I picked up an old HP 721A power supply for cheap the other day. After firing it up I found that it outputs about 10V when the voltage control is set to zero and 37 at maximum. Turning the voltage control up increases the output voltage on a seemingly logarithmic scale, as in it increases quickly as you start to turn it up and slowly towards the end of rotation.
After probing around with my ESR meter I found the filter cap C2 was open-circuit...I replaced it with a 480uf cap I had on hand, but the problem still persists. All other caps tested fine. The regulator also tested OK.
Any advice would be appreciated.
After probing around with my ESR meter I found the filter cap C2 was open-circuit...I replaced it with a 480uf cap I had on hand, but the problem still persists. All other caps tested fine. The regulator also tested OK.
Any advice would be appreciated.
Check the diode CR7, sounds like it may be shorted. It's a 7V, 400mW Zener diode.
There should be 7V across it in normal operation.
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Just checked it, it tests good. All the other diodes test ok.
Am I correct in assuming the voltages printed in the schematic are not affected by the position of the voltage control?
Am I correct in assuming the voltages printed in the schematic are not affected by the position of the voltage control?
The manual says that most voltages will change with different output voltages or current.
I do see that now in the notes...Any idea on what the position of the control was when the voltages were recorded?
Power supply fail
I dont't know you power supply but your problem seem due to the reference voltage stage. Basically you colud see your PS like an opamp with a reference voltage on input, tension to be regulated on other input and the risultance driving an electronic potenziometer (a rehostat in reality). You should have a zener, a diodes string begore the gain stage. This seem be the faulty stage!
I dont't know you power supply but your problem seem due to the reference voltage stage. Basically you colud see your PS like an opamp with a reference voltage on input, tension to be regulated on other input and the risultance driving an electronic potenziometer (a rehostat in reality). You should have a zener, a diodes string begore the gain stage. This seem be the faulty stage!
There should be a negative regulated voltage relative to the negative output terminal that the bottom end of the the voltage pot goes to so that the output voltage can be turned down to zero while the reference voltage is still there. It will not regulate below the value of the reference without this.
I don't have a circuit diagram here but I would expect there to be a fuse of some kind that is open circuit.
I don't have a circuit diagram here but I would expect there to be a fuse of some kind that is open circuit.
http://www.pa4tim.nl/wp-content/uploads/2011/11/schema721.png This is a link for schema.
If you look schema, reference is determinated by cr3 and cr4; C3,C4,C5 (40uF each); R2,R3 (330 each). This line is REFERENCE for Power Supply! I suspect condensers in short circuit or faulty. resistors and transistors should be Ok due the low current erogated and knowing well component selection in HP tradition.If reference line it's ok look at Q4 maybe it's faulty . Remember transistors are PNP gemanium type, you should reverse the abitual thincking!
If you look schema, reference is determinated by cr3 and cr4; C3,C4,C5 (40uF each); R2,R3 (330 each). This line is REFERENCE for Power Supply! I suspect condensers in short circuit or faulty. resistors and transistors should be Ok due the low current erogated and knowing well component selection in HP tradition.If reference line it's ok look at Q4 maybe it's faulty . Remember transistors are PNP gemanium type, you should reverse the abitual thincking!
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