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Hi,
My BITX40 is transmitting a transient when the PTT is pressed.? I'm concerned that its causing splatter.? Anybody else seeing this?? And what to do?
Thanks
Simon VK3ELH
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Looked into this a bit more.?Scope is telling me that there is a?carrier (when trimmed to a minimum) of?1V peak to peak. This is after it settles down from the initial transient spike?which appears to be 60V peak to peak.? All into a dummy load.??Audio from a nearby receiver suggests?that it is the carrier tone that is the spike signal.
Tried unplugging the speaker, the mic, the volume control, no difference.
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Re. the millisecond carrier burst...
Many who are new to BITX and other similar designs will discover the presence of a carrier? burst when transitioning from receive to transmit, and an audible click when transitioning? from transmit to receive.? This was subject to much scrutiny and analysis in the early? days of BITX20 and its many variants. ?
It seems that the culprit is the coupling capacitor between microphone amplifier and the? balanced modulator, and the coupling capacitor between receive audio amplifier and the? balanced modulator.? In either transmit or receive mode these capacitors get charged? to a particular value.? When a transition is made from transmit to receive or from receive? to transmit this charge causes a few milliseconds of bias upset in the balanced modulator. ?
Several design changes have been proposed to "fix" this situation but most seem to add? more problems than they fixed.? This situation is a trade-off for simplicity and low cost? versus complexity and high cost. ?
Arv K7HKL _._
On Fri, Nov 18, 2016 at 7:15 AM, simon.jack68@... [BITX20] <BITX20@...> wrote:
?
Looked into this a bit more.?Scope is telling me that there is a?carrier (when trimmed to a minimum) of?1V peak to peak. This is after it settles down from the initial transient spike?which appears to be 60V peak to peak.? All into a dummy load.??Audio from a nearby receiver suggests?that it is the carrier tone that is the spike signal.
Tried unplugging the speaker, the mic, the volume control, no difference.
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Thanks Arv.? That puts my mind?to rest even though there doesn't seem to be a fix
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After a couple of searches and some reading from the archives, I got a thought. ?Normally that's a dangerous thing, but I'll toss it out for the real experts just in case. hi hi Would it be possible (or useful?) to add another relay and a slight delay so that when the PTT is pressed, the RX circuit is broken and there's a pause before the TX circuit is made? I'm not concerned about the speaker noise, but rather the carrier pulse when PTT is pressed. ?Maybe giving the caps a couple of miliseconds to discharge when changing states would fix all the symptoms? ?Or maybe not? Thanks, Andy KG5RKP
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I think the "cap
discharge through a transistor" approach essentially achieves
the same outcome?
On 27/01/2017 8:08 AM, AndyH wrote:
toggle quoted message
Show quoted text
After a couple of searches and some reading from the archives,
I got a thought. ?Normally that's a dangerous thing, but I'll
toss it out for the real experts just in case. hi hi
Would it be possible (or useful?) to add another relay and a
slight delay so that when the PTT is pressed, the RX circuit is
broken and there's a pause before the TX circuit is made?
I'm not concerned about the speaker noise, but rather the
carrier pulse when PTT is pressed. ?Maybe giving the caps a
couple of miliseconds to discharge when changing states would
fix all the symptoms? ?Or maybe not?
Thanks,
Andy KG5RKP
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Can't say - I have no idea what the "cap discharge through a transistor" approach is, and haven't yet found anything about it in searches.
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Adding another relay apparently isn't a new idea, but one that apparently works. I'm still trying to find details of the mods Uli, DF5SF created. ?I haven't yet found details in further message searches or in the files section here or on the Yahoo group. /g/BITX20/message/3270?p=Created,,,20,1,0,0 There also appears to be the 'reed relay' mod: /g/BITX20/message/10973?p=Created,,,20,1,0,0::Relevance,,ptt+noise,20,2,0,4103578 I still can't find details of the 'transistor across the cap' mod. Message archeology in progress...hard hat area... ?hi hi Andy
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Try post 20828: "Farhan mentioned this during his interview with Tom,?. He felt it was due to the cap C114 around the audio preamp Q16, holding the amp on for a bit when that rig is switched into transmit. I wouldn't be surprised that C110 might also be holding U1 on as well. ?He thought about placing a transistor across the cap to discharge it when going into transmit. Drive the transistor from the xmit?voltage."
On Fri, Jan 27, 2017 at 10:17 am, AndyH wrote:
I still can't find details of the 'transistor across the cap' mod.
?
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Thanks Jerry - I guess I was looking too far back. Can anyone link to an example of how to implement such a mod? ?(Can anyone confirm that the transistor and associated bias bits needs to be installed around both C114 and C116 to do the job? ?Or are there others? ?My initial 'fear' was that I'd have to 'neutralize' caps in all of the bi-directional amps.) I have great respect for the experienced radio builders, engineers, and old Hams in the community. ?I'm none of those, however. ?What's 'obvious' to the pros doesn't yet trigger a memory of an example on page 12 of one of DeMaw's books. I'm workin' on it! ?:) Heading back into the bowels of the corporate knowledge...
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Jerry, The archeology might have paid off. ?I think this matches what Farhan mentioned and what others call the 'transistor across the cap' fix. ?I'll give it a shot. Picture in our archive from K5DNL: ?/g/BITX20/photo/2855/5?p=Name,,,20,1,0,0 The parts references appear to be from the QRPKits BITX20A schematic - page 33 from here: ? Looks like break the base connection of Q12 in that schematic (the base of Q5 in the HFSigs BITX40) and install a 2N7000 FET along with a 4K7 and 10uF. Thanks all - off to order some 2N7000s... 73
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On Fri, Jan 27, 2017 at 07:32 pm, AndyH wrote: Jerry, The archeology might have paid off. ?I think this matches what Farhan mentioned and what others call the 'transistor across the cap' fix. ?I'll give it a shot. Picture in our archive from K5DNL: ?/g/BITX20/photo/2855/5?p=Name,,,20,1,0,0 The parts references appear to be from the QRPKits BITX20A schematic - page 33 from here: ? Looks like break the base connection of Q12 in that schematic (the base of Q5 in the HFSigs BITX40) and install a 2N7000 FET along with a 4K7 and 10uF. Thanks all - off to order some 2N7000s... 73
? I just completed this mod and it didn't work. ?The radio appears to function normally, but the audio click and RF power spike remains unchanged.
Can anyone recommend a fix for this PTT audio spike that works for the BITX40 boards please? 73 Andy
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On Fri, Mar 3, 2017 at 08:24 pm, AndyH wrote: On Fri, Jan 27, 2017 at 07:32 pm, AndyH wrote:
Jerry, The archeology might have paid off. ?I think this matches what Farhan mentioned and what others call the 'transistor across the cap' fix. ?I'll give it a shot. Picture in our archive from K5DNL: ?/g/BITX20/photo/2855/5?p=Name,,,20,1,0,0 The parts references appear to be from the QRPKits BITX20A schematic - page 33 from here: ? Looks like break the base connection of Q12 in that schematic (the base of Q5 in the HFSigs BITX40) and install a 2N7000 FET along with a 4K7 and 10uF. Thanks all - off to order some 2N7000s... 73
? I just completed this mod and it didn't work. ?The radio appears to function normally, but the audio click and RF power spike remains unchanged.
Can anyone recommend a fix for this PTT audio spike that works for the BITX40 boards please? 73 Andy
?Next step... On Fri, Jan 27, 2017 at 10:22 am, Jerry Gaffke wrote:
Try post 20828: "Farhan mentioned this during his interview with Tom,?. He felt it was due to the cap C114 around the audio preamp Q16, holding the amp on for a bit when that rig is switched into transmit. I wouldn't be surprised that C110 might also be holding U1 on as well. ?He thought about placing a transistor across the cap to discharge it when going into transmit. Drive the transistor from the xmit?voltage."
I left the 2N7000 across the base of Q5 as it didn't seem to do anything. ?Next I used the same 2N7000 circuit around C114 to short the cap when switching to transmit. ?The RF spike increased (about 400 mW) and now I have a strong click when PTT is activated and a smaller click when PTT is released. I'll try C110 next. 73
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On Sat, Mar 4, 2017 at 12:55 pm, AndyH wrote: On Fri, Mar 3, 2017 at 08:24 pm, AndyH wrote:
On Fri, Jan 27, 2017 at 07:32 pm, AndyH wrote:
Jerry, The archeology might have paid off. ?I think this matches what Farhan mentioned and what others call the 'transistor across the cap' fix. ?I'll give it a shot. Picture in our archive from K5DNL: ?/g/BITX20/photo/2855/5?p=Name,,,20,1,0,0 The parts references appear to be from the QRPKits BITX20A schematic - page 33 from here: ? Looks like break the base connection of Q12 in that schematic (the base of Q5 in the HFSigs BITX40) and install a 2N7000 FET along with a 4K7 and 10uF. Thanks all - off to order some 2N7000s... 73
? I just completed this mod and it didn't work. ?The radio appears to function normally, but the audio click and RF power spike remains unchanged.
Can anyone recommend a fix for this PTT audio spike that works for the BITX40 boards please? 73 Andy
?Next step... On Fri, Jan 27, 2017 at 10:22 am, Jerry Gaffke wrote:
Try post 20828: "Farhan mentioned this during his interview with Tom,?. He felt it was due to the cap C114 around the audio preamp Q16, holding the amp on for a bit when that rig is switched into transmit. I wouldn't be surprised that C110 might also be holding U1 on as well. ?He thought about placing a transistor across the cap to discharge it when going into transmit. Drive the transistor from the xmit?voltage."
I left the 2N7000 across the base of Q5 as it didn't seem to do anything. ?Next I used the same 2N7000 circuit around C114 to short the cap when switching to transmit. ?The RF spike increased (about 400 mW) and now I have a strong click when PTT is activated and a smaller click when PTT is released. I'll try C110 next. 73
I installed the 2N7000 across C110. ?There was still a ~400mW RF pulse on key-down. ?This also killed all of the audio from the receiver except for the key-down pop. ? ? ? I've removed all three mods and, as far as I'm concerned, the "transistor across the (insert part here) fix" is a bust. ?Relay(s) next, I guess. ?Or just live with it. ? ? ? 73 ? ? ? ?Andy
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I tried shorting C114 and C110 and didn't do a thing. As a quick check I desoldered one leg of C114/110 and the kerchunk was greatly reduced. Still had some change over noise but no where near what it normally is.
73
Ken VA3ABN
On Sat, Mar 4, 2017 at 3:55 PM, AndyH <ahecker@...> wrote: On Fri, Mar 3, 2017 at 08:24 pm, AndyH wrote:
On Fri, Jan 27, 2017 at 07:32 pm, AndyH wrote:
Jerry, The archeology might have paid off.? I think this matches what Farhan mentioned and what others call the 'transistor across the cap' fix.? I'll give it a shot. Picture in our archive from K5DNL: ?/g/BITX20/photo/2855/5?p=Name,,,20,1,0,0 The parts references appear to be from the QRPKits BITX20A schematic - page 33 from here: ? Looks like break the base connection of Q12 in that schematic (the base of Q5 in the HFSigs BITX40) and install a 2N7000 FET along with a 4K7 and 10uF. Thanks all - off to order some 2N7000s... 73
? I just completed this mod and it didn't work.? The radio appears to function normally, but the audio click and RF power spike remains unchanged.
Can anyone recommend a fix for this PTT audio spike that works for the BITX40 boards please? 73 Andy
?Next step... On Fri, Jan 27, 2017 at 10:22 am, Jerry Gaffke wrote:
Try post 20828: "Farhan mentioned this during his interview with Tom,?. He felt it was due to the cap C114 around the audio preamp Q16, holding the amp on for a bit when that rig is switched into transmit. I wouldn't be surprised that C110 might also be holding U1 on as well.? He thought about placing a transistor across the cap to discharge it when going into transmit. Drive the transistor from the xmit?voltage."
I left the 2N7000 across the base of Q5 as it didn't seem to do anything.? Next I used the same 2N7000 circuit around C114 to short the cap when switching to transmit.? The RF spike increased (about 400 mW) and now I have a strong click when PTT is activated and a smaller click when PTT is released. I'll try C110 next. 73
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?Has anyone tried changing C115 and C122 from 1uF to 100nF??? (C115 for receive pop, C122 for carrier issue?)
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