Complete Re-Cap of a TI99/4a

 

In this entry I am doing a complete re-cap of the TI99/4a. I managed to purchase direct replacement capacitors which were mainly axial with the exception of one 470µF radial capacitor.

 

This clip features a disassembly of the TI99/4a followed by replacing all the capacitors with pictures of locations and vias to de-solder.

 

Equipment required:

Cutters

Pliers

Soldering Iron

Tweezers

Prying tool

De-Soldering Gun or Solder Sucker

Solder Wick

 

Components required:

Power Supply Board Electrolytic Capacitors

2 X 47µF 16V Axial

1 X 1000µF 25V Axial

1 X 3300µF 35V Axial

1 X 4.7µF 50V Axial

1 X 470µF 12V Radial

 

Motherboard Electrolytic Capacitors

You will also require some Heatsink Compound

And some IPA

 I have included an embedded YouTube clip of the whole process at the end of this entry.

Before we can replace the capacitors we must get inside the computer; this was my first attempt at it so I will include the stripdown as part of this entry.

Remove the switch slider from the front of the TI994a; It came out easily with the use of a prying tool.

After that, remove the 7 case screws with a small cross headed screwdriver; the ones highlighted in Yellow were smaller in length than the ones highlighted in Green.

Then remove the 2 fixing screws for the Power Supply PCB.

The external power connector simply lifts free from the case.

Now disconnect the connector with the 4 brown wires which take the required voltage to the Motherboard +5 -5 +12 and 0V

Then remove the PSU PCB

Now it is time to remove the motherboard; remove the three highlighted screws. It took me some time to realise that the one in the middle had to be removed as well!

Lift the motherboard to about 90° from the case.

And carefully remove the keyboard connector from the motherboard; I gently wiggled it from side to side until it slid off the pins on the motherboard.

Now remove the sideways edge connector PCB.

Then slide off the two metal clips from the sides of the RF shield.

Next, remove the three nuts blots and washers which fix the RF shield to the motherboard.

The above locations have longer screws than the picture shown below.

Now remove the RF shield.

Finally we can start to Re-Cap the PCB’s; I started with the PSU PCB first.

Before we do so, I would like to show you how to identify and read the markings on electrolytic capacitors. Failure to solder them in the correct orientation can and will result in either damaging the capacitor as they are prone to exploding as well as damaging the circuit and PCB (see my YouTube clip for a demonstration)

Below are the typical markings and orientation found on most axial capacitors.

And the picture below features the typical negative side markings found on most radial capacitors.

It is very important to replace the capacitors in exactly the same orientation!

I decided to replace the Capacitors on the PSU PCB in their numerical order and below is a photo of all the capacitors required for the PSU.

 

I did have most of them in stock but I had to pay about £5 for the 3300µF 35V smoothing capacitor… it really is a beast of a capacitor.

To make life easier it is a good idea to replace the capacitors one at a time. Starting with C7; the picture below shows the location of the capacitor on the component side of the PCB. I have also marked the orientation of the capacitor but the silkscreen printing on the PCB also marks the positive side of the capacitor.

We also need to know which vias to de-solder in order to remove the capacitor so I have marked their locations in the picture below.

With the information shown above, de-solder the two highlighted vias; I used my de-soldering gun for this but a simple cheap and cheerful solder sucker will work just as well.

I find it easier to de-solder after freshening up the solder on the vias; this is done by melting the original solder on the via and adding some new solder.

Take care when de-soldering as it is very easy to damage traces on the PCB during the process. A wee tip when using a de-soldering gun is to place it over the component leg, let the solder melt (typically 2-3 seconds) give the nozzle of the de-soldering gun a ‘wiggle’ whilst pulling the trigger and removing the solder.

When I attempted to pull the capacitor out from  the PCB I felt some physical resistance. This meant that not all of the solder had been removed when using the de-soldering gun. Forcing the capacitor out at this stage would damage the traces on the PCB so I resorted to a tried and tested technique. Simply heat the leg and via of the capacitor on the component side until the solder melts and gently pull the leg free from the PCB.

Once the capacitor has been removed, it is a good idea to inspect the vias for damage or any signs old solder which will prevent you from placing the new capacitor into position. I found that there was a slight blockage on one of the vias so I used solder wick to draw the solder up from the via.

 

Solder wick is relatively easy to use but if used carelessly can result in damaging the traces. I find the best way to use it is to place it flat over the via in question and simply lay the tip of the soldering iron on top of the solder wick and as close to the via as possible. Try and avoid trailing the soldering iron across the solder wick as this can sometimes lift the traces underneath.

Most of the original capacitors on this PCB had their information and orientation markings hidden from view. It is a good idea to bend the legs of the new 47µF 16V capacitor into shape using a pair of pliers and in such a way that the markings can easily be read when looking down on the component side of the PCB after fitting.

And fit the new capacitor in the same orientation as the original.

Below is a picture of the orientation of C7

Once fitted, simply solder the capacitor and trim the excess legs of the capacitor using a pair of cutters. When trimming the legs, try and avoid cutting into the solder joint as this will weaken the solder joint. I tend to trim just above where the solder stops.

Now we repeat the process for the remaining five capacitors.

 

Starting with C8, 1000µF 25V

Here is the position and orientation of C8 from the component side.

And below are the positions of the vias to de-solder.

De-solder and refit the new 1000µF capacitor in the correct orientation as shown above.

Then replace C9, the 3300µF 35V capacitor; to remove this you will have to remove the cable tie which holds it into position on the PCB. Use a pair of cutters to snip the cable tie and then remove it.

Below is a picture of the position and orientation of C9 taken from the component side of the PCB.

And the picture below shows the positions of the vias to de-solder on the solder side of the PCB.

De-solder and fit the new 3300µF capacitor in the correct orientation.

The next capacitor to replace is C10, 47µF 16V.                                                                        

Below are the pictures of the orientation and locations of the capacitor and its vias.

After replacing C10, replace C12 which is a 4.7µF 16V capacitor.

The pictures below show the orientation and locations of the capacitor and its vias.

We now come to C15, 470µF 50V.This is the only radial capacitor to replace; however you could replace it with an axial capacitor if you de-solder the via circled in black on the picture showing  the location of the vias on the solder side of the PCB.

You will also have to remove the cable tie which secures it to the PCB.

After replacing the final capacitor you should have a PCB looking like this.

All that needs to be done now is to fit a cable tie around C9. You could also clean the PCB with some IPA to remove any flux residue and dirt. As I am planning a deep clean of the system I will do this at a later point.

 

Now we come to the motherboard; unlike the PSU PCB, the motherboard silkscreen does not have information about the orientation or numbers of the capacitors.

So I started out replacing the cluster of five capacitors in the area shown below.

Below is a close up of the area in question.

I started with the 100µF 16V capacitor shown below.

Then after replacing that one I moved on to the capacitor directly below it which was another 100µF 16V

The next capacitor I replaced was the 10µF 50V shown below.

Then I replaced the 22µF 16V capacitor found in the position below.

The last capacitor in this cluster was another 22µF 16V and can be found here

The next capacitor I replaced was another 22µF 16V and can be found in the area shown below.

Here is a close up of the orientation and location of the capacitor and its vias.

The next capacitor is a 100µF 16V and can be found in about the middle of the PCB.

Here I have highlighted the orientation and location of the capacitor and its vias.

The next capacitor to replace is a 22µF 16V and can be found to the right of the previous capacitor.

The final capacitor to replace is another 22µF 16V and can be found in the highlighted area shown below.

Here is a close up of the position, orientation and location of the vias for the final capacitor.

After replacing the capacitors you should have a PCB looking like this.

Now it is time to reassemble the TI99/4a!

 

The first thing we need to do is to remove the old dried out heatsink compound found on the V.D.C. Chip (TMS9918A) with some IPA and a cotton bud.

Then remember to clean the old heatsink compound from the metal heatsink found on the RF shield.

Apply fresh heatsink compound to the V.D.C. chip; do not use too much heatsink compound for this. I recommend only using a single even stripe of heatsink compound down the middle of the chip as shown below.

Then refit the RF shield.

Screw the RF shield together using the three nuts, bolts and washers.

 

I started with the smaller one first.

Then fitted the two longer screws.

After that, refit the right angled edge connector in the position shown below.

Then refit the two metal clips to the sides of the RF shield as shown below.

Now refit the keyboard connector.

Then secure the motherboard to the case of the TI99/4a using the three screws shown below.

Now refit the PSU PCB starting with the power connector (this will only fit in one way)

Then secure the PCB to the case using the two screws in the positions shown below.

Refit the main power socket towards the rear of the TI99/4a; note that this simply drops into position.

Then refit the seven case screws; note that the four screws towards the front of the case are shorter in length than the three at the rear.

Finally, refit the power switch slider; this simply pushes on.

Now it was time to test!

Set up the TI99/4a.

Switch on…

 

And…

Success!

If any of the capacitors have been fitted in the wrong orientation it may take some time for them to fail so I would recommend leaving the system on for at least 20 minutes which should be long enough for any incorrectly fitted capacitors to fail.

 

Below is a clip of this modification from start to finish.

I hope that you have found this entry useful, if you have any questions or queries then please feel free to leave a comment or email me directly at

alleged_geek@aol.com

Thank you for taking the time to read this entry.

 

Next up in my TI99/4a series is a FlashRom99 installation as well as a joystick adapter PCB.