Miata Mailing List: August 1998, Message #333

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From:	Malcolm Hathaway <MHathaway@concord.tt.slb.com>
Subject:	(none)
Date:	Tue, 4 Aug 1998 09:08:14 +0000

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I posted this yesterday, but I noticed it ended up with lots of wierd
little <br> things, so i'm reposting with a cleaner format, (I
hope).  Also, I added asterisks were the bold print is, in case the
bold print don't work...<br>
<br>
<br>
Howdy, folks!  After various discussions with other do-it-yourselfers,
especially Steve Smith, I have pulled my radio apart and done the famous,
and closely held,  Headrest Speaker Fix (which, incidentally, does
nothing physical to the headrest speakers).  This is quite doable by any
able-bodied Miateer with a small soldering iron, a couple of 250V, 0.4 uF
caps, and some patience.  It basically involves replacing two coupling
capacitors on the volume control pc board with two capacitors with larger
values.  This allows more bass and mid-range to get to the headrest
amplifier, which in turn allows one to HEAR the headrest speakers!  There
may be a few other tweaks that Mr. Anderson does to improve things even
more, but this will give the bulk of the improvement available.<br>
<br>
Here are the details:<br>
<br>
Remove the radio, (which really does require removing the center console
from between the seats).  If you do not have the factory plug to connect
the HR speakers, make sure to mark which wire goes to which plug pin.  I
inherited a setup which did not have the factory plug, but I have found
that 1/8" wide female spade connectors will slide tightly over the
speaker pins in the back of the radio.  They are connected with these,
now.<br>
<br>
At various points in the dis/assembly, you will need to carefully
de/insert some thin extender pins which connect one board to another.  Be
careful not to bend these long pins, or re-assembly will be difficult. 
Going slowly and gently will help.<br>
<br>
Remove the bottom and bottom panels from the radio (no screws here, they
are press-fit)<br>
Remove 4 control knobs from front panel.<br>
Pop off front panel (clips are at each side), again, no screws.  Some
electrical connector pins will be disconnected in doing so.<br>
Turn unit upside-down, with rear of radio facing you.<br>
Remove 4 screws securing tape transport unit.  Slide tape transport unit
out, straight up (electrical pin secure the back edge of the unit)<br>
<br>
Notice a small 1" x 4" PC board on the left side, which is
secured by a grounding screw toward the back of the unit.  This is the
volume control board.<br>
Remove 2 lock-nut from volume/control potentiometers (pots), (the light
blue/green things that the knobs were attached too.)<br>
Remove little plate from under these nuts. <br>
Remove ground screw at the rear of vol. control board.<br>
Lift vol. control board out of radio, straight up until the connection
pins are free along the left side of board (they are plugged into VC
board from the backside).<br>
<br>
The caps in question are the reddish/brown components between the two
control pots, labelled C248 and C348.  These can be replaced, or
additional (non-polarized) caps added into existing holes alongside the
existing caps after the solder is melted.  I tried adding in values
between 0.1 uF and 1.0 uF, (the existing cap values are too small to
matter).  I wasn't able to do exhaustive tests, but 1.0 uF may be too
big, so that loud passages with much bass could cause distortion.  I
think I even heard this on very bassy NPR speaking voices.  I settled on
44 uF (two .22 uF caps on each side).  The fractional value is not
critical, it was just what I had.  Try Radio Shack or some such to find
caps.  For reference, there is a huge difference between the standard
value (which is very small) and anything bigger than 0.1 uF... its like
turning off a mute button.<br>
<br>
 With the new caps in place, carefully slide the small board back into
place, **<b>being sure to re-insert the extender pins correctly**. 
Replace the grounding /mounting screw, replace nut plate over the ends of
the pots <b>**little projections from this plate face toward the inside
of the radio)**</b>, replace lock-nuts on pots, slide tape transport back
into place, replace its screws, remount the face-plate, **<b>going slowly
to ensure all electrical pin connections are okay**</b>, and put top and
bottom covers back on (the top cover has the radio security label,
otherwise the top and bottom are practically the same.) <br>
<br>
Bold print indicates slightly trickier bits, (which I screwed up once or
twice).<br>
<br>
I am lurking these days, but will reply to the list or privately to any
responses or questions.  Thanks again to Steve Smith, who figured out the
secret awhile ago.  Also, if I have left something out, feel free to add
your suggestions.  Please don't yell at me if you radio explodes, (mine
didn't...)<br>
<br>
Good luck, and happy listening!<br>
<br>
Mackyboy<br>
'90 MBRG with the OTHER Headrest Speaker Fix<br>
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