[help] troubleshooting a Marshall SV-1 Supervibe

[HeaD]

Hi, I have a problem with a marshall sv-1 supervibe. It was working well until yesterday, but suddenly it has started to work badly. The sound has too many basses, and the volume is really low also when the effect is byassed!!

I am sure that a component is damaged, but I havent a schematics of this pedal and I'm not able to understand what component is!

How can I do to find it? Thanks to everybody!

[HeaD]

No one? Maybe could I replace all the Opamps and transistors? but I would like to know if there is a way to understand what does not work  before. It works badly even if the effect is bypassed... maybe i'is a problem in the bypass circuit part. Anyone knows how a marshall pedal bypass works? Thanks!

[petemoore]

  Might not take a boatload of 'savvy', but at least 'some...
  Without the schematic...
  You can still take some voltage measurements on the opamps and maybe some other places.
  Audio probing could be done also, but...migth be hard to 'see' where your'e going with it without a schematic.
  I imagine your box has 'regular old sized' parts in it, IWCase it Is most probably fixable w/out a microscope.
  It's probably 'sealed 'stuff out, but you can use a magnaglass and close in light to look for conductive 'foreign objects'.
  The vibe probably follows some other circuits topology [some of it at least]...I've seen circiuts Retroed with just some notes and high resolution photos of the top and bottom of the PCB.
  :preliminary' voltage measurements is what I'd do first anyway.

[HeaD]

Hey thanks for your help Pete , it could be some conductive foreign object there, but I dont believe... this pedal has been never opened and has always worked well! I will proceed with the measurement of the Opamp voltage... but how can I understand if the voltages are correct or not? There are 5 TL072 and 2 MN3101, Thanks!

[no one ever]

http://www.geofex.com


read... read it again.. and then read it once more. i'm no EE, but i've learned a bunch from good ol R.G.

vibes are pretty complicated things. do you have a multimeter? do you know how to use it? read up on "when good opamps go bad" and other stuff like that.

[HeaD]

wow, thanks

[Jaicen_solo]

I have a supervibe which I LOVE! I find it's quite noisy though, so I tend to have the filter on full which cuts a lot of the shimmer unfortunately.
Either way, I think your problem may lie in the seperate bypass board. The supervibe uses a double sided board for the actual circuit, so it could be difficult to diagnose problems with that. Especially so without a correct schematic.
Firstly, you should check the voltages on the board attached to the stompswitch, should be fairly obvious if somethings wrong there. I've found that the wiring is suspect so check that, the power supply to my LED broke after a while, which is irritating to say the least!

[R.G.]

There's not a lot of detailed work that you can do without a schematic. About all you can do is to check for proper power supply voltages and proper operating voltages on ICs, transistors and such, and poke around for broken wires.

First - go read the Guitar Effect Debugging Page at GEO.

Next:
Are any wires broken?
Measure your battery voltage - is it over 8V when supplying the pedal?
Measure the voltage from the input jack ground lug  to:
- All pins of any opamps. Pin 4 on dual opamps should be 0V, pin 8 the battery voltage. Any opamps with signal going through them should have pins 1/2/3 and 5/6/7 at the bias voltage, about half the battery. If the thing produces an LFO sweep, the LFO opamp will have pins that fluctuate up and down.
- all pins of any transistors. A transistor simply must have the base separated from the emitter by one silicon diode drop (for silicon transistors) and some voltage across the collector- emitter or it won't pass signal.

[puretube]

weak battery?
dirty switch?

replace battery with a fresh one;
stomp the switch hard & quick 10 times;

[HeaD]

Ok, I've found it!!! It was a broken J202 in the Bypass section! I have temporarily replaced it with another j202 in the effect section, and now the pedal works in bypass mode. Obviously when the pedal is on it does not work, because there is a missing component...

Last question: Can I replace the J202 with another Fet? Can an Mpf102 or a J201 be right? Thanks to all!

[puretube]

googling before answering had told me: "True Bypass", and "Passive Bypass"...

[HeaD]

Ok, I've changed it with an MPF102 and it works well!!! Thanks to all again