DOD 280 compressor mod

I have here a schematic and parts list for a DOD 280 compressor. Does anyone have any suggestions of what components I could change to fix the slow pumping/random volume drops inherent in the compressor? It seems that at most settings the volume will dip if you hit the strings too hard.

Thanks!

This looks like a rather primitive circuit with very few controls on knee, attack, decay etc.

And too much depends upon the LED/LDR that you choose. Individual builds could vary based on choise of LED/LDR

If hard chords totally cut the volume out, put a reistor 100K-500K in series with the LDR.

In case you make this into a Pot and get it to the front panel, this can be your "Compressor ratio" knob. But this simple mod on this simple circuit will also change gain and you will need to readjust gain each time you change ratio.

Let me know how that goes. Then we can discuss bigger mods for Knee and attack.

Hey, thanks very much. I opened up my 280 and discovered the circuit board is arranged quite differently from the schematic I posted. I took some pictures, but unfortunately they're very fuzzy (cheap camera phone).

I'll post them here in case they could be of some value. Any idea where the LDR might be, just by sight? I think these might come out a little clearer if I try taking them from further away... I'll try that later tonight.

If it is a LDR and LED in one, it will be a four legged creature,

Try tracing forward from pin 1 and 2 of the IC, we see from the schem that the LDR is in between these pins.

Looks like your camera cannot focus closer than about 20 cms.

Okay, I've located it. You're right, it's got four legs. I was looking for something that looked like an LDR, so I didn't recognize it. Of course, it's the only component that doesn't look like a cap, resistor, chip, or potentiometer, so I guess that should have been my first clue.

Okay, I haven't found a schematic out there that exactly matches my DOD 280. There is one that comes close, but not exact. So, I mapped it out myself. This is from the component side perspective, not the solder-side perspective:

According to the closest schematic I could find (DOD 280A), these are the values, though I can't say for sure if all of them are correct:

Q1: PN4124 Transformer

Q2: PN4124 Transformer

R1: 10K

R2: ?

R3: 470K

R4: 500K

R5: 3M

R6: 100K

R7: 100K

R8: 220K

R9: 220K

R10: 4K7

R11: 22K

R12: 22K

C1: .01uF

C2: .047uF

C3: .047uF

C4: .047uF

C5: .047uF

C6: 47 uF

C7: 10uF

C8: 10uF

U1: On the schematic it's listed as TL022 or LM358 -- mine is labeled as BA4558 348 218.

U2: VTL5C2?

Here is a link to the closest schematic:

http://www.generalguitargadgets.com/pdf/ggg_opcomp280a.pdf

Originally posted by Uma Floresta

So, my question is, where exactly would I install the 100K resistor so that it is in series with the LDR? I'm obviously not well-versed in the theory of all this, but I'm handy with a soldering iron and have done a couple of DIY mods.

Thanks!

bump!

Okay, Web Surf came up with a solution to the problem. Basically you need to break a lead in the circuitboard, like so:

Now, in between those two points, solder in a 1Meg resistor. This will get rid of the random volume dip problem inherent in the stock DOD 280. This will, however, reduce the variety of sounds available in the unit. Even with the compression maxed out, the effect will be fairly subtle -- no super squishy sounds. But, it does sound natural and limits your dynamics without imposing itself upon your tone too much, which is nice.

An interesting note: if you put in a 100K resitor instead of 1M, you get regular compression at low settings, but if you crank the compression, you get an interesting fuzz tone. Kind of sounds Fuzz Face-ish. I'm thinking I might install a DPDT switch to select between the two. The fuzz sound is actually pretty cool.

1M is definitely too much, and 100k would probably be a lot for my tastes... I would make it a 100k pot wired as a variable resistor and label it as a RATIO control.

regards, Jack

Originally posted by AMZ-FX 1M is definitely too much, and 100k would probably be a lot for my tastes... I would make it a 100k pot wired as a variable resistor and label it as a RATIO control. regards, Jack

Well, interestingly, it creates fuzzy distortion with the 100K pot when the compression is cranked. A lower value (22K) made it sputter in an almost tremolo-like manner. I didn't have any resistors between 100K and 1M on hand (I need to get some more parts), but Web Surf recomended a 220K resistor in place of the 1M and a 2.7 K resistor at R1.

Let's say I wired in a 220K pot at that position. I assume the values would be from near 0 up to 220K, at the minimum and maximum settings of the pot. Is that correct?

Uma,

If you used the green layout that is posted above, then there is a problem. I think you should make a connection between the lower ends of R2 and R9 in order for the opamps to be biased properly.

Also, how is the power connected to pin 8?

regards, Jack

Okay... I'm not sure about the power, I'll check later.

Last night I was trying stuff on it and I think I shorted out the compressor pot. If I can fix that problem I may just turn it into a fuzz and buy a BYOC comp kit. I don't know, maybe not.

Okay, I'm not sure what the problem was, but it's working again. I'll try connecting the two resistor ends and see what happens.

I still need to check the power thing. I'll report back.

Actually, I looked closer, and the lower ends of R2 and R9 are in fact connected. Sorry I didn't notice that sooner! Good catch.

I'm not too tech literate, but the bottom rectangular section is some sort of plug. There five wires going out from it. The first one on the far left goes to the switch, the second and third go to the AC jack, the fourth goes nowhere, and the fifth also goes to the switch.

Hi Web Surf,

I just thought I'd let you know, that your mod DOES work as you originally indicated. I think I must have just done some sloppy soldering that resulted in the distortion. I've moved the mod into an "on - on" two-way switch, and lo and behold, it works. No distortion, and the random volume dips are gone. Now it's a nice, useable compressor. I just wanted to let you know that you were right all along -- it was just operator error. I've found 220K is just about right. 100K is an improvement, but there's still just a bit of that volume dip thing going on sometimes.

ok so what's the final verdict? i picked up the same pedal=- orange one, where do i put the 22k resistor? bt where you made the etch in the pcb?

Also, hows the battery life on these guys? Does it draw power if a cable's plugged in, even if the pedal isn't on? How about the tone suck when bypassed? Anyone true bypass one of these old dod pedals and put an led?

Also does the mod let you put the comp knob past noon? cuz right now the attack slow to the point of uslessness past noon. Would a reverse log pot help this issue?

Okay, this is the PCB, component side:

What I did was basically take a very small flathead screwdriver and I bored into the pcb at the spot with the arrow pointing at it (I did it from the underside, though this diagram shows it from the top (component side). I went until I'd gone all the way through to the other side. You could probably do this with a small drill bit, if you were careful. There's probably a much better way to do this, but that's what I did. I don't have a lot of tools.

Once that lead has been severed, you solder in a 220K resistor between the two points, making the connection where the lead once made the connection internally in the PCB. I put it on the underside of the board, which is the easiest way to go. You can do like I did, and wire it to a two-way switch that goes between the resistor and just a jumper, to go back and forth between standard and modded.

220K is what I found works best. 100K is an improvement, but doesn't totally get rid of the problem. Originally my soldering wasn't right, I had done something incorrectly (I'm not sure what) and I was getting distortion. When I re-did it, taking it off board and putting it on a toggle switch, somehow whatever I had messed up or soldered sloppily was resolved and now it works great.

Another option, as AMZ mentioned, is to hook up like a 250K linear pot in between those points, so you can sweep between different values. That'd probably be more versatile than my toggle switch.

I never use batteries, so I don't know. I hate batteries. I had to use my radioshack adapter though, because it has a small male plug input and it's positive tip rather than negative tip -- just like the old MXR pedals. You could probably mod it so it's negative tip barrel style, but I didn't bother.

Tone suck seems to be negligable. I don't really notice any, but I'm not overly picky about that. I'm sure you could also mod it to TB without too much trouble.

Yes, with this mod you can crank the comp knob all the way up, with no slow attack issues. Works well, though the compression doesn't seem quite as extreme, it's still squished enough for most peoples' needs.

Thanks a ton for the info. I'll put in that resistor tomorow. I'm pumped about this pedal. Killer find for 35 bones. I'll have to make a power supply adaptor for it. Thanks again for the info, idk how they could have released this pedal with this issue when theres such a simple fix, crazy dod bastards.

Yeah, not sure what they were thinking. It does have a nice overall tone to it, I think. Plus it just looks damn cool.

hummm weird, mines boards different then yours. Same orange casing sn 2806808. Its a much lighter board with only a small green tint and plastic pot knobs that are mounted on the opposite side of the components. I not sure exactly where to put the resistor, theres 4 legs, 2 are spread far apart, and 2 are close. Do i want to put the resistor inbetween the 2 spread apart legs or between the 2 close legs?

This can be fixed as well, I suppose.

First we looked at the minimum gain and increased it. The way we did it reduces the maximum gain.

We can always calculate a different resitor to use in parallel to the LDR to get the same maximum gain as before.

I could assist if you inform me the values that the LDR has at the beginning and the end of a guitar note ( when disconnected !!! )