Another DOD question: FX56 and FX59

[Fancy Lime]

Hi there,

following my recent fascination with the 80's and 90's DOD overdrive/distortion pedals, I had a look at the FX56 American Metal and FX59 Thrash Master. Allegedly, the American Metal is supposed to be DODs answer to the BOSS HM-2 Heavy Metal, aka. the Swedish Chainsaw, an old favorite of mine. I did not manage to find a schematic. I did find one for the Thrash Master, which shares a lot of topology features with the HM-2. At least as far as I can tell from this hot mess of an error ridden schematic:

http://4.bp.blogspot.com/-mMD0Cdhzk0E/T-_GwsLouII/AAAAAAAABas/u3jeCR9emZk/s1600/DOD%2BFX59.gif

Oh boy, that is so wrong, it is almost an art form of it's own. But you can extrapolate the most important parts, at least. Does anyone know, if FX56 and FX59 were build on the same board? Does anyone have one of these and/or a slightly less disastrous schematic?

Cheers,
Andy

[space_ryerson]

Thankfully, in the old days, DOD would supply a schematic with their pedals. Here you go

If I recall correctly, DOD used the same circuit for multiple distortions, with the main difference being gyrators for the tone circuit. This was a long time ago, so don't hold me to that

[Rob Strand]

Oh boy, that is so wrong, it is almost an art form of it's own.

So funny! 

But you can extrapolate the most important parts, at least. Does anyone know, if FX56 and FX59 were build on the same board? Does anyone have one of these and/or a slightly less disastrous schematic?

It's a common problem with DOD schematics.  The more you look the more problems you see.

I can't answer your question but I do remember an older pcb pic of the FX56 being posted and someone saying it didn't look like his PCB and that his PCB was a lot like a HM-2.   That pcb pic had no markings but the later ones may have markings.

This site has some excellent pics of DOD pedals and pedals in general,
http://mirosol.kapsi.fi/

Couldn't find those on that site but found this,
FX59 unit with FX56 B PCB; (1990 maybe as late as 1994)
https://medias.audiofanzine.com/images/thumbs3/dod-fx59-thrash-master-557332.jpg

----------------
EDIT

FWIW, I had a look at the info on that older PCB.   The main reason the poster thoughr it was different was because his PCB didn't have TL022's in it and there were some PCB layout differences,

But this link to a 1986 FX56 shows it's pretty much the same thing,
http://img-cdn.jg.jugem.jp/698/2938743/20141115_897950.jpg

I can see a few parts moved on the PCB but it doesn't change the general design.
I can also see C13 is not loaded on the FX59 pic.

[Rob Strand]

I clean-up with mind jogger document for common bugs on DOD original schematics.
--------------------------------------------------------------------------------------------------------
DOD "Original" Schematic Issues
Rob S, V1.1, 28 Sept 2019

Be aware some models change over time.   You will see FX99,  FX99B, FX99C ...
The later models are different.  Sometimes the circuit changes significantly, especially
from FX99 to FX99B.

DOD re-use circuits.  You might see the same circuit used on different models.
The part values are often the same but this might not always be the case.

The original DOD schematics have errors.

Sometimes a new version of the schematic is available and it has different errors to the
previous version.  Sometimes fixing the bugs of the previous version and sometimes not.

Most common bugs:

- Totally bogus connections on the schematic.  Very common.
  Sometimes the bogus connections occur in many places on the schematic.
  The circuit will not work if built to the schematic.

- Capacitors totally missing from the schematic.   Very common.
  A common bug is capacitors to ground are missing on RC low-pass filters.
  A tell-tale sign is a series resistor going to a chip pin.
  Other places are caps to ground to the JFET switches, or
  the cap on the CD4007 footswitch circuit.

- The designator numbers go higher than the number of the parts on the board.
  Some designators are not used - there is no part on the PCB or schematic
  for some designator values.

- Designators left off the schematic.
  A common "bug" is the designator numbers are missing on the
  schematic and are shown as R? or C?.

- Rubbed-out part values or designators.

- Misleading or ambiguous part values which are not consistent with the
  naming conventions elsewhere on the schematic.

  Common on caps.   eg 4.7K for 4.7uF,  120k for 120pF
  The same value might be used elsewhere on the same schematic and marked 120pF.

- Pot tapers not given.
--------------------------------------------------------------------------------------------------------

[Fancy Lime]

Thanks for the link Tim! That one looks like a working circuit, at least. FWIW, the schematic I dug up seems to be traced by someone. It certainly looks different from the official DOD schems in terms of style. I was wondering about the unconventional gain control but that seems to be legit. DOD designs are sometimes a bit strange, me thinks. With BOSS , I can *usually* follow the design rational and know why they did this or that in this or that way. With DOD I am frequently thrown off, thinking "why not do it the way everyone else is doing it, which would work better and use less parts?"

Andy

[ElectricDruid]

With DOD I am frequently thrown off, thinking "why not do it the way everyone else is doing it, which would work better and use less parts?"

Could be to cover that fact that several of "their" designs seem to have had a previous life in other manufacturer's pedals. The Swedish Chainsaw is definitely an example - there are several DOD pedals that are virtually identical to the HM-2 circuit. I'd guess they changed a few parts to avoid any potential problems, and then made a few random changes on the schematic too just to make sure.


Edit: Here's a link to my page about the DOD PDS-1550, which has one channel which is basically the HM-2/American Metal circuit.
https://electricdruid.net/digitech-pds-1550-distortion-pedal-analysis/