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Here is my little rant about the power supply:

I think this PS has a design problem, because when I moved it to another place in my desk, plug it again and power on, the input jack short circuit with a screw that holds the transformer. Now it is stock in 37.2 volts and the voltage and current potentiometers does not change the value.  FAIL ¡¡¡¡¡

When I open it, trying to find a fuse, I notice the problem ….. and did not find any fuse  FAIL ¡¡¡¡¡

The “manual” is a 6 page explanation on how to use the PS, very obvious, but I was trying to find a schematic to repair it, with no luck. Third strike. FAIL ¡¡¡¡¡¡

I hope the readers can help me with any suggestion to make it work. I include some pictures. (I am not an expert electronic designer, but I can read a schematic and I can use a multimeter)

Thanks for looking and thousand thanks for any reply

Luis from Mexico

I read a complaint about this particular model, 1803D.  Given how sloppy the parts layout is from your pics, you'd really need to go through the boards and layout to look for issues.  Is this the way the unit is inside, you didn't disassemble the PCBs?  Pic12 there is a PCB near the bottom of the casing beside the AC socket, if AC in on that line, it can be very unsafe.

Look first for cold solder joints, like this fellow did. 

Have you looked at the AC inlet socket?  Most have built in fuse holders. 

FWIW, I have a Mastech too and I posted pics of its guts on another thread.  The construction is first rate, but yours pictured here requires some work.

Thank you for your replay Saturation,

In picture 12, the board above the transformer is the voltage selector 120/240V. I remove it only to have a better picture shot of the "problem point" and to try to find the fuse. The AC socket is in its original location, and I agree with you, it is very unsafe in that place (it result in a total fail ¡¡¡¡)

Unfortunately the AC socket it´s not as the one you show me, and does not have a fuse. I read the link you send me and now I will think twice about buying another Mastech PS to replace this one.

Fortunately I have a backup PS with analog meters and no current potentiometer, but it will do the work

Thanks again

You're welcome.  A lot of folks from RCGroups use Mastech PSU for charging their battery packs.  But it looks like so many factories in China assemble it then sell it as one brand name, so when you buy it its hepful to open them and make sure its not one of the less well made ones.

The power resistors in pic2 are odd, the purple and big white ones; its just a subtle thing but those guys should be standing up with some clearance below it to allow better heat dispersion, also the 2.2k purple wirewound resistor is contacting R7, so if it heats up it a lot it will transmitted more watts to a low watt resistor that isn't made for such loads, not to mention potentially drifting it out of spec.  The power transistor V14 is heat sinked, but lying flat isn't the best way to disperse heat unless there is a large ground plane underneath the PCB.

Why does the IEC socket in pic10 have a whole bunch of plastic on the left side missing? You can see the PCB inside is exposed to the outside.


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