Need Help with a DOD 690 Chorus

[87camaro]

Hey Guys,

i need some help reviving my dod 690 chorus....sad512 chip was dead and the dod schematic shows that it can be replaced with a 5106 providing the bias trimmer is changed to a 100K trimpot.

The effect works, chorus is nice and full but its accompanied by wierd background noises and the signal is sligtly distorted....what would cause this?

I have attached a schematic in pdf format.

Thanks in advance!

[Mark Hammer]

I can't speak to the weird background noises, but misbiasing is a very common cause of distortion in the delay signal. You will hear it when the BBD (any one of them) is either underbiased or overbiased.

I wasn't aware that an SAD512D could be so easily replaced with an R5106 and a slight change in the biasing circuit. That's very good news indeed, given how hard it is to find 512D chips.

Given the age of the pedal, and the way these things work, I would recommend that:

1) You add some capacitance between the wiper of the bias trimpot and ground to make that bias voltage a little more immune to line noise. Almost every other manufacturer sticks some like 10-47uf in there. I have no idea why DOD didn't.

2) You replace the 470uf cap i the power supply with a new cap of some higher value, like 1000uf. Given the change in cap size since the time when the 690 was built, I'm confident it will fit the space. That poor regulator has so little rectification before it, it needs a little help from a bigger cap.

Note that Small Bear Electronics carries the 5106 ( http://www.smallbearelec.com/Detail.bok?no=264. ) Also note that the little 25pf cap in the corner of the schematic is the component that essentially determines the delay range covered. Smaller values move you closer towards flangey sounds, and slightly larger values move you towards thicker "Pat Metheny" sounds.