Gemini PS-01 Mixer Channel Noise

[ezfreebz]

Hi I have this Gemini PS-01 mixer. Sorry for all my life I cannot find a schematic anywhere.
https://www.cebuappliancecenter.com/.../gemini-ps-01/

Channel 1 has this ring that sounds like feedback. It's pre-fader so the rest of the unit functions perfectly as long as I don't use Channel 1. I can also eliminate the ring by turning the Mid EQ all the way down. I think that means it's pre-EQ and I'm just EQing it out. It's a rock solid signal at ~265Hz. The pitch doesn't change. Here's a video of the signal as I turn up the mid EQ.
https://youtu.be/a9SFm4gYdog

I've been examining the board for shorts, blown caps, anything that might be an obvious issue, but I've run out of ideas. Does anyone have any idea what I'm even seeing here? I've been searching forums but I don't even know what to call this problem, so the results have been pretty unhelpful.

Any ideas?
-E

[Jazz P Bass]

IF the Mid control affects the issue then I would concentrate there.

[J M Fahey]

Not sure WHAT are you showing us in that video.

1) what is the music for?
Are you playing it through the suspect channel or is it just "video background musical tracks"?
If the latter, drop it, we need the problem sound and nothing else.
If showing the problem, I hear no change.

2) you show us a screen waveform.
Fine, most "should" do the same.

But we have no clue on frequency, level, etc.
It looks like a continuous tone injected in that channel, but not knowing frequency .... it means nothing.

It does not look like itīs unstable and bursts into oscillation at some point, waveform looks too controlled for that.

I do not here that continuous tone appearing at the output.

A few mixers include a fixed 1kHz (or 440 Hz) reference oscillator to help setup levels.

Does the frequency change when moving any control?

[nickb]

The problem is in the EQ section as the 'high' control changes the frequency. A tough one to figure out even with a schematic. Try to at least narrow down which components are part of the EQ circuit so you can concentrate you efforts.

[ezfreebz]

OOPS! J M Fahey you are right. I re-recorded the video but with the output instead of whatever I was listening to at the time.
I played with the EQ a little more. The frequency changes slightly when I play with the low and high eq, but just slightly.
There are no signal inputs. I played with the input gain to show it has no effect.


https://youtu.be/MAFXh-2G3ZY

Not sure WHAT are you showing us in that video.

1) what is the music for?
Are you playing it through the suspect channel or is it just "video background musical tracks"?
If the latter, drop it, we need the problem sound and nothing else.
If showing the problem, I hear no change.

2) you show us a screen waveform.
Fine, most "should" do the same.

But we have no clue on frequency, level, etc.
It looks like a continuous tone injected in that channel, but not knowing frequency .... it means nothing.

It does not look like itīs unstable and bursts into oscillation at some point, waveform looks too controlled for that.

I do not here that continuous tone appearing at the output.

A few mixers include a fixed 1kHz (or 440 Hz) reference oscillator to help setup levels.

Does the frequency change when moving any control?

[J M Fahey]

Just thinking aloud and not *practical* help since we do not have the schematic:

* there is one kind of midrange equalizer based on a Wien Bridge, made out of 2 resistors and 2 capacitors R1 R2 C1 C2 which define a frequency



* there is also an *OSCILLATOR* (hint hint) based on a Wien bridge.

Exact same R1 R2 C1 C2 network, go figure.

Then, how does circuit when to select a certain frequency (in this case apparently 265Hz) for boost/cut equalization and when to oscillate?

Easy peasy , it only depends on gain stage (the Op Amp) gain.

To oscillate: = or > than 3X

To select a certain frequency: 1X or tops 2X

Check this ESP/Rodd Elliottīs Parametric equalizer:, same thing but with R1 and R2 variable to select frequency, R1 and R2 above are replaced by a potentiometer in series with a resistor.
You need a double pot but no big deal.
And in YOUR case, I guess they used that circuit, only with a fixed frequency, so no frequency pot. No big deal.


The oscillator in Fig 1 is a non inverting amplifier, gain 3X (the magical number) : (R4/R3)+1=2+1=3X

The ESP equalizer is used as an inverting Op Amp stage , Gain is: R5/R4=1X

I SUSPECT in your preamp "R5" opened and turned your EQ into an oscillator, which only stops when Boost/Cut is set to maximum Cut (which kills all the gain, even the exaggerated one which makes it oscillate).

Bass and Treble pots change sound something but only because they are all in the same EQ net and are somewhat interactive, but are not the main problem.

But we have no Schematic, so my guess is hard to confirm.

* Just 2 "emergency vaccine authorization" type suggestions to try:

1) trace board tracks from Midrange pot legs to some Op Amp which is actually doing the Tone Control thing.
Identify it and post which is it, I can suggest adding negative feedback (just 1 resistor) but need type to suggest which pins to use.

2) same thing, but blunt/crude: short pot legs from wiper to -/cold/Cut end with a low value resistor so we tame midrange gain, it might stop oscillation.

We may end with a "no mids" channel but at least it will be somewhat "usable" for some kind of program.

Only problem I see is that I expect everything to be SMT size, so tiny components, tiny tracks, maybe a double sided board, all of which complicate things.

Post some gut picture showing preamp board.

EDIT: in case it isnīt clear:

* I suspect something broke (track? pad?) and left that Op Amp full gain in mid frequencies, so it turned into a perfect waveform oscillator, thatīs not your typical motorboating - poor grounding/shield/layout oscillation.

* since it receives the most bumps and kicks, I would inspect the area around the Midrange pot with a loupe, resolder pads, maybe even replace it. ,

[ezfreebz]

Juan I really really appreciate the time you put into this. I spent some time probing the circuit for any kind of loose or shorted component. I just can't seem to find the problem. I took some pictures of both sides of the board. Each picture shows the problem channel on the left and a perfectly functioning channel on the right. In the top view (yellow) the mid-range pot is the one at the 3 o'clock position. The back side (green) looks pretty messy, but that's mostly epoxy that (I hope) hasn't affected anything.

https://ibb.co/vQhf8rp
https://ibb.co/cNH02vT