Re: Mockingboard A to Apple speaker?

[Rubywand <rubywand@swbell.net>]

Tom writes ...
> 
> Hi, the Mockingboard C I used to have had a cable (two wires) that connected to
> the Apple speaker pins in my //e (so all game music would play through the
> board). Is there any way to fix a MB A to do the same thing? I assume just
> solder a couple wires to the appropriate place on the board but I have no idea
> where, compared the two boards and they were totally different. I am a novice
> with hardware so would need detailed instructions, I could send a pic of the MB
> A, also have a pic of the MB C.
> 
 ....

     Thanks, again, for sending the Mockingboard A and C pics. They 
have been uploaded to GS WorldView's archive at ...

http://196.25.210.13/gswv/a2zine/Pics/BoardsAndSystem/ .


     It looks like the boards are using LM386 IC's for each channel's
audio output. These would be the two small 8-pin chips located near
the volume controls.

     Not easy to tell much about circuit details from the board pics.
A circuit that might work okay is something like this ...

                  ____                                ____ ____
 \   added leads |    *                           1-o|    U    |o
  \_ ____________|    *                              |         |
  | |                 *             +     +         o|         |o
  |_|____________     *<--/\/\/------|(--)|---       |  LM386  |
 /               |    *  1000 Ohm   5uF 5uF   \---3-o|         |o
/                |____*             20V 20V          |         |
                                                    o|_________|o
Speaker        1000 Ohm pot       


The LM386 is viewed from the top (chip) side of the board.

You connect a new lead to each existing Speaker lead. (So, your
Apple II's internal speaker remains connected.)  One way is to
remove a bit of insulation on each existing lead and solder a
new lead to the exposed place. Another way is to add the new
leads at the speaker.

The two new leads go to a 1000 Ohm pot-- one on each outside
terminal.

Connect a small 1000 Ohm resistor to the center terminal of 
the pot.

Make a small non-polarized electrolytic capacitor by connecting
the negative ends of a couple 5uF caps. One of the + ends goes to
the 1000 Ohm resistor.

The other + end goes to Pin 3 of one of the LM386 chips. The one
chosen will determine which channel's speaker will output Apple II
sound.

How you arrange this stuff is up to you. Most likely, it will be
easier to locate the new circuit close to the internal speaker.
Be sure stuff is placed so connections won't bump into any metal
or any connection points on the Apple II motherboard.

The lead that runs to the Mockingboard could be wirewrap wire. It
can be tack-soldered directly to the LM386 or to the pad for Pin 3
on the bottom side of the board.


None of the component values is critical. For instance a 10,000 Ohm
pot would probably be okay; and, the caps could probably be 2uF
through 20uF. However, the caps should be identical and the voltage
rating should be at least 16 Volts. Without knowing MB audio circuit
details, sticking with something fairly close to 1000 Ohms seems like
a good idea for the resistor.

I do not know whether there is any real chance of damaging the
selected LM386's input. The 1000 Ohm resistor should significantly
reduce the chances of damage.


     Before turning ON the computer, set the pot to near the center
of its range. The Mockingboard needs to be connected to its external
speakers (or speaker if it is a one-channel model).

     Turn ON the computer. You can enter CTRL-G or do Resets to get
BEEPs. The pot should let you adjust the signal going to the LM386 
so that Apple II BEEPs, etc. sound okay coming from whichever
Mockingboard speaker the LM386 connects to.



Rubywand