DOD Analog 680 Troubleshooting

[knave101]

First post.

I am in the process of replacing capacitors on all my old gear from my band days in the 80's...including pedals. And have run into an issue on my DOD 680. I understand that some may suggest leaving things that work as they are, but I enjoy the hobby of fixing things up, and I'll likely die with all these in my closet. But in this case I may have caused some damage. Interestingly, when I replaced the electrolytics on my Roland Bee Bah (an awesome pedal that was key for me when we played), many of the electrolytics were 200% out of spec...so I proceeded to attack my other pedals as well.

I had a functioning DOD 680, but after reading some posts decided to do a few mods. I replaced the caps with equal value Nichicon KZ or ELNA SilnicII caps. Immediately it stopped repeating. After adjusting the small trim pot, the unit works, but repeats only once. Any attempt to turn the "Repeat" knob over the 2 O'Clock position and loud repeating non musical sounds eminate....these are non musical distorted 'grunts'.  I've checked all joints, tried adjusting the trim, but nothing. Voltages are 15V and 7.9V at 351N chip.

I have the Block Lettering unit with the NE571 Compander Chip. Original Reticon SAD4096, 4013BPC, and LF 351N. I replaced the RC4558 Chips with NE5532P chips (and the unit sounds very clean and with much less noise than previously)

Any help is appreciated. and I hope it's not the SAD chip....from what I'm reading. Thank you.

[ElectricDruid]

No repeats implies that the feedback isn't getting back to the input. That could be the pot connections, or it could be a bad joint on one of the caps you've replaced.

[mth5044]

I can't help you out really, but would you mind uploading an image of the front and back of the PCB? I've got a weird 680 layout in mine and I'd like to compare it to yours. Thank you!

[miketbass]

I would check the orientation of the caps you have replaced. I agree that the chain between the feedback back to the chip is likely the issue. The pots are board mounted on these if I remember correctly. Typically when a problem arises in this fashion it is because of something that has been changed, less likely a new problem that has developed. It looks like there is a 10uf cap tied to ground around the repeat pot/rc4558 network. Id start there.

Heres a schematic I have.

[PRR]

> many of the electrolytics were 200% out of spec...so I proceeded to attack .... I'm an engineer (mech) and Physician....and though I love electronics, it's not my strong point...though I wish it were, but I understand the lingo.

As a real engineer, you understand using a bigger beam to reduce cost. You can buy 16kPSI steel for $1/lb or 100kPSI alloy steel for $10/lb. If size/weight is not an issue, you use a bigger chunk of the cheap steel. Likewise I can buy lumber #1 dense SS at 4kPSI rated, even select-out the densest no-knot slices which will be stronger; or I can buy a bigger stick of #2 Stud at lower price which (if it fits) will carry the same load. VERY frequently I have been putting a 2x6 stud where a 2x3 would be plenty, because I need insulation space.

Physicians are reported to say "take two aspirin and call me in the morning". In fact, one might be enough; three is not usually a problem. A doctor stabbed my gall and put in a 100ml(?) drain-bottle, when it turned out I only had 30mL to drain and if I'd had more seepage I'd just empty it more often. Yes, many drugs do need much better than +/-50% accuracy in dosing-- the human/animal body is more finely designed than any of our devices. But not (AFAIK) 5% precision.

The electronics designer can buy hand-trimmed film-caps to hold +/-1% of value for a lifetime. OR buy hasty-made low-tolerance electrolytics for a fraction of the $/uFd. In many electronics problems you only need "enough", more is not a problem until cost/size become annoying. If I am filtering wall-power hum, I set a goal and figure an ideal cap to just hit that goal. After skimming cap-prices, I realize that a -50%/+200% e-cap, picked oversize, is by far the cheapest. If I get "lucky" and end up with 4X the uFd I need to hit goal, that is fine.

Older e-caps seem to have been sized to yield more than rated uFd, because the forming process is an infinite growth and in rapid production they came out lower than expected. Hence the large tolerance. Some of them do seem to "grow" with age. The practical result is that hum is a little less, bass goes a little lower, it "beats spec" in ways which are no problem.

Yes, sometimes it matters. Some pedals throw a 5uFd in a NFB loop to set a 500Hz bass-cut. If the cap "grows" to 10uFd, bass is not cut until 250Hz, strong midbass notes muddy-up the distortion. Of course the original 5uFd choice was a matter of taste, and the particular axe and amp and playing style, so is not carved in a Government Specification.

[knave101]

I appreciate the posts thus far. I'm looking at the schematic...hopefully I've made an error that can be remedied. Here is the circuit board of mine.

[miketbass]

Can you post a photo of the trace side? Also, when switching out polarized components i always mark the orientation with sharpie either on the board or on some tpe stuck nearby. A swab with alcohol on it will clean any market off the board along with any flux.

[knave101]