DBA Robot Pedal - Need Debugging Advice

Frequent questions about dead or faulty pedals. Whether you repair, mod or build pedals, debugging is important. Threads about general debugging problems irrespective of the circuit involved...

DBA Robot Pedal - Need Debugging Advice

Postby GuitarStv » 04 Dec 2013, 18:35

Hi All,
I just recently started building guitar pedals and have been doing pretty well so far. I built an MXR Blue Box (http://johnkvintageguitars.homestead.com/Effects/Fuzz-ODs/BlueBox/MXR_BlueBox-Final-Verified.png) and Mad Professor Snow White Auto Wah (http://tagboardeffects.blogspot.ca/2013/04/mad-professor-snow-white-autowah.html) as my first pedals, and they worked great the first time around. Unfortunately I've been having a lot of trouble with my third pedal, the DBA Robot.

I'm using the rotary knob control layout from here:
http://johnkvintageguitars.homestead.com/Effects/Fuzz-ODs/DBA/01-DBA_ROBOT_VERO.png and have added a momentary switch to ground to toggle on and off vibrato (based on the spec sheet for the HT8950 I was pretty sure this should work).http://johnkvintageguitars.homestead.co ... rified.png

Anyway, I've attached the pedal to all the controls and plugged it in. The LED works, the bypass works, but the actual effect is very very very quiet. Unless I set the gain at maximum I can't hear anything. When I turn the volume knob I get some sound at around the half way position, but then turning it further in either direction makes the pedal get quieter again. I can hear the effect doing some pitch shifting and fuzz, and my momentary switch toggles the vibrato on/off as expected, but the loudest I can get the effect is about 1/5th of the bypassed guitar volume. The rotary switch seems to go through different sounds as expected.

I have been over the board layout about four times now, and am pretty certain all the components and connecting wires are in the correct location. I've tried swapping out the HT8950 with a spare, with the same results. I've measured the values of all resistors on the board and they check out as well. I've measured the values of the pots, and they check out, and sweep as expected. I re-heated each solder joint on the board to try to rule out bad solder joints.

I put one probe on ground and measured the following values on the legs of the TL072:
3.27 3.82
3.27 3.26
0 2.29
0 0

And these values on the legs of the HT8950:
3.55 3.56
0 3.55
3.55 3.55
0 3.55
1.37 1.3
1.37 1.35
3.8 1.25
0.01 3.8
3.70 1.35


Anybody have something I should try next? I'm kinda out of ideas at this point. Sorry for the long post!


Crappy pics of the board (sorry I've only got a crappy camera):
http://www.flickr.com/photos/52081428@N03/11208746166/
http://www.flickr.com/photos/52081428@N03/11208823585/
http://www.flickr.com/photos/52081428@N03/11208845836/
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Re: DBA Robot Pedal - Need Debugging Advice

Postby GuitarStv » 11 Dec 2013, 16:48

Well, I've been poking at this for a few days now. The gain and control pots seem to be working (go from very very quiet to only a little quiet, and changes pitch as expected - just very quietly). It's got to be something related to the volume control. I've noticed that when leaving the board plugged in for a while one of the components is getting very warm to the touch. I think it's the 43R one in the upper right of the diagram, although I'll try to verify this tonight. Anyone have ideas for me to try debugging-wise?
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Re: DBA Robot Pedal - Need Debugging Advice

Postby GuitarStv » 12 Dec 2013, 13:12

Well, I figured out what was wrong. The wire marked 'Ground' on the board is apparently supposed to be used to ground the 4 way switch connections . . . it is NOT to be grounded to the power supply, jacks, etc. Moving this wire appears to have fixed the effect and has stopped the resistor from overheating. Just posting this up there for anyone else who might run into the same problem.

For this message the author GuitarStv has received thanks: 2
Mrutg (25 Dec 2013, 16:26), Nocentelli (12 Dec 2013, 13:15)
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Re: DBA Robot Pedal - Need Debugging Advice

Postby Mrutg » 26 Dec 2013, 23:17

Hello GuitarStv - are you still there?
I am having a lot of problems with this bad boy. Can't even get a bypass signal which is strange because even as a total begginer i never had problem like this in simple fuzz boxes i built.
How do you ground the input, output and the main effect switch? Is the wiring of the main switch different than standard ?
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Re: DBA Robot Pedal - Need Debugging Advice

Postby okwilliam » 04 Dec 2014, 23:00

hey, I'm also having the same problem. Any solution? Is the layout correct? En how do you ground the rotary switch if not to the main ground?
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