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Thread: Fix for DM600PVR audio

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    Senior Member gw1's Avatar
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    Default Fix for DM600PVR audio

    This is a variation of the . It boosts the Dream audio up to the level of other set top boxes. The parts are readily available, cheap and easily assembled in a couple of hours provided you have reasonable soldering skill.

    For this version I chose to desolder electrolytics rather than SMD resistors. The soldering is easier this way and you don't need the magnification tools.

    The audio circuit in DM600 is very similar to genuine DM500, so the modification is almost the same as before. The only difference is I've removed the 470R current limiting resistors because they're no longer needed; the SMD resistors on the motherboard are used instead.

    Some warnings to begin with
    • This voids your warranty
    • My DM600 motherboard is Version H (I imagine it will work with all versions but can't guarantee)
    • Take static electricity precautions to avoid damage
    • Unplug all cables from receiver before soldering on it
    • Check workspace is clear before applying power to avoid shorts
    • You need a temperature controlled iron with reasonably fine tip. If you're not confident about your soldering don't risk damaging your receiver - get someone else to help, or make do with an external amplifier.


    How it works
    The DM600 uses a Wolfson stereo DAC at a 48kHz sample rate. That part has a good digital low pass filter and noise shaper (something the cheaper DACs lack) and Dream include a supplementary third order Sallen-Key filter as suggested by Wolfson. The DAC's output swing and DC bias mean a rail-to-rail op-amp such as can manage with a single 5V rail. A final RC stage in the DM600 audio design provides current limiting and helps suppress any remaining high frequency noise (which Sallen-Key ).

    This mod intercepts the audio signal at the output DC blocking capacitors C75 and C76. The LM833 op-amp I've chosen, after 2.54X gain and headroom allowance, needs around 9V as its supply. A 7809 regulator provides this. Ideally the +12V would be tapped from some point after L29, eg from C270. However it's better we avoid soldering on SMD capacitors if possible as the uneven heat might cause fractures. That's why I've suggested the cathode of D31 instead.

    A divider network provides a clean +4.5V midrail operating point for the amplifiers, with DC blocking capacitors being necessary at the input and output. The receiver's RC output stage is preserved which is why no extra current limiting is required on the amplifier board; the 470R resistor found in the DM500 mod has been replaced by a wire link.




    Construction

    The veroboard layout is shown below. As you can see it's virtually identical to the compact one I posted on the DM500 thread, except the 470R resistors are gone.



    Start by cutting the veroboard to size (14 x 9 holes) by scoring with a knife, snapping off on the bench, then cleaning with a file. It can't be much wider otherwise it won't fit between C130 and the hard disk.

    Make the 10 track cuts using a drill bit, then inspect carefully to ensure fully cut.



    I suggest soldering the three longest wire links first, then the IC, then the fourth wire link between IC pins 3 & 5 on the solder side of the board. Then fit the remaining components. Be sure to observe correct polarity of the electrolytic capacitors.

    Note that two parts have a lead passing through a drilled hole and soldered to adjacent tracks.



    Before mounting the board in the receiver it's a good idea to prepare and solder the connecting wires into the board. That way there's less chance of getting solder splashes or whiskers on the motherboard. Cut, strip and lightly tin the leads before fitting to the veroboard assembly. The black lead only needs to be about 4cm long, the read one about 6cm. For Lin/out and Rin/out you'll want pairs of IDC wires about 5cm and 8cm each.

    Installation

    Here are the steps I followed when modifying my DM600 PVR.
    1. Remove all screws on back and sides
    2. Remove cover
    3. Remove rear metal plate by popping out the top corners
    4. Remove front by gently lifting the two plastic clips on its top edge
    5. Unplug tuner module
    6. Remove two screws securing hard disk mounting plate to side of case. Lift drive assembly out and unplug.
    7. Remove three screws holding motherboard
    8. Lift motherboard out and turn over
    9. Locate C75 leads, heat, apply a little solder and gently lever C75 out. Don't force or tracks may be damaged.
    10. Repeat for C76.
    11. Check that there are no solder splashes causing short circuit.



    12. Place double-sided tape over SDRAMs U12 & U13, keeping upper side of tape covered.

    13. Place the amplifier board on the tape and connect +12V and GND wires only, at points indicated below.



    14. Check your workspace is clear of metal objects, and check the amplifier board isn't shorting on anything. Then apply power. With your DC multimeter's black lead on 7809 regulator tab, check the following IC pin voltages:
    • Pin 8 = +9V
    • pin 1 = +4.5V
    • Pin 7 = +4.5V


    15. Remove power (there should be no leads at all plugged into DM600).

    16. Solder audio flyleads onto C75 & C76 pads, making sure you get the in and out leads the right way around.



    17. Reconnect the front panel (the IDC cable red wire should align with the RHS pin and emerge on the top of the header). Temporarily plug in the tuner module and attach a cable to your dish for testing.

    19. Check again that the work area is clear and the amplifier board isn't shorting on anything. Then apply power, tune to a test channel, adjust volume to maximum and check audio levels. I used TUNE channel on Optus C1 and measured about 0.28-0.29V AC rms on the RCA output sockets. I also tuned into ABC Classic FM and JJJ for a quick listen.

    (contd. next post)



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    Senior Member gw1's Avatar
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    Default

    20. Remove power, unplug tuner module but leave the front panel connected.
    21. Return the motherboard to its case and fit the three screws.
    22. Move the amplifier board so it abuts electrolytic C130. If you now plug in the hard disk and secure to case (two screws) you'll find it's a snug fit with the amplifier board.



    23. Depending on what double-sided tape you used, you may now want to lift out the veroboard and apply a second layer to its rear for a firm hold.
    24. Reinsert tuner module
    25. Clip font panel back into place.
    26. Plug in RS232 IDC header
    27. Replace rear metal plate, taking care to fit digital audio connector through its hole. You'll need to pull apart rear corners of case slightly for the rear to pop into place.
    28. Secure tuner module to rear plate using silver screw.
    29. Fit two black screws to SCART connector but do not overtighten as it cracks easily.
    30. Replace lid and fit remaining four screws.




    Results

    The levels I observed were exactly as expected, an amplitude increase of 2.54X, making it louder than my Relook 400 but slightly quieter than the Humaxes.



    As mentioned before there's no absolute standard for line level across all devices, just conventions. If you want to make your levels a little louder, up around the Humax level, just change the two 5K6 resistors to 6K2.

    Parts List


    Besides the above you'll also need some double-sided tape.

    Have fun.

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