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

Congratulations on what you have done so far, I will quite happily work with a person with your positive attitude.

My fist gut reaction to your last posting is that the design of the +5VDC supply is well over the top if only supplying 750mA,

You probably do not need me to say this but the current regulation circuitry for the +5VDC supply is almost identical to that of  the +15VDC supply so simply comparing the voltages on all of the pins of B2 and B5 may well give a clue as to what is happening.  Given that the +15VDC supply uses two current sense resistors in parallel suggests to me that it provides a greater current than the +5VDC supply.

I find it very annoying that service manuals rarely give the current drawn from the various supplies as it is frequently very useful information.

My final point for now is that I forgot to ask you to directly measure the voltage across R11 and R27 with the supply in the instrument, from this we can work out the currents drawn.

Keep smiling

G Edmonds
RichardM:
Ok, some initial measure,ents.

R11 measures spot on with 4 wire kelvin on m HpP3478A

DC voltages on B2

Pin 1 0v
Pin 2 5.319
Pin 3 5.026
Pin 4 0
Pin 5 0
Pin 6 1.333
Pin 7 22.04
Pin 8 0

DC voltages on B5

Pin 1 0
Pin 2 15.06
Pin 3 14.81
Pin 4 0
Pin 5 0
Pin 6 2.024
Pin 7 25.35
Pin 8 0

DC voltages on B7

Pin 1 0
Pin 2 20.22
Pin 3 20.02
Pin 4 0v
Pin 5 0v
Pin 6 2.005
Pin 7 25.38
Pin 8 0v

One thing I haven’t mentioned is that one of the tracks from the MP headers (MP3) has clearly been overloaded and burnt in the past. The track was lifted so I just snipped it off a few days back. I didn’t think it was critical to maintain the link between the header and the via for MP3 based on the schematic. It may provide some insight though. Photo attached. Headers on the left are MP1 to MP10 bottom to top. I suspiciously measure some DC and AC on MP3 even though it is not connected to the trace anymore. Maybe the burn has contaminated the pcb ? Just an added complication, the board is in pretty good condition otherwise given its age.
iMo:
I would replace all those red bricks - I assume the tantalums capacitors.. Put there new with a higher voltage rating.
PS: perhaps the PSU is missing the cooling water in the copper pipe there in your shot..  >:D
xavier60:
Is there activity on B2 pin 6.
RichardM:
That is an old photo before I cut the burnt trace that was floating in the air. I have replaced all the tantalums since then.

I also replaced the low gain 2n3055 tonight. It was T2.

I made an observation by chance when testing the 5V under load using a big power resistor like I did previously. Not sure what to make of it.

If I turn on the power supply with the 5V under load from the resistor, I get zero current flow and the 5V is line zero.    If I start the power supply with no load on the 5V line and then add the load, I get current flow. Right up to 800ma no problems. So, I guess I need to swap back the old transistor to see if I observe the same behaviour. Fingers crossed I don’t see the same behaviour suggesting T2 was the problem. Somehow I don’t think it is T2.  Otherwise I will be scratching my head. I also need to get the supply back into the unit with the new transistor to see if it works. All a bit strange to me now. Keeps my grey matter active 😀
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