Boss GE-6 Graphic Equalizer - Loud rustling noise

[badoogie]

I have a Boss GE-6 which sounds fine when all of the sliders are at zero (in the middle), however when frequencies are boosted or cut a loud rustling/hissing noise is heard.  This happens on four of the sliders in particular (100, 200, 400 and 3.2k).  I have cleaned each of the sliders with Servisol, but this has had no effect.

Any ideas what I can check next?   

[thermionix]

What kind of power supply are you using?

[GibsonGM]

Did you actually open it up, and get access to the guts of the sliders? That really sounds like worn/dirty pot innards (the sliders).    How old is the unit?  Could they be worn enough to need to be replaced?

Could the solder joints be faulty?  Try to move the suspect sliders around a tiny bit without sliding them...see if you can make the noise based on the solder joints.  If so, re-flow with new solder.   

Last quick thought...if you can figure out how to get at the pot terminals at the PCB...try measuring for DC as referenced to ground.....if a cap was leaking prior to the slider(s), it could allow bias voltage to get on the control and make a scratching noise. Just a thought.

[thermionix]

Mike I think you're taking it as noise while the sliders are being moved, I thought he meant noise when the sliders are set anywhere besides zero.  It's not clear to me after re-reading.  Also, everybody has their own ways of describing noises.  To me, the "rustling sound" is usually a preamp tube or plate load resistor.  But, you know, not in a Boss pedal.

[GibsonGM]

...unless it's one of those boutique triode Boss pedals!     

Yeah, I am taking his complaint to be "anywhere but in the middle", and not necessarily moving...which is odd....suggests to me, really, some DC on the things, or wear, but....quiet if they're on "zero"?    DC would be associated with movement.   So, all I can suggest is what I did...make sure you're really cleaning them, check the solder contacts, look for DC.   Hit it with a medium-sized object to see if something aggravates it.   

I'd like a look at a schematic to see what is going on when they are at "zero", maybe something would suggest itself that way...   

[Rob Strand]

As GibsonGM aluded to there could be a DC issue.

One way this can happen is bad electrolytic caps.

The symptoms don't allow anything to be narrowed down:
- The fact it does has a problem on boost *and* cut points to an issue with the bands.
- It could be the electros used for the bands,however, the 3.2k band doesn't have an electro.
- I don't have a GE6 schematic but the GE7 show those bands on different opamps.

So there's no pattern.

Then there's the question of why there's more than one band misbehaving.

Have you ever seen that pedal working 100%?  Like did it develop those faults all together or did they propagate over time?

[badoogie]

Thanks for the replies guys. 

Sorry, I should have been clearer.  The noise is present when the sliders are set anywhere above or below zero, rather than just occuring when the sliders are moved.  I got the pedal in this condition, so I don't know how quickly the faults developed.

I opened the pedal to get to the pots.  I removed the foam/rubber dust protector from the front, sprayed in the contact cleaner, and moved the sliders back and forth to work the contact cleaner in.

Everything else in the pedal looks good.  The solder joints look fine and none of the caps look damaged.

I've been using a Boss PSA adaptor, but daisy-chaining with another Boss pedal, as the GE-6 is ACA spec.

[antonis]

Hit it with a medium-sized object

Could you be more specific, Sir..?? 

A 10Kg (22lb) hammer could be consider as "medium-sized" object..??

[GibsonGM]

I was exaggerating about a medium-sized object, Antonis......I would try to make the noise worse, to aggravate it.  If you tapped it with a pencil eraser, for example...it may show that some solder joints are not well-done, or that a resistor is cracked...something like that.     

I have to agree, this is subtle!   Try powering it with a battery, see what that sounds like.    I have the GE-7, and it is also noisy (but does not "rustle"...the noise floor is simply on the high side).  Mine is noisy because they used cheap caps, and unless you want to change them all, it is what it is.
May or may not be a factor in yours...

Is it time to audio probe this yet?  With a schematic, you might be able to narrow down some things, like "is this only at the slider?". 

[Rob Strand]

I suppose one way to go forward would be to measure the DC voltage at output pin of the opamps and see what happens when you move the pots.  Sometimes it's a good idea listen the same time as measuring so you can corellated things better.   Just don't short anything in the process.

[badoogie]

Thanks guys, I'll give it a go and let you know what I find.   

[PRR]

https://www.hobby-hour.com/electronics/s/schematics/boss-ge7-equalizer-schematic.png

Replace "C18", 33-47uFd near two 33K resistors.

That point is supposed to be be audio-quiet. If C18 is breaking-down (from age) it could make a racket. This may cancel when pots are centered, get worse as you go off-center.

It should also inject racket at IC5, so I am not sure; but cheaper to try than to think it through.

[badoogie]

I found a schematic someone has traced from their GE-6 here:

http://www.niklasdahlheimer.de/2016/08/30/schaltplan-boss-ge-6/

Using an audio probe I started working through the circuit, and I found that the noise develops on the output side of R27.  Before the resistor there is just an 'acceptable-for-an-old-pedal' level of background noise, but after it (i.e. between R27 and pin 3 of IC1) there is the rustling/crackling noise.

Could it just be that resistor causing the noise?

[Rob Strand]

At this point I'd be thinking IC1A has an issue but don't write it off just yet.

Do you get the same rustling noise on pin 2 of IC1  (and while you are there on pin 1 of IC1) ?

Can you measure the DC voltages on IC1 pins 1, 2 and 3.   Also measure the DC voltage on pin 14 IC1 and at the positive side of C18 (33uF).

[thermionix]

Maybe R27 itself?  Is it a carbon film?  Do you have a metal film to try in its place?

I'm still thinking about plate load resistors making that "rustling" or "frying bacon" sound.  Usually carbon comp or carbon film.

[badoogie]

Thanks for the replies guys.  I took some readings on the ICs...

IC1

1: 0.629mV
2: 6.04V
3: 6.03V
4: 9.11V
5: 5.25V
6: 5.35V
7: 5.42V
8: 5.50V
9: 5.50V
10: 4.58V
11: 0.2mV
12: 4.98V
13: 5.47V
14: 5.47V

IC2

1: 5.46V
2: 5.46V
3: 9V
4: 9.09V
5: 5.31V
6: 6.88V
7: 6.88V
8: 7.2V
9: 7.2V
10: 5.35V
11: 0.2mV
12: 5.34V
13: 8.21V
14: 8.21V

[Rob Strand]

IC1

1: 0.629mV
2: 6.04V
3: 6.03V

This looks very bad.

IC2

3: 9V

5: 5.31V
6: 6.88V
7: 6.88V

8: 7.2V
9: 7.2V
10: 5.35V  (maybe OK)

12: 5.34V (maybe OK)
13: 8.21V
14: 8.21V

These look bad.
For pin 3 you may have shorted against pin 4?

Can you measure the DC voltage at positive side of C18 (33uF)?
Given your supply is 9V  I would have expected 4.5V for many of the other measurements
but you are seeing voltages 5.25V to 5.50V.
None of the voltages look good except the power!

Until we get further info it's looking like both opamps are bad.

[EDIT1:  It occurred to me if the LED wires on the second board shorted to some of the other wires or PCB traces then you might see *some* of the above behaviour for IC1.  It's a long shot but worth checking.]

[badoogie]

Thanks Rob, I think you might be right about IC2, I checked pin 3 again and the reading was 4.47V.

The reading on the positive side of C18 is 5.41V.

[Rob Strand]

Thanks for the check.

The reading on the positive side of C18 is 5.41V.

OK so that means a there's lot of good voltages as well.
The faulty IC(s) might actually be causing the voltage on the positive side of C18 to rise from 4.5V to 5.41V; in that extra "fault" current is feeding into the divider.

Where to go from here?

1) Recheck all the bad voltages.  Firstly to make sure they are bad and secondly to make sure the voltages are still the same ie. the badness is consistent.
2) I'd as also look for signs of gunk on the PCB, signs of water damage or battery leakage.   If you see anything maybe clean the PCB.   The fact is has multiple and sporadic faults is a little suspicious.  This can sometimes be cause by chemical means.
[I was buying Bass off a guy once and his Boss multi-effect board wasn't working.  I had a quick look to find it was full of cats pee.]
3) With the circuit unpowered use the continuity tester to check for shorts around the areas where there a problems.  To reduce the work maybe concentrate on IC1 pins 1, 2,3.
4) I might be tempted to lift the wires 4 and 5 of JP1, the ones that go to IC1 pins 2,3.  First check those wire match the schematic.   If decide to lift them be careful not to fray them as they are a real pain to get back in.
After that recheck the voltage around IC1 pins 1, 2,3.

All this is to be sure the IC(s) are actually bad.   

The easy way out is to buy some new IC, chuck them in, and see how it goes.  However that device might not be so easy to get nowadays and if you swap them out and the fault still exists it's a real let down.  If it works then job done.

[badoogie]

I have replaced a couple of the electrolytic caps: C18, which was mentioned earlier, and C17, as I wasn't 100% sure if the gunk around the base was glue or if it had come out of the cap (it was glue).  This didn't change the sounds coming from the pedal, so I took some further readings on the ICs...

IC1

1: 0.622mV
2: 5.70V
3: 5.68V
4: 9.08V
5: 4.90V
6: 4.88V
7: 4.88V
8: 5.05V
9: 5.05V
10: 4.24V
11: 0.1mV
12: 4.57V
13: 5.04V
14: 5.03V

IC2

1: 7.71V
2: 7.71V
3: 6.70V
4: 9.08V
5: 4.84V
6: 7.11V
7: 7.11V
8: 7.20V
9: 7.20V
10: 4.92V
11: 0.1mV
12: 4.92V
13: 7.51V
14: 7.51V

Perhaps I should replace them.  I've put some TL074 and LM324 ICs in my next Tayda order, along with a couple of sockets.