My First Post - Microamp Footpedal Redux (Mistake) Project

[italianguy63]

Hope I do this right.. new to the forum.  MC

Diamond in the Rough – Turning Disappointment into Something Good.

This is an interesting story and journey of sorts.  It started with me purchasing a broken Dunlop Crybaby GCB-95 Classic wah pedal from eBay.  I planned to repair it, if possible, mod it, and either keep it or resell it.  I am handy with electronics, so I figured no biggie.

Anyway, I get this pedal in, and I open it up.  Wow!  No wonder it didn't work (actually it did).  It was not a wah pedal as I could tell—as there was no inductor.  The wiring was a jumble of old solid-core phone cord bell wire.  It looked really rough.  For kicks, I hooked up a battery, a guitar, and connected it to my practice amp.  It worked basically as a volume pedal, but the sound was noticeably "muddy."

The PCB was lying loosely inside wrapped in foam.  The board had been obviously modded based on some solder burns, parts of the board were jumpered, and some of the components were not connected-- as 2 of the 6 connections were not hooked up.  I noticed some numbers on the (old) circuit board.  It had "133-3001-101" and "MXR" on the foil side.  I ran the number from the board on the Internet, and found out it was a vintage MXR 133 Microamp (signal booster) board from the 1980's.  I also printed out some rudimentary schematics I found for it.

Next, I looked at the potentiometer.   It too had a number on it--  "024-005318-03."  Turns out this was a vintage Thomas Organ pot from a 70's vintage wah pedal.  The pot was connected to the output jack, so it was just used to dim whatever was coming out of the board; obviously not how it was designed based on the schematics I had.  It measured as a 100K logarithmic curve.

Anyway, what to do...  I obviously had already spent more than the pedal was worth.  Getting new Crybaby guts would put me further in the hole.  So, I just started playing with this, trying to "fix it right," and see what transpired without buying more expensive parts.

The values of the components on the board did not match the schematics I found, but the circuit was correct.  Later in this article, I have redrawn the schematic to match what I have, and what I did.  But, for now, I realized some of the components that were soldered on were not original.  I was able to figure most of it out, and undid most of what was boogered.  Since the components were not as they were supposed to be—I studied an article that was a discussion of the MXR-133 circuit design.  It turns out the values of the components work together in formulas and ratios.  They control namely the impedance of the input/output, the amount of gain, and the high-pass and low-pass cut-off points.  Long story short, I was able to figure it out, and fix the rest of what was wrong or missing.
 
As one example, in the high-pass output circuit, a previous owner had replaced one of the resistors arbitrarily with a capacitor of the size attempting to make it a "filter cap."  The potentiometer was downstream of this, so I was able to conclude (only had to see the bell wire to know this) that this project was indeed a hack job.
After working through the PCB components, I replaced the wiring, mounted the PCB securely, added an external DC power jack, reconnected the potentiometer correctly, put in a "true bypass" 3PDT switch, and added a "power on" LED.  More about the LED now...



I thought I had seen this, but can't find it on the Internet.  I was interested to see if I could do some kind of lighting around the perimeter of the board.  I decided to experiment, using a piece of 1/8" Plexiglas like a fiber-optic lens.  Basically shining a bright light through it and letting light emit out the edges.  I cut a piece to match the outline of the board.  I drilled the holes for the feet, and another so I could put an ultra-bright blue LED towards the back of the Plexiglas shining forward.  I Nylon-tie-wrapped the LED to the Plexiglas, and left enough wire to hook it into the bypass switch for power.  I mounted a 2nd LED up in front of the treadle in a bushing in case my "fiber optic" experiment didn't pan out.  But, it did work pretty well.  Adding the extra thickness of the Plexiglas requires lengthening the screws in the feet.  Size 6-32 by ½" work well.




The final verdict:  I hooked everything up, and gave it a run.  Outstanding!  Very clean boost.  This circuit was designed to add 20DB to 26DB of boost.  The foot pedal easily adjusts the amount of boost.  Full-on it is very heavy, clean, and loud.  It just starts to sound "overdriven" but still very clean-- basically very high-horsepower.  I LOVE IT.  It boosts the level up very cleanly and evenly, once you keep pushing more, you expect it to start to break up, or sound overdriven, but it doesn't!  It just keeps ramping.  As the title to the article says—"Turning Disappointment into Something Good," I now plan to keep this pedal and add it to my collection.



Here is the circuit redrawn for prosperity.  I am not an electrical engineer, just somewhat handy with electronics, and get along pretty well.  I know engineer types love to read and study this stuff, so feel free to make comments, but no flames please based on my limited know-how!  Again, several of the original component values DID NOT match the schematics I was able to find.
I had a friend that told me about guitar work... it just needs to "sound" good, as nothing else matters much.  He was talking about high-priced gear at the time.  Good words of wisdom I think; a lot can be accomplished inexpensively with some tenacity, experimentation, and imagination.

Mark Chiampi, October 2013

[ggedamed]

Hi, Mark and welcome!

You're right, you have seen the lighting plates. It's right on this forum (this is one example).
Also you should remember that NE5534 is compensated internally for gains higher than 3. Pins 5 and 8 can be used to adjust the compensation.

I enjoyed reading your story. Keep having fun!

[italianguy63]

Thanks!

I thought I had seen the lighting somewhere.  At least I didn't dream it or dementia.  Good deal.

I don't know what "interal compensation" means.  I will have to look that up.  Learning here.

This is really a relatively new hobby for me.  I am trying to learn to play guitar, but seem to like the electronic tinkering more.  I have already done some other work-- modded a Leslie speaker cabinet (cool project BTW), did some lighting on my amp head, and been modding a bunch of wah pedals...

MC

[italianguy63]

OH!  And building an amplified "Talk Box" from scratch.  That is basically done, but I have to replace the speaker jack I broke.  I  will post that project next...  MC

[PRR]

> redrawn for prosperity

It made you rich? "Posterity" more likely.

The circuit seems fine (part-values are not so fussy as part-ratios).

> what "internal compensation" means

The negative feedback puts output signal back into the input. You know what happens with a microphone, amp, and speaker in a room, when speaker-sound gets back to the mike too much. It howls and screams. A single amplifier is simpler than a room+speaker, but still you must be careful about phase and phase-shift. More careful at lower gain (more NFB).

The General-Purpose opamps (4558, 741, TL072) are "universal internally compensated". They have a heavy capacitor inside which makes them slow, but stable for any gain. (This cap is often 30pFd, which isn't "heavy", except it wraps the main gain stage so acts-like about 15nFd.)

709 and 301 are Externally Compensated. You figure your gain, look on a graph, and add the specified capacitance to two pins on the chip. (BTW, most dual opamps are internally compensated because they don't have the extra pins needed for external compensation.)

NE5534 is a bit odd. It is often used for HIGH gain stages such as mike-amps and mix-amps. For gain greater than 3, no external capacitor is needed. For lower gain, you are supposed to add a 22pFd cap across pins 5 and 8.

Your plan goes down to gain of one.

If you don't add the compensation cap, you should expect oscillations around 8MHz. You can't hear this, but the opamp can't be screaming full-power at 8MHz +and+ handle your audio with great care. Typically you get distortion, hiss, thump, or other mystery problems.

I have been seeing recent builds with NE5534 at low gain and no comp cap. The original problem was the slow output stage rolling-off around 5MHz. But the design is 30+ years old, and by Moore's Law our chip technology has improved 250 times since then. While they can't make major changes to the NE5534 without foiling some user's design, maybe they have diddled the design just enough so the compensation cap is not *essential*. Still a good plan to install 22pFd pin 5 to 8.

[duck_arse]

The General-Purpose opamps (4558, 741, TL072) are "universal internally compensated". They have a heavy capacitor inside which makes them slow, but stable for any gain.

does this mean "slow" as in slew rate? I been too scared to ask about the compensation caps on opamps.

[italianguy63]

Thanks Paul--

Like I said, learning here.  22pF is tiny.  Next time I am at the parts surplus house I will grab one.  Should be simple to add it.  I don't think those foils go anywhere right now.

Prosperity, posterity!  HA!  English is not my strong subject either.  

Yeah I had fun with this and learned some circuit design while I was at it.  Just a sense of accomplishment for it actually working well.  

Do you think a linear taper pot would improve this, or is the log OK?  MC

[italianguy63]

Finally got around to fixing it.  Added the cap to the foil side of the board.  Of course, can't hear any difference.  Here is the revised schematic:



Thanks again for the help!  MC