help: MXR Dist+ squeeling that varies w/ guitar volume knob

[XtraMedium]

Hi, all--I'm new here, and I'm new to stompbox building.  (I've made some non-audio circuits in the past, though).  

I've breadboarded a MXR Dist+ clone.  Here's a link to the schematic:

http://www.harpamps.com/schematics/mxrdistp.pdf

The problem is that this thing squeels when I plug in my Strat.  The squeeling changes pitch when I vary the volume knob or change pickups.  It turns into a high screech if I max the volume knob on the guitar.  Adjusting the tone knobs on the guitar has no effect on the squeel.  If I put another pedal between my guitar and the Dist+, the squeeling goes away.  There's probably a simple solution, but I haven't been able to find it in the FAQs or old posts.  Any help you could offer would be great!

Also, the pots on the effect only seem to have any effect at the end of their range.  I'm assuming this isn't normal...

[bryantabuteau]

umm.. i had issues with my first build ( a dist+ clone)  that was a strange oscillating squeal.   I just hadn't grounded my output jack properly.  Not suggesting thats your problem, cause you indicate it works with another pedal somewhere, but anyways.....

and the schematic i used for the dist+ had the pot values all weird.  evrybody recommends different values, and i think the distortion (or volume pot, i can't remember) should really be reverse log taper.  not that i can buy reverse log taper pots in NZ.  

I still haven't found correct ranges for my pot values on the D+ so that its usable over the entire pot sweep.

[strungout]

Oy.

Usually when that happens to me, I can find the culprit to be a lowpass that's not doing it's job. Check the 8-10pF cap in the feedback loop of the 741, make it a bigger value and see if the squeal goes away. It might be a good idea to use a quieter opamp too, TL07x or something like that.

Ciao.

[Phorhas]

yeah the dist. plus tends to get that way on a breadboard. try to get shorter wires and jumpers

[petemoore]

THis is the general 'barometer' I use to tell if a cct is working 'right'...if it boosts and distorts [talking dist here] and the knobs work, and the pin voltages look good, most probably it's just 'actin' lil' funny because of
 lack of RF rejaction:  [RF Rej. improves with shorter wires and shielding]
 HF Rolloff is not 'big' enough
  As mentioned above

[XtraMedium]

Thanks for all your input so far.  I spent some time tinkering with the circuit last night.  I've tried different values for the lowpass cap in the feedback loop.  Modifying it (and other components feeding the inverting input) changes the pitch/tone of the squeal, but doesn't eliminate it.  

I've wondered if the problem was because the circuit is on a breadboard.  Since I don't have it in a case, I just wired the chassis grounds on the inp and out jacks together.  Is that correct in this situation?  Are oscilations like this a normal problem building most guitar fx circuits on a breadboard, or just a problem for higher-gain ones like the dist+ ?  

Why would the oscilation go away when I put another bypassed pedal between my guitar and the dist+?  Is some sort of buffering effect going on that I could add to my circuit? (I was using an old Peavey Hotfoot Distortion II pedal.  I think it also worked with a DOD chorus, too.  Heh, old 80's pedals...    )

Wow, do I have a lot of  :?:   :?:   :?:  

[bryantabuteau]

I built mine on a breadboard with no issues whatsoever, so I don't think its inherent in a breadboard build of a dist+.  I was also using a 741.
Are the grounds you wired on the ouput and input jacks wired to the circuit ground as well?

[XtraMedium]

If the circuit ground is the negative terminal of the battery, then not directly.  But once a plug is inserted into the input jack, I believe the plug completes the connection since the (V-) is wired to the sleeve of the jack.

[XtraMedium]

 It works, it works, it works!  

My assumption in my last post was wrong (I was at the office then and didn't have the circuit with me).  My schematic had the ring and tip on the input jack reversed (as did my circuit), so the V- was actually wired to the tip of the input jack.  Several of you suspected a ground problem, and under closer inspection I thought that something just didn't look right with my old schematic.  (I got it off the Internet years ago.)  I switched the wires and presto! Now I'll play with some mods...

[Surfin7]

is this squeeling like a "woooooooo...." when you turn up de volume ?
I have this problem, got the scheme from the TonePad site, and at the build reports there someone sugested to use tantalum caps, I´m using then but the sound is still there.
Let me try to explain: with the volume at something like 60%, the dist. goes at 60% with "clean distorted" sound, if I level any of them up, theres this "wooo", going away if I turn down. To get more volume, need to level dist down, and "vice-versa".
But the "fly" just appears when the input is low, when playing hard (and fast) you almost can´t hear.

I have no 3DPT switch, the pedal is on all the time, and the ground of input jacks are wired to each other, if I gound to the circuit the pedal becomes mute.

Any sugestions ?
Thanks !  

[MartyMart]

I just had a similar problem with a "Liquid Drive" - which is almost the
same circuit, well its Dist + based.
The small "pf" cap across the op-amp needed to be increased from I
think 22pf to a 100pf !
The "squealing" has gone  
I lost a little bit of "high end" but its fine.

Marty.

[Surfin7]

thanks !    I´ll try this weekend !!!  

[Surfin7]

weel, changed the cap from 10 to 110pF but the "wooo" is still there, I recorded the sound.
the guitar connected to the box and then to PC, at the first "woo"  the volume is at 50% range, the drive goes from 0 to 100%, at the second "woo" the volume is at 100% and the drive goes again from 0 to 100%

http://www.geocities.com/ex_nevasca/images/squeel.zip
just 72kb

thanks for the help !!