Adding DeProfundis mod board to Bow Echo delay

[FootSolder]

This issue has been driving me nuts for days and I've tried many posted solutions without success. Simply, I want to add the DeProfundis daughterboard (as per the Vero layout) to a Dead Air Bow Echo PT2399 delay without the on/off/on switch - I just want full-time modulated delay. Whatever I seem to try gives me either delay or modulation (sometimes with a very short delay) but never together.

Currently one side of the LDR is connected to TIME Lug 1, the other side of the LDR connects to TIME Lugs 2/3. This gives me modulation with a tiny slapback, if I disconnect from Lug 2/3, lovely long delay. Is it possible to get long modulated delay without the switch? I'm sure I'm doing something dumb, any suggestions gratefully received. Thanks!

[ElectricDruid]

A link to the schematic for this DeProfundis daughter board and the Bow Echo would really help...otherwise we're guessing. Thanks.

However, I can guess as well as the next person! It's got an LED/LDR which it uses to add modulation to the delay, right?

In which case, you want to add the LDR across the outside legs of the delay pot, e.g. pins 1 and 3. That's the same as saying the LDR needs to be connected from PT2399 pin 6 to ground, the same as the Delay pot. It sounds like that's how you have it connected.

When the LED is off, the LDR will have a resistance in the MOhms and the delay pot won't be affected. As the LED comes on more and more, the LDR resistance will drop down to a few KOhm or even less and the pot/delay time will be heavily affected (parallel resistances).

Basically, the pot then sets the *maximum* delay time, and the modulation will make the time go from that to something much shorter. If you're not getting the longer times, that implies the LDR is always at a fairly low resistance for some reason - either because that's the resistance that type of LDR has, or because the LED is always on for some reason.

But like I said, I'm guessing without the schematics...Go on, help us out a bit.

[FootSolder]

Sorry! I didn't realize the attached images were not visible. Here are some links for the schematics and the Vero layout for the daughterboard with the 3-way switch I want to delete:

https://drive.google.com/file/d/1Cmz9CTWEuYIjFtMaeAiyy89Orej7Kzni/view?usp=sharing

https://www.dropbox.com/scl/fi/lmw9e94ivrl39ph5yljwe/PLFO-Grind-Customs-FX.pdf?rlkey=zgk4397azywmdgaemvg10rg11&e=1&st=p2dg8gyg&dl=0

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgEzZs87oR54IAtQttD6WsNGJFKtZ15GIxfQsHtmCnfV0VarLN5sdp5KHndNrR4n-5nGU6T0ovbABU1oZkCiSsUm7fX6xcQS0rCeME677PxWxyxUgK4FOogWY9arRxbgUfhtGpG7YsXZe0/s1600/Grind+Customs+FX+DeProfundis+Modulation+Daughterboard+Rev2.png

FWIW the modulation is strong when engaged (waterclear bright white LED and generic Tayda LDR) but it seems like you are saying that the LDR might be capping the delay time? I have heard examples of this modulation board with a number of PT2399 delays and they had modulation with longer delays. Maybe there is something about the Echo Bow that doesn't allow for that?

[ElectricDruid]

Sorry! I didn't realize the attached images were not visible.

No, my apologies - I opened this thread in a different browser just now, and now I see the images, so it was my bad, not yours. I guess I need to update some stuff...sigh. Sorry.

The DeProfundis board seems to be the problem:



The problem here is the depth control that pans from permenently-on to modulated. That basically gives you super-short-delay, or modulated-delay. While it might work in certain situations, it seems from your description and the schematic like you need something that is off when no modulated and only gets more on as you increase the modulation.

I would disconnect the B1K Depth pot from R6 (or remove R6 from the board) and instead connect pin 1 of the Depth pot to ground. This will stop the thing from being always-on when there's no modulation and should work better with your other circuit.

There's a lot else that might need tweaking though, so it's hard to say at a distance. It's a start.

We'll see

[FootSolder]

Okay, so the LDR is connected across the time pot on Lug 1 and Lug 2/3. Removed R6 from the mod board and moved Depth Lug 1 to ground. I now have no modulation and a very short slapback - the LED is flashing away as before. Hmmm

[ElectricDruid]

Which LED is flashing away as before?

This is important. The nameless LED attached to the Depth control you shouldn't be able to see because it should be inside the vactrol. And since I'm not seeing many other LEDs I think you could be talking about, I'm a bit confused.

Please elucidate. Thanks!

[FootSolder]

It's a 'hand-rolled' vactrol with a clear white LED and a generic Tayda LDR in heat-shrink tubing. I leave the lower end of the LED wrap slightly open so I can see it is pulsing, the LDR is sealed to the front of the LED. Kinda caveman but usually works...

[ElectricDruid]

I see. Ok, fair enough.

I think you'd probably better measure the LDR's resistance with the LED off and then again with the LED fully on and see what you get.

[duck_arse]

Whatever I seem to try gives me either delay or modulation (sometimes with a very short delay) but never together.

I don't understand this statement. you said it doesn't modulate the delay, but you get modulation. do you mean you can see the oscillator is oscillating, but you can't hear it modulating as you expect? or do you mean the self modulation from the modulation pot [2M]?

I told you I was confused.

[FootSolder]

The resistances I measured on the disconnected LDR are; OFF 58K, ON 2K7

[FootSolder]

Whatever I seem to try gives me either delay or modulation (sometimes with a very short delay) but never together.

I don't understand this statement. you said it doesn't modulate the delay, but you get modulation. do you mean you can see the oscillator is oscillating, but you can't hear it modulating as you expect? or do you mean the self modulation from the modulation pot [2M]?


I told you I was confused.

I realize the MODULATION pot on the delay is confusing, it is not part of the issue - I'm just talking about the modulation from the daughterboard.

The daughterboard is supposed to add modulation to any PT2399 delay. The original schematic includes a switch for modulation only/delay only/modulated delay - I don't want the switch, just full-time modulated delay but I can't get that to work (even with the switch). The LED for the LDR flashes and responds to the depth/rate controls and produces a good modulation when the LDR is attached across the delay TIME pot BUT the delay disappears, when I disconnect the LDR the delay returns. In short, I can't get both effects happening together.

[ElectricDruid]

The resistances I measured on the disconnected LDR are; OFF 58K, ON 2K7

Yeah, that's going to cause problems. The LDR resistance is in the same ballpark as the delay pot, so the parallel resistance even when off is going to be roughly half the full 50K value. So there's no way you can get long delays with modulation.

I was expecting something more in the MOhms range for the off value. 2K7 for the On value is ok though!!

If I were you, I'd find another LDR and make up another vactrol and repeat the test.


If you're interested, if you've got two parallel resistances Ra and Rb, the equivalent resistance is:

   R = (Ra x Rb) / (Ra + Rb)

So if the LDR is 58K and the Pot is 50K:

   R = (58 x 50) / (58 + 50) = 2900 / 108 = 26.8K, roughly half.

If the LDR is 1M and the pot is 50K:

   R =  (1000 x 50) / (1000 + 50) = 50000 / 1050 = 47.6K, basically 50K.

If the LDR value goes up further (and >10M dark resistance is not uncommon with LDRs) then the result is basically the pot's value is unchanged.

Meanwhile, at the other end, say the LDR comes down to 1K:

   R =  (1 x 50) / (1 + 50) = 50 / 51 = 980R

So in this case, the LDR value completely swamps the pot and takes over control.

[duck_arse]

I'd be inclined to say that if you can see the led in the vactrol flashing, the LDR is going to be seeing far too much ambient light. it needs to be totally isolated from your sight, to be sure it can't see any light not from the led.

[ElectricDruid]

I'd be inclined to say that if you can see the led in the vactrol flashing, the LDR is going to be seeing far too much ambient light. it needs to be totally isolated from your sight, to be sure it can't see any light not from the led.

Yes, this is also true. You won't get the full "dark resistance" from the LDR unless it's in absolute pitch dark. Perhaps the LDR you've got will be fine if it is *really* in the dark. Try it, measure it.

[FootSolder]

I went through my LDR stash and found one that went to around 2 MEG dark - nothing more than that unless I order a 9203. To address potential light leakage, I completely wrapped the vactrol and tested it in a dark room. However, it didn't make any noticeable difference, still just a *very* short single slapback with no modulation at all.

I did find a post that claimed the issue was the resistor that came before the TIME pot in some versions of a PT2399 delay, rather than after it like in the DeProfundis delay for which the daughterboard was obviously designed. I did try the suggested work around; jumper the 1K then run a 2K2 from TIME lug 1 to ground but didn't work for me. It does seem like the wiring from the daughterboard to the TIME pot and/or ground is the issue?

[ElectricDruid]

It does seem like the wiring from the daughterboard to the TIME pot and/or ground is the issue?

No, it doesn't, not when your original LDR was only getting up to 58K. That's clearly a problem.

You're saying that you've now tried it with a vactrol that goes to 2M in the dark and it's completely wrapped and you *still* get the same result?
If that's the case, then yes, I agree with you, there's something else wrong, and perhaps it is your wiring. The LDR should be in parallel across the Delay time pot. There's only two wires to connect.

Can you post a photo of what you've got now so we can take a look? Perhaps we can spot something you've missed? Extra eyes often helps.

[FootSolder]

Going apologize for the spaghetti, there is a lot of wiring as there is also a reverb circuit attached (which is functioning fine). The vactrol is bottom right and the green wire goes to TIME lug 2/3, while the other side of the LDR (yellow wire) goes to TIME lug 1. The delay board also connects to the TIME lugs 1 and 2/3 according to the Vero layout below:

[ElectricDruid]

Ok, have you got the required cuts under the vactrol? Is it the right way around? (I can't tell). Connecting the LED end to the PT2399 would explain the symptoms, and would be easily done once the thing is all covered in heatshrink...

[FootSolder]

Yes, cuts good under vactrol and correctly installed with LED cathode on bottom, only the LDR is connected to the TIME pot

[duck_arse]

switch off. cover the led/ldr as best you can. measure the resistance AT THE POT across the time pot as you rotate it. see if you can measure the resistance from the IC pin to ground as you rotate the time pot.