Very dodgy fuseholder

[BurningTantalum]

I found a Yihua-branded 15V 2A bench PSU in a bin. It seemed untouched, but the 1A mains fuse was blown and the transformer primary measured 8 ohms. It powered up OK on a bench supply so I replaced the transformer. The mains lead had been cut off so I replaced it- there was the usual cheapo Chinese single pole mains switch switching the neutral, which is a ploy to enable the termination of the mains neutral somewhere; the live being terminated on the fuseholder. As I eased the heatshrink off the end terminal of the fuseholder, the whole contact just fell out of the back of the holder. I'm not clear how the thing was supposed to be held in, and a coil spring fell out other end of the holder. The contact is made of a very thin tinplate, as are the sprung 'fingers' in the free cap, which have no pressure to hold the fuse at all. I replaced it with a Bulgin out of the parts drawer and all is well.
I thought it was worth a warning.

The PSU itself, Yihua 1502USB, is particularly horrible- with a single 2N3055 on the back panel. It does not deliver anywhere near its claimed output current at 15V, and there is a weird 'RF meter' bar graph to test mobile phone signal strength that I have not managed to get to the bottom of.
BT

[Ian.M]

Hmm, an image translate service gives:

Short-circuit automatic reset

USB 5V safe voltage limit

mobile phone RF display

for the Chinese text next to the ammeter.

I expect the intended market is 'hole in the wall' kiosk phone repair shops, and the bar graph is a simple RF amplitude display to aid fault finding.

While you've got it handy, some photos of both sides of the PCB and the general interior layout would be nice.

The original fuseholder doesn't inspire confidence, especially if you use the back contact for incoming line as one is supposed to, (to minimise the touch hazard when the cap is removed.

[BurningTantalum]

Here are some pics of the interior. One chip is a PIC; I licked my finger and rubbed the top of the 14-pin DIP to clean it and the writing disappeared !
I must lay off the gin whilst working.
It was only when looking at the pics now that I spotted what is probably an antenna in the layout near the bar graph. I held a communicating mobile near to the unit but the bar graph didn't budge. I am not at all clear what the label on the top of the case means.
The metal case is so weedy that when I drilled the mounting holes for the new transformer, I placed the case over a slip of timber in a vice and then smacked the end of the drill with my palm in a lazy man's centre pop. The drill bit went through the soft steel- 0.8mm including 2 layers of paint.
I am certain that it never worked from new and was tossed as there is no 'warranty return' for this type of item. I pulled the dry 'E & I' plates out of the transformer and the primary looked OK. I pulled off a few metres of wire until I lost interest but it looked OK.
Of course I cannot throw it away so it will live on the shelf.
BT

[Ian.M]

Yep, that's definitely on the cheap & nasty side, although its probably not the worst that has been seen here.   I guess the PIC's just for voltage readout and the bargraph, and suspect the pot is wired to control an analog regulator.   I could be wrong - if the pot wiper goes to the PIC its a clear indication its in the control loop.

Under 0.8mm of steel certainly isn't providing any worthwhile heatsinking to the 2N3055, so I would expect it to get excessively toasty at sustained full load and low output voltages.

I *really* don't like the lack of separation between the low voltage  and the mains wiring.  Sleeving the wiring to the 2N3055, + dressing the neutral wire and the line wires to the fuseholder and power switch with a few cable ties so they cant get near the rest of the low voltage wiring would be advisable.

[rsjsouza]

If the collector of the 2N3055 is electrically tied to the chassis, be very careful with ground loops. I blew a motherboard and a very expensive development kit this way.

It really does not seem to be a great power supply and I would be cautious in using it due to the hidden corners that were cut in manufacturing but are not evident, such as flammability of materials and parts, or unreliability of the main bypass transistor (a pass through fail could be fatal to the ewuipment connected to it)