Repairing the Psion WaveFinder (Revision 1.5 - June 2007)

Please make absolutely certain that there is no other reason for this behaviour before embarking on the repair. In particular, you should ensure that it is not a software problem (in particular, it has been reported that the supplied software will not work with Windows XP SP2) or simply that you're not in an area with a usable signal. The excellent WaveFinder FAQ is a primary source of useful information.

You should be aware that this repair information is not really aimed at beginners. The WaveFinder uses surface mount devices which are very small and difficult to solder/desolder without some experience. A temperature-controlled soldering iron with a very fine bit and 26swg low melting point solder are strongly recommended. Antistatic precautions should also be taken. You should also read the inevitable disclaimer before using any of this repair information.

It is also possible that faults other than that described here could occur - these are beyond the scope of this document.

Details

Unfortunately, it is also possible that the analog-to-digital converter (ADC) (a National Semiconductor ADC1173) will also be damaged - in one WaveFinder I have repaired the ADC had a short-circuit between its power rails.

Although I haven't traced the entire circuit, it looks as though the TPS76333 supplies only the ADC and the adjacent 74LCX74 dual D-type latch. I haven't needed to replace a 74LCX74 so far - but to date experience is very limited.

Note that the unregulated 10V supply from the mains unit is switched via a p-channel mosfet which is only switched on when the WaveFinder is initialized, so it is necessary to 'power up' the WaveFinder before any measurements can be made.

Key steps in the repair are as follows:

1. Unscrew the antenna rods.
2. Open case by removing the four T6 Torx screws.
3. Remove the grey plastic plate from the 'foot' by using a flat-bladed screwdriver to release the two plastic clips (be careful not to break them) and then remove the plate by sliding it down towards the cable.
4. Unplug and remove the antenna/LED PCB.
5. Lift out the main PCB by feeding the USB cable up through the casing (you might find it easier to unclip the ferrite core and slide it down the cable first).
6. Desolder (fairly powerful iron needed here) and prise up the two lugs on the small screening can where the USB cable enters. Remove the can and unplug the USB cable. Prise up and remove the lid.
7. Reconnect the USB cable to the WaveFinder, PC and mains unit. Use your favourite software to attempt to receive via the WaveFinder and check whether there is 3.3V present on the capacitor as indicated on the photo below. You might choose to omit this step if the TPS76333 has obviously incinerated itself! If there is 3.3V present on the capacitor, then the TPS76333 probably hasn't failed and you should look for a fault elsewhere. If there is 0V on the capacitor, then check that there is at least 10V on the input to the 8V regulator (at the location shown in the photo).
8. If you have satisfied yourself that the TPS76333 is defective, then (after powering down the WaveFinder and disconnecting the USB cable) desolder and remove it.
9. The TPS76333 can be directly replaced by a National Semiconductor LP2985AIM5-3.3 which comes in the same SOT23-5 package. The ST LE33CZ comes in a TO92 package and would be difficult to fit where the TPS76333 was located. Although not ideal, I positioned the LE33CZ above the 5V regulator. I connected the centre ground pin to the centre ground pin of the 5V regulator, the output to the capacitor and the input to the input of the 8V regulator via a thin piece of wire. I also soldered a 0.1 microfarad ceramic capacitor across the input and ground leads of the LE33CZ as shown in the second photo.

   It would be better to mount it somewhere where it isn't going to be heated up by the 5V regulator, but at least here it is in thermal contact with the lid of the screening can. I also stuck a small piece of tape to the lid so that the LE33CZ leads couldn't accidentally touch the metal lid. The LP2985AIM5-3.3 is a much neater replacement.

10. If possible, connect a multimeter set to measure current in series with the input to the LE33CZ. Reconnect the USB cable and power up the WaveFinder. There should be a current of about 30mA into the LE33CZ. If you cannot measure the current, then check the temperature of the LE33CZ - it should not get very hot. It will be difficult to measure the current into a LP2985AIM5-3.3, but you could monitor its temperature.
11. If the current is around 30mA (or the 3.3V regulator doesn't immediately get very hot) then you could try connecting an antenna and testing to see if anything can be received.

    If the current is very much more than 30mA (over 200mA, say) or the 3.3V regulator immediately gets hot - switch off straight away. You will probably need to replace the ADC1173 but you should check to ensure that nothing else is causing the short.

12. If you need to replace the ADC1173, you may find it easier to cut all the pins off using a scalpel and then desolder and remove them individually.

    Soldering in a new ADC1173 is not easy and I can't really provide much in the way of advice other than to use a very fine bit on a temperature-controlled soldering iron, 26swg low melting point solder and a jeweller's loupe with a bright desk lamp. I positioned the chip using tweezers and then tack-soldered a couple of pins to fix it in place before soldering the rest.

I strongly advise that you replace the standard WaveFinder mains unit with a regulated 12V unit capable of delivering 300mA. The unregulated unit that was supplied with the WaveFinder delivers about 19V off-load and since the TPS76333 has a maximum input voltage rating of 10V, I think it likely that damage occurs when the mosfet first switches on and the TPS76333 is subjected to a voltage well above its maximum rating. Note that the LP2985AIM5-3.3 has a maximum input voltage rating of 16V so the use of a regulated 12V mains unit is also a good idea.

Note that one of the other regulators fed from the same supply as the TPS76333 has a minimum input voltage rating of 10V. I assume that's why whoever designed it specified a 10V supply - unfortunately it should have been a regulated supply and not the cheap unregulated item that was supplied. Incidentally, if your WaveFinder hasn't failed yet then a regulated 12V unit is likely to prolong its life. Even though it's a couple of volts above the TPS76333 rating it will still be much safer than the original unregulated unit.

Parts suppliers and links to data sheets

The ST LE33CZ is available from Rapid Electronics as order code 82-3008. ST LE33CZ data sheet.

The ADC1173CIMTC is available from Farnell as order code 9778179. ADC1173 data sheet.

TPS76333 data sheet.

WaveFinder main PCB logic side with key locations annotated:

WaveFinder main PCB logic side showing LE33CZ with ADC and TPS76333 removed:

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