External PSU Modification

For issues with the External PSU, see PSU and Modulator Issues

SAM External PSU Modification as on Newsdisk 5

If you use your SAM with a normal domestic television set using the aerial socket, you may have experienced an annoying buzz on the sound. Some of the noise is inherent in SAM, but the rest is created by the modulator picking up radio frequency (RF) noise from the power unit.

This article details a solution that I have used satisfactorily for some months. This involves removing the modulator from the power unit and putting it into its own separate box.

IMPORTANT NOTICE AND WARNING

If your SAM is under 12 months old, then it is still under warranty. The opening of the power unit as described in this article WILL INVALIDATE YOUR WARRANTY! Please also note that the opening of the SAM power supply unit could expose you to potentially lethal voltages.

* * * * The modification should NOT BE UNDERTAKEN BY ANYONE WITHOUT ELECTRICAL EXPERIENCE. * * * *

Tools Required:

1. A small cross-headed screwdriver.
2. A medium cross-headed screwdriver.
3. A soldering iron with a small bit and some multicore solder.
4. A pair of wire strippers.
5. A drill and various bits (some large).

Components Required

CABLE ASSEMBLY

Cut 20mm of the outer sheath off and peel back the braiding. Remove the foil sheath and cotton rib. Remove 4mm of insulation from each of the wires. Tin each wire with solder.

Remove the cover from the plug body and slide the cover over the cable. Trim back each wire to 4mm and solder the blue wire to pin 1 of the plug.
Solder the green wire to pin 5, the red wire to pin 2 and the yellow wire to pin 3.

Clamp the cable into the plug shroud and solder the braid to the shroud. Reassemble the plug.

POWER UNIT

N.B. ENSURE THAT THE MAINS POWER PLUG IS REMOVED FROM THE SUPPLY

Remove the four silver, cross head screws, one from each corner of the underside of the power supply unit. Remove the power unit cover. Remove the three cross head screws securing the modulator inside the power unit and lift it out of the way.

Remove the single black, cross head screw securing the main, brown printed circuit board (PCB). Gently lever off the clips securing the two regulators to the silver heat sink inside the power supply.

Lift out the PCB. Remove about 60mm of outer cover from the new cable and unravel the braiding. Cut the braiding to leave about 20mm on the cable.Remove the foil sheath and the cotton rib.

Remove around 4mm of insulator from the green and yellow wires and tin them with solder. Remove the thin black wire (+12v) from the PCB and solder the new green wire in its place. Remove the purple wire (0v) from the PCB and solder the yellow wire in its place. Cut the blue wire to 40mm, remove 4mm of insulation and tin the end.

Remove the fat red wire (0v) and solder the new red wire in its place. Slide 15mm of heatshrink over the braid, split it into two equal parts and form it into a loop with a 2mm hole. Cut off the wispy bits of wire and touch with solder to hold together.

REPLACING THE 12 VOLT SUPPLY DIODES AND RESISTOR IS ADVISED see Hardware Bugs

Diode D4 is de-soldered and replaced with a new 1N5349. Be careful to replace it the correct way round, the silver ring to the + side. Diode D3 is de-soldered and replaced with a new 1N4001, warning as above. Resistor R1 is de-soldered and replaced with a new 33ohm 4 watt resistor. The resistor can be fitted either way round (See Tips below).

Replace the PCB, securing the new cable with the smaller "p" clip and put the loop of braid under the screw to touch the chassis. Use one of the spare longer cross-headed screws to hold the clip and braid. Replace the existing clip on the cable outlet, amd replace the clips over the two transistors, being very careful not to damage the transistor cases.

Plug the power unit mains supply plug in and switch on. Check the voltages. Connect the negative (-ve) lead of the multimeter onto the chassis and check that there is a positive voltage of +5 volts on pin 1 and +12 volts on pin 5 of the plug. All other pins should be at 0 voltage.

Replace the power unit cover, being careful not to trap either of the leads.

MODULATOR BOX

Mark one end of the box L/H and the other end R/H. All work to be done with the box facing you. L/H to the left side with the lid removed.

DRILL THE FOLLOWING HOLES IN THE BOX

1) 10mm diameter hole 37mm from R/H end and 10mm from  the top.
2) 2mm diameter hole in the base, 7mm from the R/H edge and 20mm from the front edge with hole from operation 1.
3) 2mm diameter hole in the base, 12mm from the L/H edge and 7mm from front edge with hole from operation 1.
4) 2mm diameter hole in the base, 12mm from L/H edge and 34mm from front edge with hole from operation 1.
5) 15mm diameter hole in R/H end 22mm from back edge and 16mm from top.
6) 2mm diameter hole in R/H end 10mm from back edge and 16mm from top.
7) 2mm diameter hole in R/H end 33mm from back edge and 16mm from top.
8) 5mm diameter hole in L/H end 20mm from back edge and 16mm from top.
9) 2mm diameter hole in back edge 30mm from L/H end and 16mm from top.

Countersink all 2mm holes.

Trim modulator PCB with a round file for clearance from the box top fixing posts.

Desolder the 4 small wires from the modulator board. Mark which wire is removed from which position. Push the cable through the 5mm hole into the box and resolder the 4 wires back into their original positions.

Cut the 3 plastic spacers to 9mm  long. Tilt the modulator PCB with the new cut-outs downwards and position in the box to the edge with the 10mm hole in it.

Carefully press the modulator PCB downwards until the aerial outlet socket locates in the 10mm hole.

Affix the modulator PCB with the 20mm screws from the base of the box and passing them through the spacers, affix them with the nuts. Place 10mm of heatshrink over the black wire from the modulator and solder onto pin 1 of the socket. Place 10mm of heatshrink over the red wire from the modulator and solder onto pin 2 of the socket. place 10mm of heatshrink over the purple wire from the lead and solder onto pin 3 of the socket.

Place 10mm of heatshrink over the black wire from the lead and solder onto pin5 of the socket. The socket can now be fixed with 2 10mm screws to the inside of the box R/H end.

FINAL TESTING

Connect the power unit to the mains supply and switch on the mains power. Attatch the neutral lead of the multimeter to the socket case and check the following:

1) Pin 1 +5 volts
2) Pin 2 0 volts
3) Pin 3 0 volts
4) Pin 5 +12 volts

If all is correct, proceed to the final stages. If not, return to the start and go through a wiring check.

FINAL STAGES

Connect the modulator lead into SAM, the power lead into the new modulator box and replace the aerial lead into the new modulator box outlet. Connect the other end of the aerial lead into the TV socket. Switch both on, and tune the TV into the standard MGT/SAMCO self test picture signal.

See diagram 2 and adjust the small black screw furthest away from the front L/H end of the modulator box until the least noise on sound is found. Only a small adjustment will be required. Adjustment must be made with a small insulated screwdriver. Much trial and error may be required. WARNING.. DO NOT TOUCH THE TWO BRASS SCREWS.. When all is correct, fit the modulator box cover.