LM723 - What's the Story?

tb lilley

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My Defender is occasionally resetting. The board has been modified to run 4164 RAM which uses only +5VDC, so I'm thinking there might be a problem with the +5 coming out of the linear power supply. I've already recapped the power supply and stuck a new bridge in at BR1, so that only really leaves the LM723 to go at.

The schems tell me that there are 2 LM723s on the power supply, and it's the one at IC1 that's doing some sort of job with the +5, but what exactly does the LM723 do, and is it worth replacing?

I assume that it does what it says on the tin, i.e it regulates voltage (to +5VDC), but I thought that was the job of the 2N6057 (which I also replaced a couple of months back). ..... Any advice gratefully received. Thanks.
 
The schems tell me that there are 2 LM723s on the power supply, and it's the one at IC1 that's doing some sort of job with the +5, but what exactly does the LM723 do, and is it worth replacing?

I assume that it does what it says on the tin, i.e it regulates voltage (to +5VDC), but I thought that was the job of the 2N6057 (which I also replaced a couple of months back). ..... Any advice gratefully received. Thanks.

Good question. The LM723 is the "brains" of the voltage regulation circuit, the 2N6057 is the "muscle." Both are critical to proper +5VDC power supply operation. An LM723 datasheet I found online explains it pretty well: "By itself, it will supply output current up to 150mA; but external transistors can be added to provide any desired load current." The 2N6057 is that external transistor which allows it to provide the several Amperes required. (BTW, the other LM723, IC2, is for +12VDC regulation.) If you suspect an issue with +5VDC, the regulator (LM723, IC1) would certainly on the short list of suspects. They seem inexpensive and readily available, so I'd say swap away.

PS; have you already replaced C12?
 
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Thanks for such a comprehensive answer. That's really helpful.

Yes, I've replaced the monster cap at C12 (took some finding, but greatplains stocks them).

So, after what you've said I reckon the LM723 is going to have to be replaced. Makes a lot of sense. It's sat in the middle of a new bridge at BR1, and a new 2N6057, with a new cap at C12 in play, plus the fact that the RAM is now running on +5VDC ...... the old LM723 is the likely culprit. Thanks again.
 
The LM723 is the actual regulator, while the 2N6057 is the actual Gate/Regulation transistor.

The LM723 watches the output (feedback) from the drain side of the 2N6057, while it's supply out is hooked to the gate of the 2N6057 to bias the transistor to that voltage, so the drain of the transistor puts out whatever the LM723 tells it to minus whatever the voltage drop across the part is.
 
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