TI99/4a PAL Composite Video Mod for the PHA 2036 Modulator

July 11, 2022

TI99/4a PAL Composite Video Mod for the PHA 2036 Modulator

This modification only works on the PHA 2036 Modulator (the metal cased modulator) There is a tutorial available on the TI-99 Italian User Club Website should you happen to have the plastic PHA 2030 modulator.

www.ti99iuc.it

Many thanks goes out to Sergio Vigano and the TI-99 Italian User Club for making this modification public.

My TI99/4a had a reasonable picture on my modern TV using the RF cable; it was quitter stable and the colours were pretty good as well. But as I am planning to restore and upgrade the classic computer I decided that I would try my hand at the composite video mod found in the link above.

The website is excellent, it has clear and easy to follow instructions and I did think about doing the modification exactly as shown on the site. But then I decided to try and improve the mod a little bit, so I ordered some parts from Amazon and eBay.

I have included screenshots of the exact parts I ordered for this mod during the clip.

I wanted to try and keep the modulator looking as close to the original as possible and decided that I would order a 4 pole 3.5mm chassis mount socket and a 3.5mm to RCA cable so that I could connect it to the TV using a standard SCART adapter.

I also decided to mount the socket out of the same side of the modulator as the RF socket is on.

As far as modifications go, this one was pretty easy but it does require some tooling which you may not have such as a drill. There is another procedure for this mod available on the TI-99 Italian User Club which requires less tooling.

Tools required:

Cutters

Pliers

Soldering Iron

Powered Drill (with stand if possible)

Shifting Spanner

Tweezers

Heatgun

Tapered Reamer (only if you don’t have an 8.5mm drill bit)

3mm HSS Drill Bit

8.5mm HSS Drill Bit

Flat Headed Screwdriver

Components required:

4 pole 3.5mm chassis mount socket

4 pole 3.5mm jack plug to RCA connectors cable (I would reccomend about 2M in length)

1.5mm Heatshrink Sleeving

3mm Heatshrink Sleeving (or cable ties)

Around 300mm of the following colours of 7/0.2mm equipment wire

Black

Yellow

White

I found the 4 pole 3.5mm chassis mount socket in Amazon; see picture below for the exact one I ordered.

And the RCA cable from eBay; I had bought a small 0.5M cable for another project so used this smaller cable during the modification. I have since bought a longer 2M cable which was actually cheaper than the 0.5M cable!

Before we start this mod, you need to determine if the TI99/4a’s modulator is the correct version for this type of mod.

Below is a picture of the correct modulator.

Let’s have a look inside the modulator and figure out the best place to fit the socket.

Remove the two small case screws on the modulator with a small flat bladed screwdriver.

Below is a picture of the inside of the modulator and my proposed location for the 4 pole socket.

I chose this location for a couple of reasons:

1 I could have drilled the hole to the right side of the modulator as shown in the tutorial in the TI-99 Italian User Club but upon inspection I found that fitting the connector there would be difficult due to the length of the connector.

2 I wanted the cable to come out on the same side as the original RF cable.

3 I wanted to drill the hole with the minimum chance of damaging any of the components inside the modulator such as the variable capacitor highlighted below.

So after some careful measurements I found that if I drilled the hole 60mm from the left of the modulator and 15mm from the top, this would ensure that I would have little risk in damaging any of the components.

Measure and mark the case with a permanent marker (use IPA to remove the ink afterwards)

Then punch the location of the hole with a centre punch (if you have one) I didn’t have my one to hand for this mod so I carefully scored the hole location with a scalpel.

Measure the connector to determine the correct hole size; the data sheet said that the threaded end of the connector was M8 but I decided to measure for myself just in case using my vernier callipers.

After reading the callipers I decided that an 8.5mm hole would suit but as my drill bit set only went up to 6.5mm, I would have to use a tapered reamer to enlarge the hole further from the 6.5mm.

Then drill the hole; I got to try out my new drill press stand for this (but if you are careful use a standard powered drill or hand drill) I found that the best way to hold the modulator in my vice was to refit the lid of the modulator.

As a 6.5mm hole is quite large, it is best to drill a pilot hole; this would help avoid any wobbling of the drill bit on the modulator. I opted to use a 3mm HSS drill bit for this.

Fit the 3mm drill bit to the drill and set the depth to drill the hole just below the metal plate of the modulator; this would also reduce the chance of damaging any of the components inside the modulator.

Drill the pilot hole then enlarge it with either an 8.5mm drill bit or in my case a 6.5mm drill bit. On a safety note, please wear safety glasses whilst doing this!

Once drilled, I used a tapered reamer to gradually enlarge the hole; note that I opened up the modulator again to ensure that the tapered reamer didn’t cause any damage to the electronics.

Ideally, the connector should be a tight fit but you do not want to have it too tight as you will risk damaging the plastic thread on the connector.

Once the socket fits snugly, remove any sharp edges caused by the drill with a file on the inside of the modulator and if needed use a de-burring tool on the outside of the modulator.

Now it is easier to solder the wires on the socket before fitting it to the chassis of the modulator but I wanted to double check the connections from the socket to the RCA cable so I dug out my multimeter and did some continuity tests.

The photo below shows the connections of the socket taken from the manufacturer’s data sheet.

However, I found that after testing, the actual pinout for the socket to RCA cable was as shown below.

Below is my wiring diagram for the socket; you will see that I have linked pins 1 and 2 together. Some modern TV’s will automatically put sound to both speakers when it only has a mono input but in case either of my Geek TV’s didn’t I decided to link pins 1 and 2 together which should ensure that both speakers get the same audio from the mono feed from the TI99.

For wiring up the socket to the modulator I used three 300mm lengths of 7/0.2mm equipment wire. For the Video Pin, I decided to use Yellow, for the GND, I used Black and for the Audio, I decided to use white instead or Red as I usually only use red for a positive DC Voltage wiring.

Now with the socket in the helping hands, strip twist and tin 5mm of insulation from the Black and Yellow wires.

As we are going to link pins 1 and 2 together, strip twist and tin about 10mm of insulation from the white wire.

Now place the stripped black wire through the small hole on Pin 4 of the socket and solder it; I find the easiest way to keep the wire in place is to use a pair of cutters or pliers to put tension on the wire as shown below.

Then repeat the process for the Yellow wire on Pin 3 of the socket.

Now make an ‘L’ shape with the white wire with the tinned part bent over at a right angle from the insulation. Solder the tinned part on to Pin 2 and bend the excess tinned wire over to pin 1 and solder it.

Trim the excess tinned wires away from the socket.

Cut 2 15mm lengths of 1.5mm heatshrink sleeving and fit over the Yellow and black wires; ensure that no exposed conductors are showing then shrink the sleeving with a heatgun.

And you should have a socket with the wires coming from the pins looking like this.

Now fit the socket to the chassis of the modulator; I used a shifting spanner to tighten the nut which reduces the risk of scratching the black paint on the chassis.

Now cut 4 10mm lengths of heatshrink sleeving; I prefer to use heatshrink sleeving in place of cable ties to group the wires as I think that it looks neater.

Then route the wires as shown below, fit the sleeving and shrink it down with a heatgun.

Now solder the Black, Yellow and white wires on to the following positions as shown in the picture below.

It is a good idea to tin each position before soldering the wires on to them; this is quite fiddly for pins 3 and 4 of the modulator due to its close proximity to the chassis.

Cut the Black wire to length.

Strip, twist and tin 5mm insulation from the Black wire.

Then solder the black wire on to position 3 on the modulator.

Fitting the Yellow wire to Pin 4 is exactly the same process, cut the wire to length, strip, twist and tin 5mm insulation and solder it on to position 4.

I found it easier to use tweezers to hold the wire in place when soldering the yellow wire.

Now cut the White wire to length, strip twist and tin 5mm insulation and solder it on to the lower leg of the 330Ω resistor as shown in the earlier wiring diagram.