DOD 680 Analog Delay...no delay

[piano boy]

Just plugged this up, I've had it since early 80s, and today no delay. I assume the BBD is shot, opened it up, and found a socketed SAD4096. Reading the archives, these look almost impossible to replace without scavenging another pedal. There is signal coming through, just no effect.

What should I do with this pedal? Is there any part substitution for the 4096? Is there any industrial or consumer equipment these were typically used in that might be floating around? I live in an "oil boom" area, and occasionally you see siesmograph and lab analysis gear from the 60s and 70s around (I remember the time a tube based gas chromatagraph passed by on the way to the landfill...)

[Mark Hammer]

For what has to be about the 100th time, the BBD is unlikely to be shot, and more likely to be improperly biased.  BBD's have the quirk that the incoming signal needs to sit on top of a DC voltage in order to successfully pass from the input to output of the chip.  There will be a trimpot on the board near the SAD4096 which needs to be set just right or else you will hear absolutely no delay signal at all, even though the chip is 100% functional.

As happens, when a BBD based device makes its living getting stepped on and kicked and then spends another 20 years sitting corroding, there is a high likelihood that the bias setting of the trimpot has drifted over the years, or that the wiper on the trimpot simply isn't providing decent contact.  In the majority of cases (though certainly not all of them), pedal owners who are frantically worrying about a blown delay chip find the pedal returns to its glory years once the trimpot (and sometimes trimpotS) are set right.

There will likely be a trimpot straddling pins 8 and 9 (or maybe just a single equal-value resistor away from each) of the SAD4096 to balance out the levels of the two complementary parallel paths in the chip.  Adjusting this will minimize clock noise and optimize signal quality.  This is not the bias trimpot I'm talking about.  The bias trimpot will either be directly connected to pin 14 of the chip, or be tied to one of the input of an earlier op-amp stage.  You can look at the anderflange.pdf document at my site (http://hammer.ampage.org) for an example of this in action).  While you are there, read the review of the 680 in DEVICE.

[piano boy]

Read the Device review, printed the anderflange and am reading it, will try the biasing tonight.

[mattv]

For what has to be about the 100th time, the BBD is unlikely to be shot, and more likely to be improperly biased.  

Funny you would say that! I was searching through the archives for various BBD info (just 'cuz) and you HAD mentioned that about 100 times. Then I was led to another page where the only thing said about BBD effect troubles is that the BBD is *probably* dead. So that may be where some of this certainty comes from.

Just thought I'd share my experiences.  

[Ansil]

For what has to be about the 100th time, quote]


LOLLLLLLLLLLLLLLLLL    TOO FUNNY

sorry mark i know what you feel like sometimes..  but what ever happened to the researching a topic before you asked..  but hey thats th eonly way we learn is by askign.  and rem you are like the formost bbd man.

[Mark Hammer]

Thanks for the undeserved praise.  I'm far from the foremost BBD man, just the one with the biggest mouth and fastest fingers.

You know, at a certain point it makes sense to have FAQs and refer people to them.  Beyond that point, especially as they start to turn into one long random walk (assuming one omnibus FAQ document), or multiple fragmented tidbits here and there, sometimes the most expedient thing to do is just answer the question.  
Personally I find that a month after someone has posted something that seems FAQ-worthy, someone ends up saying it better or smarter or more thoroughly, and expecting Aron and Peter to monitor every posting with an eye towards additions to the ever-expanding FAQ is perhaps expecting too much.  There also comes a point where a topic has come up for discussion and regurgitation so often that one confuses what you *think* is probably in the FAQ with what really is there.  Rather than drive someone crazy sending them to a document that doesn't say what I think it says, I answer the question.

It'd be nice if you could just say "go to the FAQ", but it's not always the best or easiest or most feasible strategy.

[Bassisst90]

So. I'm having the same problem, only there is only one trim pot on the whole board. I'm not sure what I'm supposed to bias it at and how I'm supposed to measure as I bias. I triad turning the trimpot before but nothing came of it.
Also, does anyone know where I can get a replacement transformer for it? I can't seem to find one myself (I've never looked for transformers before). The specs on the box say: 110volts, 50/60hz, 4 watts, and it all goes into a 78L15 transistor.

[rustypinto]

That trim sets the bias of the BBD.

BTW, which BBD is in there? Should have the name "Reticon" stamped on it.

What makes you suspect the transformer? Are you getting 15VDC power?

[oldschoolanalog]

From the "Reality Check Department": While I would 99.99% of the time agree with the erudite Mr. Hammer's assessment that the BBD is likely to "not be shot"; the only exception to that would seem to be the SAD4096. They seem to go bad all by themselves. No help necessary. Google around and you will read stories from much more informed folks than myself speaking of a manufacturing defect that makes them go bad over time. In my experience it seems this is the one BBD that usually is the culprit. Other BBD's do have a good reliability factor, IMHO.
And before you ask; the SAD4096 is one of those "hen's teeth" rare items to find.
If it indeed is the BBD, consider a retrofit (there is info here; UTSF).

[trjones1]

So. I'm having the same problem, only there is only one trim pot on the whole board. I'm not sure what I'm supposed to bias it at and how I'm supposed to measure as I bias. I triad turning the trimpot before but nothing came of it.
Also, does anyone know where I can get a replacement transformer for it? I can't seem to find one myself (I've never looked for transformers before). The specs on the box say: 110volts, 50/60hz, 4 watts, and it all goes into a 78L15 transistor.

The quick and dirty (and easiest) method of biasing the BBD is to

1. make an audio probe (read about it at geofex.com)
2. determine which pin of the BBD is the signal output
3. connect the probe to that pin
4. feed a signal into the effect
5. adjust the trimpot until you hear delay coming out of the BBD

I resurrecting an old "broken" Deluxe Memory Man this way.  It only took about 1/2 an hour total, and that had, I think, 4 BBDs to bias.

[Bassisst90]

O.K. I'll try that. BTW the chip is an SAD4096 and I found a retrofit, though I was hoping that it was something simpler. So if I try this and it doesn't seem to work, then I will have to try the retrofit. About the transformer, I get about 15.39 volts going to the transistor, its just that this transformer seems extremely noisy and I thought maybe there'd be something better out there. Also, I have to make a power switch for it and I thought it'd be better to do that with a fresh transformer, though I'm not sure how I'm going to do the power switch, since the photo I see here: http://files.effectsdatabase.com/pics/dod_680_002.jpg
does not reveal much and I'm not sure what transformer leads are going to where.

[Bassisst90]

So, I got it to delay, though it delays way too short. It sounds more like a reverb right now. It seems that if I turn the trim to the left or right too much, there's a huge feedback loop. I'm really confused. I don't know why its doing it. Here's a picture of the board
http://i926.photobucket.com/albums/ad103/Bassisst90/DSC01202.jpg
http://i926.photobucket.com/albums/ad103/Bassisst90/DSC01201.jpg
http://i926.photobucket.com/albums/ad103/Bassisst90/DSC01198.jpg

The circuit board shows a section which was not drilled out and I've seen some boards that had it filled with some parts. I don't know if that changes anything.

[digthisbigcrux]

isnt the delay time knobs rotation backwards on these?  left is faster repeats, right is slower? 

[Bassisst90]

It seems that way, but what I'm speaking of is the TRIM pot. If I go to far to the extremes there's a huge feedback loop. So the longest delay I get is slapback echo. Is there a way past this?

[Bassisst90]

Okay so I've tried to do a retrofit to the delay shown here:
http://www.diystompboxes.com/smfforum/index.php?topic=63905.20
However, it doesn't seem to work. I omitted the oscillator cap and resistor per the instructions, I have the trim pot and put the 10 microfarad capacitor on the board, as well as the mn3102 and mn3205, but I don't know what the other holes are for. Anyone have any idea?