Electronics > Repair
Power supply ripple on UA723 based circuit
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iMo:
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..
George Edmonds:
Hi

At this point if the 723 is in a  socket, I would replace it.

I have seen strange behaviours from power supplies when the 723’s begin to fail, if my memory is correct the 723 holds the record for the longest length of time that any single IC has been in constant production

G Edmonds
RichardM:
Replacing the ua723 at B1 has no effect. Replaced GL3 mono effect.

I replaced all the tantalum’s but still see 60mv of ripple. It is present on the emitter of T4 and base of T1.

I put a spare unit on the bench next to this one. The most obvious difference, apart from the lack or ripple on the 5V line on the spare unit, are the voltages across GL3.

On the good unit, -3.95vdc on the anode, 0vdc on the cathode.

On the unit with high ripple, +2.0vdc on the anode, +1.84vdc on the cathode.  I also measure 1.8vac on the cathode🤔
George Edmonds:
Hi

I assume you have also replaced the 741 B2,

Now you talk about the voltage on GL3, a small signal fast switching diode, being incorrect which is the output of B2 the 741.

Do you realise that the function of  B2 is to provide overload protection for the  +5V supply.  It senses the voltage across R11 a 0.068 Ohm resistor which is the power supply current limit sensor resistor and shuts down the 723 to prevent over load damage to the +5V supply.

Your scope traces clearly show that your problem is NOT power supply ripple.  It looks to me that the power supply is starting to power up and then being shut down by an external overload condition, have you checked for this?

G Edmonds
RichardM:
Yes, I have replaced B2. If it is not ripple what is it?

The power supply is working and all voltages are correct, +5, +15, -15, +20 and +28. I was just trying to eliminate the ripple, or whatever it is, on the 5V rail per specs.
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