Strange PSU noise - intermittent startup failure (with vid) - Page 1
Forum section: electronics / projects

author: nexus
author_rank: Regular Contributor
timestamp: September 11, 2021, 03:10:07 am
content: Hi all,Continuing on the path to fixing an E4446A spectrum analyzer. It has a strange power supply condition where the first 1-2 times you plug it in, it makes a squealing noise, and no standby light comes on the front panel. Pushing the mechanical power button does nothing.After unplugging and re-plugging it in a few times, it the noise suddenly goes away and the unit operates normally.Of course I've already got the PSU out on the bench. Going to desolder and inventory all the bits I want to replace (movs, electrolytics, tantalums, X/Y mains filter caps including RIFAs... think the ceramics should be fine, no carbon resistors that I can see...).Desoldering the 4 daughter boards in this thing was fun. The 2.54mm headers were basically press fit the pth drill was so small. Had to resort to the messy solder blob technique because no amount wicking would get the holes perfectly desordered. Even that just barely worked to free the boards.Thought I'd post about it with a vid here too. Maybe someone is familiar with strange power supply noises and can tell what the issue is just from listening. I suspect its a flaky capacitor that reforms after applying power several times.Full repair vid to come after its all buttoned up of course.For anyone who is curious, power supply is Keysight part #0950-4447. Cost from Keysight is USD $3,457 new.https://www.keysight.com/my/partDetail/0950-4447Used working on ebay is around $1,300 to $1,500 usd. and it is used in the following instruments:E4440AE4443AE4445AE4446AE4448AN8201AEnjoy.

author: moffy
author_rank: Super Contributor
timestamp: September 11, 2021, 06:43:11 am
content: Is there an HV section to the power supply? If so try disabling it.

author: nexus
author_rank: Regular Contributor
timestamp: September 11, 2021, 03:31:54 pm
content: There does not appear to be any HV sections in the supply. All the outputs to the analyzer are low voltage DC, the highest being 32 volts.The highest internal voltage is probably the rectified mains, after which everything is regulated down. The strange part about the noise is that it "winds down" after unplugging the mains, as though proportional to some kind of internal capacitive discharge.It looks like most of the switching controller action is on the daughter boards that I desoldered. The controllers of choice appear to be the SG3845D. The service manual says the supply switches at 121.95 kHz, and the SG3845D datasheet indicates that freq. is adjustable up to 500 kHz.Hoping that a refresh with caps and other passive will be the trick. Remember that the supply works, just not the first 2 times you plug it in cold.

author: moffy
author_rank: Super Contributor
timestamp: September 12, 2021, 08:46:51 am
content: Wierd. Replacing the electrolytic capacitors is always a good start especially any very low values like 1u.

author: jmelson
author_rank: Super Contributor
timestamp: September 12, 2021, 03:52:23 pm
content: Yup this supply likely has a high value resistor from the 340 V DC input that charges up a cap to run the control IC, and then that resistor is disabled by a transistor and an auxiliary winding on the converter transformer powers the controller after that. Most likely a small cap, like 10 uF doesn't have enough capacitance left to make the transition over to running from the transformer. So, replacing the smaller caps is likely to fix it.Jon

author: nexus
author_rank: Regular Contributor
timestamp: September 13, 2021, 05:12:26 pm
content: Got all the electrolytics (all 39 of them) dropped out. Also replacing the RIFAs (all 5 of them) and other X2 film caps, NTC thermistors, MOVs, main fuse, solid tantalums, and mains input EMI filter receptacle.To my surprise, lots of the caps are third rate brands. This is a 2004 vintage supply (made in Korea), so maybe Sam Young (Korea) used to be good... I see a lot of mixed opinions about them. Most caps are Sam Youngs and a few are Chinese (Pilkor). There are a few Nichicons, but far and few.Needless to say all replacement parts will be top shelf high reliability, high temp, low ESR Japanese (Nichicon, Rubicon, United chemi) and American capacitors (Kemet, Vishay) new from Digikey.

author: wraper
author_rank: Unknown
timestamp: September 13, 2021, 05:32:05 pm
content: QuotePilkor is Korean and was a subsidiary of Phillips until gaining independence in 1994. Also it's one of the largest capacitor manufacturers.

author: nexus
author_rank: Regular Contributor
timestamp: September 13, 2021, 05:57:41 pm
content: ah okay. I said Chinese because I stumbled across this website when searching the part # online. Also looks like a cap manufacturer named "Pilkor" but in Beijing:http://www.pilkor.net/pilkor20210909/index.html

author: nexus
author_rank: Regular Contributor
timestamp: September 19, 2021, 12:59:50 am
content: The digikey with all new psu parts arrived. In total, replaced the following tally:39 - assorted electrolytics5 - RIFA Y2 paper caps5 - Solid tantalum capacitors2 - X2 polypropylene caps3 - NTC thermistors1 - MOV1 - carbon resistor1 - cracked wirewound resistorSchmood it up to factory spec with Dowsil 737 neutral cure RTVAttached BOM if anyone is curious.Also replaced:1 - input EMI filter/receptacle1 - input fuse (just to be thorough)After many hours of careful assembly, soldering, cleaning, and more assembly... it works.No more startup problems. Powers straight up to standby and turns on normally.One less power supply in the landfill and a happy spectrum analyzer.