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I'm doing some proactive maintenance on a vintage games console from the late 70s. There are a couple of linear regulators which I thought would be prudent to replace whilst I'm at it. Unfortunately the markings on one are nearly illegible and on the other are completely gone. (They were covered by some tape which took the markings off when I removed it)

I know the marked one is a LM78M12x - the 'x' is illegible. The other is a 5v regulator confirmed with a DMM. I have searched for schematics in vain.

The question is what suitable modern replacements should I use here? Are there any other clues I can look for which might help?

Can I cause any problems by choosing the wrong ones e.g. LM7805 instead of LM78M05? Should I go with a modern switch-mode version?

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  • \$\begingroup\$ A switch-mode will add lots of high-frequency spikes to the VDD rail, and you likely will have to lots of new capacitors, while possibly facing constant unwanted "RESET" events. \$\endgroup\$
    – analogsystemsrf
    Commented Nov 24, 2018 at 12:11
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    \$\begingroup\$ Don’t fix what ain’t broke is my belief. \$\endgroup\$
    – Andy aka
    Commented Nov 24, 2018 at 12:26
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    \$\begingroup\$ Why replace them if they aren't broken? It isn't like an electrolytic (or paper and oil) capacitor that will go bad with time. \$\endgroup\$
    – JRE
    Commented Nov 24, 2018 at 12:27
  • \$\begingroup\$ Seconding what Andy and JRE said--there's no need to replace linear regulators unless they've somehow failed. These parts don't have limited lifespans, so they should continue to work just as well as they always did if they aren't mistreated. \$\endgroup\$
    – Hearth
    Commented Nov 24, 2018 at 12:58
  • \$\begingroup\$ If the game is COOL inside, then the components will have an extended lifespan. Perhaps add a tiny fan, if there is any heat buildup; this protects the ICs and the electrolytic caps. \$\endgroup\$
    – analogsystemsrf
    Commented Nov 24, 2018 at 13:12

2 Answers 2

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I would assume that's a version of LM78M12, which is a 12V regulator. Have a look at the datasheet for the pinout of your package (http://www.ti.com/lit/ds/symlink/lm341.pdf - I know it says LM341 in the link and LM78M05 in the PDF, but the layout is the same).

Once you identify which one you have (TO-220 or TO-39 package), check the pinout and measure pins 2 and 3 with a DMM. If our assumption was correct, it should read 12V. From what I gather, these were 500mA regulators, so something like a regular LM7812 should work fine.

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Assuming your gadget is working satisfactorily,Best way to identify is to measure the output of the regulators and go in for the latest equivalents. Normally these regulators will lost long unless short circuited beyond it’s short circuit protection spec.

Normally these power supply will have problems related to electrolyte capacitors drying up or (open) blown due to load current. With my experience many chronic power supply problems got solved just replacing aged capacitors and re -wetting of oxidised solder joints and wires Your proactive efforts can be focused on defective capacitors and Oxidised soldering joint due to time and humidity levels at your place.

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