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Preface: I know little about electrical engineering, I mostly used a multimeter to diagnose and measure and to make sure I made nothing obvious wrong. But I know nothing about circuits, reading diagrams etc. I'm well aware of the dangers, which is why I want to verify whether I repaired a radio correctly:

I have a Sony CMT-X3CD (CD/Radio/BT stereo) which didn't turn on anymore. With some troubleshooting and the service manual I was able to diagnose that the power board failed. As a spare board is nowhere available I researched a bit and found that I can replace the power board with a generic SMPSU (13.5v, 1.5A, 20.2W) that ends in a regular DC plug. The old power board connected with a four-wire connector to the device's main board.

According to the schematics, it supplied +13v on wires 1 and 2, and GND on 3 and 4:enter image description here

The main board accepted that connection accordingly:

enter image description here

Now, as the power supply has a DC plug with + and - connectors, I use a female adapter that can be connected to two wires. I removed the connecting wire from the original power board, twisted the two 13v wires into a single wire, and the two GND wires as well. The +13v wire went into the "+" port, the GND wire in the "-" port:

enter image description here

This works, the radio powers on again and I can play music again. Before I close it up, I want to make sure that this is a safe fix, i.e. presents no fire hazard or has the potential to blow up. I'm especially unsure about the fact that the original board only supplied +13v (i.e. no explicit -13v connection which I could connect to the "-" connection on the new PSU).

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2 Answers 2

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1 Gold Star for attaching the schematic.

Electrically, you are fine. As long as things are mechanically stable, such that wires don't come loose and touch things, you should be good to go.

There is no "-13v connection". The supply has only one output (you got lucky there), +13 V, and its return, GND. Some supplies label the return as "-something", but for a single-output supply this is incorrect. If the supply had dual, balanced outputs (common in audio power amplifiers) such a +/-13 V, then the GND terminal(s) would be the return for both of them.

To go a bit deeper, because your replacement supply is completely floating, meaning that there is no explicit connection or reference to earth ground, it can be used as either a +13 V supply with the - contact as the return, or a -13 V supply with the + contact as the return. Voltage is a potential difference between two points, and we label those two points positive and negative. But there is nothing in that definition that says which is the "output" and which is the "return". Depending on the application, point A can be defined as more positive than point B, or point B can be defined as being more negative than point A.

The vast majority so single-supply applications operate on a positive supply voltage and GND. BUT, back in the 50's and 60's, early transistor radios operated with a "positive ground". The 9 V battery's negative terminal "powered" the circuits, and the positive terminal was connected to the circuit GND.

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- means ground in this case.

The hazard I see here is more of a mechanical one. It looks like there are some stray wire strands poking out where the wires enter the connector. It would be good to have all the wires solidly inside the screw block. I'd also consider wrapping the whole connection in tape, and/or sticking it down to a certain place. If the radio is moved, jostling around the connector, and either one of the sticking out wire strands, one of the screws, the metal part where the wires push in, or the metal connector barrel touches something it shouldn't, it could create a short circuit and break the radio again.

Alternatively it's possible to remove both connectors, join the wires directly with solder and cover them with heatshrink (which you have to put on the wires before you join them!) - but your way is fine.

Congratulations on your successful repair.

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