Boss CE-5 - clean sound off AND on

[Bucksears]

I know this is a DIY/building forum, but this is my best shot at troubleshooting this issue.
I picked up a mint-in-box analog (pink label, serial dates it made in '94) Boss CE-5. Full disclosure in the ad that it was 'as-is'/not working. Confirmed with the seller that the pedal passes a signal to output when it's off and on (LED lights up), but there is no chorusing effect.
My first thought was the switching JFETs, so I replaced those. No chorus.
Then I replaced Q12, which is fed from the above JFETs. No chorus.

I haven't taken any voltage readings yet, but will try to do so tonight. I have a few MN3007s on hand, if it comes down to replacing that - I'd just like to rule out any other potential part failures first.
Any suggestions on where to start looking?

[Ripthorn]

Audio probe the output of the BBD. My guess is that you will not get anything there. You will likely have signal at the input of the BBD, but not the output. If you do get output, then your clock or LFO could be the issue, as having no change in delay time will result in just a very short static delay signal and not chorusing.

[MrStab]

Also check for voltage change at FET gates between bypass and active states (though LED lights up, so flip-flop does appear to be working).

[Mark Hammer]

The trimmer feeding the bias voltage to the input pin on the BBD needs to be set right.  If not, no signal will emerge from the output pins.

It can be tuned/set by ear, although best if the dry signal is NOT included in what you are hearing.

The bias trimmer will go from no BBD output at each extreme end of rotation, through to a very distorted and lower amplitude output as you move away from the end of rotation to something much cleaner and louder as you approach the optimum trimmer setting.  As you pass through that point, things get distorted again, and finally silent.  You won't damage anything electronically by playing with that trimmer, but they are typically not built to withstand a lot of manipulation, so try not to wiggle it too much.

[Bucksears]

I used an audio probe on the MN3007 and I'm getting chorusing; it's not subtle, almost tremolo/vibrato-like, but rate affecting the sound.
I'll start probing the IC path that follows.

[Bucksears]

As I mentioned in my original post, I replaced the 2SK118Y switching JFETs with a couple of 2SK30Y ones that I got from Tayda.
With the pedal on or off, I'm getting 4.05V on the Drain and Source, with the Gate reading all over the place.
With the audio probe, I'm not getting sound on the DGS of either switching JFET.
Could the replacements I got from Tayda be bad?

[MrStab]

I was thinking more about what's upstream, switching the gates of both old and replacement JFETs. Could you elaborate on what you mean by the voltage being all over the place?

(FWIW, removing the FETs and leaving bypass path unconnected while shorting pads of Drain & Source to/from wet path would help determine whether chorus circuitry works, but I don't want to encourage too much desoldering)

[Bucksears]

I was thinking more about what's upstream, switching the gates of both old and replacement JFETs. Could you elaborate on what you mean by the voltage being all over the place?

Sure. Testing this a couple of hours ago, the Source and Drain voltage for both JFETs was 4.2V (the power supply was putting out 8.8V). The Gate on both JFETs was just random voltages on the meter; like it would cycle up to 3.7xxxxx volts, then blip, and start over.

DON'T ASK ME WHY OR HOW, but it's working now. I was audio probing again, and clipped the ground probe to the ground lug on the footswitch; in doing so, I accidentally pushed the footswitch cube through the pedal slightly. After I snapped it back in place, I probed the Mono output (point 7) and Dry output (point 5). They both sounded like they should, so I plugged the pedal directly into the amp I was using - works fine. It's more subtle than I would have thought and the EQ isn't drastic.
I appreciate the help here, and will be using the audio probe more in the future.

[MrStab]

The gate voltage fluctuating seems reasonably normal for "On" state (as it's floating and not pulled up or down anywhere), but would have been odd if it wasn't a constant ~0.6V or so in "Off" state. There's a clue there if that was the case.

Symptoms suggest some possible silliness (e.g. intermittent connection or short) in this region, from the CE-5 service manual I found on Google:



If effect is unexpectedly weak, it could be that both JFETs are now On, and Wet and Dry signals are being summed together. Does Q4's Collector and Base voltage change when the LED goes on and off?

[Bucksears]

The gate voltage fluctuating seems reasonably normal for "On" state (as it's floating and not pulled up or down anywhere), but would have been odd if it wasn't a constant ~0.6V or so in "Off" state. There's a clue there if that was the case.

Symptoms suggest some possible silliness (e.g. intermittent connection or short) in this region, from the CE-5 service manual I found on Google:



If effect is unexpectedly weak, it could be that both JFETs are now On, and Wet and Dry signals are being summed together. Does Q4's Collector and Base voltage change when the LED goes on and off?

In that schem (I have the same one from the analog CE-5 service manual), that Q4 in the middle (bipolar silicon, not a JFET) is actually Q12 on the PCB; they have Q4 shown twice in the schematic.
I'll open it up and retest, hopefully later today.

[MrStab]

Boss are such amateurs! Pffffft!