Gutshot of the Maxon OD-820:
(http://i69.photobucket.com/albums/i44/bside2234/Maxon%20OD-820%20Overdrive%20Pro/IMG_0931.jpg)
What type of diodes are D4 and D5 in the bottom left? These are the feedback loop clipping diodes in the circuit. Their form factor looks about the same as a zener, but I've never seen diodes with a white stripe like that and there are no markings on them - at least not on the top sides, in the gut shots I've seen from two different 820s. But these are current-production pedals, so I don't think we're dealing with obscure NOS magic diodes here!
It's hard to say, but if you actually have the pedal in front of you, just measure the Vf with your DMM and find a matching one in your stock.
Are they germanium?
Do a meter comparison between the ones on the board and a a 1N4148 or 1N914. My guess is its one like that. Looks like clipping diodes to me <shrugs>
Sorry, these are just gutshots I found online... don't have it in front of me :( In the schematic (http://1.bp.blogspot.com/-jPB8pPbtAQI/T70Tr--7MdI/AAAAAAAABX8/oupbJ4uJosM/s1600/OD820.gif) it just says "Silicon" with no indication of what type.
They're silicon -- they aren't Zener. You can see how Zeners would be marked to the right. That Zener is there to protect the charge pump from overvoltage.
I've never seen a DO-35 silicon with a white AND blue stripe, but it's not inconceivable that they're a made-in-Japan 1S1588, which is the "Tubescreamer" diode. After all, Maxon would have ... a few of those.
Found a schematic online for an 880 and its listed as a 1S1588 or 1n4148, so either way we were right ;)
Cheers,
Russ
Yep, you're right. Here's a gut shot of a reissue 880:
(http://i667.photobucket.com/albums/vv31/greendogs23/IMG_4618.jpg)
Same diodes as the 820. Thanks for finding that!