Electronics > Repair

Power supply ripple on UA723 based circuit

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George Edmonds:
Hi again

Now if the R&S schematic and your measurements are correct there is a simple problem that effects all of the 741 devices.  There should be a -3.3V supply on pin 4 of them all, you show 0V.

This negative voltage appears to come from the junction of R14 and GL4, GL4 is a Zener diode type BZX55/C3.3 according to the parts list.

The 741 is an early device that is designed to operate between  + and – supplies, your measurements show NO negative supply to pin 4 of any of them..

Your photograph shows at least one brown plastic cased electrolytic capacitor, these capacitors by Roederst have a known high failure rate.

G Edmonds

RichardM:
Thanks. I always do a diode check at the start so I either missed this or it died due to all my probing. It was definitely shorted. I replaced it and there is no longer the strange start-up with the load connected :). -3.3v on the ua741 pin 4 now. Now I will connect it back up to the unit and see if the 5v is OK. Unfortunately I have to go to work first!!

Richard

RichardM:
OK, the power supply is working when connected to the SMS.  ;D

I get all zeros when it boots which is expected, but pressing front panel button seems to generate values at the incorrect places on the LED's. Hours of trouble shooting ahead :) Quite a few logic ic's on the front display PCB. Also a CD4514BE line decoder ? I have a spare to try.

Thanks for the assistance.

RichardM:
I should say I never actually fixed the original issue. I still see “noise” or 50mv on the 5V rail.

iMo:
Show the ripple/noise, plz..

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