Adjust modulation inside President JFK mainboard PA338AB ?

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rooth
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Adjust modulation inside President JFK mainboard PA338AB ?

Post by rooth » 09 Jan 2020, 21:37

Hello everybody !
Do you know how modulation at JFK President ( mainboard PA338AB ) can be increased?
I made the microphone change from the dynamic variant to the elecret. It is ok, but I would like to increase the modulation a bit.

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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by ch25 » 09 Jan 2020, 22:59

Vr12
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by cb4ever104 » 10 Jan 2020, 06:51

Thought you might have tried this first before going to the trouble of changing the element.
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by rooth » 10 Jan 2020, 18:57

@ ch25 : I'm not VR12 on motherboard .I looked better on the motherboard and I think it's VR5 ... What do you think?
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by ch25 » 10 Jan 2020, 19:30

Nope, it is PWR pot. Any AM mod/AMC labelled?
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by rooth » 10 Jan 2020, 19:42

Only SQ , DEV , RF.M and S.M .
I think it must be RF.M ...?

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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by ch25 » 10 Jan 2020, 20:23

On 338 board there is no am mod adjustment. Just found a schematic.
You can try to remove Q31 to disable limiter, but you need to adjust micgain to avoid overmodulation.
q31.jpg
Try that, if it will be too strong we try to modify AMC level other way.
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by rooth » 10 Jan 2020, 20:41

:wave: OK, I'm trying. Thanks!

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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by Black Spirit » 10 Jan 2020, 21:15

DEV is for FM modulation [DEVIATION] Dont adjust it too high though or you may over deviate.

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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by cb4ever104 » 11 Jan 2020, 08:20

ch25 wrote:
10 Jan 2020, 20:23
On 338 board there is no am mod adjustment. Just found a schematic.
Weird, or what ? Something to note for the future, and yet another reason not to buy a JFK
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by rooth » 11 Jan 2020, 12:31

@cb4ever104 : No problem, I like to experiment :D Radio is great !

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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by 14CS06 » 11 Jan 2020, 19:53

Hello

Image

Image

@+
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ch25
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by ch25 » 11 Jan 2020, 20:23

What is have to do with topic we discuss here Claude?
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by rooth » 24 Jan 2020, 18:29

The limitation with Q31 I see is for AM ... For FM I am interested in canceling the limitation.
I did not find any other references on the internet related to this ....
I think in the last instance I try to increase the power from the adjustable VR5 motherboard.
Does the power increase to the left?
Do you need any further changes?
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Re: Adjust modulation inside President JFK mainboard PA338AB ?

Post by ch25 » 24 Jan 2020, 19:10

Do not cancel FM limiter, you will become unreadable.
You can't have too many antennas...

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