Only somebody who has had suitable training should undertake this repair.
The Toshiba RD100KB is a tv freeview tuner with a 320 GB hard disc and a DVD recorder built in. This is not a HD unit.
Just after the 12 months warranty ran out, the unit failed. The fault symptom was that you could not turn it on. On applying mains power there was a click followed by another click a second later.
I was not keen to scrap it especially when it has some recorded programs on the hard disk that had not been viewed. After having unplugged everything, including the mains supply, I removed the cover. I could see the power supply area and this had extra plastic safety shielding over parts of it.
Further disassembly revealed a complex power supply arrangement with multiple output dc voltages. I could see that one section of the power supply was permanently connected to the mains input and this produced a 5 volt rail. Pressing the “power on” button caused a relay to close a pair of contacts on the power supply board and produce the multiple dc outputs require for the logic and various devices. As I said the relay only closed the contacts for a moment and then dropped out again. I decided to entirely remove the psu board from the unit and this meant disconnecting quite a few ribbon cables and some mechanical disassembly. Once I had got the psu board onto a flat and non conductive surface, I applied a link to short out the relay contacts. This meant that mains would be applied to the main power supply and I could chase down the fault. It seemed that the power supply would only work properly when the other bits were connected as a load. However I was able to determine that the 12 volt rail was missing with everything connected.
After breaking everything down again, and with the mains supply unplugged , I probed around the 12 volt supply components looking for short circuits. Indeed there was a short circuit. I removed a capacitor as they are the usual suspects, but this was not the problem. Cutting a track threw suspicion upon a couple of Schottky diodes that were in parallel. I de-soldered the end of one and lifted it. The short circuit disappeared. This particular diode was a short circuit. The diode is a SK3B and is D2204 on the board. I tested the board again with just the remaining diode in circuit. It was now working. I re-assembled everything and removed the link from the power supply board.
A functional test showed everything in working order. The Schottky diode has a 3 amp rating and as there were two in parallel they must be expecting currents of up to 6 amps. For this reason I ordered some replacement diodes so that I could change them as a matched pair.
It was pleasing to get this unit working again. Modern electronics are usually reliable but there do seem to be a lot of fake components around these days.
This experience makes me wary of Toshiba products. It was certainly not designed for ease of servicing and I could not get hold of a circuit diagram.
Personally I prefer to see a proper mains earth for the metal case.
Peewit
