First of all, a few caveats.
This instructable shows you some ways to repair a Series 3 TiVo PVR. However, anything you do is at your own risk - at the very least, you're going to void your warranty, and I can't take responsibility for anything you do. Please be confident that this is within your ability before starting; there exist companies (in the USA) who will happily take your money and do these tasks for you if you feel unable. However, if you are confident using a soldering iron and plugging in a cable, then you might be able to fix your broken TiVo for less than $3 in parts.
Next, remember that there is HIGH VOLTAGE inside the TiVo, and power supplies can retain capacitance for several minutes. Unplug the device before you open it. Be VERY careful about what you touch on the power supply - even when unplugged, as it can retain charge for some time after being disconnected.
The photographs are for a Series 3 Australia/New Zealand model, though they are likely very similar to the US Series 3 and other models. The steps involved in fixing those models are very similar, but components are located differently and the software is different, so be aware of this.
The instructable deals with the two most common causes of failure -- the Power Supply, and the Hard Disk. Other things may potentially fail but they are not addressed here.
Tools you will need:
T10 Torx screwdriver - to open the case and remove hard disk
T8 Torx screwdriver - to remove power supply
Electrical voltage tester - to test power supply. Not essential but it helps.
Soldering iron and solder - to replace failed capacitors
Solder removal tool or solder wick - to replace failed capacitors
Desktop PC with SATA disk controller, and sufficient cables to connect to additional drive - to repair hard disk
winmfs.exe software - to repair hard disk. Get it from here
Replacement parts you may need:
Replacement capacitors ( 4 x 2200uF 16V and 2 x 47uF 50V capacitors )
Replacement SATA hard disk, at least 360GB or up to 1TB, NAS-rated or PVR-rated.
UHF signal booster (if problem with MPG artifacts and signal)
Step 1: Identify the Problem
What is wrong with your TiVo? This might help identify where we should start.
Q. Does the TiVo power on at all? When switched on, does the 'Starting up' messages appear?
If NO, then the power supply has failed completely. Fix it or replace it.
If YES, continue.
Q. When powering up, does the TiVo reach the 'Almost there' screen?
If NO, then it may be a power supply problem, or a disk failure. Listen very carefully to the power supply (DO NOT open the box to touch it!). Is it making a 'tick-tick-tick' sound? This is a strong indication that it is a power supply capacitor at fault. If not, it may still be the power supply, or maybe a disk failure. You can test the power supply using a multimeter if you have one - it should be providing 12V and 5V to the motherboard - if you see 4V, this indicates a capacitor failure.
If YES, continue.
Q. Does the TiVo reach the initial TiVo video, but then crash either during the video, or a little later?
In this case, it is a disk problem. Either the disk has failed, or has failed parts, or is corrupt. If you are lucky then you might be able to take a backup before it fails entirely (see later); else you'll need a fresh image.
Q Is the startup fine, but you have problems with the signal being lost; sound being lost; or too many MPEG artifacts on screen?
In this case, it might be that your UHF signal is too weak. The TiVo needs about 80db of signal and often you just don't get enough. In this case, install a powered aerial signal booster between your aerial and TiVo input; these can be cheaply bought from any Audio/Visual shop.
Q Do you have problems that occasionally the TV will not have any output form the TiVo and just a blank screen? Switching the TV off and on again can sometimes correct the issue.
The TiVo puts out a very weak HDMI signal but some older TVs require a stronger one. In this case the TV will simply not recognise the signal and will show a blank screen. You might try changing to using the component video signal instead of HDMI; use a different TV; or try and find an HDMI booster (possibly a powered switch box would do the job). This is a bit harder to manage as there is no way to tell the TiVo to increase the HDMI signal.
Q Does the fan make a loud buzzing noise?
You might be able to fix a buzzing (unbalanced) fan by simply cleaning it. Shut down the TiVo and unplug it, then remove the fan. Use a damp (not wet) cloth to gently clean the dust off of the fan blades, before drying and refitting. This might solve the issue of noise. If not, then you may need to replace the fan. You should be able to find an identical 12V fan from Jaycar, Maplins, Radio Shack, or similar shops - simply swap it out and you're fine. The motherboard connector is a standard computer motherboard fan plug.
Choose next step
At this point, you have to choose whether to attempt a power supply fix first, or a hard disk replacement. Generally, unless you're getting a full bootup before crash, you're better off working on the power supply first.
Step 2: Power Supply
So, we're going to try repairing the power supply.
There are two things you can do here - either completely replace the power supply unit with one from WeaKnees, or assume the problem is failed capacitors and replace the bad capacitors yourself.
Removing the power supply
The power supply board is to one side of the unit. Open the TiVo using the T10 screwdriver and you should be able to identify it. You can remove the power supply from the unit by removing the T10 screws holding the board down, the two T8 screws holding the connector to the backplane, and then disconnect the wires by pulling the plugs from the motherboard and hard disk (see first image).
Checking the power supply
Take a look at all the capacitors (see third image). Do any of them appear to be convex slightly on the top? Are any leaking electrolyte? Have any exploded (this is a dead giveaway)? Even if none appear to be convex, they may still need to be replaced. It can be a good idea to simply replace all 4 of the main capacitors (the blue-green circles left of the 3rd image) as they only cost about 20c-50c each anyway. These are labelled C501 - C504 on the motherboard.
Once you know which capacitors you are going to replace, get replacements. Get the same number of uF and the same or higher voltage rating (eg, you can replace a 10V 2200uF with a 16V 2200uF if you want). The capacitors are all labelled on their sides with the rating; the most common ones you need are the four 10V 2200uF; you may also want to change the two 50V 47uF capacitors (C801 and C113).
Remove the old capacitors
Desolder the old capacitors from the board, remove them, and clean the holes of and blocking solder. You may need to remove the white glue holding them in place.
Install new capacitors
Place the new capacitors in location. Note that the -ve wire of the capacitor goes in the shaded hole - don't get the back to front or they will not work. Solder them into place, and finally clip off the spare wire.
You have now repaired the power supply! Maybe...
If you have a multimeter, you can test the voltage output is a steady 12V and 5V. Or, you can reassemble the unit, start it up, and see if it boots correctly.
If it does not boot up, then either you messed up your soldering, or the problem lies with the hard disk.
Step 3: Hard Disk
So, you think the issue might be the hard disk. It may have failed entirely, or it may be starting to fail, or it may have corrupt system data. The first thing to do is to test it and back it up if possible.
WARNING: Hard disks can be fragile, and are static sensitive. Treat the disk gently; do not drop it, place it gently on the table. Be aware of static - if you do not have a static wristband, then avoid static - do not place the disk on the carpet, don't wear nylon leggings as you work, and so on. If you are gentle with the drive there should be no problem.
If you do not have a PC with SATA cables to test the drive, then you can still buy a replacement that is pre-installed with software from WeaKnees.
Remove hard disk
To remove the hard disk, remove the three torx screws holding the metal disk tray in place. You then need to disconnect the power/data connector from the back of the hard disk.
Having removed the tray, turn it over, and you will find four more screws holding the disk to the tray. Remove these to get the disk out.
Connect the disk to your PC
Shut down your PC first!
Obviously you need the primary hard disk of your PC to remain connected, but if necessary, you could temporarily disconnect your SATA CD-ROM or secondary hard disk while you work to provide the required connectors.
Link the SATA power and data to the disk from your PC. Book up the PC and it should spot the hard disk, but not identify any filesystems on it.
DO NOT USE WINDOWS DISK MANAGER. IT WILL CORRUPT THE DISK.
Install and run the WinMFS program. This should identify the disk as a TiVo disk. If it cannot spot the disk, it is likely the disk has completely failed and it will need to be replaced.
If WinMFS can spot the disk, run a backup of the disk immediately. Use a Truncated backup unless you desperately need to keep your old recordings. If the backup fails or hangs halfway, the disk has failed and needs to be replaced.
Once you have a backup things are a bit easier. You can try repairing the boot partition (default.tbp is attached). You can also try doing a low-level disk scan and repair - this might help, or might not. If your problem is with a failing (but not yet completely failed) disk, then you may well be able to get a working backup image that you can copy to your new replacement disk. Otherwise you'll need to obtain a clean fresh image.
Set up replacement disk
If you find this hard, or cannot obtain the install image file, you can always buy a pre-configured disk from WeaKnees.
Shut down your PC so that you can disconnect the old disk, and connect up the new one. Boot up and start WinMFS, which should be able to spot the new blank disk.
If you have a backup from your original disk, you can now try to write it to the new disk. The new disk must be at least as large as the old one; if it is larger, after restore you will be prompted to expand the data partition to use the whole disk (say yes). If you are restoring a truncated backup, then any previous recordings will be lost; they will still appear in your TiVo 'Now Playing' list, but clicking on them will give an error. After restoring you will need to manually delete all the items in the Now Playing.
If you do not have a backup image, you need a fresh install image to place on the new disk. Getting one of these is a bit troublesome; you can ask on the Tivo Community forum or in other places. You need one for your particular country's model -- for example, AU/NZ people need the Series 3 Australia/New Zealand image -- and specifically a WinMFS image. If you have problems, send me a message. I might have one though I cannot post a link here for copyright reasons. When you have the image, write it to the disk, and extend to fill up all the space.
Note that TiVo OS cannot support disks bigger than 1TB. If your disk is larger, do not increase the TiVo filesystem bigger than 1TB or it will not work.
When you have your new disk, shut down your PC, disconnect, and reinstall it into the TiVo. When you start up, you should get to the normal start. If using a fresh install image, then you will start in the guided setup as you did when you first bought the device; complete setup as you did back then (it may take a little time as it will also need to update the entire program guide)
If your TiVo starts up, but the recordings page says "A hardware problem has been detected which needs your attention", mentioning "Error 51", then you need to perform a complete reset and reconfigure. This is done under the Settings menu using the "Delete Everything" option.