DOD 670 flanger sound/tech debugging?

[spoony]

Hello,

I came across an old DOD 670 flanger that had some problems that I mostly addressed by changing the electrolytic/caps out. For good measure I threw in a new NE571 compander.

The flange is awesome and everything works great until the width/depth control goes past 3 o'clock then it starts and get increasingly garbled. My question is whether this was in the design of the pedal? The voltages look to all be within range. I do have a schematic that I found on the web which shows a bunch of resistors being changed out and maybe that was to address something. These changed out resistors have been added-hand drawn into the schematic.

Any help would be most appreciated. Though I can live with the limitations its hard to believe that DOD designed this pedal to work this way.

Thanks,

Spoony

Here's a link to the schematic,

http://music-electronics-forum.com/t5850/

[Mark Hammer]

Do you know that the pot is functioning properly past the 3:00 position?  Maybe there is need for a cleaning.

[spoony]

Yeah I cleaned all the pots while replacing the caps. I also checked the sweep on the pot and it appears to be working properly.

[Mark Hammer]

Okay, one less question to consider.

Back to cogitation.

What is shown as the Width control is analogoyus to the Depth control on the Boss BF-2, in that it adjusts the balance between the LFO and a DC offset.  The combination of the two should not be greater than a certain amplitude, or else it will exceed what the clock generator can manage.  The resistor pair consisting of 470k and 33k, tied to the wiper of the Width control, act as a voltage divider to drop that combined DC+LFO down to something sutable, which the subsequent op-amp feeds to the clock generator.

Three pieces of information I'd like from you:

1) Do the resistor values on yours correspond to the pencilled in values or what is scratched out?

2) Does the clockwise/3:00 position correspond to more LFO, or less LFO?

3) Does the problem still occur if you set Depth to minimum and then turn up Width?  It may be that the pedal simply has no built=in protection against accidentally high LFO+DC coltages, and just assumes you'll adjust things appropriately.

[spoony]

Mark,

1) The resistors are the same values as the scratched out ones on the schematic.

2) Clockwise = more LFO.

2) So I just went back and listened a little closer. This is what I have noted. It appears the noise/hiss seem to begin at 12:00 and increases incrementally when the manual control, aka depth, drops below 12 o'clock and the width control does the same only in the reverse, 12 and above, order.



Thanks for helping me trouble shoot this,

spoony

[Mark Hammer]

I'm going to suggest that you consider, as an experiment, replacing the original pair that's in there with the 470k/33k pair penciled in.

Actually, just out of curiosity, are ANY of the penciled-in components and component values installed on your unit?

[spoony]

 Mark,

I was wondering if changing the components/resistors that are on the board to the ones that were penciled in might be a fix to this or some other problem? Its hard to say whether those changes were made by someone at the factory or someone performing some type of modifications I'm unfamiliar with. Though the board is very easy to work on I didn't want to just start changing resistor values out with out a little feedback.

Yes, as far as I can tell the ones on the board are all the same values as the original schematic before the drawn in changes. A couple of them are a little unclear under the marking but all the ones I could make out are clearly in line with the original schematic.

Do you think DOD actually let these out to the public like this? I will change out the resistors you mentioned. It will be a couple days before I get around to it but I'll let you know what happens. Do you have any idea where the scratched in values may have come from and whether or not I should change all the penciled in resistors?

Thanks again,

spoony

[spoony]

Hi Mark,

I did as suggested and changed out the 470k/33k. That went so well, though it didn't completly get rid of my noise issue, that I changed out all the other penciled in resistors. Now the flanger sounds awesome. I will now self oscillate, something it didn't previously do, and it has become pretty quite. Overall this is a great sounding flanger and I am very happy I came across it.

Thanks again for the help,

spoony

Still hard to believe DOD let these out this way?