Mosfetti Forum

Not every repair starts with a perfect Digi-Key cart. Sometimes the honest inventory is a drawer of salvaged passives, a suspect electrolytic, and a stubborn piece of test gear that still has one more life in it.

Dave’s EEVblog repair of a Keithley 2302 battery simulator using junk-bin parts is entertaining, but it is also a quiet masterclass in disciplined improvisation.
https://www.youtube.com/watch?v=D3c3UVDSKTg

What “junk bin repair” really requires

Improvisation without method is how boards become worse. Useful junk-bin work still needs:

• A theory of the fault before swapping random parts
• Measurement of rails, references, and control loops
• Understanding which substitutions are electrical vs only mechanical fits
• Awareness of voltage, ESR, ripple current, and tolerance — not just “same µF”

When substitution is smart

• Passives in non-critical timing roles with clear margin
• Second-source semiconductors with compatible pinout and ratings
• Temporary parts used to confirm diagnosis before ordering exacts

When substitution is reckless

• Safety-critical isolation barriers and crowbars
• Precision references you have not characterized
• Power FETs or sense resistors where SOA and TCR matter
• Anything lithium-battery adjacent without understanding protection circuitry

Mosfetti bench habit

Keep two mental bins:

• Diagnosis parts — temporary, expendable, for proving the fault
• Final parts — correct spec, reputable source, documented in your notes

The junk bin is for the first. The second is how a repair stays fixed six months later.

Discussion

What is the most “wrong on paper, right on the bench” substitution you have made that still worked years later — and what would you never try again?