DOD FX75 Redrawing

[Baran Ismen]

Hi everyone.

Out of boredom, I've started to redraw this schematic on EasyEDA with the hopes of creating a PCB for it. A couple of questions;

1- Is this done before, I wouldn't want to spit in the wind 
2- I'm unsure about some of the capacitors, such as 0.0023, it's non-polar indeed, but shall I read it in a format of uF or nF or what? An unusual value, tho, the closest one I think would be 27, but on which value?
3- What's the difference between FX75 and FX75B? Is it for bass ?
4- Is there anything else I should consider?

[antonis]

All caps in micro Farads (μF or uF) unless stated otherwise (e.g. 25pF, 120pF etc)
Electro polarized caps are marked on their positive leg with a (+)..

[Baran Ismen]

All caps in micro Farads (μF or uF) unless stated otherwise (e.g. 25pF, 120pF etc)
Electro polarized caps are marked on their positive leg with a (+)..

Thanks for the information antonis.

I've just come to realize that there's also a schematic for FX75B. This one is nearly done right now and I'll try to make a tracing for it, let's see how successful EasyEDA for this matter.

Meanwhile, I'd like to know which model is better overall.

[Chillums]

For that 2n3 (.0023) I'm sure a 2n2 will be fine there.  2n3 is indeed a bizarre value.

[ElectricDruid]

Are we sure that schematic is ok? There's some stuff which looks a bit weird to me. Perhaps someone else can take a look and explain it? My problems are:

1) The "Regeneration" pot seems to be in the dry path to the output mixers
2) U1B doesn't have an input resistor to mix the signal to the -ve input

thanks,
Tom

[amptramp]

The lower output op amp does not have a resistor in series with the input to the inverting input.  This input is driven directly by the output pin of the upper op amp.

The upper op amp appears to have its input shorted to the Vcc/2 supply.

Is the 100K resistor across the J113 correct?  Where is the rest of the output switching?


I may not have found everything, so check the circuit, there may be more anomalies.

[ElectricDruid]

+1 agree with Ron. There's some weirdness going on with that schematic - likely errors from whenever it was traced.

I certainly wouldn't go directly to a PCB design from the schematic just yet. Draw the schematic out and then breadboard it first, because it's going to need some tweaks before it works and that's easier on a breadboard rather than hacking at an expensive PCB.

[PRR]

There are many images for FX75s, not all the same, though I do see some oddities propagating through history.

While the modern low cost for PCB makes it not TOO painful to get bogus boards, still maybe better to prove your plan (breadboard?) before the acid hits the copper.

[Rob Strand]

Some tips about DOD schematics:

- The DOD factory schematics often have significant errors, so there's a difference between a redraw and a correct schematic.

- DOD pedals change over time.  Sometimes there are also different models like A, B, C versions - which can have minor to major changes.  So you need to know what version you are tracing.

- To get an accurate schematic you often have to find PCB pics of the correct version and retrace the schematic.

This version looks like an earlier factory schematic,

https://electronicscheme.net/wp-content/uploads/2011/07/DOD-fx75-scheme-diagram.jpg

You can see for example that the 0.0023 caps are actually 0.0027 (2n7); resistor is 27k.    The hard to read cap on U2 is probably 6n8.
(For 6n8:  f1 = 1/(2*pi*27k*2.7n) = 2.18kHz and  f2 =  1/(2*pi*10k*6.8n) = 2.34kHz  which matches.)

[ElectricDruid]

That one makes quite a bit more sense, Rob, thanks. It adds a 47K to that U1B mixer for a start. And the regen and signal routing doesn't seem so messed up.

[Rob Strand]

That one makes quite a bit more sense, Rob, thanks. It adds a 47K to that U1B mixer for a start. And the regen and signal routing doesn't seem so messed up

I'm fairly certain that one is a factory schematic.  Makes more sense but no guarantees it's correct through.

The schematic posted by the OP could be a later factory schematic or it could already be a redraw by someone else.  Obviously errors crept in during the redraw process.  The schematic looks like it's drawn on the old DOS based Orcad schematic package.  I'm pretty sure DOD used that for later schematics.  (OP mentioned it's schematic for a FX75B, so maybe DOD was using CAD by then.)

[amptramp]

It looks like that second schematic picked up the issues I noticed in the first schematic.  Of course, that doesn't guarantee that it is identical to the unit you have.  Running changes are often not documented - the cost of changing paperwork to match the current production model is often just not worthwhile to the manufacturer.

[Baran Ismen]

This discussion perplexed me a bit, to be honest. It's not something like TS9 that you can find an answer anywhere easily when experienced a problem.

At a local shop on Saturday, I found the DOD FX75B for quite a good price (35$) at first I was uncertain, then I decided to buy it after seeing the Reverb prices   when I was back tho, the shop was closed  I'll check it next week.

I know that they're both different pedals, but 75B seems a bit more tweakable and flexible. I'll trace it and make a PCB of it for the community, then I may sell it for at least 3x of profit      or maybe not.

[Rob Strand]

This discussion perplexed me a bit, to be honest. It's not something like TS9 that you can find an answer anywhere easily when experienced a problem

At the end of the day, no one on the forum is responsible for DOD's crappy schematics.  That's the hand we are dealt.

I've got notes on versions and corrections to some of DOD's schematics but I have no idea which ones, as it's all achieved away.   There are posts on this forum with corrections for DOD schematics as well.

The same issue from a 2002 post!
https://www.diystompboxes.com/smfforum/index.php?topic=4433.0

[Baran Ismen]

This discussion perplexed me a bit, to be honest. It's not something like TS9 that you can find an answer anywhere easily when experienced a problem

At the end of the day, no one on the forum is responsible for DOD's crappy schematics.  That's the hand we are dealt.

I've got notes on versions and corrections to some of DOD's schematics but I have no idea which ones, as it's all achieved away.   There are posts on this forum with corrections for DOD schematics as well.

The same issue from a 2002 post!
https://www.diystompboxes.com/smfforum/index.php?topic=4433.0

Now I see. It seems risky to do this, then; at least for someone who has not much experience or knowledge about the matter like me. I'd better stick with the real deal and go with confirmed DOD's for now