Juno 6 Fault Tracing / Hanging notes

Synth Repair
1

Hello there fellow synth nerds! This is my first post here so thanks in advance for taking a look at my issue and helping if you can.

I have a Juno 6 that has hanging notes regardless of settings and am at a loss as to what is going on. On boot up it hangs all voices like it has several keys held, I thought this might be the HOLD button needing a clean but after replacement of the button with a known working one and testing to see if it might be a transistor related issue it turns out to be neither.

On top of this that issue the KEY/TRANSPOSE led is constantly lit and has also so far received a replacement button and transistor.

I have traced the KEY/TRANSPOSE, T0, T1 and TOTAL GATE lines and unplugged the 4 pin, cleaned both the pins and the connector with deoxit but still no change.

Using some jumper wires I connected just the T0 and T1 lines and partial success, I can play it but missing a voice. If I enter test mode all 6 voices are present when all 4 pins are connected (Still hanging) but not when TOTAL GATE and KEY TRANSPOSE lines are connected.

So far I think the issue is within the panel board but I cannot be sure since I have stripped, cleaned and tested every component on that board and found no faults other than a dirty fader or two which I have rectified, Not even the switches are faulty.

I checked the voltage of each rail and they are damn near perfect +/- 15v and +5v. the only issue before this happened was the bender board took a beating several years ago and has since been repaired and is working as intended.

Just so it is known I have see two other owners with this fault but looking through forums it looks like the issue was never solved for them either.

Any help on the matter at hand will be greatly received and thanks in advance.

2

Try using the schematic and measuring what is going on at the test points:

Given the oscillators in many synths free run, it’ll be a chip holding its state and not letting the VCA close. Whatever is providing the gate signal.

Buy you might be lucky in that it just needs some calibration.

3

I’d check with the scope what is coming from the outputs of IC50 (4099) on the mainboard. If this looks like the correct voice individual gates, then I’d replace the two 4011 (IC 48 and IC49). In the original they are unbuffered versions. I replaced them with buffered ones (CD4011BE).

4

I measured the gate test points and gate 1 is low and gates 2-6 are high. I supect it’ll be either a 4011 or the 4099 that is at fault. Thank you.

5

So as I mentioned I checked the gates, 1 is low 2-6 are high and none are being triggered low. I checked the data into the 4099 and there is strong data, reset is low, A-1/2/3 (Binary Inputs) seem normal but pins 1 and 9-15 (Outputs) look to be irregular. I’ve had a search through parts I have spare and have found 5 NOS TC4099BP and a whole tray of CD4099B. I’ll start with replacing that and report back. Many thanks!

1 Like
6

Yesterday morning I got around to changing out IC50 and almost everything is back to normal, I replaced it with a NOS TC4099BP. This morning I thought it a good idea to calibrate and tune and discovered a (supposedly) new fault.

I went through the calibration process and noticed a problem with C4, All other keys are unaffected but it sounds and looks like (on the scope) that just C4 has less width in a square than every other key. Anyone have a clue as to why?

7

Test whether it is really C4 or whether it is a dedicated voice. Play repeatedly:
C4 D4 E4 F4 G4 A4 B4. In this way you will play the C4 each time with another of the six voices.

8

I was doing just that when the notification came through! :laughing: So it is just C4, I have probed about and regardless of what voice is triggered by C4 it still has half the width on both saw and square, this can be adjusted by PWM until the note almost dissapears completely, ruling out that part of the circuit. I have however noticed some odd behaviour on IC58 which I’m taking a look at now. I have a spare so I’ll socket it and drop a new one in but I can’t fathom how that would cause this.