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Mark
Hi Frank,
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Good idea, I think it should work -- the ptt jack is close so the connection should be easy.? I think I did test for muting using the diode either way with Vcc and Ground and both worked fine.? I also tried different diodes (silicon, germanium, schottky) to see if the different drops changed anything, but I couldn't tell a difference by ear anyway. Maybe Arv can compare the two methods since he has a board on the bench! Mark - N7EKU --- On Tue, 9/13/11, Frank <w4nhj@...> wrote:
From: Frank <w4nhj@...> Subject: Re: [BITX20] A simple mod for the ptt pop squeal problem... To: BITX20@... Date: Tuesday, September 13, 2011, 11:23 PM ? Mark, Arv, Would reversing the diode and picking up the PTT ground work? That way the audio amp would be muted before the transmitter was turn on. This might eliminate the pop all together. Frank - W4NHJ On 9/13/11 6:29 PM, Mark wrote: Any time Arv, Let me know how it works for you... 73s, Mark - N7EKU --- On Tue, 9/13/11, Arv Evans <arvid.evans@... <mailto:arvid.evans%40gmail.com>> wrote: From: Arv Evans <arvid.evans@... <mailto:arvid.evans%40gmail.com>> Subject: Re: [BITX20] A simple mod for the ptt pop squeal problem... To: BITX20@... <mailto:BITX20%40yahoogroups.com> Date: Tuesday, September 13, 2011, 7:48 PM Mark N7EKU Nice mod. I will try it on the BITX20A that is lying on my workbench. Thanks for the information. Arv K7HKL _._ On 09/13/2011 09:44 AM, Mark wrote: Hi all, I was googling around a bit, thinking of changing the gain on the lm386, and found a simple solution for the popping problem. I found in some reading that pin 7 can be used for muting the amplifier. Muting will occur if either 0V or Vcc is applied to pin 7. So I simply installed a 1N4148 diode between the transmit supply line and pin 7 of the lm386 so that it conducts Vcc to the pin upon closing of the ptt switch. There is still a small pop occuring after this mod, but it is reduced in magnitude by a great amount. I feel this is a nice mod and in keeping with the KISS theme of the BITX line :-) I will put some photos up of my pcb after the mod, but it pretty easy to do on the backside as the 4-pin readymate connector header is just next to pin 7 on the lm386 and a diode reaches very easily between the two. I also read that it is recommended, when using increased gain on the lm386 (like we are), to have a 1-10uF capacitor installed between pin 7 and ground in order to reduce power supply noise problems (see the second graph on the datasheet). This in itself helped a little bit with the pop problem (I tried this mod first) in that the pop sound changed from a "squeal" to a "crunch" which was a bit less unpleasant hi hi. This mod is also easy to do on the backside as a small radial lead electrolytic or tantalum cap can be laid down along the pins of the IC and leads soldered between pins 7 and 2 (which is grounded). Cheers and have a great day everyone! Mark - N7EKU [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] |
