DOD Chorus 690
Analog Chorus Pedal
In mid-2017, I repaired this classic analog chorus pedal, made in 1980 by the original DOD Electronics company of Salt Lake City, Utah. As of this writing, this job remains my most extensive pedal repair, and I challenge you to find an even more laborious example! Despite its small size and relative simplicity, there were so many things wrong that by the end of the job, I nearly felt as though I'd built a new pedal. It must have experienced some catastrophic event, such as a severe electrostatic discharge, or a line voltage surge. What else could cause so many problems, yet cause no visible damage?
To give a brief background, I bought this pedal for very little at a local music shop in mid-2016. It was (and still is) missing the bottom cover, which should've been an indication that all was not well, but I was told it worked. Of course, it was too good to be true—the pedal passed no distinguishable sound, giving only crackling noises. Alright, before we go on, have a look at the inside (after I finished repairs, since I forgot to take a shot before):
While not as simple as a fuzzbox, that's a pretty simple and serviceable pedal—it's a beautiful sight compared to the surface-mount abominations now dominating the market. I would not have attempted this repair if not for the spacious through-hole board and overall high quality, as well as the promise of a great sound. In any case, you may be able to spot some of the repairs done already, so I will simply list them all, followed by an account of how they came about. The repairs:
Also, for reference, you can find the schematic here. It is not an exact match, since it uses the SAD512D bucket-brigade chip instead of the SAD1024A, but it is close enough.
Now, I'll explain the process behind these tasks. As always with things coming in so completely dysfunctional, the first thing to check was the power supply. It is an extremely simple circuit, giving only one DC voltage: +15V, regulated by a 78L15 chip. Viewing on an oscilloscope, this 15V rail looked terrible, switching frequently and sharply between around 6V and 15V. The 78L15 was getting very hot, indicating that it was being loaded down during the periods of low voltage.
Nothing else was heating up much, so it was time to think. Since the correct voltage and decreased voltage were two distinct states and thus "digital" (with near-instantateous transitions between states), I suspected the only digital IC on the board: the 4013 dual flip-flop. Sure enough, removing it restored the 15V rail to a constant 14.7V. I then replaced it with a new TI part (CD4013BE), installing a socket for ease of future replacement. Since the 78L15 had been stressed, and was giving slightly low output, I replaced it as well as a precaution.
Two parts replaced, and still, the pedal wasn't passing any signal. A feeling of dread began to manifest as it dawned on me that the SAD1024A bucket-bridgade chip may be at fault. This chip hasn't been produced for decades, and is now highly sought after, so the prices are ridiculous. My fears were given weight when I probed the first delay line of the chip, and although signal was reaching the line, no signal was passing through it... it was dead! This is the point at which many would've given up on this pedal, since they wouldn't want to bite the bullet of getting new chip. After much deliberation, I went ahead and ordered the chip from an eBay seller, choosing a new-old-stock part in an Archer (Radio Shack) package that included the datasheet.
A few weeks passed before the new chip arrived. When it did, I carefully installed it, and... barely any signal was coming through the first delay line, and the output was extremely distorted. I was momentarily paralyzed, but this was not the end of the world, since if you look at the board and schematic, you'll see something near the chip that's important:
That white 100kΩ trim resistor is used to adjust the DC bias of the signal fed into the first delay line. The datasheet gives the "Optimum Input Bias" as 6V, and sure enough, when I adjusted for about 6V at pin 2, the chip passed the signal much better. In the end, I adjusted the trimmer to allow the highest possible peak-to-peak input signal without distortion at the output. I also checked to make sure the original SAD1024A chip wasn't just biased incorrectly. Nope, it was definitely dead.
If you guessed that signal still wasn't passing all the way through, you'd be right. The 4558 op-amp between the SAD1024A and the expander section of the NE571 compander was giving no output, and was getting warm—not normal. After replacing that with a socket and installing a NE5532 (another dual op-amp with the same pinout), finally, signal passed all the way through! And it sounded like utter garbage, with extreme amounts of distortion. Below shows the board with the new op-amp installed.
Good signal was going into the compander chip, and bad signal was coming out. Given that 3 of the 7 ICs were faulty so far (not even counting the 78L15), you'd expect the compander chip to be bad too, right? So, I ordered a new compander, swapped it (it was already socketed from the factory), and... the output was still distorted in the exact same way. Well, the compander chip has only a few supporting components, so it was easy to check them (which I should've done to start with). The resistors were good, as expected, but sure enough, one of the electrolytics was unmeasurable on my ESR meter, indicating it was open/dried-out/low-capacitance/etc. The bad cap was a 0.47µF 16V made by ECI, one of the few on the board with black wrapping instead of yellow. I replaced it, as well as another of the same type as a precaution (both of these with metalized film caps), and also one 10µF with excessively/unmeasurably high ESR.
At last, the pedal passed signal, and was indeed sounding great! Lastly, I replaced the input jack since it was intermittent even after cleaning, and the power plug since it was missing the ground pin. I added a ground wire to the input jack sleeve.
And then it worked perfectly. Here's a picture of all the parts replaced in the end:
I should mention that there are two remaining issues (not related to functionality) that have not been addressed, which are the missing bottom panel and one broken-off screw. Unfortunately, a simple sheet metal panel would not work, since the transformer and jacks protrude further than the bottom edge. The original cast metal bottom would've cleared this. In any case, the owner of the music store continues to look for the panel, so one day it may be found. The screw can probably be drilled out.
So, I hope this article has been interesting, and that it will help in the repair of more of these sorts of pedals as time goes on. If you have a broken Chorus 690 or similar that you don't want to repair yourself, I could repair it for you; see the Repairs page for more details.
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