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ubitx - purpose of relay contacts between Q70 and U1 (receive audio path)?

 

I wish to understand the use of the relay contact pair (appears to be closed during receive) in the receive audio path between the first receive audio amplifier (Q70) and the audio power amplifier (U1). Is it to deal with leakage through Q70 during transmit (thinking the potential problem is mic audio getting into the audio power amplifier which appears to have very high gain), or is there another purpose behind the relay contacts in the path?

I ask this because I'm rendering part of ubitx into a PC board layout and am not otherwise using any relays (there is no transmit power amplifier section in my implementation), so I am considering alternatives to the relay (perhaps use a FET to ground the same junction during transmit?)

Thanks for any advice,

73 Dave WB0GAZ wb0gaz@...
 

I have no idea where Q70 is, not in?this most recent schematic: ?

I've always assumed the relay just before the LM386 was there to somewhat alleviate the pop in the headphones when switching between transmit and receive. ?Haven't tried it, but I'd think this solution would remove any need for the relay: ?/g/BITX20/topic/5142279
Now I could be wrong, note that the relay opens on TX, the new FET closes to short that node to ground. ?Perhaps a SPDT analog switch would be better than just a FET??

The diagram is a bit confusing, as "FromVolumControlWiper" and "Volume1pin2" should be the same point in the Bitx40 schematic.
But he says R3 and R4 are only needed for CW sidetone injection, so apparently we are just left with the "FromVolumeControlWiper" connection
coming in from the left, which just gets shorted to ground during transmit by the FET. ?I'd guess that if you do want to inject a CW sidetone you would cut the trace to pin 3 of the LM386 and tie his "Volume1pin2" into that LM386 pin 3. ?R4 at a low 1k should have little effect during receive, During transmit R4 forms a voltage divider with the 100k R3 to reduce a 5v square wave "Sidetone" from the Raduino to something less than ear shattering.

This post references a bunch of older posts regarding the tx and rx pops: ??/g/BITX20/message/28880

Jerry, KE7ER


On Tue, Jun 27, 2017 at 07:33 pm, David Feldman wrote:
I wish to understand the use of the relay contact pair (appears to be closed during receive) in the receive audio path between the first receive audio amplifier (Q70) and the audio power amplifier (U1). Is it to deal with leakage through Q70 during transmit (thinking the potential problem is mic audio getting into the audio power amplifier which appears to have very high gain), or is there another purpose behind the relay contacts in the path?
 

Jerry,

He means ubitx - first audio amp stage.

Cheers
Raj

At 28-06-2017, you wrote:

I have no idea where Q70 is, not in this most recent schematic:??

 

They are meant to cutoff the audio during TX. There has been some discussion on the RX pop fix and if you implement that then
this relay disconnection may not be required.

Emacs!

Raj, vu2zap

At 28-06-2017, you wrote:

I wish to understand the use of the relay contact pair (appears to be closed during receive) in the receive audio path between the first receive audio amplifier (Q70) and the audio power amplifier (U1). Is it to deal with leakage through Q70 during transmit (thinking the potential problem is mic audio getting into the audio power amplifier which appears to have very high gain), or is there another purpose behind the relay contacts in the path?

I ask this because I'm rendering part of ubitx into a PC board layout and am not otherwise using any relays (there is no transmit power amplifier section in my implementation), so I am considering alternatives to the relay (perhaps use a FET to ground the same junction during transmit?)

Thanks for any advice,

73 Dave WB0GAZ wb0gaz@...
 

it is to disconnect the audio amplifier from the mic during transmit. it also cures the pop from the audio preamp.
- f

On 28 Jun 2017 4:41 p.m., "Raj vu2zap" <rajendrakumargg@...> wrote:
They are meant to cutoff the audio during TX. There has been some discussion on the RX pop fix and if you implement that then
this relay disconnection may not be required.

Emacs!

Raj, vu2zap

At 28-06-2017, you wrote:
I wish to understand the use of the relay contact pair (appears to be closed during receive) in the receive audio path between the first receive audio amplifier (Q70) and the audio power amplifier (U1). Is it to deal with leakage through Q70 during transmit (thinking the potential problem is mic audio getting into the audio power amplifier which appears to have very high gain), or is there another purpose behind the relay contacts in the path?

I ask this because I'm rendering part of ubitx into a PC board layout and am not otherwise using any relays (there is no transmit power amplifier section in my implementation), so I am considering alternatives to the relay (perhaps use a FET to ground the same junction during transmit?)

Thanks for any advice,

73 Dave WB0GAZ wb0gaz@...
 

Was posting kind of late, need to remember to read subject lines.
Had been assuming that removing power from the mike amp would mostly kill mike audio, but there is a 47uF cap to keep it going there for awhile.
If shorting with a FET is not enough, the 74LVC1G3157 is ?$0.07 each by the reel at Digikey and Mouser in a SOT23.

Jerry, KE7ER


On Wed, Jun 28, 2017 at 04:11 am, Raj vu2zap wrote:
They are meant to cutoff the audio during TX. There has been some discussion on the RX pop fix and if you implement that then
this relay disconnection may not be required.

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