Reverse Engineering the Grey DOD 250 Overdrive

[Steve Mavronis]

Somehow I think more and more people are after a perceived, EXPECTED TONE of a certain 'mythical' circuit, instead of actually copying the circuit itself.  

I'm not sure how accurate to the gray 250 versions these layouts are or if they contain any wishful "user mods" or not. My original 250 PCB layout was drawn studying actual PCB pictures, reading the color codes on resistors and capacitor codes, etc. That's why I call this reverse engineering a gray 250. I started with an approach of not trusting any published schematics or layouts. The only wierd thing was the gray 1979 version with the extra 100 ohm resistor that the first yellows also had; which by the way except for the color had identical PCB (and parts?) until the actual layout changed to a new pattern when the dual op-amp IC chip was used.

[Speeddemon]

I'm not sure how accurate to the gray 250 versions these layouts are or if they contain any wishful "user mods" or not. My original 250 PCB layout was drawn studying actual PCB pictures, reading the color codes on resistors and capacitor codes, etc. That's why I call this reverse engineering a gray 250. I started with an approach of not trusting any published schematics or layouts.

Then we understand eachother. And that probably means you came to the same conclusions as me, right? I've checked over a dozen original grey PCB's now and NONE of them feature the changes from the '77 schematic. I take it you found that out too, otherwise your own design would have added these parts of values.

That's why I find it strange that GGG (amongst others) will keep these (IMNSHO *wrong*) schematics/layouts alive. Unless they're purposely adding to the confusion about what exactly is a proper greybox version.

[Steve Mavronis]

There is confusion between the '77 version and the "Grey Specs" version looking at MarkM's layouts for example. On his 1977 layout the part values are different that I'm using, of which mine mirrors his grey specs layout instead. I'm heard the terms grey specs and 1977 grey used interchangebly adding to the confusion. For those collecting grey 250's on ebay they probably don't care what's inside of them as long as it's gray and has the LM741CN chip!

Check out the images from this new 250 ebay ad. Here is a photobucket gallery of the gray 250 for sale and it has closeups of the PCB with that extra 100 ohm resistor. I just sent an email the seller and see if he knows the date of manufacture or is willing to take the knobs off to remove the PCB to get the date codes off the pots.

http://s521.photobucket.com/albums/w337/chalie1/?albumview=slideshow

Sample screen capture from one of the photos:



Also I'm reading that the first yellow 250 was introduced in 1981. So was there a 1980 version of the gray 250?

[Toney]

O.k., maybe I'm missing something, but does anyone have any gutshots from a grey '77 then? 1N4001's? Never seen them in the DODs. The 2nd 1M resistor instead of 470k, never seen it either, not to mention the added cap near the input, the 2nF near the diodes, the 22uF coupling electrolytic...it's all fine and dandy, but *I* never saw a grey box with those parts, period. 

Yes, I have been wondering this too.
Has anyone verified this configuration -  sure it's not someone's personal mod?

Not that it particularly matters - its all about what sounds good, but I am kinda curious to know if these were every released by DOD as some kind of transitional Grey version.

[Steve Mavronis]

The ebay seller of the gray 250 with the extra 100 ohm resistor (gut shot pictured in my last post) wrote back to me and he says:

I've added 2 more pictures of some numbers that were inside. On the back cover there is a sticker with 2501155. Under the PCB it has the numbers 42
22 4-7 4-9, i dont know what these mean maybe you can tell me.

The number on the pots is 1378008.

I hope this help, any other info you would like let me know.

The CTS (137) pot date codes indicates they were made 1980 (80) in August (08) so the gray pedal here has to be at least as old as the pots. The yellow 250 first came out in 1981 so this pedal is most likely a 1980 version. Bare in mind I have seen another gray gut shot showing an RCA LM741CN chip which supposedly DOD only used in 1979. So the extra resistor was used in the 1979 and 1980 versions of the gray DOD 250 Overdrive.

[Toney]

It's just a 100ohm current limiting resistor from 9v.
It will make sweet Fanny Adams difference.

I assume, like many manufacturers, DOD went with what they had in the parts bins at the time.
See EH threads for the more info on this.
There are probably several variations over any given year.

My interest is kinda piqued by the unusual speced '77 with the 4001's/extra input cap to ground though - I don't really care whether it was an original release by DOD or not.  If it sounds good, it IS good.
So, how did this come to be? It seems that member JHS had the skinny on this. From earlier threads, it looks like he may have advised MarkM (Pcb's above) and Ulysses (Vero).
I wonder where this started..

[zombiwoof]

It's just a 100ohm current limiting resistor from 9v.
It will make sweet Fanny Adams difference.

I assume, like many manufacturers, DOD went with what they had in the parts bins at the time.
See EH threads for the more info on this.
There are probably several variations over any given year.

My interest is kinda piqued by the unusual speced '77 with the 4001's/extra input cap to ground though - I don't really care whether it was an original release by DOD or not.  If it sounds good, it IS good.
So, how did this come to be? It seems that member JHS had the skinny on this. From earlier threads, it looks like he may have advised MarkM (Pcb's above) and Ulysses (Vero).
I wonder where this started..

Does that current limiting resistor have anything to do with the fact that the old DOD power supplies were 10 volts?.  I noticed this on the label of my older DOD pedals.

Al

[Toney]

.
I don't know the DOD adapters- are they regulated?  Either way the 100 ohm resistor wont make any relevant effect on the voltage.
As you know a fresh 9v can easily be approaching 10v when new. If you want to mess with the voltage, just try Beavis' battery reducer pot thing and tune it in.

[Scruffie]

.
I don't know the DOD adapters- are they regulated?  Either way the 100 ohm resistor wont make any relevant effect on the voltage.
As you know a fresh 9v can easily be approaching 10v when new. If you want to mess with the voltage, just try Beavis' battery reducer pot thing and tune it in.

Yeah I assume because these pedals were old, it was from a time before 9V became standardised and they rounded up the 9.6 output of batteries.

And you're right about the 100R, don't sweat the small stuff, aslong as you get the general effect true to the original, these little additions to the circuit that were added were probably added for good reason and will have no ill effect on the unit.

[zombiwoof]

.
I don't know the DOD adapters- are they regulated?  Either way the 100 ohm resistor wont make any relevant effect on the voltage.
As you know a fresh 9v can easily be approaching 10v when new. If you want to mess with the voltage, just try Beavis' battery reducer pot thing and tune it in.

I have never had one of their adapters, I just noticed that on the back label of my old DOD pedals it says to use the correct 10 volt DOD adapter.  I thought it was strange that it was 10 volts.  I figured they regulated the voltage inside the pedal.

Al

[Speeddemon]

It's just a 100ohm current limiting resistor from 9v.
It will make sweet Fanny Adams difference.

in a Rangemaster circuit, it made all the difference for me! WIth a powersupply that is; without the 100R, noise, WITH the 100R, quite a bit less noise.
Ofcourse, the RM already had that 47uF electrolytic in there for decoupling.

[Steve Mavronis]

This is interesting thinking about the purpose of 100R as current limiting and noise reduction. It's been a while so excuse me if I totally flub this up, but Ohm's law is V=IR or Voltage equals Current times Resistance. The current going through that 100 ohm resistor (I=V/R) would therefore be 0.09 amps (from a 9V battery) feeding the Op-Amp. Hard to think what the current is without any resistor there but if we make the resistance approach closer to 0, say 1 ohm, then the current would be 10 times higher at 9 amps! So that 100R definitely limits the current for sure. I'm not sure what effect this has on the Op-Amp and general circuit yet. Less heat and noise? I'll have to dig out my 80's college course book on Operational Amplifiers and review. Even though it's pretty basic this is a cool circuit to study what is going on.

[Steve Mavronis]

My reasoning is bad. My dad informs me I can't go by the supply voltage. I have to consider the load of the entire circuit. So I better hit the books!

[Steve Mavronis]

Hey check this out. There's a guy on another forum who posted a message wondering about my clone pedal and said he owns a gray 250 in case there was any info he could provide to help my quest. So I asked him to post some pictures and tell me what the CTS pot day codes says, in his case July 1979.

What is this big diode on the solder side in the bottom photo? I've never seen that before so can't say it it is original or not:

[Speeddemon]

Looks like a 1N400x. As you can see it runs from ground to +9V, as a polarity protection. My money is on 'not original'.

[Steve Mavronis]

Looks like a 1N400x. As you can see it runs from ground to +9V, as a polarity protection. My money is on 'not original'.

Yes it was surprising to see, plus a nice clear look at the trace side. The traces are run a lot cleaner than it looks like as shadows seen from the component side.