A Simple 5V, 12V Power Supply to Fix an Internet Radio

I have a Sanyo internet radio that's about 8 years old. It went on the blink, literally, at first the display was blinking on and off and the radio did not work, then it would not power up at all.

Taking a guess that the issue was the power supply, I opened it up and determined that the internal power supply on a separate small board was indeed the issue. The 5V supply was oscillating slowly between 3 V and 5V and the 12V supply was oscillating between 10 and 12V. It appears to be quite a complex switching power supply with a lot of surface mount devices. Pretty difficult to fix without a circuit diagram or any clue how it works.

This radio has never functioned very well, it would often pause and buffer or loose connection to the internet. So possibly the power supply has been an issue for a long time. Also I noticed some pretty dodgy solder connections, many are not shiny as they should be, but instead sort of crystalline looking as you get with a cold solder joint. Many had big globs of solder on them, so not very expertly done. Given this, my first attempt to fix it was to reflow the solder on all the connections on the power supply where I could confidently do so, given the small size of many of them. That did not work. In the next section see the photos of the existing power supply and the inside of the radio.

This Instructable will show you how to build a simple supply like this that can be used to repair any number of things or power any number of projects.

Step 1: The Existing Power Supply and the Inside of the Radio

In the photos above you can see the small switching power supply pulled out of the radio, as well as some photos of the inside of the radio with some dodgy looking solder joints and what looks like possibly a repair or modification done on the radio pcb when it was manufactured.

Step 2: Circuit Diagram of the New Supply

From the initial investigation I determined that a +5V supply running at about 250 ma and a 12V supply running at about 100ma was what was required by first connecting the radio to two benchtop supplies and measuring the DC current. Likely they used a switching supply for its efficiency, but considering the low current draw this was not a concern for this old radio.

In the circuit diagram, you can see this is a much simpler linear supply which uses a transformer to step down from 120 VAC to 12 VAC, then a bridge to rectify the AC, coupled with 3 x 470 Uf electrolytic capacitors to filter this provides about 20 VDC before regulation. The LM7805 5V regulator and LM7812 12V regulator provide the exact DC voltages required.

Step 3: New Power Supply Build

The original power supply was built on an approximately 1.75 x 2.75 " piece of PCB. Here I used a similar sized pcb, mounted the new parts on it and used point to point wiring and solder to connect the parts. Then drilled holes as required to match the existing mounting posts in the radio. I used a PCB mount ATC-Frost Magnetics FT 1764-2 transformer to limit the size and was easily able to cram all of it into the existing Chassis. If you have plenty of room in your project, you can use pretty much any 120 VAC to 12 VAC transformer with up to a 2A rating depending on your current requirements.

The connections on the FT 1764-2 are a little odd looking as the drawing on the top of the transformer shows two windings for the primary, one inside the other. I tried both of these and they both work at 120 VAC. I used the pin 3 and 4 winding on the primary. I assume the other primary winding is for use with 240 VAC. This transformer is not centre tap and I took the stepped down 12 VAC off pins 7 and 12 on the secondary. There does not seem to be any documentation for this transformer online.

Also, I removed the DC power connector and pigtail along with ferrite ring from the existing dead supply and repurposed this to provide power from the new supply. The original AC Power connector cable along with ferrite ring were also repurposed to feed the new supply.

The label inside the radio notes that the existing fuses should be replaced with the same value, but I could not see any fuses, if they are there, they are well hidden. So for safety, I added a chassis mounted fuse holder by drilling a hole in the radio rear panel and used a 1A fuse.

Step 4: A CAUTIONARY NOTE

This project deals with connecting a circuit to mains voltages at 120 VAC. These voltages can be lethal. Do not attempt this project or one like it unless you know what you are doing and are familiar with connecting to mains voltages.

For safety sake, insulate all 120 VAC connections with heat shrink tubing and never touch any part of the circuit when it is plugged in.

Step 5: Fitting It to the Existing Chassis

Once the supply was built, I wired it into the radio and placed everything inside screwing the new supply firmly in place and cable tying all wires in place.

Turned it on and hey presto it works!

Step 6: Parts List

1 - PCB 1.75" x 2.75" PCB. (Radio Shack)
1 - 1A 5 x 20 mm glass Fuse.
1 - 5 x 20 mm chassis mount fuse holder.
1 - ATC-Frost Magnetics FT 1764-2 120VAC to 12VAC, 500 ma transformer. (Sayal) or other 12V transformer.
1 - 1A, 200V bridge DF02M-2 (Sayal).
3 - 470 Uf 50V electrolytic capacitor.
1 - LM7805 5V, 1A regulator.
1 - LM7812 12V, 1A regulator.
2 - 1 Uf 50V electrolytic capacitor.

Solder
Heat Shrink tubing
Wire