Re: Amplifone +/- 24v regulation

On Tue, 07 Jul 1998 14:37:35 -0400, Joel Rosenzweig <joel-r@an.hp.com> wrote:

>Has anyone considered using large capacitors to hold the +/- 24V output
>from the voltage regulators, specifically to help keep the screen from
>expanding/shrinking during the Death Star explosion?
>
>Would it be better to just add a beefier regulator?
>
>I was just thinking that we could do just what the car stereo fanatics
>do when they want to keep their amps running well under heavy demand.
>Seems to me that we could do something similar for the HV section here.

The unregulated voltage is not dropping too low, so bigger caps won't help (and
if that were the case, better regulation wouldn't help either). What happens is
that the 7x24's can't supply enough current. You could lower the value of the
50 ohm pass resistor, but that will reduce the 7x24's ability to regulate and
the vectors will not be as stable. The thing to do is add pass transistors that
beef up the current capacities of the 7824/7924.

The new pass transistors must be heatsinked (so you'll have to find a heatsink
to mount the transistors to). The ideal transistors would be the standard
2N3716 and 2N3792 already used in the WG for this. Just leave the 7x24s where
they are, remove the 50 ohm power resistors, and connect the base and collector
of the pass transistors in place of the 50 ohm resistors. (The base goes to the
input of the regulator, and the collector goes to the output.)

Cut a trace in the PCB just in front of the input of the 7824/7924 and jumper
the cut with a 5 ~ 7 ohm 1/2 resistor. Connect the emitter of the pass
transistor to the side of the cut that's not connected to the regulator, and
walla, a mucho beefed up regulator.

ASCII art:

(Old Circuit)
                              50 ohm
                        +-----/\/\/----+
                        | |
                        | ------- |
     +unreg ----------+----|7824 |---+---- +24v reg
                             -------
                                |
                                |
                               ---
                                -

(New Circuit)

           +------------+--- ----------+
           | \ / |
           | V / 2N3792 |
           | 6 ohm 1/2 watt ----- |
           | +--/\/\/---+ | |
           | | | | ------- |
+unreg ---+--+---X X----+-----+----|7824 |---+---- +24v reg
                   ^ -------
             Cut trace here |
                                       |
                                      ---
                                       -

Do the same thing with the 7924 side, but use the 2N3716 as the pass transistor.

I haven't done this to an Amplifone, but the circuit is right out of the
National Voltage Handbook. (I have used this circuit before, and it works
great, I just haven't done this to an Amplifone yet.)

-Zonn

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Received on Tue Jul 7 15:10:07 1998