Fixed linear voltage regulator failure modes

[davidwenrich]

     So... long story short. I forgot my cardinal rule, never trust what anyone tells you. I bought a DC electronic load "for parts not working".
No history. So step one was to hopefully track down information. Four score and seven years later, I have a schematic and somewhat of a manual. Has Theory of operation at least. Step two, check power supply voltages. The technical support guy told me to check for +12 and -12 at capacitor so and so. I did, it was, next step. Chase my tail for a few weeks. Then while checking this and that I notice the part number for the regulators end in XX08....not XX12 (My fault for not looking and assuming tech support told me correctly, That brings me to my question... long intro I know, but bear with me. Is there some strange phenomena that would cause BOTH regulators to fail, with the same "fail ratio", for lack of a better term... ie, both 7808 and 7908 fail, and both are outputting 11.5-12 volts? positive and negative? Well enough to seem to be working correctly, no excess heat, stable output....etc. New regulators ordered. Hurry up and wait.

[tunk]

Don't know anything about failure modes, but I think I would measure the
input voltages to the regulators. And what's the markings on the regulators?

[redkitedesign]

I can think of one fail mode: Fail short. And the inputs are +12 and -12?

[Brianf]

There is, of course, nothing to stop you regulating +/-12V using 7x08 regulators as long as you stand the middle pin on 4V.

[wn1fju]

On the slim chance that this is your problem, put a scope on the input and output pins and make sure there is no excessive 120 Hz AC ripple or high frequency oscillations.  You sometimes can get fooled by simple DC measurements (which are low bandwidth averages).

Like any semiconductor, 78xx and 79xx can fail open, short, cracked in half, etc.  I once had a 7805 in a piece that was producing no output voltage.  After I replaced it, I put the old one on the bench and hooked up an external power supply.  Put in 9 volts and the 7805 was outputting 5 volts as expected.  Scratched my head for a while, then turned up the power supply.  At >10.5 volts, the regulator output dropped to zero.  And sure enough, the equipment that it originally was in was supplying 10.6 volts!

[davidwenrich]

BrianF nailed it. inputs are +/-24v. however, I got lazy and didn't move my ground to the pin of the regulators. they both were grounded center tap from transformer, which just happened to be +4 from the other circuits ground. Found the problems though. About 60 out of 84 pass transistors are failed nearly shorted. was trying to avoid removing their "trays" and removing 84 transistors from circuit to test. parts incoming.