DOD 670 Flanger Repair

[Elvis Cocho]

Hi Everyone,
I am in serious need of advice in repairing this old 670 flanger that I decided to try to repair. I bought it in non-working condition from Reverb from a seller in Europe. My initial guess was that it wasn't working because it was plugged into the higher 220V and it caused everything in the pedal to die.

The issue that I'm having with this pedal is that the output is lower in volume than what I put into it, and it is very distorted. Here's what I've done to it so far:
I replaced all electrolytic capacitors, including the big power filter capacitor. I have also gone and replaced all chips in this pedal including the Reticon SAD512D. The reticon chip does appear to be working properly as I can get a clean output from it when I bias the input to around 4 to 5 volts.

I noticed that, in the schematic,  the output is supposed to be connected to the cathode of the electrolytic capacitor just before it and it was connected to the positive side. I flipped it and I pretty much have everything on the pedal matching up with the schematic.

I've followed the signal through the circuit with an oscilloscope and audio probe, and everything seems to be fine coming out of the reticon, then to the 2nd half of the 4558, chip and into the 2nd half of the compander. It all goes to hell at the output of the compander chip. I've tried swapping out the compander, but I get the same results.

Is there any advice anyone can provide me to put me on the right path? I'll provide the schematic and anything else anyone asks for. I'll snap photos of the pcb and post that all little later today. Thank you in advance for any assistance!

[Govmnt_Lacky]

Some of the 'big box' DOD effects had a transformer that could be wired for 120V or 220V AC input. I would make sure you have it wired for your proper power input before moving any further.

[Elvis Cocho]

Sure. Great point. So, that part is good to go as far as I can tell. There is a solid 15V coming out of the regulator after the transformer and power filtering network. All chips are fed a healthy 15V Vdd.

Here's a few more details... The oscillator and clock pulse chips are good. The LED does flash in time with the speed of the effect and I can hear a nice flanging effect both at pin 6 and at pin 3-4 of the reticon chip. Things appear to be okay until it gets to the second half of the NE571 compander chip.

One more detail is that I have replaced the stomp switch with a 3pdt switch so I have everything working for true bypass. I'll provide a gut-shot a little later.

[ElectricDruid]

From the symptoms, it sounds like you've narrowed it down pretty well to the expander stage of the 571. That could mean that the envelope level on that chip isn't right. I'd check the soldering on the caps on the 571 if you've replaced those - a bad joint would lead to the envelope not being right, and that might give you a low level and distortion (no smoothing of the envelope).

You say you've eliminated the 571 chip itself, so we're basically looking at the few component around that expander stage. Suspect anything you messed with in particular, but examine everything closely.

HTH,
Tom

[Fender3D]

Compander's pin 16 to ground kills chip output signal, actually it's used as bypass, if the associated capacitor is short or leaking it will kill wet signal. Check it

[Elvis Cocho]

Fender3D, ElecDruid, great ideas. I did check connections and solder joints around the compander chip yesterday evening and all looks fine. I touched up all the joints  and ensured there aren't any solder bridges between pins or copper traces. .

The 0.47uF capacitor tied to pin 16 going to ground will be my next thing to check. The cap in there now is new off-the-shelf, but perhaps it's rated too low and running near its max capacity if it's rated at 16V. I'll admit that I didn't closely check the rating on my electrolytic although none have popped from being overloaded yet. Perhaps that may have something to do with the symptom I'm experiencing.

Thank you for the suggestions, I hope they keep coming!

[Kevin Mitchell]

16V capacitors should good enough since VCC is 15v - but it's cutting it close. It should be OK for now.

Probably unlikely to be the issue, but have you inspected the foot switch at all?
Edit: oops. Of course you have 

[Fender3D]

for debugging pourpose, it will work even without the cap.
you can take it out and check if it works

[Elvis Cocho]

Hey Fender3D, you totally nailed it! I checked the 0.47uF cap going to ground and it was rated at 16V. It was too close to the 15VCC and it was running at complete max capacity and distorting. I replaced it with a 0.47uF rated at 25V and it solved all my problems. This pedal is running completely great now. No issues whatsoever.

This has been the project from hell for me; I've been trying to solve this circuit for about 2 months now and it was driving me insane. Thank you so much for your advice! I'll post a couple of gunshots of my work on this beast.

[ElectricDruid]

Moral of the story: 16V isn't enough headroom for a 15V circuit!!

Glad to hear you got it sorted. That's great, nice work. It's always so good when the damn things start working!!

T.

[Elvis Cocho]

Yeah, definitely some lessons learned here.

[Fender3D]

Thank you so much for your advice!

Glad to be of help!
Enjoy it and especially cuddle that SAD chip 

[CheapPedalCollector]

Sorry for the necro, but I wanted to thank Fender3D for his advice on the problems with the 670. I was able to fix not one, but two of them with his advice about the .47uf caps. I used some 100v film caps instead of the electrolytics and the distortion went away.

I want to note the schematic floating around on the net has some errors in it. Two caps that are marked 10uf are actually 4.7uf in the unit. One is the cap off pin 5 of the NE571 after the 33k resistor, the other is the one is connected to the 100R resistor off the +15v rail. The scribbles in the schematic about changed values in the oscillator circuit are incorrect and the ones on the original schematic are correct. The connection between pin 2 on the 4558 and the 62k resistor off pin 3 of the 4558 does not exist in the unit. The cap between pin 15 and pin 7 of the NE571 is unclear on the schematic, it is 4.7uf. The BIAS trimpot is 1K on one of my units. The resistor off pin 12 of the NE571 is in fact 10k and not 10uf LOL.

So thank you again Fender, much appreciated.

I wonder how many SAD512's people dumped thinking they were bad when it was the .47uf caps leaking...

[Fender3D]

...
So thank you again Fender, much appreciated.

I wonder how many SAD512's people dumped thinking they were bad when it was the .47uf caps leaking...

You're welcome, as I said I'm glad to be of any help.

PS
The same trick may work with Digitech PDS effects and TC Electronic Flangers

[CheapPedalCollector]

Oh yes, I found most DOD pedals have bad caps. The two that I didn't replace (the .47uf's) were the ones. I should know better.

I have cleaned up and corrected the schematic here for others to use.

[CheapPedalCollector]

A follow up on this, changing the 22K resistor on the output of the SAD512D to 100K will bias the succeeding op amp stage a bit better if you get no Flanging and you are sure the BBD device is functioning properly.

Some other things, make sure you do any soldering with the chip removed, it is also heat sensitive (ask me how I know...  ). Also start with the bias trimmer turned all the way counter clockwise and slowly bring it up, until the signal is loudest with no distortion. If you have distortion and haven't changed the caps on the NE571, you should. If you still have distortion, the chip is likely had to much current drawn from the stupid design of the succeeding filtering stage that is direct coupled.

I have tried decoupling the filter through a 1uf cap and using a bias resistor on the opamp, unfortunately it changed the sound too much and it became a bit weak. I used a 100k to VREF, so perhaps I should have used a larger value like 470k or 1meg.

There is notes about using R5106 in these pedals on the schematics floating around, if you put one in be sure to change the 78L15 to 78L12 or you can damage the chip permanently.

[Rob Strand]

For the Flanger, insert additional comments from Reply #20 from this thread,
https://www.diystompboxes.com/smfforum/index.php?topic=130205.20