Re: Radio or speaker problem

[Pcarselle <pcarselle@worldnet.att.net>]

Mike Arman wrote:
> 
> At 10:02 PM 7/30/1998 -0400, you wrote:
> >Mike Arman wrote:
> >>
> >> >Subject: radio or speaker problem
> >> >
> >> >Yesterday--suddenly (things gotta start sometime, I suppose)--the Bose
> >> >stereo system (in '91 200q) began emitting a thundering, low-frequency
> >> >noise (thump-thump-thump-....). <snip>
> >> >
> >> >
> >>
> >> This noise is called "motorboating" and usually appears when one of the
> >> power supply filter capacitors has gone open. This applies to AC-line
> >> powered radios, and I don't know what's inside the Bose - it might have an
> >> inverter and some rectifiers if anything in the radio needs more than 12
> >> volts.
> >>
> >> 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!)
> >>
> >> Best Regards,
> >>
> >> Mike Arman
> >Mike:
> >Filter capacitors on a superb 12v supply ???
> >Is more filtering needed. Please illumninate
> >Thanks,
> >Pablo
> >
> >
> 
> Hi Pablo -
> 
> Yes, no and maybe.
> 
> "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
Mike: 
Thanks for your guidance.
I was wrong in assuming that rectifiers provide half or full wave (base
to 1.41 peak)- as opposed to DC as you mention, or sinusoidal input to
filter capacitors and that the filtering function was a result of the
two distinct  different filter seen time constants (the charge and
discharge time constants). I seem to remember it took 5 or so RC time
constants to achieve filtered DC levels,  and  that when the discharge
constant is very "quick" an interesting AC componente is fed thru the
B+.
For an automobile circuit, assuming no resistance between the +12v
supply and the subject loads, filtering was totally unnecessary and the
12v supply could be considered stable for practical purposes. Obviously
any high resistance element in the path of this assumption would
introduce a severe lack of capability to decouple the drive of a power
hungry device. I think this is why some amplifiers (high output)
strongly recommend direct connections (dedicated) to the battery. I have
heard of decoupling filters used but I believe these where used to
ground AC components in a hurry (Mr. Alternator etc...) Typically a Tank
circuit filter tuned to the expected Ac component noise (ussually a coil
whose inductance presents a high impedance to the high frequency
components you mentioned, paralled by a capacitor to ground what is
left). The oscillation you mention I presume (correct me please) is
feedback thru the B+. which without an schematic I assume is common to
the amplifiers you mention. I agree. Therefore I would go after
decoupling the 12v (very NEAR to the device) Same I would do for grounds
and their possible unwanted loops..
Component derrating or changing in parameters would push me more in the
direction of distortion ( if audio)
drift (if tuning circuit) or erroneous sampling in the CD circuits for
instance. It can of course be the amps as you said; if internal, all
speakers would play..... if speaker located.. I am with you !!!
I appreciate your message 
Regards
Pablo