Help With MXR D+...Pictures Included

rickortega · February 16, 2006, 07:52:17 PM

I asked a friend to fix it a while back and it came back without the original Distortion pot and lots of modded stuff. I'd like to get it working again; but, after I removed as much of the mod nonsense as I could see, I tried the pedal and here's what happened.

When I turn the Distortion pot past about 10 o'clock, the pedal cuts out completely. I reviewed the wiring as best as possible; however, all I really have are pictures from eBay of a few boards. Can you see anything in these two pictures? I have more if they would help.

This is my 1st post in a long while...so, Hello!! and thanks!

Hazy

tiges_ tendres · February 16, 2006, 07:59:41 PM

one of the lugs on your drive pot isnt connected to anything, also looks to be the wrong value according to the project file at tonepad.com.

back of pot says 1 m should be a 500k reverse log

rickortega · February 16, 2006, 08:03:21 PM

Can you help with the wiring on the lug? What did I miss?

Also, I thought that the original Dist pot was 1 Meg.

Thanks,
Hazy

tiges_ tendres · February 16, 2006, 08:06:52 PM

the wire is supposed to run from that empty lug, to a junction that is connected between R3 and pin 4 of the IC

here is a project file if that helps.

http://www.tonepad.com/getFile.asp?id=4

johngreene · February 16, 2006, 08:27:51 PM

The unconnected lug on the distortion pot isn't going to make any difference. Neither is the fact that it is a 1 Meg instead of a 500K.

I can't really tell from the pictures but it looks like the ground from the distortion pot connects to the PCB but nowhere else. Both the distortion pot (center lug) and the PCB need to connect to the input jack to connect the ground to the battery. I would also put some tape over the purple wire bare spot.

--john

rickortega · February 16, 2006, 09:33:35 PM

Quote from: johngreene on February 16, 2006, 08:27:51 PM
I can't really tell from the pictures but it looks like the ground from the distortion pot connects to the PCB but nowhere else. Both the distortion pot (center lug) and the PCB need to connect to the input jack to connect the ground to the battery. I would also put some tape over the purple wire bare spot.

I have the center lug on the distortion pot connected to the outside lug of the output pot (black wire) and that is jumpered to the ground on the board (also black wire). I just can't find a clear picture of the old wiring layout to follow.

I will re-wire the purple wire because it is severely sub-par.

Any ideas on the distortion pot cutting out the pedal? Any other wiring mistakes?

Thanks again...

rickortega · February 16, 2006, 10:59:07 PM

I changed the purple wire, replaced the diodes, re-checked all of the solder connections and cleaned up the solders around the socket. The pedal still cuts out as you increase the distortion knob past about 10 o'clock. I've swapped the orientation of the diodes and that doesn't seem to have any effect on the pedal cutting out.

Does anyone have a clean wiring diagram for the old MXR D+? I know I could fix this thing if I just had the layout...not the schematic.

Thanks,
Rick

tennisdude · February 16, 2006, 11:02:41 PM

i would try swapping out the pot. maybe try a 500k reverse log one just for kicks? it sounds like the pot is having issues.

scaesic · February 16, 2006, 11:03:38 PM

yah, probably the pot.

rickortega · February 16, 2006, 11:05:30 PM

Quote from: tennisdude on February 16, 2006, 11:02:41 PM
i would try swapping out the pot. maybe try a 500k reverse log one just for kicks? it sounds like the pot is having issues.

I just replaced the pot...it's a new 1meg reverse log pot that I got from SB electronics yesterday. Brand new.

rickortega · February 16, 2006, 11:38:12 PM

Could one of the little caps be causing the problem? When I turn up the distortion knob, there is a very loud scratchy sound. I've see this problem with pots on amps but I can't remember what causes it or what is the solution.

BTW, I measured the new reverse log pot and it's 855k at full range.

rickortega · February 17, 2006, 07:52:38 AM

Any other ideas...I have new resistors and could pick up some caps. However, from my amp work, this feels like something else is going on. That scratchy pot sound is a clue...I just don't know.

Rick

flies · February 17, 2006, 11:24:42 AM

scratchy sound on a new pot? i thought the scratchy sound you got when turning a pot meant that the pot was dirty...but if the pot is new then it shouldn't be dirty at all....

rickortega · February 17, 2006, 04:24:14 PM

Quote from: flies on February 17, 2006, 11:24:42 AM
scratchy sound on a new pot? i thought the scratchy sound you got when turning a pot meant that the pot was dirty...but if the pot is new then it shouldn't be dirty at all....

On Marshall amps (and probably others), the Presence pot can sound scratchy and be perfectly fine as far as the cleanliness and the condition of the pot are concerned. Other components on the board can make the pot sound scratchy.

Here's what one guy said on another forum: "The scratchy pot/s may be stray DC voltage, which typically is caused by a leaking cap/s. You can easily check for leaking caps by checking the cap leads closest to the volume and tone pots, there should be none with exception to the presence pot which will carry about 12 vdc. Also, check for whiskers around your terminals to make sure they're not getting shorted."

This pot's not dirty...but, i still can't figure out why the distortion pot shuts off the pedal as I turn that pot up close to 12 o'clock.

Any layouts out there?

johngreene · February 17, 2006, 05:35:44 PM

Can you take some voltage measurements with the unit working and again when the pot is turned up so it cuts out?

--john

rickortega · February 17, 2006, 05:38:59 PM

I correctly suspected something in the distortion pot loop and it was the 0.5uf cap. I replaced it with a .047uf mylar cap...my D+ is back.

johngreene: I was going to take some voltage measurements but decided to replace that .5uf cap 1st. It was a good 1st guess...thanks for chiming in.

Rick

News:

Help With MXR D+...Pictures Included