KX1 speaker output with superimposed oscillation

[Francisco Barros]

Hi to all,

The scope shows a superimposed high frequency oscillation just on the
negative peaks of the audio signal.
It produces a distorted and annoying audio when the signal peaks, specially
when listening a loud station on the external speaker.
This happens when the volume is close to or at maximum volume. I fails just
with Panasonic RP-HT60 20 Ohm headphones and external 8 ohm speaker (both
work fine with my K2).
It works OK with Panasonic earbud type headphone and with an external 8 Ohm
load !!.

I have re-check every component all around LM386 audio amp: values are OK,
soldering fine.
It doesn't matter if the KX1 is working with external power (7-12 V) or
batteries.
At 12 V the RX current drain is about 35 mA (minimum volume). I raises to 71
mA at maximum volume with the external 8 Ohm resistor, and 75 mA with the
external 8 Ohm speaker.

Hope somebody can help me. Thanks in advance.

73,
Paco Barros

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[Don Wilhelm-4]

Paco,

That certainly sounds like the strong station is overdriving the LM386,
and that the overdrive point changes depending on the load to the
LM386.  Try reducing the RF gain to see if the condition is improved.

The LM386 characteristics are different than the LM380 AF Amp used in
the K2, so the fact that your headphones and external speaker work fine
with the K2 is not really a good indication.

The load to the AF Amp in the KX1 changes as the AF Gain is changed, so
the fact that you notice the distortion only at the max settings is not
an indication of a problem.  It may just be that you cannot use the
RP-HT60 headphones nor your 8 ohm speaker at max AF Gain with your KX1.  
A 4 ohm speaker or an amplified speaker may be the best solution,
although you could reduce the gain of the LM386 to prevent the strong
input signals from driving it to the zero voltage rail.

73,
Don W3FPR

Francisco Barros wrote:

> Hi to all,
>
> The scope shows a superimposed high frequency oscillation just on the
> negative peaks of the audio signal.
> It produces a distorted and annoying audio when the signal peaks,
> specially when listening a loud station on the external speaker.
> This happens when the volume is close to or at maximum volume. I fails
> just with Panasonic RP-HT60 20 Ohm headphones and external 8 ohm
> speaker (both work fine with my K2).
> It works OK with Panasonic earbud type headphone and with an external
> 8 Ohm load !!.
>

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[Francisco Barros]

Hi Don

First of all, thanks for your answer.

Well, I have checked it also with a signal generator: the output signal at
R3 (AF Gain) & R5 junction is always a clean 2,2 Vpp 600 Hz tone, with RF
levels of 10 uV till 500 uV, except at close or maximum volumen. I this
case, even with a 10 uV RF level, the strange oscillation is present, just
on the negative peaks, as I told on my first post.
At RF levels above 500 uV the signal is clearly distorted (almost square,
even at minimum volume) due to overload, I think this is normal. At levels
below 10 uV, the audio level is lower also, so no problem with this
artifact.

It is quite strange cause it works fine with a resisitive load of 8 Ohm, but
not with a 8 Ohm speaker. ??

Again, many thanks.
73,
Paco

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