Modifying the 5508W Cable TV receiver

These receivers were used by Pay TV providers such as Austar in Tasmania for service delivery via microwave transmission. They have built in to them a tuner that can easily be modified to operate on channels lower than UHF 28
If your unit is still working & can change channels, then a simple modification will be all that is necessary
If your unit doesn’t work properly, but instead comes up with an error code such as o4 then more modifications will be needed.

Theory

These units use a microprocessor to control a 4 band TV tuner to receive TV signals on VHF and UHF bands. An IC in the tuner receives a data stream from the CPU and then controls a VCO (voltage controlled ocililator) in the tuner. The VCO operates from a 32 volt supply which is reduced to what ever is required to tune in the programmed station. For SBS on UHF channel 28 this voltage is 4.1 Volts. What we are going to do is to remove the 32 volt supply from the tuner and then provide our own voltage (0.52V) directly into the VCO to force the tuner to the channel we want. Since the tuner is a 4 band tuner, we will still need to select a channel number that is in the UHF band (such as the channel SBS would normaly be on)

Preventing your receiver from disabling itself
Apparently these units will disable themselves if no pay TV signal has been received for a certain period of time. By placing a diode between pins 5 & 16 of U502 it may be possible to prevent the unit from disabling itself.

The modification
Start by removing the 4 screws that hold on the top cover. This can be done using a pair of wire cutters to grip the screw heads (operate the wire cutters like a screwdriver to unscrew them) or a 4mm socket (nut driver) will also work. The 4mm socket is the easier method of the two.

Next remove the left side cover from the tuner unit (the shiny tin can). This should just pop off.
Solder a wire to the second last connection point near the rear of the tuner module as shown in the picture. Feed this wire through a hole in the cover plate & push the cover back on.

Next, cut link W209. This link most likely has been cut in the factory to add a resistor and a large capacitor. Cut the link on the side closest to the front of the unit, or remove these components altogether.

Method 1 - where there is a strong signal to receive
Connect a 10K and a 1.2K resistor together. Connect a wire(we will refer to this as the yellow wire) to the end of the 10K resistor. Connect the red wire (from the VCO control line) to the junction of the two resistors. Connect the other end of the 1.2K resistor to the metal can of the tuner (ground). The yellow wire connects to 5 volts which can be obtained from link E207 (see picture in method 2). This simple arrangement will provide a VCO control voltage of about 0.53 volts, which will work well for stronger receive signals.

Method 2 - where it is desirable to be able to fine tune the receiver
Make up a small circuit using a 10K resistor, a diode and a trim pot (about 10K to 47K will do, but must be more than 1.2K)

The diode is connected across the trim pot and it’s cathode (the end with the band) is on the right and will be connected to ground
The wire you have connected to the tuner unit is connected to the centre connector on the trim pot (red wire in the picture)
The 10K resistor is connected to the Anode end of the diode and connects to another wire (yellow in the picture) that will be soldered to link E207 on the board

The modification fits with the cathode of the diode soldered to the tin can of the tuner module

Now turn the receiver on and attach a multimeter to the wiper on the trim pot. Adjust it to read 0.52 volts.

From now on the tuner when set to channel 3 (or whichever channel was previously used for SBS on UHF 28) will now pick up ATV transmissions on 444.25 Mhz. You can also fine tune the trim pot when receiving an ATV signal, but the voltage should remain somewhere between 0.5 and 0.55 volts.
If ATV transmissions are on another frequency in your area then you will need to experiment with the trim pot settings. As a guide channel 28 (SBS) equates to 4.1 volts. If you need a voltage higher than 0.6 V then use just a trim pot without the diode and the extra resistor. 32 volts can also be obtained from the front end of W209 which we cut in an earlier step.