# Maxon OD808 problem



## Gene Machine (Sep 22, 2007)

Hey all. I've had my OD808 for over a year now, and something strange started happening. There is a digital noise coming from it, almost sounds like morse code and it is repetative.

This happens with both the battery and an adapter, and for both on/off modes while plugged in.

Any experience, ideas? I've never had a pedal do this before. Other pedals in my chain were not doing it, so it's not a room noise/EM noise thing.

cheers.
Gene


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## amagras (Apr 22, 2015)

I doubt it's digital because that's a 100% analog pedal but wait to hear what the other guys say, @nonreverb and @mhammer could have an idea.


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## ezcomes (Jul 28, 2008)

Ill bump for a learning experience!


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## Gene Machine (Sep 22, 2007)

amagras said:


> I doubt it's digital because that's a 100% analog pedal but wait to hear what the other guys say, @nonreverb and @mhammer could have an idea.


Good point. I just checked the schematic. Strange, it sounds like Morse code. and The same pattern over and over.


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## mhammer (Nov 30, 2007)

Is it this one?








Or is it this one?








The upper is the original, which uses op-amps for the in/out buffers, while the lower one is the current reissue, using transistors for buffers.
If it's a more recent model, then something like a bad capacitor would be well down the list of potential problem sources. If it was an original, there is the outside chance that one or more of the 35 year-old caps have dried out. Although the noise you describe does not generally arise from dried out caps. Plus, the largest-value cap is 10uf.

And just to confirm, you get the noise when it is the only pedal in the chain?


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## Gene Machine (Sep 22, 2007)

mhammer said:


> Is it this one?
> 
> And just to confirm, you get the noise when it is the only pedal in the chain?


I have the newer one, just a year old. I haven't had the chance to open it yet, since it uses those stupid little allen keys with the hole in them. FEK. couldn't just be allen or phillips.

yes, it also makes that noise when isolated, and with a battery. so it's not the adapter or another pedal.

I'll head to princess auto tomorrow, and look for some tools that will work.

p.s. thanks for the input


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## mhammer (Nov 30, 2007)

So when you say a "Morse code" kind of noise, you mean a sort of "bee-bee-beeep-beeep-bee-beeeep" intermittent chirping, rather than any sort of noise like an AM radio tuned between stations?


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## Gene Machine (Sep 22, 2007)

More like intermittent chirping, but it is not a solid tone per se, more like high frequency noise.

I got a tamper-proof torx bit, so I am taking it apart tonight to check some values... I found a schematic. Let’s see what’s going on in there


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## chuck_zc (Dec 6, 2009)

Subscribed.


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## greco (Jul 15, 2007)

chuck_zc said:


> Subscribed.


Same here.


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## Gene Machine (Sep 22, 2007)

Ok, nothing obviously wrong, nothing burnt or loose. All obvious stuff looks good, grounds, Vcc, etc.

Will check capacitors tomorrow.


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## Gene Machine (Sep 22, 2007)

Ok, more info:

Noise is here : 




It doesn’t appear when there is no input. Once a plug is inserted to input(i.e applying power) the noise happens while either disengaged (buffered bypass) or engaged through guts of the pedal. 

The 100uF cap on power supply holds a steady 9V. No fluctuations I can see on the DMM. All the grounds seem good.

I found one schematic that looks close, but not exact. Right now I am suspecting the buffered input which I think is common to both modes.

Thoughts?


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## mhammer (Nov 30, 2007)

Thanks for posting that. It will help pinning down the source.
If the pedal circuit is always on (as is true of all e-switched pedals), and the noise only occurs when something is inserted into the input jack to turn on the power, AND the noise occurs whether the power is external or battery, AND the 100uf cap seems fine, THEN our attention turns to D1, which is used as a wrong-polarity protection diode, going to ground. They can get fried when people plug in a supply with the wrong polarity.

The diagnostic test would be to desolder one end of that diode, so that it is effectively out of circuit. If I'm right, the noise should abate. Since it is not in the audio path at all, there is no requirement to find an exact replacement. It can be easily replaced with a 1N4001.

Fingers crossed.

EDIT: I should add that the diode is not required for the pedal to function. It is only there to assure the pedal keeps on working, and doesn't get damaged by plugging in the wrong type of adaptor or too high a voltage. So, as long as you apply battery or external power properly, lifting the diode won't wreck anything.


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## Gene Machine (Sep 22, 2007)

Coolio mhammer. Thanks. I will. Check tonight.


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## Gene Machine (Sep 22, 2007)

mhammer said:


> THEN our attention turns to D1, which is used as a wrong-polarity protection diode, going to ground. .


Just to be sure, are you talking about the diode in parallel with the 100uF cap? Because that is labelled D5 on the board. D1 and D2 are connected to the op amp.


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## mhammer (Nov 30, 2007)

Yes, the black one with the white stripe, not either of the two smaller orange glassy ones. I was looking at a different schematic, to confirm the diode went to ground and wasn't in series with V+, forgetting what the part was labelled on the actual board.

Thanks for noticing the error.


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## Gene Machine (Sep 22, 2007)

mhammer said:


> Yes, the black one with the white stripe...


Ok, it was good, but since it was already off I replaced with a brand new diode. Same issue.


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## mhammer (Nov 30, 2007)

Jeez that's a puzzler. I don't want to start suggesting to change this or that component, because I don't want you to risk damaging the board.

I'm going to cogitate a bit more. I'll refer a few of the expert minds I know to the video, and get their take on it. I'll be back.


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## fretzel (Aug 8, 2014)

It is you’re WiFi/cell phone. Can’t recall which but try isolating those. Same thing happened to me a while back.


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## Gene Machine (Sep 22, 2007)

fretzel said:


> It is you’re WiFi/cell phone. Can’t recall which but try isolating those. Same thing happened to me a while back.


It’s not my cell phone, i’m aware of that one. This is isolated to this specific pedal.

I’m thinking that it may be a noisy transistor.


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## fretzel (Aug 8, 2014)

I thought they sounded pretty close. Drive me crazy when it happened to me as my amp was fairly new at the time. But out of warranty. Good luck.


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## Gene Machine (Sep 22, 2007)

I have found a couple of incorrect circuits schematics, ever seen a proper one?


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## Gene Machine (Sep 22, 2007)

Update: I think it is the input buffer transistor, a FET part no. K246 

Got some on order, more to follow.


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## mhammer (Nov 30, 2007)

It probably didn't _need_ to be a 2SK246, given that it's just a unity-gain buffer. But you've gone and ordered them, so no turning back now.

FWIW, you wouldn't think that silicon transistors age, but they do. Well, it's not so much "aging", in the sense of changing properties even if they just sit there in a bin, as much as prolonged use. I have a Heathkit TA-16 amplifier. The thing was noisy when I bought it. I changed the input transistors for new ones (and they weren't even substantially different one) and the noise level dropped substantially. Weird.

I hope the folks on the other forum were helpful for you. They generally are.


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## Gene Machine (Sep 22, 2007)

So I made a mistake... I found the proper schematic and the FETs are actually for the switching circuit. It uses a C1815 BJT NPN for the buffers. No prob, my FET order is only $2 lol. I have several 2n3904, but the pin order isn’t the same. EBC vs ECB

Anyway, i’ll Probably order a couple, they are only $0.20 each or so...


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## butterknucket (Feb 5, 2006)

I have no electronics experience so I'm just putting this out there, but do you have a phone in your pocket that could be causing some kind of transmission to your pickups and it's messing with the pedal?

Stranger things have happened....


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## mhammer (Nov 30, 2007)

Gene Machine said:


> So I made a mistake... I found the proper schematic and the FETs are actually for the switching circuit. It uses a C1815 BJT NPN for the buffers. No prob, my FET order is only $2 lol. I have several 2n3904, but the pin order isn’t the same. EBC vs ECB
> 
> Anyway, i’ll Probably order a couple, they are only $0.20 each or so...


We'll be in town at the end of May for our son's graduation from Dal. We'll be staying in res at Mount St. Vincent, so I'll be handy. If you want some 2SC1815s, I have plenty. I can bring you some if you want.


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## Gene Machine (Sep 22, 2007)

butterknucket said:


> I have no electronics experience so I'm just putting this out there, but do you have a phone in your pocket that could be causing some kind of transmission to your pickups and it's messing with the pedal?
> 
> Stranger things have happened....


Thanks, but way beyond this in the troubleshooting flowchart.


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## Gene Machine (Sep 22, 2007)

mhammer said:


> We'll be in town at the end of May for our son's graduation from Dal. We'll be staying in res at Mount St. Vincent, so I'll be handy. If you want some 2SC1815s, I have plenty. I can bring you some if you want.


Thanks, but I hope to have it working by then.


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## Gene Machine (Sep 22, 2007)

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.


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