1N4004 from GPE. About 20 for $1 last I bought.

Yes, cheap over kill for 99% of your pinball diode needs. (but not high wattage rectifier needs)

Don't forget to replace those crappy disc caps on many of the switches.

They are often bad. If you see one with a single leg cut, you KNOW that one is bad.

I always replace them if doing a playfield swap. Diodes are much less likely to go bad.

You can get 100 of those caps for .90 cents:

http://www.taydaelectronics.com/capacitors/ceramic-disc-capacitors/10-x-0-047uf-50v-ceramic-disc-capacitor-pkg-of-10.html

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Vid is correct, diodes rarely fail. It's always the caps. If a diode is bad, more the likely it's been physically abused.