On the bench today is a HP 5385A frequency counter which seems to have a short to ground on the 5+ rail. Someplace….. Hunting.
Fixed: The 5v over voltage crowbar protection circuit was being switched in, because the 5V rail became noisy/ripple (+/- 700mv ripple voltage). Replaced the aged smoothing capacitor on the 5v switch mode power supply output. That fixed the ripple and stopped the over voltage issue the zener diode was tripping on.
This is a nice counter for sure, and the functions this counter has which my Racal-Dana does not, is manual control over the gate time.
Also, this counter cleaned up beautifully, even smells nice, and says HP on it….