Original 70's Maxon Dirt Box, Anyone?...

[SolderBoy]

Hi.  This beaten up, repaired and (badly) resprayed pedal came to me this week.  The vol pot needed replacing and the rest of the wiring was a bit of a mess, but it was simple enough to fix.  I also rewired it to true-bypass, as it had a DPDT stomp switch in it (looked original, a Japanese made switch), but only one pole was used originally.

The thing is, I couldn't find any schematics online, and few references to it.  And the ones I did find were not the same as this.  I scribbled out a schematic.  It is a super simple circuit.  No input or output buffers.  No led, DC jack, or polarity protection.

The bass roll-off at the gain circuit, and the whole tone control were exactly as a TS808, but the parallel (silicon) diodes go to earth, as in Rat, Fat Cat etc etc.
After some TLC, it worked just fine, but I don't even have a guitar amp these days to give it a good listen.  (I work on bass stuff mostly.)


So, my pedal guru friends, what can you tell me about this?  Is it the grandfather of the TS808?  Here in Australia these old pedals are pretty rare.

[SolderBoy]

It looked like this:



(But in even poorer cosmetic shape.)


And this:



But the resistors were dark brown, instead of light beige.


And this:



"MP-DB701", But the word Maxon wasn't tinned, and the solder, in general was greyer...

[DFX-PCBS]

Cool, looking at the inside pics online I bet you could trace the circuit and easily make a schematic

[SolderBoy]

Umm, well yeah.  As I mentioned, I did trace the schematic, but not from online pics.  These are just google images I found that look the same.  I would've posted actual pics of the one I mended but you can't upload them on this board.

I traced the schematic from the one I pulled apart in my workshop this morning.


So... Yeah...  Anyone know what this is?  The schematics I've seen for Ibanez dirt box, OD855, and OD850 are not like this pedal.  Perhaps this is an early model...?

[Rob Strand]

Look here,
http://tonemachines.blogspot.com.au/2012/02/ibanez-dirt-box-1976.html

"Well upon closer inspection it turns out that it is the same circuit as the Ibanez Overdrive II! Which itself was the precurser to the Tube Screamer. "

That would the narrow box Ibanez Overdrive II.
I thought that one had germaniums but the pic show silicons.

You have to really careful about some of the Ibanez pedals in that era.  There were quite a few variants.  Some pedals had the same names and completely different circuits.

[Edit Details
Circuit identical to:
Ibanez OD-855, Overdrive II
First Run, 1976/1977, no LED
PCB:  Maxon MP-DO701
"The units have flying fingers on the side,
a round button, and the vintage Ibanez logo.
The middle knob is Balance and has a smaller knob."
(no LED)
Circuit is same as the following one except no 100k on the Balance pot and no LED stuff, no protection diode.

This one is a later version which has an LED and the modern Ibanez logo.
Schematic is close but not the *exactly* same to prev; differences mentioned above.
http://www.6v6power.ru/inf/Distortion/Ibanez%20OD-855%20Overdrive%20II%20.jpg

After that the OD-855 has a square button and is completely different.
]

[Edit 2: changed some text in that last part.]

[thermionix]

SolderBoy, can you share the schematic you traced out?  I'm curious because I'm looking at pics that show pots as 500k (dist), 20k (tone), and 50k (vol).  The schematics I've seen for the Ibanez show 50k (dist) and 10k (vol).  So obviously some significant variation.  I've never heard one of these but it looks easy to breadboard, I probably have all the parts already.

[SolderBoy]

Look here,

Thanks man.  That's a great post. 

You are absolutely right.  That hand-drawn circuit is almost the same as mine.  Along with the absence of led, led resistor, zener, and protection diodes, and DC jack, mine also has no 100k output resistor.  Plus the volume (balance!) pot is audio taper, not linear.  Oh and yeah, silicon clipping diodes (the red ones with a blue band).

Because it was resprayed (also in a fairly amateurish way), I have no way of saying what it actually was, but I can confidently tell the owner that it was either a "The Dirt Box" or an early "Overdrive II"...  Right?

[SolderBoy]

SolderBoy, can you share the schematic you traced out?  I'm curious because I'm looking at pics that show pots as 500k (dist), 20k (tone), and 50k (vol).  The schematics I've seen for the Ibanez show 50k (dist) and 10k (vol).  So obviously some significant variation.

[Rob Strand]

[ I have no way of saying what it actually was, but I can confidently tell the owner that it was either a "The Dirt Box" or an early "Overdrive II"...  Right?]

Not much doubt in my mind.  Especially if the PCB has that marking.

I don' t know what to make of the audio taper balance pot.

I have a schematic which has mark-ups for the first model.  It looks like an original Ibanez schematic.
It shows a Linear Balance pot.

The pics for the Dirt Box also show a linear Balance pot.

So it's not possible to work out the difference from the Balance pot.

The Ge vs Si diode has always been an issue.  I don't think I ever seen one of those with Ge diodes on the PCB.

[Rob Strand]

The schematic looks good.

I believe the sustain pot is a C taper (reverse Audio).

[Rob Strand]

One thing I forgot to mention.   The Ibanez boards of that era often have the dark brown resistors.

[Rob Strand]

Here's what looks like an original Ibanez schematic:

[SolderBoy]

I don' t know what to make of the audio taper balance pot.

Well that pot was not original, and the soldering to it was pretty amateurish, so it may not be correct.  It needed replacing and I put in the same.  To me, an audio taper seems appropriate for this.  But I guess a linear taper gives you finer control of that top range.

I guess adding that 100k output resistor would greatly drop the output by a fair amount....  9 or 10dB I'd think.

I believe the sustain pot is a C taper (reverse Audio).

Yes you are right.  My mistake.  It has to be reverse log since it is at max gain with low resistance.  (Unlike the TS where the pot is in the feedback loop!).  I'll change it and repost...

[SolderBoy]

Here's what looks like an original Ibanez schematic:

Well there it is!  Nice one.

Thanks for all your help Rob.  I think I'll put in a linear pot instead, just for the sake of originality...   

[SolderBoy]

Oh, and those wire colours are the same as I wrote down too...

[Rob Strand]

Thanks for all your help Rob.  I think I'll put in a linear pot instead, just for the sake of originality.

No problem.  Thanks for clearing up the pot taper.   Sometimes there's so many variations with this stuff it's hard to know what is modded and what is a variation.

(I've only just realized I can post images on this forum!)

[Rob Strand]

Oh, and those wire colours are the same as I wrote down too...

The wire colors and notes were added by the author.

At some point I checked them and also checked quite a few part values from pics.
The fact it agrees with your trace, and the later schematic, pretty much confirms it's correctness.

[SolderBoy]

Thanks for clearing up the pot taper.

Actually I've noticed that G taper is a classic TS schematic mistake.  It's oftten marked as linear.  Do you know where I can get a 16mm G20k?  The pot works, but it has some cracks in the phenolic wafer, so I'm not sure for how long!  I'd think a smaller value linear taper would sweep OK, but I really should keep a rare pedal like this as original as possible...


And for those following along at home, I've reposted a correct schematic.

[Rob Strand]

Actually I've noticed that G taper is a classic TS schematic mistake.  It's oftten marked as linear.  Do you know where I can get a 16mm G20k?  The pot works, but it has some cracks in the phenolic wafer, so I'm not sure for how long!  I'd think a smaller value linear taper would sweep OK, but I really should keep a rare pedal like this as original as possible...

You could try Small Bear Electronics.  Not sure what types or values he keeps for G tapers (ie. sizes, shaft type, detents etc.)

Linears will work but they have only a small effect in the central region and a sudden boost/cut at the extremes.  It's "OK" for the tube screamer ckt but at the end of the day you can't beat the right tapers.

[digi2t]

How about a w20k pot?