Diagnosing a DOD FX90 analog delay

[irnmadn88]

Rob Strand 14 Feb 2023
Images converted to links due to malware warning - perhaps old site is now redirecting.



DIYFreaque steered me here...

I just dug out a bunch of effects along with my guitars that have been in a box for several years...

I have a DOD FX90 delay pedal that is not working. I get signal whether bypassed or on. The DELAY, REPEAT, MIX knobs do noting to change the sound. I am hoping to diagnose the pedal as alive or dead, repairable or not. If dead and not repairable, I have an enclosure to build a simple effect...

I am operating on the assumption that the MN3005 BBD is toast... I can get a couple of MN3007's from a friend (and maybe a socket) but I don't trust my soldering skills for that yet...

I do not know if I messed with the trim pots on the inside before putting the pedal away years ago... but assume I did.

I have found a REV A schematic here: http://www.schematicheaven.com/effects/dodfx90_delay.pdf and my pedal is a REV H. I am also trying to lay out a schematic using bancika's program for comparison but that is proving to be time consuming...

DIYFreaque noticed right away that the op-amp is different from the above schematic and the gut shot here:

http://www.jeeps-offroad.com/attachment.php?attachmentid=12377&d=1231950199

And the other side...

http://www.jeeps-offroad.com/attachment.php?attachmentid=12376&d=1231948686

[theehman]

First thing to do is check the clock circuit for the BBD and the input/output of the BBD.

[irnmadn88]

First thing to do is check the clock circuit for the BBD and the input/output of the BBD.

Thanks!

I noticed on your website that you offer a schematic for the DOD FX90 "Dated: 8-23-87" where as the one I linked above is for a REV A dated 12-2-85. My FX90 is a REV H.

Is yours different?

[theehman]

First thing to do is check the clock circuit for the BBD and the input/output of the BBD.

Thanks!

I noticed on your website that you offer a schematic for the DOD FX90 "Dated: 8-23-87" where as the one I linked above is for a REV A dated 12-2-85. My FX90 is a REV H.

Is yours different?

I don't know.  I'll take a look tonight.