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”
- 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
- 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
Keep two mental bins:
- Diagnosis parts — temporary, expendable, for proving the fault
- Final parts — correct spec, reputable source, documented in your notes
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?