HP 6266B Power Supply - Page 1
Forum section: electronics / repair

author: nickbenjones
author_rank: Newbie
timestamp: June 26, 2015, 03:34:37 am
content: So i recently acquired a power supply off of ebay that i am attempting to repair. Thinking it would probably be a simple cap job; however, such is not the case. I have checked all of the reference voltages they seem to be good. I also checked the crowbar resistor all appearing normal. The power light will flicker but the power supply will not turn on. It turned on when i first unboxed it but the + voltages were non-existent and the negative rail showed an under voltage of about 15 volts. Any ideas where to go from here?

author: oldway
author_rank: Super Contributor
timestamp: June 26, 2015, 07:47:15 am
content: https://www.eevblog.com/forum/repair/hp-6266b-%2840v5a%29-crowbar-repair-and-tear-down-pictures/Look first if the rear panel connections (TB1) are right.

author: nickbenjones
author_rank: Newbie
timestamp: June 26, 2015, 02:17:03 pm
content: Rear panel straps are set properly according to the manual and the diagram on the chassis.

author: oldway
author_rank: Super Contributor
timestamp: June 27, 2015, 10:19:52 am
content: I have two HP 6296A, they are quite good power supplies...First check the reference circuit: you must measure 12.4V between - of C10 (100µF 50V) and base of Q15 (25) and 18.6V between + and - of C9.Measure also ripple upon C10 and C12.Check diodes of preregulator circuit, replace C15, C16, C17.Set voltage and current to max values.Desolder one side of R102 (0.125R 5W) to open crow bar circuit.Is the power supply working or not ?If not,Measure output voltage (pin6), (Test point 16) of constant current comparator Z1.Measure output voltage (pin 12), (test point 91) of constant voltage comparator Z1

author: cncjerry
author_rank: Unknown
timestamp: June 28, 2015, 03:50:53 am
content: Many of the HP power supply problems are related to the sense circuit / connections not being correct.

author: nickbenjones
author_rank: Newbie
timestamp: August 09, 2015, 12:19:18 am
content: Sorry for the late response, i do appreciate the help though unfortunately work had taken me away from my hobbies. Replacing caps 15, 16, and 17 resulted in the power supply returning to normal operation with the one flaw of the volt meter reading about ten volts below the actual output. The amp meter is dead on. Unfortunately the voltage meter cannot be calibrated back into range. I am suspecting more bad caps and will probably replace all of the capacitors minus the two main filter caps.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 09, 2015, 12:50:10 am
content: Hi,I did a quick analysis of the voltmeter circuit:If I assume that the circuit was designed with the adjustment set in the middle. Then the meter is a 1mA meter with a resistance of about 110 Ohms.I would check the meter and the values of the resistors in the meter circuit. You can adjust the values of the resistors if the meter cannot be calibrated.I would also measure the output of the power supply with an oscilloscope if you have one. Look for excessive ripple.Regards,Jay_Diddy_B

author: nickbenjones
author_rank: Newbie
timestamp: August 09, 2015, 01:36:11 am
content: do not have an oscilloscope at home, but i did measure for ripple using my volt meter in ac mode. I do not know if that is a quantifiable measurement, but given the circumstances it was all i could do. The resistors in the volt meter circuit are all within spec.

author: nickbenjones
author_rank: Newbie
timestamp: August 09, 2015, 01:38:05 am
content: the meter itself is measuring 96 ohms....