Marshall Guvnor Oscilattion problem PLEASE HELP (SOLVED)

[Atodovax]

Hello Everyone. I have a problem, i have a Marshall Guvnor and for some reason when i turn the gain over 3/4 i get this strange high pitch oscilattion that changes the "pitch frequency" when the volume knob is rolled off. I tried swapping the tl072 but no luck, also replaced the power suply capacitor (10uf) but the whistle is still there. It is so loud that the guitar signal is not even audible. All i can hear is a crazy oscilattion. Also i can read an AC voltage at the input jack (TIP - Sleeve) .. Any ideas where i should start? Voltages at the IC are correct . When i put the GAin knob at 3/4 or more i can see how the red clipping LEDs get latched ON and they never turn OFF if i dont lower the gain. Also if i disconect the guitar and touch the cable jacks i can hear strange noise like changing the pitch.
Thank you very much!

[Rob Strand]

Is it a commercial unit or a DIY build?

[antonis]

I should tell you to check for open/missing/bad joints NFB caps (those placed between pins 1-2 & 6-7) but what's puzzling me is the AC voltage measurement on Input jack..

Is that AC also present on power supply..??

P.S.
As Rob said, more info about pedal's origin and AC voltage amplitude, plz...

[Atodovax]

Is it a commercial unit or a DIY build?

It is a comercial unit. Sometimes it works fine but whenever i unplug the cable from the guitar with the pedal on i start hearing a strange feedback high pitch.
Sometimes it works fine and sometimes with to much treble and gain it will stay latched on a high pitched oscilattion.
Does it makes any sense? The voltages are correct compared to the ones in GGG layout ( Substracting the voltage drop across  the protection diode between + and - ) i checked all the cables and everithing seems just fine

[Atodovax]

I should tell you to check for open/missing/bad joints NFB caps (those placed between pins 1-2 & 6-7) but what's puzzling me is the AC voltage measurement on Input jack..

Is that AC also present on power supply..??

P.S.
As Rob said, more info about pedal's origin and AC voltage amplitude, plz...

Well i am using it with a 9v battery so i should be meassuring any voltage at the input right? I can see 0.200 volt in AC with my DMM and also i can see 0.500 DC when i check with the probes on the tip and sleeve of the cable conected to the pedal. That voltage changes when i roll the gain pot. Is it normal to have voltage at the input?

[GibsonGM]

The DC on the jack is a problem, Atodovax - no, that is not normal. It usually means you have a missing or bad input capacitor...likewise if you pick up DC on the output (output capacitor).   In this case, it may signify that something has shorted on the PCB, or is 'dead'.  I would look for the input cap and see what's up with that.

The AC you measure...perhaps that is the oscillation itself?  No, without an input signal, you should see no AC.   I'd make sure the input cap is not dead.

[Atodovax]

The DC on the jack is a problem, Atodovax - no, that is not normal. It usually means you have a missing or bad input capacitor...likewise if you pick up DC on the output (output capacitor).   In this case, it may signify that something has shorted on the PCB, or is 'dead'.  I would look for the input cap and see what's up with that.

The AC you measure...perhaps that is the oscillation itself?  No, without an input signal, you should see no AC.   I'd make sure the input cap is not dead.

Thank you very much!! That did it! Now i have no Dc volt anymore and no more whistle!!

[GibsonGM]

Excellent, Atodovax!  Glad to hear it, enjoy the build and the forum     

I wonder if somehow somebody input a voltage into that pedal higher than the rating of the cap - that would ruin it.  Not sure about it failing as a short, but maybe it did, and the new one fixed it, great!    Should be 10nF, I believe...those little caps are usually rated pretty high....but it does happen. 

[antonis]

Excellent, Atodovax!  Glad to hear it, enjoy the build and the forum

IS THAT ALL..?? 

(no proposal for MosFet booster on the output, nothing..??) 

[GibsonGM]

Excellent, Atodovax!  Glad to hear it, enjoy the build and the forum

IS THAT ALL..?? 

(no proposal for MosFet booster on the output, nothing..??) 

He is not yet ready for the magic of Mosfet, Antonis!!!  We do not dare to unleash that sort of power at this time. 

But soon he will be ready....oh, he will be      

[antonis]

 

As you can realize, Atodovax, nobody leaves this room without singing the MosFet blues..!!! 

[GibsonGM]

{ Atodovax: The inside story is that i often recommend to people that they build some sort of Mosfet boost if they need more level, or if they want to improve their sound, or try a new project.  Antonis likes to get on me over that, ha ha.  I am very fond of Mosfets, they sound much like tube circuits to me, but don't require the high voltage supply       Some time when you want a new, easy build, you may want to check out something like the AMZ Mosfet Boost, or ZVEX SHO - I think you'd love that after the Guvnor! }

Happy now, Antonis?   

[antonis]

I think you'd love that after the Guvnor!

True & Correct..!!
(but better a buffer before Guvnor's tonestack..) 

Happy now, Antonis?   

I say no more or I put myself in the forum's potentially banned spammer tank..   

[GibsonGM]

I DO like the idea of the buffer for the tone stack!   

I would simply use the booster after the pedal.    Or use many gain sections, and make an entirely new preamp, like the Box of Rock 

You would not be banned, Antonis, you are too valuable! 

[antonis]

That reminds me the need for drilling an extra hole on the enclosure for a (3rd) switch between A & B to get output booster "independently" engaged.. - pedal is dismandled for over than 2 years for paint fresh..
(till now, it was only set "on" when tonestack was engaged..)



P.S.
As you can see, it is NOT a MosFet one..

[GibsonGM]

VERY nice, Antonis!  I always have thought the Guvnor was missing something.  The opamps push a lot of gain into the tone stack, but that is the end, and the output impedance is not resolved.  I like the way you have set up the gain recovery, and I agree that a "boost" switch would make this much more versatile!

I use BJTs also, I like all of the usual gain-producing devices.  I really only talk about Mosfet because I like entire preamps made from them, or JFETs.   My favorite thing in the world is a cranked tube amp, but of course they can be difficult due to high B+ voltages and heater supplies...I think the Mosfets and Jfets can come close, and are much much easier to use!  The fit in smaller space!   And a mosfet boost into a tube amp does very nice things    It is probably not so nice into a solid state amp, or a tube preamp that does not have enough headroom   

[marcelomd]

Well, you can always use an opamp stage to recover signal from the tonestack. Which leaves you with an extra stage that you could use to... hmmm... clean blend? 

[GibsonGM]

...or make an LFO with it, add a vactrol, and create a stutter effect for the Guv'nor...

[thermionix]

They do make singles you know.

[antonis]

They do make singles you know.

With only four pins and half of the spacing..??