[Laser] WB9AJZ laser RX problems

[W2MXW]

It's pins 1 and 8 on the '386.
I've never had a problem with oscillating '386's after the following design
measures, but plenty before, and what you are experiencing sounds like it
(called "motorboating" for obvious reasons). Do the following:
Use the Zobel network as suggested by G0MRF; I personally use .047 uF and 10
ohms in series. I use an audio output capacitor of either 100 uF or 220 uF
(whatever is on hand).
Use a 10 uF from pin 7 to gnd. (+ to pin 7.)
Use a 100 uF from pin 6 (Vcc) to gnd.
With regard to the gain control pins (1 and 8), leaving these pins open
gives a gain of 20; using a cap. of 10 uF across these pins gives a gain of
200, and a resistor and cap in series as suggested by G0MRF gives an
intermediate gain between these values dependent on the resistance value
(e.g., 1.2k gives a gain factor of 50). Naturally, using higher gains
invites the threat of oscillation so do at your own risk.

73 es good lasering :-)

Jon W2MXW

-----Original Message-----
From: G0MRF@aol.com <G0MRF@aol.com>
To: laser@mailman.qth.net <laser@mailman.qth.net>
Date: Sunday, December 08, 2002 7:34 AM
Subject: Re: [Laser] WB9AJZ laser RX problems


>Hi Wendell
>
>
>The LM386 can self oscillate:
>
>1) If there is insufficient capacitance at the supply pin to ground.
>
>2) If you leave out the series resistor and capacitor from the audio output
>pin to ground. (zobel network)
>
>In my RX I use a LM386 with 2.2 Ohms in series with 100nF to ground and a
>1000uF out to the speaker. The 2R2 / 100n should be right at the pin, not
>across the speaker.
>
>I've also used a 1k in series with a 1uF non polarised capacitor to reduce
>the open loop gain. This goes between the two gain contol pins.
>(OK I can't tell which ones they are as I've built it deadbug style and
it's
>upside down. But its either pins 1 and 8        or possibly 4 and 5.)
>
>
> Good luck
>
>
>73
>
>David  G0MRF         www.g0mrf.freeserve.co.uk/laser.htm
>
>
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