Another Power Supply - Page 1
Forum section: electronics / repair

author: OpenCircuit
author_rank: Regular Contributor
timestamp: March 27, 2017, 10:02:57 pm
content: Stopped work on the other PSU since this one has a problem I can actually observe. One of the MOSFETs on the input side of the DC-to-DC main step-down transformer (red circle) released the magic smoke and had a blown fuse. Replaced the MOSFET (red circle and the adjacent MOSFET) along with the fuse and it repeated itself. I suspect now there is a short somewhere on the other side of the transformer or within the transformer itself (now removed). I did find a short on the two diodes just to the right of the main transformer (annotated in yellow)-will remove from circuit to confirm. There appears to be some thermal damage on D27 above J71. D27 and the two next to it are Low drop power Schottky rectifiers (stps30l45ct). Each Schottky rectifier has a complimenting rectifier on the other side of the aluminum heat sink for a total of 6. No testing on them yet. Was wondering if anyone can explain the purpose of these six? Thanks.Click image once then again on the next page for a full size image:

author: OpenCircuit
author_rank: Regular Contributor
timestamp: March 28, 2017, 12:26:11 am
content: Black diode (D11) read 534/1 when removed from circuit. ZD2 still shorted after being tested out of circuit. Main cap indicates thermal damage where one spot directly corresponds to Th1. D35 also shorted. Seems this PSU is a mess as well.

author: BradC
author_rank: Super Contributor
timestamp: March 28, 2017, 01:42:43 am
content: Quote from: OpenCircuit on March 27, 2017, 10:02:57 pmYou need to make yourself a dim bulb tester and stop needlessly torturing semiconductors.Just get a mains lead, put a bayonet cap socket in series with the active wire and plug in a small wattage lamp somewhat larger in value than the idle current of the device under test. You'll get the lamp flash brightly as the main filter cap(s) charge up and then it'll settle down to a nice dim glow. Of course, if you have a semi-frying fault the lamp will generally continue to glow at or near full brightness, but limiting the current to a value that generally will prevent the smoke coming out.Also good for reforming capacitors on old Valve kit that has been idle for more than a couple of years, and with the addition of a diode, making a "get me out of the poo" battery charger.I have an assortment of lamps from about 5W up to 100W. Much nicer than hitting the power switch and ducking for cover as things explode.

author: OpenCircuit
author_rank: Regular Contributor
timestamp: March 28, 2017, 02:11:46 am
content: Quote from: BradC on March 28, 2017, 01:42:43 amYes, no more tests w/o implementing this light bulb trick and everything else that is faulty.

author: OpenCircuit
author_rank: Regular Contributor
timestamp: April 05, 2017, 10:40:16 pm
content: What to replace Q18 with (lower-middle)? Transistor, but...?

author: rmacintosh
author_rank: Regular Contributor
timestamp: April 06, 2017, 04:36:34 am
content: I believe that's part of a gate drive cct something like the attached...So q19 which is readable looks like the pnp making up that circuit, get the npn variant for q18Looks like the markings on the transistors could be readAlso, looks like zd1 (just to the left) also had a bad day...?

author: OpenCircuit
author_rank: Regular Contributor
timestamp: April 09, 2017, 03:33:51 am
content: Q19, Q17 and Q16 read "K2F Y8"Q14 is "K1P Y9"Last 2 are just the date code.Searching for "K2F diode (MMBT2907A)" I get this data sheet:http://datasheet.octopart.com/MMBT2907A-7-F-Diodes-Inc.-datasheet-7840139.pdfIndicates that "...complementary "NPN Type Available (MMBT2222A)."Go with that?Not sure what happened to ZD1, but I replaced it.

author: OpenCircuit
author_rank: Regular Contributor
timestamp: April 09, 2017, 06:33:45 pm
content: Quote from: blueskull on April 09, 2017, 04:58:56 amPic of PSU is in the first post (800watt SMPS for a desktop computer). Main controllers are the PS229 and a CM6800TX