| Electronics > Repair |
| Power supply ripple on UA723 based circuit |
| << < (7/7) |
| ondraN:
They are at Reply#9 |
| ondraN:
Look at the pin6 of LM741 (output). It shoud bee some overcurrent issue. |
| iMo:
--- Quote from: RichardM on November 19, 2024, 11:56:58 am ---I should say I never actually fixed the original issue. I still see “noise” or 50mv on the 5V rail. --- End quote --- --- Quote from: ondraN on November 19, 2024, 12:47:18 pm ---They are at Reply#9 --- End quote --- There is none picture of a 50mV ripple at 5V in this entire thread.. So it would be great if the OP would show us the 50mV ripple or noise at the 5V line.. The picture in #9 is a signal at the cathode of the GL3, 3Vpp large, it is basically output of the 741. Read the posts.. --- Quote from: iMo on November 15, 2024, 11:09:42 pm ---That is not a ripple from the diode bridge, the stuff oscillates.. Look at C2, C3 (is that a tantalum 470n?).. Is your 5V output loaded? Btw, the oscillation at the GL3 cathode is 3Vpp when I look at the shots.. That may mean the 741 (the current limiter) senses an overcurrent at the R11 (the shunt) and limits the 723's output (when the 741's output goes up), so it oscillates. It could be the C3 is "shorted" somehow, or a short or overcurrent downstream the 5V line.. --- End quote --- |
| RichardM:
I have taken some update measurements on the 5V line with and without the keyboard/display pcb. This all started because I was and still are getting gibberish on the front display leds and also no consistency in responses to key presses. I measured the noise/ripple on the C6 and. C7 at the time and was seeing 50-60mv. Now after recapping and fixing the Zener on the power supply I see the following. Am using a fluke87v and a Leader LMV-186A along with my scope. Leader pic attached :) - 4.8mv AC between chassis and MP2 header (+5v) on power supply with and without the keyboard/display pcb connected - R67 on display pcb (+5v line) to chassis 23mv AC. Keyboard pcb obviously plugged in. - R67 on display pcb (+5v line) to C7 -ve side 30mv AC. Keyboard pcb obviously plugged in. Scope shot also attached. - R67 on display pcb (+5v line) to C6 -ve side 24mv AC. Keyboard pcb obviously plugged in. - ST20 pin 16 to pin 11 or chassis, 0.1mv AC. This is where the 5V comes into the front keyboard/display pcb. This is with the display NOT plugged in. Clearly the 5V in at the ST20 ribbon connector to the keyboard is low ripple, not the same when the keyboard pcb is connected? |
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