HP 6643A Power Supply Repair — I tracked down the faulty circuit, but need help - Page 1
Forum section: electronics / repair

author: Eth7an
author_rank: Contributor
timestamp: February 15, 2021, 05:37:10 pm
content: Hi all,I recently acquired an HP 6643A power supply that I was planning to upgrade my current power supply with. Unfortunately, it doesn't work, so I embarked on my first repair mission.The symptoms: Power supply triggers OV protection circuit when turned on. The voltage rises up to about 48 (its a 35v supply) before the crowbar clamps down and the protect circuit triggers. Interestingly, while the output is off (soft output on/off button) it produces about 22-24v on the output.I managed to confirm the secondary interface circuit that provides the analog CVProg signal (from 0-10v, which is supposed to control the full range) functions properly, so I moved on to the output circuits and found that one FET, Q204, gets mildly hot while the other 7 stay completely cool. Right next to Q204 is R209, which is browned and burnt (reading 800ohms for a 1k resistor).So I'm certain that there's something off with this output circuit, specifically the subsection relating to Q204, but I really have no idea what to do next.If anyone can help, it would be much appreciated!Thanks!EthanService manual:http://ridl.cfd.rit.edu/products/manuals/Agilent/power%20supplies/CD1/Service/654xAser.pdf

author: EHT
author_rank: Frequent Contributor
timestamp: February 15, 2021, 10:46:41 pm
content: I'm thinking that given R209 is seeing excessive voltage to burn it out, either R210 is short or U201 is internally short. Make a few more resistance measurements comparing the bad circuit to one of the good ones. If you can't work it out you might have to remove U201, Q204 and test them out of circuit.BTW, I was wondering what the Power NPN transistors XQ20* were. Quick Google search found a parts cross-ref sheet showing they are MJ15003. Nice to have standard parts! I love that it is built like a tank. Thespecs sheetsays the NPN transistors are rated at 20A/250W each and there a 7 of them in a 180W, 3.5A PSU (6644A)!! Maybe the MJ15003 is higher rated than the original part (?), but nonetheless seems incredibly solid. This is a great PSU.I have a 6622A (luckily working fine) for which they put the output stage in a hybrid which I suppose is unobtanium / unfixable. Fingers crossed it has many years left!

author: Eth7an
author_rank: Contributor
timestamp: February 16, 2021, 12:45:48 am
content: Thanks EHT!I think I've figured it out...Under the heatsink (which I had to desolder each transistor to lift), I found that R206 was also blown. Then I tested XQ203 out of circuit and found that it's shorted! I think that's the problem.I found a few claimed replacement transistors, and I'll have to replace the other burned components. Hopefully this will fix the power supply and get it back to fully working.One thing I'm not sure of however is transistor matching. Do you know if they need to be matched in any way? I'm guessing not since they each have their own feedback circuit, but this is really over my head.Thanks,Ethan

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 16, 2021, 06:32:45 am
content: take f206 out & test if ok, then move to diagnosing that defective left brach

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 16, 2021, 06:37:00 am
content: if you are starting in this electronics repair, maybe just replace U201, Q204, R209, C202 and check the remaining resistors around Q204if you are intermediate level, we can guide you step by stepregards, pierre

author: Eth7an
author_rank: Contributor
timestamp: February 16, 2021, 12:30:50 pm
content: Thank you perieanuo!I've just ordered a replacement for the blown transistor, and will replace that along with the damaged resistors later this week.At that point, I will let you know if I still need any assistance to get the power supply back in working order.Ethan