each voice, and then everything gets mixed prior to reaching the final VCA. That's where I'd be looking. I think all of that is on the module board.


Exactly, thanks a lot for the input. Yes, both the square and sub are fine, it has to be something in the saw mixer path. Suspects are IC16/19/20 and Transistor 23. I'll probably start replacing the transistor because it's the easiest.
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Re: Juno 106 issue: no saw wave

Postby Mooger5 » Fri Dec 16, 2016 9:30 pm

One circuit that´s common to all voices is Saw On/Off. I´d check for a change in voltage at pin 12 of IC40 and emitter of TR23 while switching Saw On and Off.
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Re: Juno 106 issue: no saw wave

Postby polysixer » Mon Dec 19, 2016 1:30 am

Mooger5 wrote:One circuit that´s common to all voices is Saw On/Off. I´d check for a change in voltage at pin 12 of IC40 and emitter of TR23 while switching Saw On and Off.


That's a great idea! Completely flew past me that section in the service manual. However pin 12 and TR23 emitter display a constant 4.83V regardless of the button selection. Should I guess the IC is bad after all?

Taking a close listen with maximum volume and square/sub set to off I noticed the faint saw wave heard is always there being triggered by the keypresses, regardless if the ON/OFF saw button state, and in some notes it triggers 2 simultaneous notes. I haven't done the acetone process on the wave chips, as the square/sub sounds perfect and I don't really know if it's advisable. Doesn't this look more like a MUX problem on the other hand? Great comments so far, thank you.
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Re: Juno 106 issue: no saw wave

Postby Mooger5 » Mon Dec 19, 2016 11:37 am

Sorry, I meant TR24, not 23. A reading of a constant +5V at pin 12 should tell you that IC40 is probably faulty. There should be a logic signal at TP16. If it checks OK, I´d replace IC40.

If it´s related to amplitude of signals it´s not a mux problem, as that one is only for pitch control. What you´re hearing at high output levels is probably transistor leakage and crosstalk between adjacent voices.
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Re: Juno 106 issue: no saw wave

Postby polysixer » Mon Dec 19, 2016 2:06 pm

Mooger5 wrote:Sorry, I meant TR24, not 23. A reading of a constant +5V at pin 12 should tell you that IC40 is probably faulty. There should be a logic signal at TP16. If it checks OK, I´d replace IC40.

If it´s related to amplitude of signals it´s not a mux problem, as that one is only for pitch control. What you´re hearing at high output levels is probably transistor leakage and crosstalk between adjacent voices.


Yep, I was actually checking TR24, sure. I'll replace IC40 today and tell you how it goes, thanks.
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Re: Juno 106 issue: no saw wave

Postby Mooger5 » Mon Dec 19, 2016 3:53 pm

And actually the mux has to do with the waveform amplitude in the case of the junos dcos, but inthis case its not the culprit since there is nothing wrong with the square output. Sorry.
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Re: Juno 106 issue: no saw wave

Postby polysixer » Tue Dec 20, 2016 9:17 am

And it turned out IC40 and was at fault, of course :) All perfectly working and gorgeous sounding! Feels great bringing back this baby from the dead. There is some crosstalk at top volume but that's true of all Roland vintage gear I've tried, not too worrysome. I'm waiting for two analog renaissance chips and it's good to go!

Still remaining question: is it advisable to remove the epoxy from the wave generators if they're still working fine?

Again, thanks for your invaluable help!
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Re: Juno 106 issue: no saw wave

Postby Mooger5 » Tue Dec 20, 2016 10:48 am

No problem. Please mark the thread as "solved".

You could minimize some noise by calibrating the VCAs.

As for the wave generator chips, you must know the saying: "if it´s not broken..."
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