Repairing a power supply

[russellnash]

I have a power supply that came out of a Samsung Home Theatre, about 10 years old.

powerupply.png

The original problem was that the fuse at 1, removed in the picture, had blown. I bridged it and it blew a hole in the transistor 3.
I replaced the fuse and the Transistor and it then run for about 30 seconds then blew the Thermistor at 2.
I replaced the rectifier and the Thermistor. It then blew the Thermistor again and the Zener diode at 4. I replaced the Thermistor and Zener diode but they both blow after around 30 seconds. I replaced the white ceramic resistor next to the Zener diode, which is connected in series, because it tested as closed circuit. It had R200 printed on it, which I assumed meant it should be 200R, although the R is the wrong way round. All done without load because I didn't want to cause any damage through the wrong voltages coming out of it.
I am running out of ideas what to do next. Anyone see anything or have suggestions?

[sporniket]

All done without load because I didn't want to cause any damage through the wrong voltages coming out of it.

There exists electronic load, you should get one.

Also, do you have a schematics of the supply ? Seems to me you poke in the dark.

[russellnash]

That is correct, I am poking in the dark. No, there is no schematic, that would be too easy. I am assuming that not connecting a load wouldn't cause the symptoms that I am getting due to too much current running through it, but i don't know that. Initially I found short circuits, but after eliminating them it still doesn't work properly. I have never tried to fix a power supply before, but it is worth a try and it is always entertaining when components go bang.

[6502addict]

I found some test load on aliexpress not a professional tool but something very cheap around 25€
I used it to build the power supply for my SWTPC

can also be found on amazon: https://www.amazon.co.uk/Constant-Curre ... B08HM2LBCD
it"s not the exact model I'm using

[rubber_jonnie]

I've come to this thread a bit late, but my advice would be to never ever bridge a fuse. They are there for a reason and for safety. Sure the transistor blew but it could have been a lot worse.

Power supplies can be really dangerous if you don't know what you're doing (Ask me how I know) so if you are unsure, don't work on them, find somebody to do the work on or replace the PSU with something else suitable.

If you're in any doubt have a look at the main bulk capacitors, they're probably rated at 400v (If you're in the UK as I am), which is why I have a capacitor discharge tool and if working on a PSU it's the first thing I reach for.

If you do carry on working on it, my advice is to test everything such as transistors, caps, rectifiers etc even before you apply power. This may involve removing them, but some you can test in place with a multimeter in diode mode.

All I'm saying is take the approach of safety first with PSUs.

[6502addict]

to avoid having to repair power supply I often rebuild one from a 19.5v 6A portable computer power supply
and LM2596 chinese modules for the positive and I invert with ICL7662 for the negative....
most of the time the negative only require a low intensity for serial ports
this way I never have to touch components connected to 230v

that's how I build my SWTPC power supply and it works fine
I can send from 17v to 30v and the output are really stable
+8V / + 15v / -15v

I also created a power supply to power commodore 720 motherboard
this board also require a 50 or 60 hz signal produced by an avr tiny

[russellnash]

Thanks for the replies. I haven't touched it for weeks, but I did get it to not blow up any more. Such things for me are quite funny as long as you don't really care about what you are working on. I wouldn't want to blow a hole in an ST, for example. When I started I just thought it might be worth a look, it could be something simple like capacitors or maybe just the fuse. I have had many electric shocks, mostly 230 V AC, which I find usually no big deal, but a couple of times 500V DC. That really hurts. :D
My house has a PEN electrical system and when I first arrived I kept getting the typical 120V AC electric shocks from the dishwasher and tap because the protective earth in the socket it was connected to was not bridged to neutral. It didn't really hurt, but it was enough to notice.

Power supplies can be really dangerous if you don't know what you're doing (Ask me how I know) so if you are unsure, don't work on them, find somebody to do the work on or replace the PSU with something else suitable.

I agree. How do you know?

If you do carry on working on it, my advice is to test everything such as transistors, caps, rectifiers etc even before you apply power. This may involve removing them, but some you can test in place with a multimeter in diode mode.

I did exactly that.

to avoid having to repair power supply I often rebuild one from a 19.5v 6A portable computer power supply
and LM2596 chinese modules for the positive and I invert with ICL7662 for the negative....
most of the time the negative only require a low intensity for serial ports
this way I never have to touch components connected to 230v

If you could maybe give me some more information on how you build that some time, that would be interesting.

[rubber_jonnie]

I agree. How do you know?

I have had many shocks over the years, but the worst of them have them have come from power supplies, notably bulk capacitors. That's why I now own a capacitor discharge tool.

I did once as a child get a mains shock at 240V and that was how I learned to fly. I also blew all the fuses in my old workshop when I learned that accidentally bridging the pins on a Mac Classic mains bulk capacitor with my hand was both painful and something I'd rather not repeat.

There are other instances, but I have to keep some back for later. Our hobby can be slightly terrifying at times.

[6502addict]

This is the power supply I made for my SWTPC 6800
not an original but a rebuild from peripheraltech
it's almost identical except for the 1Mb RAM build with 2 alliance 512k chip

a real SWTPC is consuming around 10A on 8V

a SWTPC 6809 with the custom memory board uses less than 2A on 8V

I've chosen a 19.5v 6.15A portable computer power supply
the red led indicates that the 19.5v is present

I use 2 chineses LM2596 Module 1 to produce 8V and the other one to produce 15V
BEWARE that you have to tune the LM module to produce the right tension
uses a power resistor to tune the modules

never connecte the power supply to the final computer
without having tuned the LM2596 and fully tested the stability

in my experience then output tension are stable from 17V to 30V at the entry
for the -15v I' m using an ICL7662 to invert the +15V
they are stackable with my board I can put up to 3
for the SWTPC only one was necessary -15v is only used by the serial port

I added a few leds to be sure that the output tension were present..
green led 8v
yellow led +15v
blue led -15v

Screenshot 2025-03-17 092057.png

[russellnash]

Thanks for the information.