Author Topic: Power supply ripple on UA723 based circuit  (Read 1743 times)

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Offline George Edmonds

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Re: Power supply ripple on UA723 based circuit
« Reply #25 on: November 18, 2024, 12:15:20 pm »
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
 

Offline RichardMTopic starter

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Re: Power supply ripple on UA723 based circuit
« Reply #26 on: November 18, 2024, 09:16:12 pm »
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
 

Offline RichardMTopic starter

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Re: Power supply ripple on UA723 based circuit
« Reply #27 on: November 19, 2024, 11:33:38 am »
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.
 

Offline RichardMTopic starter

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Re: Power supply ripple on UA723 based circuit
« Reply #28 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.
 

Offline iMo

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Re: Power supply ripple on UA723 based circuit
« Reply #29 on: November 19, 2024, 12:20:10 pm »
Show the ripple/noise, plz..
Readers discretion is advised..
 

Offline ondraN

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Re: Power supply ripple on UA723 based circuit
« Reply #30 on: November 19, 2024, 12:47:18 pm »
They are at Reply#9
 

Offline ondraN

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Re: Power supply ripple on UA723 based circuit
« Reply #31 on: November 19, 2024, 12:49:41 pm »
Look at the pin6 of LM741 (output). It shoud bee some overcurrent issue.
 

Offline iMo

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Re: Power supply ripple on UA723 based circuit
« Reply #32 on: November 19, 2024, 05:42:55 pm »
I should say I never actually fixed the original issue. I still see “noise” or 50mv on the 5V rail.

They are at Reply#9

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..

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..
« Last Edit: November 19, 2024, 06:55:58 pm by iMo »
Readers discretion is advised..
 

Offline RichardMTopic starter

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Re: Power supply ripple on UA723 based circuit
« Reply #33 on: November 20, 2024, 09:05:30 am »
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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