Hey guys,
as always, I have a problem with a build. And as with my last view builds, I’ll put out a reward for the person that gets me to a working pedal: You get a pcb shipped to your door. The pcb was also included in my BOTY package in the hopes I get it working, so there's that.
So this is my take on the DBA Echo Dream 2. I basically took the schematic that is out there, redrew it in Eagle (it’s basically identical) and made a layout. You can find the „original“ schematic, the redraw and the layout in the zip file linked below. Several people commented on effectslayout and tagboardeffects that they managed to build it on vero, so the schem should be fine and the project doable.
It generally works, all the functions do what they are supposed to except for at long delay times. I get this weird „dog farty thing“ (© cooder) going on, where the delay chip is motorboating like crazy. It is dependent on the settings of the VCO chip. You can hear the problem in the clip below.
In addition, I got a serious white noise problem, which I didn't notice at first but is very strong at high fuzz and vol settings. (it is possible that it wasn't there in the beginning, but I can't say for sure) could likely be improved by boxing it, but I doubt it would remove it entirely. More likely this is related to the AC ripple I found on the DC supply, which is amplified. More on that in the pdf.
Things I’ve tried so far:First, I had fake XR2206 chips, but got decent ones and the oscillator is working (checked via oscilloscope). Also got some original ones from the 70s from a coworker. They are a bit different, but still lots of noise. Overall, I think the chips I’ve got are decent
I switched out the PT2399 and tried 3 overall, all with similar results
Checked my soldering
Checked component values and schematic on original schem, tagboardeffects and effectslayout.com and compared it to mine. Seems to check out.
Checked my actually soldered components for value and orientation.
Brian suggested to increase R28 (on my schematic) to around 270k. Played around with it, but couldn’t eliminate the noise.
He also said that R18 makes more sense to go to pin 13 of the Pt2399 instead of the junction of R16/17. Tried that, didn’t make much difference.
Also tried the green LED from pin 7 of IC3 (Pt2399) to GND. Didn’t do the trick either.
I populated 2 pcbs overall and both behaved similarly.
The voltages were probed with a DMM and an oscilloscope and can be found in detail in the pdf in the zip file along with the schematics, board layout and pics. In short: the DC input is straight DC, but there is a slight AC overlay after the input resistor, when the VCO chip is inserted. The noises are highly dynamic depending on the settings, but generally are related to depth, rate and delay time settings (amplified by fuzz and vol obviously).
Obviously, I could not find the culprit after about 5-6h of debugging. This is were you come in. I would really appreciate it, if someone finds the time to compare schematics, check the layout for obvious design rule mistakes, give some tipps on where to search, etc.. I'm busy at work again and hardly motivated for this atm to be honest, but having no idea where to start and always ending without any progress doesn't help either.
Imho, it is likely either a systematic error that I did twice and did not see or a design mistake with the pcb layout. This is my first delay and they are tricky to do right. Especially since DBA is known for their notorious out of spec parts and childlike layouts, this is ok for me. It could just well be that the 8Vpp swing of the VCO is coupling into some other nets or similar stuff. I should have given the VCO it's own seperated ground plane to begin with.
ZIP-FILE thanks for looking!