Juno 106 - wierd distortion fault

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Juno 106 - wierd distortion fault

Postby psygoat » Mon Oct 06, 2014 6:47 pm

Hi.
I've got a juno 106 in for repair.
I've done quite a few junos but this is a strange one -

Symptom - distorted sound on all of the voices apart from 1.
If I check with a scope all the outputs from the VCF on the voice chips show a clean signal.
If I put it in test mode and cycle through the voices - the signal at the VCA outs from the voice chips are distorted except for voice 1.
The distortion isn't clipping distortion, but more like an intermodulation distortion.
I've replaced one of the voice chips, which made no difference.
The power supply checks out ok - all the correct voltages are there and there doesn't seem to be excessive ripple.

Has anyone come across this before?

Thanks. Mark
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Re: Juno 106 - wierd distortion fault

Postby synthparts » Tue Oct 07, 2014 1:37 am

The first thing I would do is remove all the 80017As. Faulty ones can cause all sorts of problems - I've even seen them shorting the power rails to ground. Then re-install them one by one to see if one or more of them is causing the prob. You might also give them the acetone bath and remove the coating while you're at it. The BA662A VCA could also be at fault. If you need one I have them. Usually when they fail they just don't pass any signal though...
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Re: Juno 106 - wierd distortion fault

Postby psygoat » Tue Oct 07, 2014 4:53 pm

I can see that - I'll have to think hard about that one as the voice chips are a bit tricky to get out in one piece. Thanks.
I can always pull them and put a socket in -that'll make things easier later.
What do you think is the best method for getting them out btw?
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Re: Juno 106 - wierd distortion fault

Postby sam » Tue Oct 07, 2014 8:28 pm

I usually do 2 all six chip replacements per week...so I have a lot of experience.

You need a good iron and solder pump.

Iron on the pin for 3 to 5 seconds. Then use the pump.

Do this on all the pins then clean of any solder off.

Wiggle the chip and make sure the pins come loose...usually with the iron on the pin it comes free with wiggling.

Pull it out and clean the pcb. With pure alcohol or product.

You will have a chip which can be ace toned.
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Re: Juno 106 - wierd distortion fault

Postby psygoat » Tue Oct 07, 2014 8:36 pm

Thanks - I'll let you know how I get on.
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Re: Juno 106 - wierd distortion fault

Postby sam » Tue Oct 07, 2014 8:47 pm

Take your time always ..be careful not to break the pcb tracking.

I see this from people that have a go without any soldering technique..I have to sort out many disasters.

Good luck and do it proper.
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Re: Juno 106 - wierd distortion fault

Postby synthparts » Thu Oct 09, 2014 12:37 pm

A desoldering iron with built-in vacuum pump is the really the way to go with these. I highly recommend the Hakko 808.
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Re: Juno 106 - wierd distortion fault

Postby polysixer » Wed Oct 24, 2018 3:40 pm

This is exactly the problem I'm seeing in a Juno 106 now. Intermodulation distortion even with the voices, PSU and chorus perfectly calibrated. Even bypassing the chorus and going to something like -3 with all waveforms on I can get it to distort. Comparing to another 106 from a friend, mine seems much louder, distorting with 3-4 notes played depending on the frequency, filter position and waveforms selected/PWM position. I'm tracking the issue in the VCA/Chorus/Jacks board but so far no luck. I'm just hoping it's not the MN3009s. Any experiences or ideas welcome while I check the VCA and compare it to my friend's.
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Re: Juno 106 - wierd distortion fault

Postby Mooger5 » Wed Oct 24, 2018 6:12 pm

IIRC on mine this fault it was caused by the hpf chip, IC3 on the chorus board. One voice sounded clean, two or more just distorted no matter what. A new CD4052 replaced the old one and the problem disappeared
But it could be any other culprit in the signal path. On the main board there´s tp1 and tp2. Injecting an audio signal with increasing amplitude should tell if the culprit is for instance IC1a, or if the fault lies ahead.
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Re: Juno 106 - wierd distortion fault

Postby polysixer » Thu Oct 25, 2018 8:25 am

Mooger5 wrote:IIRC on mine this fault it was caused by the hpf chip, IC3 on the chorus board. One voice sounded clean, two or more just distorted no matter what. A new CD4052 replaced the old one and the problem disappeared
But it could be any other culprit in the signal path. On the main board there´s tp1 and tp2. Injecting an audio signal with increasing amplitude should tell if the culprit is for instance IC1a, or if the fault lies ahead.


I wish it was that simple. I’ve replaced IC3 already cause the HPF was my first suspect and those old 4052 tend to fail, but the problem remains. I have already tested all the ICs in the board using a proper working 106 chorus board and all of them are good, even the MN3009s so that’s good at least. I’ve also recapped all polar electrolytics and some bypolars too. The problem is coming down to some transistor or resistor degradation or cold solder joint. I wish the problem was easy to see under the scope cause there doesn’t seem to be clipping in the signal in the audio path of the board. More tests today.
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Re: Juno 106 - wierd distortion fault

Postby Mooger5 » Thu Oct 25, 2018 10:01 am

If bypassing the chorus doesnt change the problem then the problem is not related to the bbd chips. It seems that it could be the final output stage, or the muting transistors, or any other thing in that area, but since the distortion is present at low final vca settings, its not in that area. It lies before the final vca, or that vca itself, not ahead of it.
If its not the 4052 then look into ic1 in the mainboard, the m5218l chip. And the wiring connecting both boards.
Good luck.
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Re: Juno 106 - wierd distortion fault

Postby polysixer » Thu Oct 25, 2018 12:31 pm

Mooger5 wrote:If bypassing the chorus doesnt change the problem then the problem is not related to the bbd chips. It seems that it could be the final output stage, or the muting transistors, or any other thing in that area, but since the distortion is present at low final vca settings, its not in that area. It lies before the final vca, or that vca itself, not ahead of it.
If its not the 4052 then look into ic1 in the mainboard, the m5218l chip. And the wiring connecting both boards.
Good luck.


Thanks a lot for your help again. I ruled out IC1 summing out on the mainboard as swapping a chorus/jack board completely removes the issue. It's definitely located in that board. I'm looking into the transistors TR13 and 14 now, just before entering the chorus and out of the VCA. Muting transistors at the final output stage TR7 and 8 were also some of my first suspects and they're already replaced too with no results. I'll report my findings tonight.
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Re: Juno 106 - wierd distortion fault

Postby Mooger5 » Thu Oct 25, 2018 3:35 pm

Dont bother to replace the transistors. They are in the chorus audio path. I was being rethorical, or trying to.just take a more indepth look at the schematic. Two lines go from the output of the vca buffer: one goes to that filter where the transistors are and onto the bbd etc. Thats the wet signal. The other line goes straight into the final opamp with nothing in the way but the series resistor. Thats the dry signal. The dry signal is always present. When chorus is engaged it gets mixed with the dry signal at the summing node of the final opamp stage. Since with either chorus on or off the distortion occurrs, anything in the chorus path can be ruled out.
I ll post some suggestions later.
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Re: Juno 106 - wierd distortion fault

Postby polysixer » Thu Oct 25, 2018 4:19 pm

Mooger5 wrote:Dont bother to replace the transistors. They are in the chorus audio path. I was being rethorical, or trying to.just take a more indepth look at the schematic. Two lines go from the output of the vca buffer: one goes to that filter where the transistors are and onto the bbd etc. Thats the wet signal. The other line goes straight into the final opamp with nothing in the way but the series resistor. Thats the dry signal. The dry signal is always present. When chorus is engaged it gets mixed with the dry signal at the summing node of the final opamp stage. Since with either chorus on or off the distortion occurrs, anything in the chorus path can be ruled out.
I ll post some suggestions later.


Yes, I've also seen what you say and it works like that. My doubt however is that I'm not sure with there is some of the signal is still passing some of the chorus circuitry even if bypassed. If it's completely dry there's still a TR9 at the output of the final VCA that can be checked. Either that or some diode failure in that part of the circuit. Keep in mind we're looking at something that causes intermodulation distortion and not signal clipping. TR13 and 14 was a good hypothesis cause they seem to buffer/phase offset the signal and could explain that. Thanks again.
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Re: Juno 106 - wierd distortion fault

Postby Mooger5 » Thu Oct 25, 2018 9:18 pm

No problem. OK, I was indeed thinking about clipping distortion, and was about to suggest testing the power rails near the opamps, decoupling caps and also the non polarized blocking caps, as any of those could cause asymmetrical distortion.
Can you post an audio file? Is the distortion you hear present on both channels? If so, any possible culprit must be common. For instance, if there´s some leakage from the wet to the dry signal when off is selected, the suspects would not be the fets tr11 and tr12, as both would have to fail at the same time, but a leaking tr4 instead, that controls them. Anyway such situation I think would cause just a very faint chorus, audible when disengaged.
I dont know about tr9, doesn´t look like a suspect, but tr10 otoh is regulating the positive supply of the final opamp stage IC6. For some reason they found it necessary. I call IC5 the final vca btw.
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