Dirty fix for DOD Fx90 analog delay

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Dirty fix for DOD Fx90 analog delay

Postby allesz » 28 Oct 2016, 22:25

I got one in good conditions for 50 Euros, it's a really nice analog delay... since you do a plug and unplug test (that I did not do in the shop :oops: ) and discover it sucks noticeably volume and trebles, even when bypassed.

The schematic is online, it's not too difficult to locate the 470k resistor that goes from the 100n input cap to the input of the first IC, a TL072. This resistor sets the gain of the input buffer. On the pcb everithing is on the left bottom corner, you can't mistake the TL072, it's the only one.

I soldered a 2,2M res in parallel with the 470k one (so we have around 380k total). The volume was almost right, but still the signal got too dark. So i took a 100p capacitor and a 10k resistor, joined them in series, and put the couple in parallel with the input resistor too.

Now it sounds right bypassed and working. The delay got a little noisier, but nothing too bad for an analog delay.
A nice and different delay to me, and strange too: the time pot works bakwards, and the mix knob can pass only the delayed signal, giving interesting options. The feedback can be set (via the pcb mounted dedicated trimmer) for a very nice infinite repeat, always on the verge of feedback, but never actually getting there; my AD9 can't do this, so I will do a swap on the pedalboard to see how it woks "on the road".
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