Help needed for debugging EQD Rainbox machine clone

[okuhtfesa]

Hi guys,

I need your help to debug my EQD rainbow machine clone I bought on pcbguitarmania, called Rainbow device. The problem is the effect sounds to work but at a very very low volume. I really run out of idea to find the problem.

I did an audio probe and it sounds like the effect is working as expected as pin 27 and 28 (L OUT and R OUT) of FV1 sounds good, so I assume there is an issue with IC3 (TL072) as I get a very low volume on pin 1 of the TL072. I tripple checked every resistor and solders. It all looks good. Here are the voltages I get on IC3 (and also IC1 TL072) :

4,5 V - 1      8 - 9 V
4,5 V - 2      7 - 4,5 V
4,5 V - 3      6 - 4,5 V
0 V    - 4      5 - 4,5 V

Could a faulty capacitor be involved ? Any idea ? Thanks for your help.

[bartimaeus]

The first thing I notice is that it's missing capacitors on the FV1 output to create a lowpass filter. each 1k resistor on the output should have a 1nF or 2.2nF cap to ground connected after it. See the example in the FV1 datasheet: http://spinsemi.com/Products/datasheets/spn1001/FV-1.pdf

The other filters in the datasheet aren't necessary, but the lowpass filters are needed to remove high frequency noise that could cause issues with stuff later in the circuit.

Just to confirm, the dry sound works correctly?

Your TL072 voltages all look correct to me tbh. If it was a problem with the input buffer or output mixer, it would affect the dry signal too.

Try removing the 1uF C17. Combined with R18 at 10k, that adds a lowpass filter at 15Hz, which would suck the life out of any signal. Perhaps the author meant 1nF, but forgot because they used so many 1uF caps elsewhere.

Looking again, I think the author REALLY wanted to use extra 1uF caps lol. C14 and C16 are both unnecessary if the First and Second pots were connected to Vb instead of ground.

[okuhtfesa]

Thank you for sharing your time and knowledge bartimaeus. I really appreciate. Everything sounds good now !
Indeed, the problem came from the wet sound being completely filtered by the 1 uF cap. I replaced it with a 1nF.
Also I did put a 2,2 nf from both FV 1 outputs to ground as recommended in the datasheet.