Marshall Bluesbreaker Motorboating

[John P]

Hello-Built B.B. pedal years ago, never got it to work. only "mods' were using germanium clipping diodes (IN34s?). Works as boost, but turn up the Drive past 10 O' clock, and when you hit a note, starts motorboating like crazy. Any clues?? TIA.....

[David Schwab]

Sounds like feedback. Are your input and output wires away from each other? Could be the board layout too. Might also be an electrolytic cap with the polarity reversed...

[ozgur]

Well....B.B is one my favourite stompbox and i never had such a thingy before...As in all stompbox pre-launch check ups you should check the batt or power conn.s,look for dry joints or "cool" solder...İf it all looks ok then perform this little test like thing...(umm..so pro!)

assuming you have a voltmeter check voltages from the op-amps 4th and 8th supply rails..Your op-amp should be a dual op-amp like 5448 or TL072 or what ever it is...Lets go back to day you soldered your op-amp...if you're using a socket for op-amp check if it's clean,no flux no dust no sand not even a dead mice in it bla bla bla...if you did NOT used a socket (as i always did...8 Blues Breaker clones for friends...) for the opamp there might be something defective in it due to staying tooooo hot while soldering...Then check the Bias voltages...should be aroud 4.5 VDC around the voltage divider...Then check the voltages after biasing resistor aroun 1 Megaohm i guess..If everything seems okay remove the clipping diodes.But before removing check the polarities ...Another source of extreeme feedback...They might be defective cracked (1n4148 usually...) Check if you can still read what it says on diodes...More than providing an eye test, this shows us if any extreme current have reached the diodes anyway...well...Another point...if feedback or motorboat what ever happens when you unplug your guitar from the input this may lead us to a weird problem...No ghosts nor aliens...There might be a kind of hum around 50 Hz to 2Ghz which is very well collected by miles of coil wires by the guitar causes itself (the hum) to leak into B.B...Also if you're around a kind of powerfull transmitter or any other wireless device which is said to be compatible to FCC rules might not be working as it's thought to be...Sooooo...What we got? the hum come to pickups than to b.b...At the same time the hum reaches to B.B. itself...Also a little before the hum reaches from the guitar...At the end we have a phase shifted sine wave madly turning around b.b.
How to reduce this...Shielding...The key to humless fx...You might use a fully metal box also connected to chasis of the instrument...Or you can wrap the b.b with NON conductive thingy or dip in epoxy (no turning back!!!!!) and wrap the isolation with a piece of foil...And also connect this to ground...you can use a piece of shielding material around....Anything conductive will work...

I guess this is too much to say about b.b.But i love it...I hope these will work...Or just kill your bb and build a new one...

[Satamax]

Hello-Built B.B. pedal years ago, never got it to work. only "mods' were using germanium clipping diodes (IN34s?). Works as boost, but turn up the Drive past 10 O' clock, and when you hit a note, starts motorboating like crazy. Any clues?? TIA.....

Sounds like you've made some kind of oscilator, if the motorboating varies with the pot being moved check the circuit around the 47pF,3.3 and 47 KΩ If it doesn't vary the motorboating, then check your feedback loops.