I need help...QW-MS305D - Page 1
Forum section: electronics / repair

author: myster1086
author_rank: Contributor
timestamp: October 24, 2018, 02:13:36 am
content: OK I have a cheap adjustable power supply. Qw-ms305d is the model.When I power it up, it jumps to 62.8v, really does not a display error. I have it opened up but haven't done anything because frankly I'm lost as to what it could be. The dials are reading correctly when I test with my multimeter. Everything seems fine. When it spikes it dwindles down to 0, even unplugging it the voltage creeps. Can anyone help walk me through fixing this?

author: cnqhdszq
author_rank: Regular Contributor
timestamp: October 24, 2018, 03:27:38 am
content: Check the main output regulate transistor first ,then the IC who controls the transistor . My English is not good . Hope you can understand !The PSU is not good ,to look for a better one .

author: myster1086
author_rank: Contributor
timestamp: October 24, 2018, 08:14:34 am
content: I know its not good, far from it. But it was a gift and to put it to good use would be awesome.From what I gathered check the main output transistor, then the ic. Sorry I know I should know all this thanks for your time

author: myster1086
author_rank: Contributor
timestamp: October 24, 2018, 02:05:46 pm
content: So I realized I an no good at testing I get confused by the terms used on different sites. My meter in diode mode buzzes with pins 1-3 not 2-3 or 1-2. I'm uploading a bunch of pictures. Maybe that would help

author: cnqhdszq
author_rank: Regular Contributor
timestamp: October 24, 2018, 03:31:10 pm
content: You should cut the wires open link the main transistor .and where are the pictures?

author: myster1086
author_rank: Contributor
timestamp: October 25, 2018, 06:20:03 am
content: Oh shoot they never uploaded?? I hate chrome on Android.I'll put them up with this comment sorry.I have a bunch of old, can't Tell if old old or just old, dell sever PSU. They have a ton of components. Currently the lm324n is what's on the adjustable unit, and the dell has lm339n as far as I can tell they are the same, slight differences in speed but idk if that matters here in this case? Can I swap them for each other? Basically looking thru all old boards to see if I have parts to replace or even rebuild, maybe into a better safer unit? The other power supplies are 1999 astec ac/dc 263w units. I'll upload a few snaps as well. Going to hop on computer and add in 5 minutes

author: myster1086
author_rank: Contributor
timestamp: October 25, 2018, 06:41:27 am
content: https://drive.google.com/open?id=1PkAF2Kqg5KDX2Y9FiLL0mWvFFxWF8dkZwont let me upload, heres a drive link i uploaded all there no need to download you can view with link

author: cnqhdszq
author_rank: Regular Contributor
timestamp: October 25, 2018, 09:54:04 am
content: There are no Google's services in China . so I can not see the pictures.324 is quad operational amplifiers;339 is quad voltage comparators;They are not the same chip at all . You can neither replace a 324 with a 339 nor replace a 339 with a 324 .Can you speak/read Chinese ? if you can ,we can "talk "in Chinese .

author: cnqhdszq
author_rank: Regular Contributor
timestamp: October 25, 2018, 09:58:35 am
content: 你首先测量调整三极管的电压，E C B三个电极间电压，正常情况下Ube 约0.6V Uce应该在几伏到几十伏之间。确认调整三极管正常后再检测主控芯片是否正常。每一部分都应该先查供电电压，再查二极管 三极管等半导体器件，最后是电容 电阻 。

author: tpowell1830
author_rank: Frequent Contributor
timestamp: October 25, 2018, 10:15:36 am
content: Quote from: cnqhdszq on October 25, 2018, 09:58:35 amGoogle translate:You first measure the voltage of the trim transistor, E C B between the three electrodes, under normal circumstances Ube about 0.6V Uce should be between a few volts to tens of volts. After confirming that the trim transistor is normal, check whether the main control chip is normal.Each part should first check the supply voltage, then the semiconductor device such as the diode transistor, and finally the capacitor resistance.

author: myster1086
author_rank: Contributor
timestamp: October 25, 2018, 10:19:27 am
content: feel free to email Google can translate. i had a friend in Korea debug a midi for a program he makes. then i could send pics idk why i can't upload. bobbyp.freely same as drive, gmail

author: myster1086
author_rank: Contributor
timestamp: October 28, 2018, 04:51:00 pm
content: OK how about I take the lm324N, the two irf840, ka7600b, p75nf75, and the th1602ct and replace them all with new ones. Ideally that should fix it right?

author: coromonadalix
author_rank: Super Contributor
timestamp: October 28, 2018, 08:13:40 pm
content: No it may not repair your unit, if you dont do a proper diagnostic of the problem(s) you may end with more problems by changing tons of parts for nothingNormally smps supplies fail at the main mosfets and pre regulator sections ...Notably the irf840 and related resistors may have blown, normally the p75nf75 should be ok, i hope (this one can be costly to get)

author: myster1086
author_rank: Contributor
timestamp: October 30, 2018, 04:08:33 am
content: i have 1 of 2 irf840, the lm324n, and p75nnf75. but if its likely the irf840, i should have many many more, have a 10lb box of transistors. i can try those first, any others that have parameters nears the same i should look for? say i dont have another 840?

author: coromonadalix
author_rank: Super Contributor
timestamp: October 30, 2018, 10:09:36 am
content: Inner capacitance, rds, wattage, voltageThe rds is very important, usually the lower the better, it will affect the disspipated heat, if its an irf840, put back an irf 840, its been designed wth that part.Sometimes you have some surrounding resistors between 20 to 100 ohms for polarization check them if they are burned ?I have one psu who blown many parts, diodes, Irf840, polarization resistors, beceause of shitty build quality, still under repairs even with schematics, something broke elsewhere ?I Ebay bought and received an 70v 5 amps linear psu to replace /substitue the switching section to see if the rest is still working ?Remove the transistors irf840 and p75nnf75, test them outside the circuit ... i have been played trick because of the drive circuit impedances ...

author: myster1086
author_rank: Contributor
timestamp: October 30, 2018, 06:19:45 pm
content: Tested the Irf840 are good its the p75nf75 and th1602ct that test out. I an looking now at a surplus store with everything

author: myster1086
author_rank: Contributor
timestamp: October 30, 2018, 11:40:16 pm
content: no go on the replacement, guess they have everythung but what i needed...anyone got a good site to search a chip and suitable replacements?

author: coromonadalix
author_rank: Super Contributor
timestamp: October 31, 2018, 12:06:47 am
content: your th1602ct is : ssth1602 ct ultra fast recovery diodehttps://www.st.com/resource/en/datasheet/stth1602c.pdfmouser :https://www.mouser.com/ProductDetail/STMicroelectronics/STTH1602CT?qs=EYQuxaDwR79f08%2FpXfZzUw%3D%3Dp75nf75 metal tab or plastic tab, this one is a hard to get, no substitutes that i know, big lead timemouser :https://www.mouser.com/ProductDetail/STMicroelectronics/STP75NF75?qs=sGAEpiMZZMshyDBzk1%2fWi5%252bqLKYs9z%2fzYMqFvhS6cG8%3dormouser :https://www.mouser.com/ProductDetail/STMicroelectronics/STP75NF75FP?qs=sGAEpiMZZMshyDBzk1%2fWiwFIPwWo0IAPPilL8yaD3zk%3d

author: myster1086
author_rank: Contributor
timestamp: November 01, 2018, 08:46:19 am
content: I found both on mouser as well no lead time. Its metal the p75nf75. Only 12 for the pair that's including shippingI'll update when they arrive and I swap

author: NeilFraser
author_rank: Newbie
timestamp: April 27, 2020, 07:48:07 pm
content: My QW-MS305D power supply failed in the exact same manner. I ordered replacements for all four heat-sinked semiconductors from AliExpress for $3.37. Replacing the P75NF75 is what did the trick. I posted a description and video here:https://neil.fraser.name/news/2020/04/26/

author: myster1086
author_rank: Contributor
timestamp: April 28, 2020, 03:21:26 pm
content: Same here I actually got 5 of each free from the manufacturer of the components. Still going to this day, although I swapped out knob for current, now its either full go or not. 50k was original 410k newer, trying to find a 50k that fits