In my previous post about the Roland stringers, I mentioned that they all used the Reticon SAD512D chip, but the second iterations of all these designs (circa 1980) switched over to Panasonic chipsets. I thought I would mention a bit about why this happened, and why a dead SAD chip make you, well, sad.
Reticon was actually an early pioneer in the use of CCDs, both in BBD (Bucket Brigade)delays and video sensors. Although co-founder Gene Weckler wasn't part of the Traitorous Eight who left Shockley and co-founded Fairchild, he followed a similar path, working for Shockley straight out of university, then Fairchild, then onto his own startup in 1969.
Their products included the SAD (Sampled Analog Delay) chips, including the SAD1024 with two independent 512 stage delay units, and the SAD512 which was basically a SAD1024 where one half of the chip had failed testing (a 1000-transistor chip in the early 70s would have been a biggish deal to produce in quantity and yields would not have been great). To understand how these worked, one cannot think of them like a digital memory chip, or even a digital delay line like the Princeton PT2399 (their distant descendant), where every location can be in use simultaneously.
The reason these were called Bucket Brigade was that they consisted of a line of capacitors and electronic switches connected to each other. As a voltage came into the chip at one end it was progressively passed through the entire chip, but instead of being done so in a ripple arrangement, where one voltage is passed out and each other voltage shuffles forward one by one, half the storage locations would be empty at any given time, and each next cycle would either sample an incoming signal and shuffle existing locations forward by one, or output a delayed voltage and shuffle existing locations forward. This may seem inefficient, but it removed the need for complex circuitry to address each delay location individually, as well as ensuring that all locations moved forward simultaneously within a single clock cycle. Indeed, all one had to do was connect the gates of half the bucket transistors to a clock input, and the gates of half the bucket switches to another, and send alternating signals to each clock input.
What the SAD512D did was no longer make the 512-stage version of the delay line an afterthought, but shrink it down into a compact 8-pin package. It also included some circuitry to generate the alternating clocks from a single square-wave input, thus making the chip very guitar pedal-friendly.
So sunshine and roses, right? Well, in 1977 EG&G bought Reticon, and it appears they really wanted them for their optoelectronic tech as all further refinement of the SAD BBDs seems to have ceased. Enter Panasonic, who soon developed rival products and continued refining their performance and voltage requirements, and by 1980 had convinced Roland to switch over to their products, even though the Panasonic MN series BBDs always required two external alternating clocks. Before too long both the SAD and Panasonic MN chips were discontinued in the face of digital tech, and became very expensive on the aftermarket. Eventually Uli Behringer's CoolAudio reissued the MN series chips, leaving the SADs high and dry and consigned to history's dustbin. What did not help either is that the SAD chips have a reputation for dying (ie my RS09) and this has further constrained supply and made second-hand parts suspect.
However.....
It appears that SAD512Ds are available again-not officially of course. The Chinese semiconductor market can be a wildwest of recycled scrap parts, sometimes labelled to be that what they are not, or even non-working chips which have been labelled to con desperate and gullible Westerners. Worse still are US Ebay sellers who import this junk and resell it offering it an air of legitimacy at a higher price. Still...when used SAD512D chips that are probably on their way out fetch triple-figure prices, it's tempting...
And indeed I was tempted. So after spending $A15 on AliExpress, this arrived...
The total lack of antistatic packaging was not promising.
And no mistaking the engraved printing for a new-old-stock 1980s manufacture (ink printed) chip. Still, testing time!!!! First up the testing rig:
Followed by removing the precious original from the pedal board for safe storage (after disconnecting the battery)
Then the unknown chip in circuit.
Results: both chips were non-functional. As static damage to both chips in transit is unlikely, one can only assume they were duds to begin with. Lesson learned. On to the repair!
