re: Radio or speaker problem

[Mike Arman <armanmik@n-jcenter.com>]

Subject: re: Radio or speaker problem
Author:  Phil Rose <pjrose@servtech.com> at ENRON-Internet
Date:    7/31/98 9:39 AM


>Date: Thu, 30 Jul 1998 20:33:18 -0400 
>From: Mike Arman <armanmik@n-jcenter.com> 
     
>This noise is called "motorboating" and <snip>
>
>It isn't a speaker problem - sounds like a bad power amp chip if it is on 
>one side or one end of the car only, but if it is everywhere, it's the 
>power supply.
>
>Upgrade time . . . (boo!)
     
Mike,
Yes, "motorboating" is exactly the word I thought to use in my original 
post. You say "it isn't a speaker problem", but do you mean to rule out (as 
the problem) the amplifiers that are built into each speaker? If so, why?


No, I wouldn't rule out the individual speaker amps - I wasn't aware that
Bose put the amps in the speakers (or with them) - I'm used to dash-mount
stereos that have everything built-in, so if you did have a problem, it
would have to be either electronics, in the head unit, or mechanical, in
the speaker. The Bose amp-speaker combination is therefore subject to
either type of problem!

It would NOT be the loudspeaker itself - the speaker is a passive component
and only gives off what is put into it - only exception to that is garbled
sound from failed surround, or scratchy sound from rubbing voice coil -
neither of which is loud enough to match what you describe.

Anyway, I think the following post should answer the question of how to fix
it:


Date: Fri, 31 Jul 1998 10:50:58 -0500
From: jtessman@enron.com
Subject: Re[2]: Radio or speaker problem

     Phil;
     
     I went through the same scenario with my V8Q, one of the rear 
     speaker/amp/eq systems went south. I hunted around town until I found 
     a shop with some experience on Bose OEM systems. They pulled the unit 
     (rears are very easy) and systematically swapped out the capacitors 
     until the problem went away. Heat and age seems to be the biggest 
     culprits in the demise of these electrolytic caps.
     
     The whole thing took 2 days and cost me $40! Problem hasn't reappeared 
     since. I would have done it myself with a handfull of caps from 
     Radioshack but they use a heavy adhesive to protect the parts from 
     vibration and it is tedious to remove. Anyway, it was the best $40 I 
     ever spent on an Audi!
     
     Cheers...jon
     
     Jon Tessman


Now, for further information, here's a response I sent to another lister
discussing your problem with me: 

>
>"Motorboating" is a form of positive feedback or oscillation. In the older
days, the most common cause was failure (going open) of electrolytic filter
capacitors in the power supply. The heat from the vacuum tubes dried them
out, and they died, sometimes partially, and reduced their capacity.
>
>(Basic electronics: A capacitor looks like a short circuit to alternating
current, and simply isn't there for direct current. Example - output of
most rectifiers is a DC voltage overlaid with AC components of line noise,
alternator whine, motor starting surge, etc. The capacitor shorts all this
garbage to ground, leaving clean, filtered DC for the rest of the device.)
>
>This positive feedback (howl at high frequencies or motorboating at lower
frequencies) can also be caused by other capacitor problems elsewhere in
the radio. Almost all car radios now use IC power amp chips, and there are
LOTS of components inside those chips. There are negative feedback loops
and positive feedback loops in these amplifiers, and if the value of the
wrong component changes just enough, you get motorboating or howl, and you
need to find - that's the trick! - and replace the affected part. If the
radio is new, it is probably just one of the amps - if it is older, the
rest of them will probably soon fail in exactly the same way as the one
that's bad now . . . 
>
>This can also sometimes be a ground problem - I'm assuming that all the
easy things were checked first before diving into the radio.
>
>
>Best Regards,
>
>Mike Arman