DOD FX90: max delay and volume adjustment?

[tootsMcgee]

I picked up what I think is an original FX90, no bloody -A, -B, -C, or -D. OK, it's possibly Rev A. It's based around a MN3005+MN3101 chip and NE570 compander.



Schematic (Rev A?)


The specs I've found say that it has a max delay time of 300ms. Even adjusting the little 1M trim pot, I can only get it up to about 250ms. The first echo is noticeably quieter/more filtered than the original sound.

Is there any way I can boost the level of the echo a little past what it is now and/or make the delay time longer? Any components that look suspect (caps)? Or am I expecting too much of this circuit?

[ElectricDruid]

Is there any way I can boost the level of the echo a little past what it is now and/or make the delay time longer?

You could try replacing the 25 pF/nF (?? can't read it) cap in the MN3101 circuit with 33 pF/nF. That should push the clock frequency down a bit and make the delay longer. The trouble is that you risk getting clock whine coming through.
That part was probably 20% tolerance when it went into the circuit 30 years ago, so God himself would be guessing what its value is now.

For the delay volume, there's a 5K trim by the NE570 that might do it - far RHS of the schematic.
You might also be able to improve it by playing with the BBD bias trim. Sometimes a slightly-off bias can give a signal but not such a great level. If it's too far wrong, it'll stop the signal altogether.

Any components that look suspect (caps)?

No, not to me. It's pretty tidy for its age, I'd say. And doesn't seem to have been mucked with either.

Or am I expecting too much of this circuit?

Well, if you're expecting long, clear delays, then yes. If you're expecting short, dark delays then you should be ok! The MN3005 only gets out to 204 msecs with a clock frequency around 10KHz, so if they've pushed the delay out to 300msecs, the clock must be down around 6-7KHz. That's seriously annoying if you can hear it, and you can certainly kiss your treble goodbye.

[tootsMcgee]

You could try replacing the 25 pF/nF (?? can't read it) cap in the MN3101 circuit with 33 pF/nF. That should push the clock frequency down a bit and make the delay longer. The trouble is that you risk getting clock whine coming through.
That part was probably 20% tolerance when it went into the circuit 30 years ago, so God himself would be guessing what its value is now.

Confirmed 25 pF on the board. Unrelated, do you have any advice for measuring capacitance? I have another project (ENGL Fuzzyhead which appears to be two NE5532's boosting the hell out of each other, no caps involved) that I've traced with a friend but have no way to measure the SMD caps on it.

For the delay volume, there's a 5K trim by the NE570 that might do it - far RHS of the schematic.
You might also be able to improve it by playing with the BBD bias trim. Sometimes a slightly-off bias can give a signal but not such a great level. If it's too far wrong, it'll stop the signal altogether.

The top trimpot slews the clock rate a bit, the bottom one messes with the clock waveform, and the third one appears to be oscillation volume. You can get some neat self-oscillation out of it. I haven't decided if I want it right at that point or backed off a bit.

No, not to me. It's pretty tidy for its age, I'd say. And doesn't seem to have been mucked with either.

Honestly I think I'll leave it as stock as possible. I'm OK at soldering but not great and this board seems relatively delicate. I have some other delays I can use for longer Pink Floyd-esque stuff. MXR Carbon Copy (really nice for the price) and an Aion Elysium/Ibanez Echomachine that I built.

Hey, hang on, aren't you the Tap LFO guy? I have a tap trem board from MusicPCB stuff nearly done that uses one of your chips. I'll probably be playing with some of your other components and kits at some point in the future.

Well, if you're expecting long, clear delays, then yes. If you're expecting short, dark delays then you should be ok! The MN3005 only gets out to 204 msecs with a clock frequency around 10KHz, so if they've pushed the delay out to 300msecs, the clock must be down around 6-7KHz. That's seriously annoying if you can hear it, and you can certainly kiss your treble goodbye.

Great news, I have a scope and I'm not afraid to use it. (I don't know how to use it.) I probed pin 2 of the MN3005 (CP1) and played with the knobs, here's what I've noted.




1. The Bias trimpot seems to affect the leading edge of the clock pulse. From 0-50% travel or so, it's squiggly and rounded. From 50-65% travel, it cleans up into a nice square wave. From 65%-100%, there's no change, but I did notice that the echo effect stops working if you turn it up too high. I'm assuming that somewhere right around when the square wave flattens out and stops improving is optimal? I can't find where in the circuit to see what else is going on.

2. The 1Meg clock trimpot goes from 0 ohm to 900KOhm measured in place with a cheap DMM. It appears to be in series with a 2.7Meg resistor to adjust the clock rate. At one end of travel I'm getting 59.52KHz between transitions, and at the other I'm getting 56.05KHz. I'm not an EE, just a guy with too much curiosity, so I don't know if I'm measuring the clock rate or twice the clock rate (is the clock frequency measured every transition or only on every leading edge?)

My guess is that since I'm getting faster than expected repeats, the clock is running faster than it should be. The datasheet for the MN3005 says 40Khz. The question is if the FX90 is using those recommended specs or not. Or if the components have drifted/were super out of spec in the first place.

I love circuits that run components deliberately out of specification or in weird ways. My favorite relatively normal one so far is the Echomachine circuit that (ab)uses a digital delay chip's clock frequency to adjust the delay instead of using the speed pins/serial control input (which only goes up to a few hundred ms). Although even things as simple as the Red Llama's CMOS abuse tickle my fancy too.

Thanks for the detailed reply!

[tootsMcgee]

Spotted a minor difference (or handwriting misinterpretation): the 2.7Meg in between the trimpot and the MN3101 is actually a 2.4Meg on mine. It was reading funny in place and my multimeter only goes up to 2Meg. I removed it and put it in parallel with a 1Meg and lo and behold it was right on the money in parallel resistance for 2.4Meg.

I'll probably not mess with it much. I was very careful soldering but it fried my nerves. I like the PCB the way it is.

Clearly the solution is to buy/build more pedals...

[Matthew Sanford]

If you need a delay, the Druid has a clean 4-second one

[tootsMcgee]

If you need a delay, the Druid has a clean 4-second one

That sounds fantastic for doing long "overlay" style stuff. I was tempted but I think I'm going to pick up the flange kit first since I only have one flanger (another DOD actually, I should photograph the innards...)

I'm sure I'll pick up the clean delay eventually...I'm feeling that urge to build things again and I don't know what kits to pick up next.

[Matthew Sanford]

Have Druid delay and flange boards waiting. A friend wants this FL99 flange as it's his fave, not sure I want to Vero dive on it. Once I get my Druid stuff done I'll put samples for you to consider, though at my rate ETA...sometime in 2027...

[tootsMcgee]

Have Druid delay and flange boards waiting. A friend wants this FL99 flange as it's his fave, not sure I want to Vero dive on it. Once I get my Druid stuff done I'll put samples for you to consider, though at my rate ETA...sometime in 2027...

No rush

On the MN3101/MN3005: I think I'm misreading. That's twice the clock frequency, so I'm actually seeing 28-30kHz.

Skimming the MN3005 datasheet, it says the long 205ms max delay happens at 10kHz clock frequency.

Let's measure clock frequency again at each end of the trimpot and delay knob...

  0% delay: 49.05kHz - 47.16kHz ( 43ms -  42ms)
100% delay: 10.86kHz - 9.057kHz (183ms - 222ms)

Which lines up pretty well with the data sheet, I think. In fact, getting the clock as closed to 10kHz as possible on the scope yields a measured delay of 207ms, which is pretty damn close to the advertised 204.8ms that I should be getting according to the datasheet. (Close enough for jazz.)

It's interesting to see either the clock or the scope (probably the clock) drift very slightly over time/temperature. Since I measured the 207ms delay sample, it looks like the clock has drifted to 10.08kHz

So this has me wondering. The manual says 60-300ms. I'm assuming that means my clock overall is skewed fast. I don't use extremely short delays often and the ~12ms bonus difference at the low end of the delay knob is useless to me. I'm wondering if the 25pF cap has some "interesting" tolerances, or if the 2.4Meg resistor actually should be 2.7Meg for realsies, or both.

Is something like this "close enough" for measuring into the 10s of pF? https://www.amazon.com/Holdpeak-Handheld-Resistance-Capacitance-Multimeter/dp/B07RGMRVDB (I feel like this would be great for the other pedal I'm working on with SMD caps...) Real tweezer multimeters are expensive but I'm not doing rocket surgery here.

[ElectricDruid]

Measuring small capacitances in the tens of pF range is difficult, and in circuit it's probably impossible. The rows in a breadboard have a capacitance not far from this between them, simply by virtue of being two bits of metal in parallel. Mostly no-one cares (10pF is *tiny*).
So yes, it's quite possible that a 25pF cap that was only +/-20% when new is giving you a value that's higher up now (and/or higher up back then too). If you can find a *really* small cap of a few pF, you could patch something in parallel with it. Or replace it with either something new and more reliable or something slightly larger.
I'm not sure I'd really trust any meter to read a value this small. Presumably there's some bit of expensive test equipment that could do it, but I wouldn't pay for such a thing, and if I got something for cheap, I don't think I'd trust its accuracy! (Catch 22?!?)

It sounds like the clock is giving you the frequencies it's expected to give, so that's all fine. It's just not getting quite as low as it might have done with a better cap. I completely agree that at the high end of the clock frequency range the differences in delay are going to be so small as to be prety much imperceptible, so I don't think you need to worry there.

[tootsMcgee]

Parallel cap is a great idea for a minimally invasive mod. I just happened to find a bunch of small caps from a Sparkfun kit. I don't know if they're X7 or C0G MLCCs but since this isn't directly in the audio path I'll take the chance.



The now "35"pf cap has lowered the clock down to the 7.8kHz range and I can eck out 285ms of delay with that. I could push it a bit more, but I'm happy enough now. I think the 380ms I was aiming for (for Run Like Hell) is probably a bit out of the question. But as luck would have it, I have everything I need for my Echo Base now except for the enclosure, so that'll fill that gap nicely.

I'll post about the pedal I'm tracing in another thread. Some of the caps are big enough to be measured with those things I think (input/interstage caps) but there are a few that I worry are tiny, and of course SMD has no markings on them.

There's probably some basic circuits I could build that I can measure with my scope to calculate the ballpark value, but I haven't looked into it too much.

[Rob Strand]

You shouldn't put all the blame on the cap value.  If you analyse the entire oscillator in detail you will find a large number variables.   

IIRC, I did a curve fit on the some of the curves in the datasheet for the MN3xxx series devices and deduced there is series resistance between the clock input pin and some internal clamping diodes.   Output impedances of the chip can come into play as well.

To get an accurate model of the oscillator you might even need to factor in the effect of the transistor TR2.

These oscillators are not precise.  The chip specs are not precise or detailed.   At the end of the day if the oscillator is too fast, manually adjust the speed by tweaking the cap or the VCO current.   If the range of the oscillator is an issue then you might need to make other mods. say to the resistive divider feeding TR2.  I suspect you could even tweak the resistors and leave the cap value as is.

As it stands the oscillator has no compensation for VBE shifts due to temperature on TR2.   Some other oscillators use diodes or transistors as part of the VCO's biasing to help.   Maybe tolerances in the VBE between different transistors cause the oscillator to cover different ranges.

It would take a lot of analysis and measurements to work out *why* you are seeing what you are seeing.    And once that it done the solution could is still be to tweak the 47k/510k/1M resistors feeding the base of TR2.   To some degree there is an assumption the built units meet the published specs - maybe not a good thing.   You might find most units 20% off spec in one direction anyway and your unit has some extra tolerances which is pushing it off further.

[tootsMcgee]

All great points. I don't know what I'm doing exactly (I know just enough to be dangerous and have developed a steady hand with an iron.) And I don't consider this a high-fidelity or accurate circuit. I don't mean that in a disparaging way. I mean that in the "this is why we like analog delay" way: imperfect and interesting.

I'm still suspicious about that 2.4M (actual) resistor in the 2.7M (on schematic) spot; that would also have an effect. I don't think it's just my unit either. Here's another pic I found that looks pretty close to the red/yellow/green/gold stripe I have:


source: https://media.karousell.com/media/photos/products/2021/10/23/dod_delay_fx90_1634994348_5b78f5d6_progressive.jpg

Maybe it's not the right schematic and/or one of the many revs between Rev A and Rev H (!) or whatever the latest one is.

Clearly this sounds like I need to get another broken pedal or two off Ebay for comparison purposes...

[Rob Strand]

I'm still suspicious about that 2.4M (actual) resistor in the 2.7M (on schematic) spot; that would also have an effect. I don't think it's just my unit either. Here's another pic I found that looks pretty close to the red/yellow/green/gold stripe I have:


source: https://media.karousell.com/media/photos/products/2021/10/23/dod_delay_fx90_1634994348_5b78f5d6_progressive.jpg

Maybe it's not the right schematic and/or one of the many revs between Rev A and Rev H (!) or whatever the latest one is.

Yes it definitely 2.4M on the PCB.   DOD schematics are notorious for having errors.   A 2.4M on the built units vs 2.7M on the schematic doesn't even raise an eyebrow.

Maybe there's other differences!

Clearly this sounds like I need to get another broken pedal or two off Ebay for comparison purposes...

If it's the same as yours you know that's how things are.  However, if it's different to yours you end up with more questions.  Questions which are hard to answer!

FWIW, there are a few posts on this forum and on other forums.  Sometimes you will find someone has posted some measured info from their unit.

[tootsMcgee]

Yeah, I'll dig around and see what I can find. Thanks again for all your help. This has been very educational.

I found an Arduino sketch for measuring capacitance based on the RC time constant. Clever--I've never thought to use an Arduino to actually source any appreciable amount of current. I wanted to see how far I could get clean readings on a breadboard, keeping in mind what you said. https://docs.arduino.cc/tutorials/generic/capacitance-meter/

I ended up with a 10M resistor for charging and modified it to use microseconds instead of milliseconds for timing. I have jumpers going all over which is obviously not good for measuring capacitance But! I could get stable readings into the high hundreds of pF. 330 pF was where I'd consider it not particularly reliable. Great for nF and above though. A fun little thing to tinker with.

[Rob Strand]

Yeah, I'll dig around and see what I can find. Thanks again for all your help. This has been very educational.

I had a different angle to get a delay time estimate without buying more units.

I took this video clip and recorded the audio, then I looked at the waveforms with a waveform viewer and measured the delay time.   I used samples from around 1:00,
https://www.youtube.com/watch?v=hbwGSqXFieg

You obviously need samples where the delay pot is set to full.

I ended up with a delay time estimate of around 350ms.

From that and your unit, maybe 300ms +/- 50ms.   That looks reasonable.   However for something with a trimpot to set the delay time you would expect a lot tighter tolerance.  Unless of course the units are not adjusted at full delay in production.  That would open up the possibility of the full delay time having higher tolerance, still, perhaps not +/-50ms.

I suppose if you do the same for a number of videos you can build up enough statistics to get a mean and spread of the delay times.
This video,
https://www.youtube.com/watch?v=y_d2AyTAbBE

Samples around 3:00, I'm estimating about 210ms.

So if I haven't screwed up these things have quite wide tolerances on the maximum delay.

[tootsMcgee]

Ayoo, TheSuperFunAwesomeHappyTimePedalShow. I've been binging that channel since I've started getting DOD pedals. I've never seen the other channel but I've added him to my binge list.

So neither of those are actually my unit! The TSFAHTPS pedal is closer to mine, with the centered logo, embossed footswitch, left-justified model number and name, but the delay pot on mine is wired completely backwards from both--max delay is full leftward rotation. Like this: https://youtu.be/IXjFNtwgib0?t=156

I wonder if those two are all the later revision(s)?

This looks similar to mine, but my serial number is way smaller. https://www.mercatinomusicale.com/mm/a_dod-fx90-analog-delay-made-in-u-s-a-prima-serie_id6411412.html?stkn=z2UCzrUroK291J8v The serial number here is in the high 474,000 range while mine is in the 278,000 range. Of course, DOD serial numbers are legendarily @#$%y, but according to https://serial-number-decoder.com/dod-pedals/dod-serial.htm it is likely my pedal was made within "a couple of months of September of 1984".

...hang on. That schematic I posted above. That was December 1985. Signs are pointing a bit to this being a very early (pre rev A?) unit. Interesting. However, it's possible the SN does not match the unit as they are only on the backplate. I bought this as a lot of 2 off eBay and both of them came with non standard DM-1900esque knobs and a few soldering iron scars. So who knows. Now to see if I can find an original schematic...that would explain the mismatched resistor...

I recorded some sound demos. I think this pedal suits me best as a slapback, especially with a bit of crunch. Either due to inherent design or (more likely) due to me messing with the bias and being unable to find the optimal point again, the delayed signal distorts fairly easily, even with shorter delay times (i.e., not below 10kHz clock). It doesn't sound as nice as the way the Echomachine purposefully overdrives the tails.

Faux chicken pickin': https://soundcloud.com/toots-mcgee-632927513/chicken-pickin-your-nose
Clean Strat: https://soundcloud.com/toots-mcgee-632927513/lenny-i-hardly-know-her

I'll get some external pictures tomorrow once I've cleaned up the chassis a bit. It's a bit battered and/or well loved depending on how you see it.

[Rob Strand]

...

So neither of those are actually my unit! The TSFAHTPS pedal is closer to mine, with the centered logo, embossed footswitch, left-justified model number and name, but the delay pot on mine is wired completely backwards from both--max delay is full leftward rotation. Like this: https://youtu.be/IXjFNtwgib0?t=156

I wonder if those two are all the later revision(s)?

This looks similar to mine, but my serial number is way smaller. https://www.mercatinomusicale.com/mm/a_dod-fx90-analog-delay-made-in-u-s-a-prima-serie_id6411412.html?stkn=z2UCzrUroK291J8v The serial number here is in the high 474,000 range while mine is in the 278,000 range. Of course, DOD serial numbers are legendarily @#$%y, but according to https://serial-number-decoder.com/dod-pedals/dod-serial.htm it is likely my pedal was made within "a couple of months of September of 1984".

...hang on. That schematic I posted above. That was December 1985. Signs are pointing a bit to this being a very early (pre rev A?) unit. Interesting. However, it's possible the SN does not match the unit as they are only on the backplate. I bought this as a lot of 2 off eBay and both of them came with non standard DM-1900esque knobs and a few soldering iron scars. So who knows. Now to see if I can find an original schematic...that would explain the mismatched resistor...

It can take some work combining all the relevant features to work out what model you have.  The chip dates are a good starting point but  the manufacture dates can be upto 2 years after that in some cases.

I'm not up on all the fine details of every revision and model of DOD pedals.  There's a lot of pedals and in some cases many revisions.  On top of that are possible production changes which aren't given a revision.

It's almost impossible to find factory schematics for each revision.  They may have existed at DOD at some point but they certainly have not got on the web over the last 25 years or so.

I found this unit.  1984 chip dates.  The thing I noticed is the timing resistor is 3.3M but there's also a 470k, which I'm assuming are either production changes or what was used on a pre A unit.   I'm assuming the 470k is the 510k across the pot on the schematic.   The trimpot is set to about 11 O clock, say 400k, so the total resistance is 3.7M.



There's a pic here as well,
https://www.freestompboxes.org/viewtopic.php?t=31762
In this case the clock resistor is 3M9 but I can't see the value for resistor across the pot.  What you can see is the 1M trimpot is set to 0 ohm.
Turns out the value across the pot is 470k, see here,
https://www.diystompboxes.com/smfforum/index.php?topic=128533.0

You can see how 2M7 with the trim pot set to 1M, totalling 3.7M, might work except in extreme cases.  The extreme resistor values and setting put that resistor at 2M4 to 4M9,  not allowing for the 470k vs 510k across the pot.      As far as centering the resistance goes, the middle of the extremes is (2.4M + 4.9M)/2 = 3.65M.  With the trim pot set to 12 O clock, the trimpot value would be 500k, so the centered resistor value would be 3.65M - 0.5M = 3.15M.   So 3.3M would be likely on average.

I noticed the FX585 delay has no 1M trim pot and the resistors are chosen on test.  One unit used 3M0.  However, the voltage divider on the clock transistor/delay pot is different and the unit is 18V.  All these changes are enough to justify a change in resistor value and it's not clear how to map all that back to the fx90.

Using the sound clips on any revision is just an indicator of the spread of maximum delay time.   Yes, it's possible some revisions were adjusted differently in production even though the spec stayed the same.

I recorded some sound demos. I think this pedal suits me best as a slapback, especially with a bit of crunch. Either due to inherent design or (more likely) due to me messing with the bias and being unable to find the optimal point again, the delayed signal distorts fairly easily, even with shorter delay times (i.e., not below 10kHz clock). It doesn't sound as nice as the way the Echomachine purposefully overdrives the tails.

Faux chicken pickin': https://soundcloud.com/toots-mcgee-632927513/chicken-pickin-your-nose
Clean Strat: https://soundcloud.com/toots-mcgee-632927513/lenny-i-hardly-know-her

I'll get some external pictures tomorrow once I've cleaned up the chassis a bit. It's a bit battered and/or well loved depending on how you see it.

Sounds pretty good to me. 

If you get distortion it can be due to a non-optimal BBD bias trimpot setting.
FWIW,  I looked at some other units.   A couple were hard to decipher the delay time from the audio but it looked like the delay was a long as 400ms on some units.

Keep in mind, we are dealing with old pedals.  You don't know if someone has messed with the trimpots over the years.   Since DOD have no set-up procedures anyone tinkering isn't going to set it up like the factory.

[amptramp]

You could make the range adjustable by using a trimmer capacitor for the low-value cap.  The ordinary compression trimmer would be OK but there are better trimmers that look like small variable capacitors with a rotary screwdriver adjust setting and a ceramic base.  Similarly, if you only need a small range of adjustability, you can put piston trimmers in parallel with fixed capacitors.

[Rob Strand]

FYI, I did a quick simulation of the circuit to see where the numbers ended up.

For a 300ms delay time I needed a low value for the timing resistor.
Something in the order of 2.4M *total* ie. resistor + trimpot.
I'm not surprised it's off from the real unit but at the same time
it's in the ball park.

I haven't investigated any deeper than that.

[tootsMcgee]

You could make the range adjustable by using a trimmer capacitor for the low-value cap.  The ordinary compression trimmer would be OK but there are better trimmers that look like small variable capacitors with a rotary screwdriver adjust setting and a ceramic base.  Similarly, if you only need a small range of adjustability, you can put piston trimmers in parallel with fixed capacitors.

Woah, I didn't know they made trimpot caps. That's a great idea. Time to pick up a few.

It's almost impossible to find factory schematics for each revision.  They may have existed at DOD at some point but they certainly have not got on the web over the last 25 years or so.

I wonder a bit if anyone out there still has them. I saw a name pop up on another thread (and of course I've forgotten) but it sounded like there was bad blood when Digitech took over and I don't feel like opening old wounds. I might be completely misreading the situation though.

I found this unit.  1984 chip dates.  The thing I noticed is the timing resistor is 3.3M but there's also a 470k, which I'm assuming are either production changes or what was used on a pre A unit.  I'm assuming the 470k is the 510k across the pot on the schematic.  The trimpot is set to about 11 O clock, say 400k, so the total resistance is 3.7M.

I have the 470k too, as well as a 47k next to it, tucked under where the trimpot is laid down flat. That's the 47k/470k/1M around the pot in the upper corner.

You can see how 2M7 with the trim pot set to 1M, totalling 3.7M, might work except in extreme cases.  The extreme resistor values and setting put that resistor at 2M4 to 4M9,  not allowing for the 470k vs 510k across the pot.      As far as centering the resistance goes, the middle of the extremes is (2.4M + 4.9M)/2 = 3.65M.  With the trim pot set to 12 O clock, the trimpot value would be 500k, so the centered resistor value would be 3.65M - 0.5M = 3.15M.  So 3.3M would be likely on average.

I'll see if I can dig up a 3M3 for experimentation. With all this variation, I'm thinking the trimpot might simply be there to allow for different part values. I remember reading somewhere that at least one company's explanation for differing part numbers was "it was the most economical batch we could get", and the 1M pot is a nice big range to account for variations.

I noticed the FX585 delay has no 1M trim pot and the resistors are chosen on test.  One unit used 3M0.  However, the voltage divider on the clock transistor/delay pot is different and the unit is 18V.  All these changes are enough to justify a change in resistor value and it's not clear how to map all that back to the fx90.

Speaking of voltages, I read in yet another thread that the NE570 might not be biased optimally, because the original circuit surrounding it was taken from an 18V circuit. I measure 8.7V across V+ and ground (pins 8 and 4 I think). The pin in question, pin 10, is hovering right at 4.34V, which is nearly perfect. So maybe that's not the case, but I do wonder: what voltages ARE these supposed to run at? The power supply call for 10V. I'm using a battery adapter and 9V. I don't want to just start throwing voltage at it for risk of frying it. More headroom would certainly be cool though.

If you get distortion it can be due to a non-optimal BBD bias trimpot setting.

...

Keep in mind, we are dealing with old pedals.  You don't know if someone has messed with the trimpots over the years.  Since DOD have no set-up procedures anyone tinkering isn't going to set it up like the factory.

Yeah some idiot started messing with them as soon as he got the unit (/s).

On that note though. I found what I think is the right bias point or thereabout for the MN3005. The Bias trimpot affects the output pins (3 and 4) of the MN3005.

I injected a 1kHz signal using my phone and found the top and bottom voltage levels where the signal bottoms and tops out, and then used my scope as best I could to find the midpoint, or at least the point where the top and bottom aren't clipping too much. Man, I need a signal generator...



I ended up with this imperfect waveform. I know the MN3005 and its ilk aren't perfect reproductions, but the asymmetry looked a bit more than I was expecting. Still, I found a point where my signal wasn't quite starting to flatten out at the top and bottom, and the crossover point is roughly in between the two extremes. Maybe this isn't the right way to do it but it seems like it's roughly where I got the bias point by ear (trimpot is just shy of exactly halfway, FWIW, almost precisely where it is on the pics you've dug up, so that's great to know).

I wonder if what I am hearing as distortion is just this asymmetry, and if there's anything to be done about it. But like I've alluded to, I want to try and keep this as-is. It's an old, imperfect pedal. It's a cool piece of equipment with a long piece of history and I want to keep it running as long as possible.

EDIT: Just noticed your 2.4M post, so I might try a few different values, but I also don't want to fry the board with all this soldering. Might be time to leave it alone lol