Author Topic: Pladask Elektrisk Bandorg Delay - will only produce one repeat  (Read 159 times)

moid

Hello everyone

I'm building a Pladask Elektrisk Bandorg Delay pedal on a PCB from Pladask Elektrisk. Everything works on it *except* it will only produce one single repeat regardless of what the feedback pot is set to. It is a PT2399 based delay, so I'm wondering what pin(s) on the PT23999 control the amount of repeats heard. My guess (from looking at other PT2399 schematics) is that it is something to do with the relationship between pin 14 and pins 15 and 16. I suspect that pin 14 should get the output of lug 3 of the feedback knob (in this case a 25K pot) and so if measured with a DMM would report 25K on that pin? Then assuming the feedback pot is set halfway around its travel, (say 12K) that value should be present at pins 15 and 16. For me I get 25K at pins 15 and 16 and I suspect that is wrong, and that is why I only get one repeat. Does that sound likely to anyone here? Presumably lower resistance values would enable more repeats?

I will post some images tomorrow; the circuit has an extra level of complexity due to there being a momentary stomp switch that generates maximum oscillation / repeats that I'm unsure how to wire - but I'll need to show images for that. I need to get some sleep now. Thanks for any help you can offer!

Knutolai has kindly agreed that I can post some images of the PCB so there'll be pictures soon :)
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anotherjim

Re: Pladask Elektrisk Bandorg Delay - will only produce one repeat
« Reply #1 on: August 16, 2019, 04:18:51 AM »
The chip can't organise repeats in any clever way. The first repeat is simply the input recorded into the delay being played back after that delay. It is always recording new input, even if that is silence and that always overwrites and replaces older recordings. So in its basic function, it can only ever produce one true repeat.

To give the illusion of multiple repeats, the playback, after converting back to an analogue signal, must be fed back to the analogue input. Losses in tone in and level in the whole process mean it can never really "hold" a repeat, despite being digital. It will either decay to silence or build up into flat out distorted mush unless the feedback loop had perfect fidelity and exactly unity gain. If it is expected to record new audio at the same time as keeping repeats going, there is a problem, since combining the 2 audio streams will boost the recording level by an average +6dB. To avoid any distortion from that, the signal levels must be reduced which means the repeats must inevitably fall in level on each trip so cannot be sustained for long.
https://www.electrosmash.com/pt2399-analysis
The feedback path will be from pin14 and mixed via resistors back into pin16. The pot is usually acting as a volume control but you might expect some additional resistors and capacitors in the way. I would need to see a schematic to help more.
"Designing with PLLs in production circuits is reserved to those with the courage of the thoroughly uninformed."
Nuts & Volts
"Turtles have the short legs, not for the walking."
-Kenji "Damo" Suzuki

moid

Re: Pladask Elektrisk Bandorg Delay - will only produce one repeat
« Reply #2 on: August 16, 2019, 06:26:48 PM »
Thanks anotherjim - I didn't realise the chip didn't actually keep the audio circulating inside itself...  I guess that's because it has so little onboard memory (that link you posted shows it has almost nothing; kind of amazing it even works!). So presumably to stop the repeats from building up on top of each other into digital mush the repeating part of the circuit goes through resistors / capacitors to cut the high frequencies and also lower the volume of the signal so that it doesn't swamp the next incoming notes / music / audio... and this part is the feedback path from pin 14 to 16. So this makes me think that if I'm only getting one repeat, this might be due to a component in the feedback path being incorrect and lowering the volume of the repeats to below audible threshold? Does that sound feasible?

Sadly I don't have a schematic for the circuit. I was hoping it would be similar to a pedal that Knutolai did post a schematic for a few years ago, but it seems quite different (to me at least). I did take photos of the PCB before and after populating it with components, obviously if that's imposible to follow then I'll try to draw a schematic.

This is the bare PCB (obviously)




Here is my populated version, note the DPDT momentary swith is detached at present because I'm unsure of the best way to wire it to the circuit (the original design needs a SPDT momentary and I don't have one and was hoping I could use this instead?)




Finally the offboard wiring (which is where I'm confused regarding the Max, Pot and Feed cables and how they would connect to a DPDT - just using one half of a DPDT doesn't work - I did try connecting Feed to Pin 2, making a jumper from pin 1 to 6 and then connecting pin 5 to Max and that worked, but then I wondered what the spare cable at Pot is for?)



Any thoughts on where to go next would be great thanks :) 

Edit: the images should be smaller now! And if you click them should get bigger.
« Last Edit: August 16, 2019, 06:29:45 PM by moid »
Mushrooms in Shampoo -  NEW ALBUM Wandering by Moonlight

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anotherjim

Re: Pladask Elektrisk Bandorg Delay - will only produce one repeat
« Reply #3 on: August 17, 2019, 11:51:59 AM »
Max/Pot/Feed switching is I guess...
SPDT on-on or on-off-on type.
Max = full feedback level, guaranteed to runaway/howlround. Maybe lug 3 of the feedback pot.
Pot = a variable feedback level for the usual echo feedback level control from zero to max. This is probably the wiper of the feedback pot.
Feed = the input for the selected feedback via the switch. An on-off-on toggle could be used so a zero feedback setting is instantly available.

"Designing with PLLs in production circuits is reserved to those with the courage of the thoroughly uninformed."
Nuts & Volts
"Turtles have the short legs, not for the walking."
-Kenji "Damo" Suzuki