HP 35665A Power Supply - Page 1
Forum section: electronics / repair

author: Johnny10
author_rank: Frequent Contributor
timestamp: October 17, 2019, 12:54:44 am
content: Just received a HP 35665A with faulty power supply.No power at all.After watching Dave's HP 35660 Tear Down I believe he said no schematic available for power supply.Just hoping someone on the forum may have more information.Thanks

author: wn1fju
author_rank: Frequent Contributor
timestamp: October 17, 2019, 01:56:40 am
content: Don't have a schematic, but for what it's worth, I fixed my 35665A power supply solely by inspection. In my case, the thing blew a fuse immediatelyand was already drawing 2A when I supplied 8VAC into it via a Variac. It's a bit of a pain to disassemble, but an ohmmeter showed shorts in the mainswitching transistor, the driving transistor, three 1N4148s and three 5.6V Zeners. Quite a bit of carnage, but all easily replaced, and it fixed the unit.So open it up, look for burnt parts, shorted parts, the usual stuff.

author: Johnny10
author_rank: Frequent Contributor
timestamp: October 17, 2019, 02:44:35 am
content: Yes, that is what I started doing.So far found 3 bad Diodes.

author: Johnny10
author_rank: Frequent Contributor
timestamp: October 19, 2019, 01:40:20 pm
content: I am going to have to do more investigation or just buy a new power supply.TRW 095-10097The AC line fuse has not blown on my unit. ButI am finding full 155VDC voltage on VCC pin of UC3842 IC and gate of Power transistor BUW12A, doubtful if much of anything is going to work in the future.

author: vtp
author_rank: Regular Contributor
timestamp: October 19, 2019, 05:17:13 pm
content: Some years ago rebuilt this power supply from my 35665A.I do not think there was a single good electrolytic in this. I have also at least partial schematics drawn somewhere but they are not in electronic format and likely to be scattered over several pages of notebook. I do remember that there was something interesting about the switching transistor(s), IIRC it was emitter switched BJT with a lower voltage MOSFET. Or some other not so common arrangement.Will see if I can find my notes.

author: Johnny10
author_rank: Frequent Contributor
timestamp: October 19, 2019, 07:45:06 pm
content: Thanks for your help.I keep wanting to put this aside but I keep researching!!!Let it GO ! :-)I just picked this up Thursday.Someone else has been in here before me.

author: dom0
author_rank: Super Contributor
timestamp: October 20, 2019, 05:24:41 pm
content: Quote from: vtp on October 19, 2019, 05:17:13 pmMOSFET+BJT cascode for switching was sometimes used in ye olde times to work around voltage limitations of old MOSFETs. Basically using the lower MOSFET as a fast switch and the upper BJT to keep the high voltage from the MOSFET. I believe this arrangement was even made in an integrated form for some time.

author: Johnny10
author_rank: Frequent Contributor
timestamp: October 20, 2019, 07:55:08 pm
content: I have been tracing out one of the boards.It is as you said, UC3842A Current Mode PWM IC to BUZ71A MOSFET driving BUW12A High Power Transistor.Have a lot of blown diodes, MOSFET, Resistors.

author: greasemonkey
author_rank: Contributor
timestamp: August 31, 2021, 08:38:53 pm
content: I repaired mine three times already.The first time (when I bought it) it was just the three zeners (D13, D14, D15).The second time was a freak occurrence. Somehow the frame of the ceramic power resistor (R29) shorted with the trace under it.The third time one of the main capacitors kicked the bucket.I documented the repairs here:https://btbm.ch/hp35665a-power-supply-repair/https://btbm.ch/hp-35665a-power-supply-repair-no-2-trw-p-n-095-10097-hp-p-n-35672-67501/And don't forget. Despair is optional.

author: Johnny10
author_rank: Frequent Contributor
timestamp: August 31, 2021, 09:56:13 pm
content: I'll check it out tonight.I put it away, Wow, looks like a couple of years ago!I bought a second one thinking I would use it for comparison but it worked so well I never got around to working on the first one again.

author: Reprobyte
author_rank: Regular Contributor
timestamp: May 13, 2023, 03:00:45 am
content: Quote from: greasemonkey on August 31, 2021, 08:38:53 pmHello and thank you very much for your two blogs, I have read them and they helped me get an idea what could be wrong with my own unit.My R29 resistor is brown on the PCB but I have no clue where to find that resistor, did you have a link for that 39kohm 7w ceramic thing? Mine tests ok from what I can see but I dunno.I replaced all the diodes and zener diodes you mentioned in your blogs so I can rule those out. I haven’t changed any caps yet but they all measure fine for capacitance and ESR too.My issue is my power supply just clicks a couple times a second and that’s all that ever happens, the diodes didn’t help.I was hoping you could give me a push to what I should replace next. This is a pretty big repair for me so I appreciate you documenting it!Thanks again!

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 19, 2023, 04:13:30 pm
content: Quote from: Reprobyte on May 13, 2023, 03:00:45 amVery old electrolytic caps (> 30 years) have often high leakeage current and it is wise to replace them even if capacitance and ESR is OK.The 'AWA leakage tester' is a nice TE dedicated to that purpose :https://www.eevblog.com/forum/testgear/i-built-the-awa-capacitor-dc-leakage-tester/Furthermore, old radial electrolytics can leak and spill out their electrolyte which is absorbed by the PCB. This leads to nasty low impedance paths between tracks. Unsoldering the caps allows to see if they leaked and contaminated the board.