Mutron BiPhase ticking noises (recording and schems included)

[tomas1808]

Hi, I bought a "dead" BiPhase knowing I could fix this thing. Turns out the internal ribbon cable was connected incorrectly and now works!

The only problem I am seeing now is that Phaser A (left) has some ticking noises during part of the sweep and also some continuous "electrocution sound" for about 1 second in the slowest speed setting (you'll understand what I mean when you hear it hah).

Phaser B (right) just has a portion of the sweep where there is a bit of hiss for about a second as well, but this is fainter an not too noticeable in the recording.

This was recorded with no input into the phaser, stereo setting, using my phone.

Also attached the schematics.



Any ideas on what could be the cause and how I could go about cleaning the sweeps?

Thanks!

 

[NoisyIndividual]

Interesting how all those clicks etc are pretty much exactly a second apart, and repeat every 12 seconds.
This is not random noise but something seemingly based on a slow clock.
Could this be due to something external to the phaser?

Unless it is something to do with beating between 2 oscillators.

 

[NoisyIndividual]

Do these one second based timings change with the sweep frequency?

 

[tomas1808]

Do these one second based timings change with the sweep frequency?

Thank for chiming in Noisy (fitting name!).

The noise is indeed coming from the phaser and they follow any change of speed.

There is no beating because both phaser channels are linked to the same oscillator.

I think this is related to the phasing circuits and not the oscillator because noises happen at different moments in both phasers while using the same OSC. So I guess it is related to the "sweeping" itself and not the oscillator. I have no idea how to proceed though.

 

[NoisyIndividual]

Ok, I've just listened again and the slow sweep oscillator does seem to be on a cycle of around 12 seconds.

Yes, weird how it is so consistent.

I have no idea either.

 

[NoisyIndividual]

I did repair one of these about a year ago, there was no feedback.
Turned out it was due to the shonky way they attach the pots to the PCB to the chassis, and one of the feedback pots legs had snapped its solder joint.
Might just be worth checking this isn't something similarly mechanical - a grounding issue perhaps.

They don't seem to be well made, or at least the one I had wasn't.

Good luck.

 

[tomas1808]

Thanks! Yeah, the PCB quality is not very good. The hardware is good I think.

Actually I did repair one of the depth pots, the ground pot leg snapped on mine so the depth range was vastly reduced.

 

[NoisyIndividual]

They do sound rather good though, I remember that well.

 

[NoisyIndividual]

Ok, the fact that the clicks seem to be based on a divisor of 6 of the sweep half cycle, and the fact that there are 6 series op-amp circuits in each phase circuit, seems to indicate that yes indeed the noises are coming from individual sections of the phase circuit(s) as you suspect.

Maybe you could scope the 6 op-amp outputs to see if there are any spikes corresponding to one of the obvious audio clicks, then you could swap components between those that have no clicks and see if the problem moves with them.

 

[tomas1808]

That's a good idea. I'll check them out!

 

[tomas1808]

I ended up shotgunning all opamps in the phasing board and also reflowed all solder and now it is noise free. Also someone jumped pins 1 and 2 of first opamp of channel B reducing the gain of that channel, also fixed that. Now it is working as if it were brand new! Thanks NoisyIndividual!