The marking on the resistors is R960 which mean 0.96 ohms; 3 in parallel gives 0.32 ohms. I suspect that the MosFET has gone short which has then vapourised the resistors.
Ah! That makes sense, as the R indicates the decimal position on bigger resistors - so this one starts at 0.xxx? I've only ever dealt with resistors above 1ohms, so these are new to me. Thanks for the knowledge!
@@tychosis fused input doesn't matter. Main filter capacitors hold enought energy to blow up both current sense resistor (resistors) and switching transistor. It short circuit it still there, a fuse should blow (or circuit breaker -open) then.
I noticed that you touched the tops of the e-caps. I already knew that e-caps can hold dangerous voltages, for extended periods. But, I have just recently learned that they can sometimes be wired in such a way that their bodies/exposed tops are conductive. I always thought that you had to touch one of its pins, pads, or traces, but that is not always the case. Do you have a "cap stinger?" It is used to discharge caps. They are easily DIY'd from a bamboo chop stick, a resistor, a header pin, an alligator clip, a piece of good wire and some heat shrink. YT has lots of these projects. They are a fun, quick, easy build and you end up with a really useful bench tool.
On camera I use a screwdriver and just short the pins because sparks make content (such as in my recentish Xbox Crystal repair ua-cam.com/video/ThBoMuv_7xM/v-deo.html) If I do need to get into something I've just unplugged though, I'll use a collection of chonky wirewounds to bring them down safely. Good call out though! This unit had been disconnected for a couple of weeks at time of recording, so I wasn't fearful of any zaps... this time...
That Mosfet is likely used for regulation, there are usually 2 sets of resistors used to fix the output voltage, one set is what you found as charcoal, the other is the potentiometer on the panel. Good channel and now I'm subbed😁
Interesting! The other set must be controlled somewhere else, as the panel uses a rotary encoder and I'm assuming some kind of smarts rather than a regular pot. Damn all these interesting comments make me want to tear it back down and take a better look!
Rest assured, nothing goes to waste in my home! I'll salvage anything useful, though to be honest I didn't think about salvaging the case... Great idea, surely able to put something fun in here! Would make a nice slimline desktop amplifier for my speakers 🤔
@@silvestronsbitsandbytes Great thinking... Expanding on your idea... That case appears to be about 12" long. That would be about right to house a pair of small speakers to make a stereo computer sound bar. The case already has some controls. The UA-camr JohnAudioTech has a great little LM386 audio amp circuit. It is super easy and has amazing performance. Power it with a transformer-based wall wart. Make sure the wall wart has voltage regulation (meaning its voltage does not droop under a load.) If not, you could put a LM780x circuit in the case, as well. Just spitballing and sharing. Oh, one last tip... V regs may have a dropout voltage as low as 1 volt. However, they require 3 volts of overhead to attain their best ripple rejection performance. A close read of the data sheets and app notes is necessary to catch this detail. Most EE hobbyist aren't aware of it. I wasn't. Thanks for all you do for the EE community.
Interesting, i can't say i've ever seen a "cheap" variable PSU with an APFC boost section before, which is what has failed here. Odds are, if the transistor has blown, so has the controller IC and any components (usually resistors) connected between it and the transistor, and the fact that the transistor has blown could be caused by the three primary side capacitors drying up, as APFC circuits put a lot of strain on those capacitors, and unless they're using good quality, low impedance capacitors (which doesn't appear to be the case here), they can dry out quickly and fail. Now, theoretically, rather than trying to repair the APFC boost section, you could bypass it and possibly run the PSU without it, i say theoretically because while some PSUs will work fine without this section, others will not.
I really know nothing about this type of power supply, so it's really interesting to see such a detailed response! I am furiously googling APFC 😅 I also don't really have much interest in fixing this one up... it's been getting on my nerves for quite a while - bad rotary encoders for the voltage selection make it hard to dial in settings, the only way to configure current limit is by shorting leads then changing and so on. I've got an Owon SPM6103 now which is much nicer and fits my requirements of easy to use and small, but also a lot more powerful!
The marking on the resistors is R960 which mean 0.96 ohms; 3 in parallel gives 0.32 ohms. I suspect that the MosFET has gone short which has then vapourised the resistors.
Ah! That makes sense, as the R indicates the decimal position on bigger resistors - so this one starts at 0.xxx? I've only ever dealt with resistors above 1ohms, so these are new to me. Thanks for the knowledge!
Resistors became fuses. (Do these benchtop power supplies typically have a fused input? I can honestly say I don't think I've ever checked mine...)
I completely agree, the resistors saved the rest of the board and I'm sure that's why they were put in place. MOSFET failed first
@@tychosis fused input doesn't matter. Main filter capacitors hold enought energy to blow up both current sense resistor (resistors) and switching transistor.
It short circuit it still there, a fuse should blow (or circuit breaker -open) then.
I noticed that you touched the tops of the e-caps. I already knew that e-caps can hold dangerous voltages, for extended periods. But, I have just recently learned that they can sometimes be wired in such a way that their bodies/exposed tops are conductive. I always thought that you had to touch one of its pins, pads, or traces, but that is not always the case. Do you have a "cap stinger?" It is used to discharge caps. They are easily DIY'd from a bamboo chop stick, a resistor, a header pin, an alligator clip, a piece of good wire and some heat shrink. YT has lots of these projects. They are a fun, quick, easy build and you end up with a really useful bench tool.
On camera I use a screwdriver and just short the pins because sparks make content (such as in my recentish Xbox Crystal repair ua-cam.com/video/ThBoMuv_7xM/v-deo.html)
If I do need to get into something I've just unplugged though, I'll use a collection of chonky wirewounds to bring them down safely. Good call out though!
This unit had been disconnected for a couple of weeks at time of recording, so I wasn't fearful of any zaps... this time...
"I can't make out what they WERE" is probably more accurate, lol
💯 definitely cratered 😅
That Mosfet is likely used for regulation, there are usually 2 sets of resistors used to fix the output voltage, one set is what you found as charcoal, the other is the potentiometer on the panel.
Good channel and now I'm subbed😁
Interesting! The other set must be controlled somewhere else, as the panel uses a rotary encoder and I'm assuming some kind of smarts rather than a regular pot. Damn all these interesting comments make me want to tear it back down and take a better look!
At 8:53 there is an 8 pin IC on the left hand side and a HUGE blob of solder possibly shorting pins 1 and 2.
Wow you're right.. that's so shoddy! There's some splatter to the left of that as well... and it's been working for years like that! Great spotting :)
The mosfet probably shorted and took out the resistors.
It's very possible, I didn't check the MOSFET for shorts at all... I should do that!
Well, don't throw it away... Salvage the components and repurpose the case...
Rest assured, nothing goes to waste in my home! I'll salvage anything useful, though to be honest I didn't think about salvaging the case... Great idea, surely able to put something fun in here! Would make a nice slimline desktop amplifier for my speakers 🤔
@@silvestronsbitsandbytes Great thinking... Expanding on your idea... That case appears to be about 12" long. That would be about right to house a pair of small speakers to make a stereo computer sound bar. The case already has some controls. The UA-camr JohnAudioTech has a great little LM386 audio amp circuit. It is super easy and has amazing performance. Power it with a transformer-based wall wart. Make sure the wall wart has voltage regulation (meaning its voltage does not droop under a load.) If not, you could put a LM780x circuit in the case, as well. Just spitballing and sharing. Oh, one last tip... V regs may have a dropout voltage as low as 1 volt. However, they require 3 volts of overhead to attain their best ripple rejection performance. A close read of the data sheets and app notes is necessary to catch this detail. Most EE hobbyist aren't aware of it. I wasn't. Thanks for all you do for the EE community.
The R at the start may suggest 0.91 ohms.
Interesting, i can't say i've ever seen a "cheap" variable PSU with an APFC boost section before, which is what has failed here. Odds are, if the transistor has blown, so has the controller IC and any components (usually resistors) connected between it and the transistor, and the fact that the transistor has blown could be caused by the three primary side capacitors drying up, as APFC circuits put a lot of strain on those capacitors, and unless they're using good quality, low impedance capacitors (which doesn't appear to be the case here), they can dry out quickly and fail.
Now, theoretically, rather than trying to repair the APFC boost section, you could bypass it and possibly run the PSU without it, i say theoretically because while some PSUs will work fine without this section, others will not.
Its only 81W output too, super weird to have the huge PFC section. But it is not cheap and in house brand (jaycar MP3842).
I really know nothing about this type of power supply, so it's really interesting to see such a detailed response! I am furiously googling APFC 😅
I also don't really have much interest in fixing this one up... it's been getting on my nerves for quite a while - bad rotary encoders for the voltage selection make it hard to dial in settings, the only way to configure current limit is by shorting leads then changing and so on. I've got an Owon SPM6103 now which is much nicer and fits my requirements of easy to use and small, but also a lot more powerful!