The hazards of modding. rrrrrrrrrrrrrrrr!

[aron]

OK, remember that Tube Driver I was modding?

Here's what I did, I changed the input cap from .047 to .01uF (to thin out the input a bit).

Then I tried to get clever (and here's where I made a mistake).

TEST YOUR MODS ONE AT A TIME!

So I then "cut" the trace on the PCB between pins 1 and 2. What I wanted to do is increase the level of this stage by putting a trim across the two pins. hmmm I soldered a trim pot right on the op amp pins.

The end result? I put the pedal together and there's MASSIVE hum.

I have no idea what I did that could cause this. If I turn the volume all the way up, I can hear the pedal and I can play the guitar and hear it. I tested my input cap mod for continuity and the fact that I can play the guitar and hear a bit beyond the hum points to the fact that the mod to the op amp section must have screwed up the circuit.

I wonder how? hmmmm

[Joep]

Hi Aron,

Are you sure it's no something else you incidentally messed up? I changed the opamp in my Sonic Distortion recently and it didn't worked after that. After the 3 (!!) time I looked a noticed a loose wire......

AlsoI think you need to have a resistor and cap coming from pin 2 to Vref or ground. This resistor sets the gain together with the pot.

Bye,

Joep

[puretube]

yes: a R betw. output & -in, and a R from -in to gnd.
Tthe gain will depend on ratio "sum of both Rs" to "gnd-R" (always>1);
Wire your trimpot (e.g.47k or 100k): ends to pin1 & gnd, wiper to pin2...
(and hope you have not blown poor old tl072)

[ExpAnonColin]

I cut the trace on a line 6 modeler when I was modding it.  Luckily it was pretty out "in the open" and since line 6 uses that funky method where they sort of tape down wire instead of using copper traces I just pulled off some of the "tape" (I know it's not take) and soldered a super tiny wire (it was 1 thread of a threaded wire) back onto it.

-Colin

[aron]

Oh yeah! Thanks you guys for reminding me that I didn't wire that trim pot correctly anyway. In any case, I removed the trim bridged the 2 pins again and that didn't make a difference at all.

Interesting.

Yes it is acting like a wire came out or something, but I took pictures before I did the mod and the wires are all in the same place.

I will work on it some more tonight.

Thanks!

Aron

[ian87]

amen, brother. last night i spent 8 hours trying to "improve" and rehouse an old MXR Commande Series OD. it would work, stop working, start working again, seemingly randomly. :-/

"one at a time" is the modding mantra....

[aron]

I need to measure the op amp and at least see if the voltages look correct.

It seems so unlikely that the mod I did (and reversed) would cause this massive hum.

It's like DC is leaking into the output signal. But how??