Subject: [NEXTEL-1] New MCS motorboating

From: Marcel <[email-masked]>
Date: Thu, 02 Jan 2003 20:43:47 -0500

-------- Original Message --------
From: "Randy Wendel" <[email-masked]>
Subject:  new MCS motorboating (update)

If you have a new dual-head MCS2000 in stock with dual-head, heads-up.

Just an update regarding the MCS2000 amplified rear speaker motorboating
problem I've been pulling my hair out on...

Quick History:  About a month ago, FD got 2 new MCS2000's with dual-heads.
One is "regular" fire truck, other is chief's Ford Expedition.  With
rear-amplified speaker volume turned *completely down* (usually the case in
the chief's Expedition), and radio is turned on, an approx. 5 seconds of
motorboating/growling sound emits from rear speaker.  When mic PTT is
enable, it growls again, and when mic PTT is released, it growls again.
(from either front or rear mic).  When rear speaker volume control was
turned up about 20%, growling would go away.  The amplified speakers,
HSN-4035A, in the vehicles, were reportedly to be replaced with B-versions
due to noise issues.  I came back with the B-versions...same problem again.

Today, I pulled out of the box a new MCS which was slated for an install.
Hooked up on the bench, which included the new B-version speakers.  SAME
THING!

I called Motorola today and opened up a case.  They think there may be some
defective B-version (HSN-4035B) speakers out there.  (and I ended up with
them apparently).  I've ordered 3 more B-versions speakers...we'll see.

Technical notes:  I played around with the new radio on the bench today.  I
found if I varied the main supply voltage to the radio between 11-14 volts,
I could get the rear speaker to motorboat.  I measured the resistance
between the ground of the power connector at the radio and the ground at the
amplified speaker, there was about 3/10 ohm difference.  If I connected the
main power supply A- ground to the amp-speaker (in the speaker) ground
("bypassing" the ground between the speaker and the radio), problem went
away.  I also measured the resistance between the the amp-speaker ground and
the ground of the audio input pair in the speaker, had about 1-ohm
difference.  If I shorted the two grounds together in the amp-speaker,
problem was about 99% gone.  Sounds like a ground-loop type problem.  The
motorboating was more inherent at a lower supply voltage, say less than 12.5
down to 11.0.  Since the problem doesn't occur with amp-speaker volume
turned up slightly, I measured the vol pot resistance and at about 540 ohms
above ground is when the motorboating stopped.

Meanwhile, the fire chief is graciously being patient.  More to follow as
needed.

Randy km0d
Bloomington MN

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---

From: Marcel <[email-masked]>
Date: Thu, 02 Jan 2003 21:00:37 -0500

I  had the same problem with a MCS2000 on a boat, Moto sent out a technical bulletin that
corrected the problem. I don't have the # but it should be on file in S-burg.

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