Electronics > Repair
Power supply ripple on UA723 based circuit
RoGeorge:
I'll check the value of the power resistor R11 (the current sensor). Resistance can increase sometimes for old power resistors. Check that it has 68 milliohms indeed, and it is well soldered. Check the voltage drop on it, too (does the voltage drop corresponds with the expected load current?).
If all OK, I'll check the value of the other passive components around that 741, and the diode, too.
George Edmonds:
HI
I would suggest reading and fully understanding section 4.14 of the service manual first.
G Edmonds
RichardM:
Fully understanding section 4.4 is beyond my skill set I’m afraid.😟 I may be able to simulate the circuit is LTSpice as a learning exercise.
After all my prodding and replacement of capacitors etc the ripple has improved but the 5V rail goes into current limiting mode with anything more than about 250ma. One step forward, 3 steps backwards. 😀
George Edmonds:
I agree that section 4.14 can be daunting, possibly because it is a translation from German.
Do you have the facilities to run the power supply external to the instrument?
This maybe a long shot, Please check the voltages and let me know what they are on B2 pins 2,3,4 and 7, also do the same on B5. If possible do this with a DMM and a scope. B2 pin 7 should be 20.5V and pin 4 should be -14.8V
From the measurements you have done it looks like one of two things is happening (1) The rest of the instrument is putting an excess load on the +5VDC supply causing the +5VDC overload protection to activate (IC B2) or (2) The +5VDC supply protection circuitry is faulty. Basically the current sensor is R11 which is connected directly across the inverting and none inverting inputs of B2. The load current at which the supply starts to limit is set by R7, R8, R9 and R10.
A good starting point with any power supply is to check the high voltage and high current parts first so start with T1 the 2N3055, the schematic suggests that it is in a socket so possibly a poor connection exists. The prime suspect must be R11 so visibly check it and if you can make 4 Wire (Kelvin) resistance measurements check the resistance from T1 Emitter to output pins 1, 2, 15 and 16, is should be just over 0.068 Ohms.
You are not helping yourself by calling the problem ripple as this usually implies mains frequency.
G Edmonds
RichardM:
Thanks for hanging in there with a hobbyist.
Yes, I have the power supply separated from the instrument on my bench. It's a pain to remove the TO-3 transistors on the heatsink to properly desolder components properly. I have done it about 5 times already. I did check the 2N3055 transistors, they varied in gain under low current using a Peak Atlas tester, 23, 63 and 108. I can try a new 2N3055 if need be.
I can definitely make the measurements you suggest and will post the results.
Fyi, I ran a separate test by applying 5v to the instrument without the power supply as I also thought the 5v may have been shorted. I used the schematic and 24pin dip flex cable usually attached to the power supply. I started inserting each card individually and the 5v rail was taking about 750ma in total. Seems reasonable given all the ICs on the boards?
Ok, no more mention of ripple 😀
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