An odd little noise mystery

[Mark Hammer]

A fellow on another forum has been experiencing a different sort of noise on his Maxon OD808 pedal (commercial-build not DIY).  We thought it might be the power protection diode, but some diagnostic  testing indicated that was not the case.  Here's a Youtube he posted of the noise.  Any suggestions are deeply appreciated.

[YellowBoy]

Reminds me of a bad plate resistor in a tube amp.  Maybe something performing a similar role in this circuit?  Bias resistors maybe?

[Mark Hammer]

It's a more recent-issue pedal, that uses transistors for the input and output buffer, rather than the pair of dual op-amps used in the original.

[Rob Strand]

To narrow down the problem it helps to know if it happens on Effect and/or bypass.  Also how the pots affect ticking (as this can isolate what stage the ticks come from).   Sometimes problems are present on bypass but you have to crank the amp to hear it.

Also, what's it powered from?   Does it do it in another room?

Could be anything at this point:
- bad connection: connector, pot, or cracked PCB solder joint.
- bad electrolytic cap
- weird opamp or transistor failure

[Gene Machine]

Hi, Mark posted for me. It is a newer Maxon OD808 Reissue pedal.

The noise happens as soon as the pedal has power, either battery or wall adapter, doesn't matter. It also is present either with the pedal engaged, or bypassed. This makes me think it is the buffer circuit.

There is nothing loose, or burnt, or smelling funny. The pedal used to work fine, it just started making noise with no apparent reason I.e. I didn't drop it or plug it into the wrong voltage.


Btw, that other YouTube clip is not my noise, Here is a clip of my noise.

https://youtu.be/7MYvqeAKws4

[Rob Strand]

The noise happens as soon as the pedal has power, either battery or wall adapter, doesn't matter. It also is present either with the pedal engaged, or bypassed. This makes me think it is the buffer circuit.

Does it get worse when you adjust the gain and/or volume?   That would point to the input buffer more than the output buffer.   (At this point, I would still consider some of the other items in the list.)

Bad mains wiring could do it but then you would expect the problem to be present on other effects.

[GGBB]

Sounds just like a noise I have in one channel of my tube amp.

[idy]

Sounds staticy. I vote noisy transistor in the buffer (answer question from Rob Strand to determine which one, or (better) audio probe.

Sounds like a "fire red fuzz (big muff variant) I saw that "ticked." New Q1=no problem.

[Fossilshark]

I have a similar problem with my diy guitar amp at the moment. Im sure this was already mentioned, but to me it sounds like a bad connection/solder joint as that was my issue.

[R.G.]

Flicker noise, denoting an intermittent contact. As Rob says, it's one of bad solder joint, cracked PCB, failing component (electro cap is more likely, but could be a mechanically cracked resistor or cap of any kind) or damaged transistor.

Quick and dirty fix attempt: re-melt all the solder joints with a little bit of fresh rosin core solder on each and see if it goes away. If not, leave it open on the bench and plugged so you can listen to it, and poke/prod at components one by one to see if any of them changes the sound. If one or more do, you've found an intermittent part or junction.

If that doesn't help, try blowing a hair dryer across it while listening. If the sound changes, you can maybe find a bum part by thermal means.

If all this fails, you have two options. First is the analytic method of getting out the oscilloscope and tracing signal, looking for where the noise first starts in the signal chain. Second is to get out the metaphoric shotgun and replace the opamp, transistors, diodes and electro caps. On such a simple circuit, remanufacturing is faster than analysis.

[Mark Hammer]

If that doesn't help, try blowing a hair dryer across it while listening. If the sound changes, you can maybe find a bum part by thermal means.

My first PC as a grad student, back in 1982, was an Acorn Atom.  Great little 6502-based single-board machine, but whoever assembled mine hadn't attended to heat dissipation very well.  The on-board 5V regulator had overheated the board and created some fractured traces that I was never able to locate.  It would partially boot, but freeze.  As a result, I kept a hair dryer in my grad-student office, and kept the chassis unscrewed so I could blow hot air on the board.  I must have been the only person in my department whose computer had to literally warm up before the fractured traces would expand and acquire continuity.

[Gene Machine]

The noise happens as soon as the pedal has power, either battery or wall adapter, doesn't matter. It also is present either with the pedal engaged, or bypassed. This makes me think it is the buffer circuit.

Does it get worse when you adjust the gain and/or volume?   That would point to the input buffer more than the output buffer.   (At this point, I would still consider some of the other items in the list.)

I will check tonight. Note, this used to work fine, for over a year. And I didn't drop it or anything so likely a component that failed. But I will look at solder traces...

[Gene Machine]

Ok. Noise gets louder as level/gain increased.

I went poking with my plastic trim pot screwdriver, made no difference, nothing seems loose.

For a few bucks each, I am going to order a couple K246 FETs, and try one in the input buffer.

[Rob Strand]

For a few bucks each, I am going to order a couple K246 FETs, and try one in the input buffer.

Not a bad plan.  When you exhaust the possibilities you have to go somewhere.

I've lost track of what the circuit looks like on the re-issue but it should work with a BJT for the buffer.  You could use anything you have on hand (just make sure you map the pins correctly).  If the noise goes then put the order through.

[Gene Machine]

FIXED!

I replaced TR1 input buffer transistor, and voila... all better.

Tough parts of fix: getting actual schematic. I managed to find one. If you find yourself needing one, let me know. Also, finding replacement parts. Not a lot of places left in town these days, everything has to come on the slow boat from China.

Thanks everyone, especially mhammer for helping troubleshoot.

[Mark Hammer]

Congrats!  Gotta love those happy endings.  And I'll bet you learned stuff along the way.

[Rob Strand]

FIXED!

Cool.

I wonder how it failed.  Maybe ESD?   If you live in a dry region the chances of ESD goes up enormously.  However, shoes and carpets is have a wide variability.