| Electronics > Repair |
| Power supply ripple on UA723 based circuit |
| << < (6/7) > >> |
| 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.. |
| Navigation |
| Message Index |
| Next page |
| Previous page |