Zener diode replacement in vintage gear

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I own a couple units of electronic audio gear from the 1970s which are unmodified and have all the original parts. Both units use zener diodes as the voltage regulator, and as I plan to replace all the electrolytic capacitors in each unit I wonder if I should also consider replacing the zeners.

I am largely ignorant of zener diodes and their service life. Should I consider replacing the zeners as well as the capacitors? Or, if I replace the caps and measure the same voltages listed in the schematic should I not be concerned about the zeners? I checked at Digikey and Mouser, and they do not appear to stock the zener part numbers listed in the schematic, nor do I know how to determine any equivalent part.

Thanks for any opinions and information.
To see if a zener is OK, and if you can get a laboratory power supply with current limit capability, set the current to 10mA more or less, and starting from zero volts, increase the voltage command until current starts to flow. This is, in reverse biased junction, and almost one pin unsoldered from the DUT. This will give you an idea of the zener voltage. If readable the part number, compare this value with a data sheet. ALWAYS start from zero voltage whichever be the power supply you are using, and limit the the current with a resistor. Lab PSU's has internal capacitors at the output that -when charged- can destroy smaller diodes, 5.1, 5.6V. etc. I use this method when repairing, and the label has been erased and usually in industrial devices there is no kind of schematics, because of proprietary secrets.
Yes, why pick on the zeners? All the other semiconductors are just as old.

While you COULD pull the zeners out and set up a test circuit on the bench, I think it is perfectly valid to just measure the voltage across them in the circuit. Your units do work, right? So they should either have good zener voltages or not.
The only time you replace zeners (other than when they measure bad) is when the board around them is thoroughly cooked from heat. FR4 boards that have been blackened are suspect even if they still work now. They might not a year from now. If a zener, high power resistor in series with it, or both cooks a board, I replace those parts and try to arrange them for better air flow. The affected solder joints need to be re-done anyway even if you keep the original parts, and while you’re at it it’s just as easy to put in fresh ones that haven’t been run up to 200C TJ for the last 10 years. But if there is no evidence of cooking you leave them be.
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Replacing "cooked" parts as wg_ski pointed out is a good thing, however, it also shows poor design by the manufacturer/builder.
These things happen from time to time, I've seen my share over the decades.
Asides from elevating the parts from the board, I'd also research the reason why they need to get so hot, and perhaps make a design change to lighten their load.
Sometimes, just increasing the dropping resistor a bit is enough, other times some serious troubleshooting down the line is needed.
Thanks to all for the replies. Both units were manufactured in the years of large circuit boards and hand soldering and show no evidence of heat damage. Your comments have eased my concerns so I will go ahead with the recap, measure the voltages, and not worry about the zeners.

DIY Audio is a such a great resource, with knowlegeable members who are ready, willing, and able to help. Thanks again.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.