Boss DD-3A Distortion Problem

[dorrisant]

This pedal works but has a distortion when using the longest timing mode. When I sample the LFO wave at pin 5 of it looks just like like the square wave from the service manual. It works fine in the two faster modes  (S=50mS and M=200mS), but has a crackling distortion present at the beginning of each LFO wave only in the longer range (L=800mS).

I was able to adjust the minimum and maximum LFO frequencies to the 12uS and 50uS specs in the service manual. I could even offset these frequencies until they overlap, so to speak... where the fastest time of mode M was faster than the slowest setting of mode L (after adjustments of the trimpots between changing modes).

While experimenting with this, i can never hear any distortion in mode S or M, no matter what the D Time pot is set to, only in mode L. In mode L, any setting of the D Time pot produces this distortion. Would anyone know where to look to figure this out?

I am reffering to the pdf found here: https://www.synthxl.com/boss-dd-3/

[Slowpoke101]

This may seem a bit strange but the problem is most likely that IC6 has a few faulty memory locations. IC6 is a 64Kb x 4Bit Dynamic RAM (as used in the late model Commodore C64C computers, etc). The distortion that you are hearing is probably that and those locations would not be used with the faster delays.

However, I may be wrong of course. Other things that can cause similar problems are much simpler. Check C027 and C019 (both 47uF 6.3V capacitors) as these capacitors can cause write errors to the memory. Also check C034 and C014, both 0.68uF Tantalum capacitors associated with the NE570 compander IC. These capacitors cause really strange problems with the audio. Now any 1uF 50V or 10uF 16V capacitor in the audio path can cause some really odd distortion problems or complete loss of audio. All this depends on what condition the pedal is in. A pedal that has seen a lot of use will be most likely full of failing capacitors due to heat. A pedal that has had little use and that has be well cared for should be fine.

With your pedal I think that the problem will be with the memory chip. But do be suspicious of any 47uF 6.3V capacitor that you find as that value is notorious for failing anywhere. Low voltage electrolytic capacitors are not really very reliable. They are used because they are small. One last thing - If you have a suitable and known to be compatible power supply or a very good battery (which will not last long in a digital pedal - they are too current hungry), try it. Your problem may be just a power problem.

[ElectricDruid]

If it only happens when all the memory chips are involved, could it be a bad connection on higher address bits? When they're zeroed, you wouldn't hear it, but an intermittant connection on a high level is going to jump from one location to another - that'd produce "crackling distortion".

Edit: If there are any socketed chips in the pedal, my first move would be to remove them, clean the legs, reseat them, remove them again, and reseat them again. That kind of behaviour tends to remove any tarnish that might have built up over the years and can often fix bad contacts. It's easy to try, and it might be enough.