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Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 09 Jan 2020, 21:37
by rooth
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.

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 09 Jan 2020, 22:59
by ch25
Vr12

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 10 Jan 2020, 06:51
by cb4ever104
Thought you might have tried this first before going to the trouble of changing the element.

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 10 Jan 2020, 18:57
by rooth
@ ch25 : I'm not VR12 on motherboard .I looked better on the motherboard and I think it's VR5 ... What do you think?

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 10 Jan 2020, 19:30
by ch25
Nope, it is PWR pot. Any AM mod/AMC labelled?
Chris

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 10 Jan 2020, 19:42
by rooth
Only SQ , DEV , RF.M and S.M .
I think it must be RF.M ...?

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 10 Jan 2020, 20:23
by ch25
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.
Chris

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 10 Jan 2020, 20:41
by rooth
:wave: OK, I'm trying. Thanks!

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 10 Jan 2020, 21:15
by Black Spirit
DEV is for FM modulation [DEVIATION] Dont adjust it too high though or you may over deviate.

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 11 Jan 2020, 08:20
by cb4ever104
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

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 11 Jan 2020, 12:31
by rooth
@cb4ever104 : No problem, I like to experiment :D Radio is great !

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 11 Jan 2020, 19:53
by 14CS06
Hello

Image

Image

@+
Claude

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 11 Jan 2020, 20:23
by ch25
What is have to do with topic we discuss here Claude?

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 24 Jan 2020, 18:29
by rooth
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?

Re: Adjust modulation inside President JFK mainboard PA338AB ?

Posted: 24 Jan 2020, 19:10
by ch25
Do not cancel FM limiter, you will become unreadable.