Guitar Pedal Repair Journey: DOD 690 Chorus Pedal
Hey y’all!
This is my first foray into doing something like this — generating guitar electronics and repair-related content for the greater good of all humankind (he said cheekily) — and I appreciate you taking the time to read what I’m about to write.
I have been studying electronics repair vigorously for the past year and have been mentored by two repair persons that I deeply respect. This writing is as much for my edification as it is for anyone that may visit this page in the future. If I can help you with a repair in any way feel free to shoot me an email at volticelectronicdevices@gmail.com
DOD 690
The pedal in question is 1980s DOD Chorus 690 that was sent to me by good friend and Youtuber Cody Jung ← click here to be taken to his Youtube channel. Though a relatively uncommon unit, the DOD 690 is a great sounding chorus that akin to the legendary BOSS CE-1. It features 4 knobs which control depth, width, speed 1 and speed 2; users are able to toggle speed settings via footswitch. The pedal also features stereo output as well as an LED indicator that is synced to the units LFO. The DOD 690 is buffered bypass and contains some very sought after ICs including the RC-4558P op-amp and SAD512D delay chip.
WHAT I DID
I’m going to provide a short list of the repair work that I performed before diving into the “how” and “why” of said work:
- Replace Carling on/off illuminated rocker switch
- Replace “Out A” jack
- Replace NE571N with Cool Audio V571M
- Replace ALL electrolytic capacitors (stock 16v) after testing (new min. 35v)
- Replace two carbon film resistors with metal film
- Replace one film capacitor with smaller size/same value
- Change 470uF 35v filter capacitor to 1000uF 50v
I’ll be linking the more uncommon replacement parts that I found throughout the article for future readers.
THE BAD
The pedal came to me in non-functioning condition and the current owner had never known the pedal to work. This can be a risky proposition for a repairperson but, as mentioned, Cody is a very good friend and this pedal was his dad’s so I was more than happy to, at least, open it up. Upon unboxing the pedal, it was pretty immediately clear that the on/off illuminated rocker switch was totally busted. To be sure, though, I checked continuity with a multimeter — suspicions confirmed. I was luckily able to find an exact replacement for the original Carling switch through Sager — these were difficult to find and not available via Mouser or Digi-Key at the time.
After installing the switch and ensuring the unit was getting power, I grabbed my guitar and amplifier, turned everything on and…
BRRRRRRRRRRRRRRRRRRRRRRR
Absolutely no guitar signal was coming through the pedal. All I could hear was a loud buzzing/distortion sound. Quickly, I unplugged and disassembled the unit to begin checking voltages. I checked:
- The switch — 120v. Good.
- Transformer-side of the LM78L15A voltage regulator 24v. Good.
- Circuit-side of the LM78L15A voltage regulator 24v. Good.
- IC chip power rail 14.7v. Good.
Everything was getting power, so, what could the issue be? This particular unit uses carbon film resistors which are a good material, though metal film resistors are better when dealing with voltages so I set out to check the resistors that were along the power rail. The 4.7k in series with the 5k trim pot ended up being greater than 10% out of spec so I swapped it for an equivalent metal film resistor. This is an important component because it is attached to the trim pot that provides the bias to the unit’s SAD512D chip. Additionally, I checked to make sure that the NE570 and SAD512D were properly seated in their respective sockets and reflowed a few suspect solder joints.
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At this point, I’m getting a clean signal in both play mode and bypass mode which, to me, is a gigantic improvement over the previously bricked unit, despite not yet getting any chorusing sound. While playing my guitar through the pedal I notice that the “Out A” jack is extremely noisy. I attempt to clean with De-Oxit and, when that doesn’t work, I opt to replace it with a switchcraft mono jack. Perfect. Now I have a clean clean signal, instead of an intermittent one. Frankly, at this point, I’m feeling a bit beat up — the IC chips in this pedal are pretty expensive (a “real” SAD512D can run in the hundreds of dollars) and I don’t want to make a, potentially, expensive mistake.
SO WHAT DID I DO?
As I mentioned earlier, I have two great mentors that I’ve been working with for the past year and I decide that, instead of just asking them what to do, I’ll devise a plan and then pitch it — you know, work out some brain muscles. I think, in the simplest terms, about how a chorus pedal works: a clean signal blended with a vibrato signal. This led me to think that the issue must be in the IC where the delay and chorus meet — the NE570. I make an order for ALL of the chips, just in case it’s something else.
They agreed.
I replaced the NE570 chip, tried out the pedal and I’m still not getting any chorussing effect. Eventually, I end up replacing (and socketing those that aren’t socketed) all of the IC chips in the circuit.
And then it worked, right? … Right?
Wrong. The pedal was still only passing a clean signal in bypass and play mode, despite an entirely new set of ICs. At this point I used a signal generator, probe and guitar amp to trace the chorus signal throughout the pedal. I found it coming out of the SAD512D, albeit quite weakly, and managed to trace it all the way to the depth knob which, if we look at the schematic, is one of the final components before the output stage — this was a good sign! The chorussing effect was traveling through the .047uF capacitor to the potentiometer leg but wasn’t continuing along the circuit path. At this point I was looking at the components attached to the other side of the potentiometer; we have a 10uF electrolytic capacitor, 10k resistor, two 100k resistors, as well as ½ of our output section RC-4558P.
In my mind, I already replaced the RC-4558P, so I was good there. The next most likely culprit was the 10uF electrolytic capacitor. I carefully minded the component’s polarization and desoldered it. Checking the capacitor on my ESR meter revealed that it was leaking DC and had lost its capacitance — instead of 10uF it read closer to 300pF. Replacing this capacitor allowed the chorussing signal to come through the resistor. However, I still wasn’t hearing the effect when the unit was plugged into a guitar amp. Admittedly, at this point I should have checked every single electrolytic capacitor BUUUT, being a rookie, I didn’t (lesson learned).
Over the next day I scour the internet for videos and articles on DOD 690 repair. I found two pieces of content (this will be the 3rd — YAY SEO!). This video which is good but wasn’t helpful for my issue, and this article from Crasno which talked about my exact issue: no chorus sound in play mode or bypass mode. I’ll let you read the article yourself if you so choose but, needless to say, every single electrolytic capacitor was more than 30% out of spec and leaking DC — some upwards of 30%. After testing and replacing all of the electrolytic capacitors and chips, I was finally ready to turn on this pedal with confidence that it would actually work.
VOILA!
It worked. It really worked. I was getting a chorussing signal. However, I was also getting something else: distortion. It sounded really bad. I opened the pedal back up, checked the DC voltage at the trim pot — 6v. The Crasno linked article above called for this bias voltage, however, he was using a 14 pin version of the SAD512D chip. I went ahead and, after checking the data sheet, lowered the bias voltage down closer to 4v. The pedal was absolutely humming (in a good way) — the chorus sounded lush and full and all of the knobs, switches and jacks worked perfectly. Still, being a curious person, I had a question:
What if the issue was only the capacitors, could all the IC chips still be good?
At this point, since I installed sockets for all of the ICs it was super easy to swap the original components back in. The result? They sounded glorious. The only chip that I wasn’t able to swap back in was the original NE570 IC chip because it’s packed between a bunch of capacitors and, quite frankly, since it is a pretty stressed component I figured it best to keep the new one in. The original owner had absolutely no problem with this solution. That being said, I will, of course, be sending the original NE570, as well as the original electrolytic capacitors, film capacitor and resistors back with a ziploc baggie.
DID I LEARN ANYTHING?
I sure did! Always always eat your vegetables and ensure that you desolder components carefully in case it turns out that they’re not actually bad and if one electrolytic is really obviously bad then maybe (just maybe) check the other ones too, especially if they’re made by the same company.
D’OH!
In all seriousness, though, this was a truly rewarding repair. As far as effects circuits go, repairing a modulation pedal is not for the faint of heart — I’m really glad that this had a happy ending. It’s all the better that I was able to do this for such a good friend and his dad.
If you’ve made it this far, thank you so much for allowing me to be a part of your day. These articles are as much for me to keep a record of my progress and gather my thoughts as it is for all of you. If this article helped you repair your DOD 690, or if I can help you in anyway, please don’t be afraid to reach out to me at volticelectronicdevices@gmail.com — you can also DM me on all social media platforms at @volticelectronicdevices — THANK YOU!
