DOD 680

[daydreamdelay]

I've got a couple of old DOD 680's which have the same date on the inside sticker and look identical inside.. one works perfectly and the other has a very faint signal when on and the repeats don't get into oscillation. I assume since it delays the SAD4096 chip still works, I've had a few friends agree and suggest I look elsewhere for the problem. I figured I could use the working one to gauge the other but, before I start poking around with my MM I'd like to get an idea from the more experienced crowd where I should look first. I tried the trimmer which controls the bias for the BBD but there doesn't seem to be any difference until full up which only causes it to distort.

any ideas?

[theehman]

If you're having to turn the bias trimmer all the way up then your BBD is on it's way to a dirt nap.  It should have "sweet spot" around the center of the travel and turning it to either side should shut the chip off.
Try swapping the BBDs between both units and see if the problem follows the chip.

[daydreamdelay]

I'm not having to turn it up.. I just tried to. That said, there's no difference in sound from the start of it, to the point where it was set (middle), or after until the absolute end where it just distorts the repeated note. I have two extra BBD's I picked up a few months back just in case but was told by the seller (former MXR tech) and a couple of people I know that my suspicion about the BBD still working was correct. With that in mind I didn't want to stick in a good BBD into a bad circuit.

so... there's a chance the BBD is the issue even though it still delays the signal?

[pinkjimiphoton]

i think ron's right. if the trimmer still works when cranked, but is distorted, it's an issue with the bbd.

i have analog chips that do that...they work, but they aren't right.

[daydreamdelay]

I'll try one of the other BBD's then and report back 

[pinkjimiphoton]

good luck!
it may be ok...if ya get a "sweet spot" in the middle of the trimmer, and ya get almost unnoticeable "drive" to the "edge" of the sound, you may be ok.