Topics

#uBITX Front end overload protection #ubitx

M Garza <mgarza896@...>
 

Hello All,
Looking at the schematic, it doesn't look like the diode front end receive protection was added to the uBITX.  
Could someone better than me confirm this?  This would probably be a necessary mod to include to prevent
the front end from being overloaded and blowing transistors as with the BITX40.

Thanks,

Marco - KG5PRT

Jerry Gaffke
 

Not there.
Good to add two back-to-back 1n4148's from relay K1 pin 12 to ground to limit the AC voltage at the base of Q90
if the uBitx should ever be plugged into an antenna while some 100W transmitter is using a nearby antenna.
Or a nearby lightning bolt.


On Tue, Dec 12, 2017 at 11:24 am, M Garza wrote:
Looking at the schematic, it doesn't look like the diode front end receive protection was added to the uBITX.  

Ashhar Farhan
 

the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f

On 13 Dec 2017 2:29 am, "Jerry Gaffke via Groups.Io" <jgaffke=yahoo.com@groups.io> wrote:
Not there.
Good to add two back-to-back 1n4148's from relay K1 pin 12 to ground to limit the AC voltage at the base of Q90
if the uBitx should ever be plugged into an antenna while some 100W transmitter is using a nearby antenna.
Or a nearby lightning bolt.


On Tue, Dec 12, 2017 at 11:24 am, M Garza wrote:
Looking at the schematic, it doesn't look like the diode front end receive protection was added to the uBITX.  

M Garza <mgarza896@...>
 

Thank you for the clarification!

Marco - KG5PRT 

On Dec 12, 2017 7:44 PM, "Ashhar Farhan" <farhanbox@...> wrote:
the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f

On 13 Dec 2017 2:29 am, "Jerry Gaffke via Groups.Io" <jgaffke=yahoo.com@groups.io> wrote:
Not there.
Good to add two back-to-back 1n4148's from relay K1 pin 12 to ground to limit the AC voltage at the base of Q90
if the uBitx should ever be plugged into an antenna while some 100W transmitter is using a nearby antenna.
Or a nearby lightning bolt.


On Tue, Dec 12, 2017 at 11:24 am, M Garza wrote:
Looking at the schematic, it doesn't look like the diode front end receive protection was added to the uBITX.  

Trent Trent <vk7hrs@...>
 

Speaking of diodes has anyone got the wire up with the swr indicator

On 13 Dec. 2017 12:44 pm, "Ashhar Farhan" <farhanbox@...> wrote:
the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f

On 13 Dec 2017 2:29 am, "Jerry Gaffke via Groups.Io" <jgaffke=yahoo.com@groups.io> wrote:
Not there.
Good to add two back-to-back 1n4148's from relay K1 pin 12 to ground to limit the AC voltage at the base of Q90
if the uBitx should ever be plugged into an antenna while some 100W transmitter is using a nearby antenna.
Or a nearby lightning bolt.


On Tue, Dec 12, 2017 at 11:24 am, M Garza wrote:
Looking at the schematic, it doesn't look like the diode front end receive protection was added to the uBITX.  

John P
 

For what it's worth, I used the BitX-40 on field day this year with high power transmitters and antennas in close proximity without any problems. My rig doesn't have any extra front end protection. 
--
John - WA2FZW

Jerry Gaffke
 

Though others have reported a blown Q13 on the Bitx40v3 when hooked up to an antenna with a nearby QRO station operating.
Luck of the draw.  
Depends on antenna types, how close, orientation, bands, use of external tuner on Bitx40, power levels, feed lines, etc

Farhan's reasoning on the uBitx seems right.
Would be interesting to remove C80 (to avoid blowing Q90) and measure the AC voltages into L1
when driving the antenna port with various signals.

Jerry, KE7ER.


On Tue, Dec 12, 2017 at 06:45 pm, John P wrote:
For what it's worth, I used the BitX-40 on field day this year with high power transmitters and antennas in close proximity without any problems. My rig doesn't have any extra front end protection. 

Gordon Gibby <ggibby@...>
 

​So, regarding Q90 and protecting it from strong signals.....


the base-emitter junction of this transistor already puts one diode effectively in the input of the rig.

not exactly sure how putting one more would raise the level of spurs tremendously?


presumably what blows the transistor isn't Forward bse emitter current, but instead a reverse voltage that destroys the junction.  


I'm not smart enough to know exactly how the ring modulator would guarantee to protect from this....


but perhaps just putting one more diode, reverse to the base-emitter junction of Q90, and right AT the base of Q90 to ground.....would serve to reduce the chances of blowing this transistor???


If incredibly concerned, one might put a bit of resistance, say 50 ohms, in series with c80.   


It would reduce the current through the base-emitter junction and through the reverse diode in the event of a really strong signals...and I don't think it would reduce the gain of the stage by that much.....




From: BITX20@groups.io <BITX20@groups.io> on behalf of Jerry Gaffke via Groups.Io <jgaffke@...>
Sent: Tuesday, December 12, 2017 9:59 PM
To: BITX20@groups.io
Subject: Re: [BITX20] #uBITX Front end overload protection
 
Though others have reported a blown Q13 on the Bitx40v3 when hooked up to an antenna with a nearby QRO station operating.
Luck of the draw.  
Depends on antenna types, how close, orientation, bands, use of external tuner on Bitx40, power levels, feed lines, etc

Farhan's reasoning on the uBitx seems right.
Would be interesting to remove C80 (to avoid blowing Q90) and measure the AC voltages into L1
when driving the antenna port with various signals.

Jerry, KE7ER.

On Tue, Dec 12, 2017 at 06:45 pm, John P wrote:
For what it's worth, I used the BitX-40 on field day this year with high power transmitters and antennas in close proximity without any problems. My rig doesn't have any extra front end protection. 

Albert Morris
 

I may be wrong, but Q90 is a transmit stage and is isolated from the rx path by C80.  The ring diode mixer (D1 & D2) essentially are reverse diode pair acrosd the input to T2 secondary offering a limiting factor for Q22 in the rx stage.  Actually those diodes are the first stage device (mixer) in the rx.

Again , I may be incorrect as that is how I interperste that portion.

73 WB8FEQ



Sent from my Verizon, Samsung Galaxy smartphone

 

At K1 pin 12 add two diodes parallel but opposite polarity to ground.

You can do that at K3 pin 14 alternatively.

Raj

At 16/12/2017, you wrote:

So, regarding Q90 and protecting it from strong signals.....

M Garza <mgarza896@...>
 

Directly from the man, himself.  Message #3 in this thread:
"the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f"

On Fri, Dec 15, 2017 at 11:39 PM, Raj vu2zap <rajendrakumargg@...> wrote:
At K1 pin 12 add two diodes parallel but opposite polarity to ground.

You can do that at K3 pin 14 alternatively.

Raj

At 16/12/2017, you wrote:

>So, regarding Q90 and protecting it from strong signals.....





Gordon Gibby <ggibby@...>
 

Ray --


I was quite aware that Ashhar had stated

"the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f"

However, I didn't understand how a diode ring mixer, on the other side of a transformer, and further on the other side of a low pass filter, could protect Q90.....

which is precisely  why I wrote
"I'm not smart enough to know exactly how the ring modulator would guarantee to protect from this..."

and also why I theorized the actual mode of destruction of Q90 as follows:
"presumably what blows the transistor isn't Forward bse emitter current, but instead a reverse voltage that destroys the junction.  "

and as Ashhar had not commented on the existence ALREADY of ONE DIODE i the front end, namely, the base-emitter junction of Q90, I wrote the following:

"the base-emitter junction of this transistor already puts one diode effectively in the input of the rig.

not exactly sure how putting one more would raise the level of spurs tremendously?"



Therefore,  based on the above points, I suggested simply adding a REVERSE diode directly across base-emitter of Q90, designed to protect that transistor from being destroyed by a high REVERSE voltage (negative on base) as C80 becomes charged up by the half-wave rectification caused by the base-emitter junction of Q90 in response to a large input RF waveform.....  [Q90 essentially performing half-wave rectification and charging up C80.]

The base-emitter of Q90 will conduct when the input waveform is POSITIVE, charging C80 in such a direction that when the input waveform goes NEGATIVE, C80 will add to the voltage and contribute to destroying the base emitter junction of Q90 --- adding a reverse diode across the base-emitter junction would prevent that, and presumably do little to add any additional spurs, if spurs are indeed created by the existence of the base emitter junction of Q90 on the other side of C80 already.

Albert opined:
"I may be wrong, but Q90 is a transmit stage and is isolated from the rx path by C80.  The ring diode mixer (D1 & D2) essentially are reverse diode pair acrosd the input to T2 secondary offering a limiting factor for Q22 in the rx stage.  Actually those diodes are the first stage device (mixer) in the rx."

And agreed Q90 is a transmit stage, but the concern is that just as the corresponding transistor in the BITX40 version, it will NOT be protected by the series capacitor C80, but rather destroyed by it, due to the half wave rectification resulting from Q90's base-emitter junction in the face of a strong incoming waveform.   Agreed that the diode ring mixer are a reverse diode pair across the a transformer secondary, but it is not clear (to me at least) how they offer protection to Q90 which receives PRE Low Pass Filter radio frequency signals.   


Jerry's suggestion for a test, 
"Would be interesting to remove C80 (to avoid blowing Q90) and measure the AC voltages into L1
when driving the antenna port with various signals."

implicitly contains the assumption that Q90 is  **NOT** protected in the situation of high AC voltage applied the incoming antenna port, which was the impetus for my suggesting an alternate solution to the perceived problem.


Hope all that verbiage addresses the issues raised,
73,
Gordon Gibby







From: BITX20@groups.io <BITX20@groups.io> on behalf of M Garza <mgarza896@...>
Sent: Saturday, December 16, 2017 1:01 AM
To: BITX20@groups.io
Subject: Re: [BITX20] #uBITX Front end overload protection
 
Directly from the man, himself.  Message #3 in this thread:
"the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f"

On Fri, Dec 15, 2017 at 11:39 PM, Raj vu2zap <rajendrakumargg@...> wrote:
At K1 pin 12 add two diodes parallel but opposite polarity to ground.

You can do that at K3 pin 14 alternatively.

Raj

At 16/12/2017, you wrote:

>So, regarding Q90 and protecting it from strong signals.....





 

Its true but from my experience with BITX40 - it can handle only 2mV at the antenna before it distorts
at even at minimum volume. I have not tested the uBitx in detail but I expect it to be similar as there is no AGC
in the RF stages. When I get back to town I will report on this.

The only case I see is strong out of band signals WILL cause problems, if you don't have nearby commercial/ham
transmitters then you should have no problems with diodes. Any signal over 50mV will be more than 60db over S9..

Raj


At 16/12/2017, you wrote:
Directly from the man, himself.  Message #3 in this thread:
"the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f"

On Fri, Dec 15, 2017 at 11:39 PM, Raj vu2zap < rajendrakumargg@...> wrote:
At K1 pin 12 add two diodes parallel but opposite polarity to ground.

You can do that at K3 pin 14 alternatively.

Raj

At 16/12/2017, you wrote:

>So, regarding Q90 and protecting it from strong signals.....

 

Hi Gordon,

I have a ham next door- two houses away and I am his elmer. His signals were so strong with SB220s that I got splatter
all the way to 145Mhz.. One day I connect a 60mA dial lamp across my antenna and it was glowing.. thats what is a problem.
There has been reports here that when one transmits on say 20M or 15M from another rig into another antenna then
the signal picked up by the BITX40 40M antenna is strong enough to fry Q13.

If the antenna signal from your own other antenna or a neighbors is really strong then even the mixer will fry IMHO.

The reverse voltage for BE on 3904 is 6V, it is a zener. The current/power that the junction handle is the issue.

If the scenario that I presented does not apply to group members then no need to do the mod.

Raj

At 16/12/2017, you wrote:

Ray --


I was quite aware that Ashhar had stated
"the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f"

However, I didn't understand how a diode ring mixer, on the other side of a transformer, and further on the other side of a low pass filter, could protect Q90.....

which is precisely why I wrote
"I'm not smart enough to know exactly how the ring modulator would guarantee to protect from this..."

and also why I theorized the actual mode of destruction of Q90 as follows:
"presumably what blows the transistor isn't Forward bse emitter current, but instead a reverse voltage that destroys the junction. "

and as Ashhar had not commented on the existence ALREADY of ONE DIODE i the front end, namely, the base-emitter junction of Q90, I wrote the following:

"the base-emitter junction of this transistor already puts one diode effectively in the input of the rig.

not exactly sure how putting one more would raise the level of spurs tremendously?"


Therefore, based on the above points, I suggested simply adding a REVERSE diode directly across base-emitter of Q90, designed to protect that transistor from being destroyed by a high REVERSE voltage (negative on base) as C80 becomes charged up by the half-wave rectification caused by the base-emitter junction of Q90 in response to a large input RF waveform..... [Q90 essentially performing half-wave rectification and charging up C80.]

The base-emitter of Q90 will conduct when the input waveform is POSITIVE, charging C80 in such a direction that when the input waveform goes NEGATIVE, C80 will add to the voltage and contribute to destroying the base emitter junction of Q90 --- adding a reverse diode across the base-emitter junction would prevent that, and presumably do little to add any additional spurs, if spurs are indeed created by the existence of the base emitter junction of Q90 on the other side of C80 already.

Albert opined:
"I may be wrong, but Q90 is a transmit stage and is isolated from the rx path by C80. The ring diode mixer (D1 & D2) essentially are reverse diode pair acrosd the input to T2 secondary offering a limiting factor for Q22 in the rx stage. Actually those diodes are the first stage device (mixer) in the rx."

And agreed Q90 is a transmit stage, but the concern is that just as the corresponding transistor in the BITX40 version, it will NOT be protected by the series capacitor C80, but rather destroyed by it, due to the half wave rectification resulting from Q90's base-emitter junction in the face of a strong incoming waveform. Agreed that the diode ring mixer are a reverse diode pair across the a transformer secondary, but it is not clear (to me at least) how they offer protection to Q90 which receives PRE Low Pass Filter radio frequency signals.


Jerry's suggestion for a test,
"Would be interesting to remove C80 (to avoid blowing Q90) and measure the AC voltages into L1
when driving the antenna port with various signals."

implicitly contains the assumption that Q90 is **NOT** protected in the situation of high AC voltage applied the incoming antenna port, which was the impetus for my suggesting an alternate solution to the perceived problem.


Hope all that verbiage addresses the issues raised,
73,
Gordon Gibby

K9HZ <bill@...>
 

Yuk!  Sounds like a good case for adding front-end sharp (tunable) band pass filters. 


Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

Owner – Operator

Villa Grand Piton - J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com


email:  bill@...

 


On Dec 16, 2017, at 7:19 AM, Raj vu2zap <rajendrakumargg@...> wrote:

Hi Gordon,

I have a ham next door-  two houses away and I am his elmer. His signals were so strong with SB220s that I got splatter
all the way to 145Mhz.. One day I connect a 60mA dial lamp across my antenna and it was glowing.. thats what is a problem.
There has been reports here that when one transmits on say 20M or 15M from another rig into another antenna then
the signal picked up by the BITX40 40M antenna is strong enough to fry Q13.

If the antenna signal from your own other antenna or a neighbors is really strong then even the mixer will fry IMHO.

The reverse voltage for BE on 3904 is 6V, it is a zener. The current/power that the junction handle is the issue.

If the scenario that I presented does not apply to group members then no need to do the mod.

Raj

At 16/12/2017, you wrote:

Ray --


I was quite aware that Ashhar had stated
"the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f"

However, I didn't understand how a diode ring mixer, on the other side of a transformer, and further on the other side of a low pass filter, could protect Q90.....

which is precisely  why I wrote
"I'm not smart enough to know exactly how the ring modulator would guarantee to protect from this..."

and also why I theorized the actual mode of destruction of Q90 as follows:
"presumably what blows the transistor isn't Forward bse emitter current, but instead a reverse voltage that destroys the junction.  "

and as Ashhar had not commented on the existence ALREADY of ONE DIODE i the front end, namely, the base-emitter junction of Q90, I wrote the following:

"the base-emitter junction of this transistor already puts one diode effectively in the input of the rig.

not exactly sure how putting one more would raise the level of spurs tremendously?"


Therefore,  based on the above points, I suggested simply adding a REVERSE diode directly across base-emitter of Q90, designed to protect that transistor from being destroyed by a high REVERSE voltage (negative on base) as C80 becomes charged up by the half-wave rectification caused by the base-emitter junction of Q90 in response to a large input RF waveform.....  [Q90 essentially performing half-wave rectification and charging up C80.]

The base-emitter of Q90 will conduct when the input waveform is POSITIVE, charging C80 in such a direction that when the input waveform goes NEGATIVE, C80 will add to the voltage and contribute to destroying the base emitter junction of Q90 --- adding a reverse diode across the base-emitter junction would prevent that, and presumably do little to add any additional spurs, if spurs are indeed created by the existence of the base emitter junction of Q90 on the other side of C80 already.

Albert opined:
"I may be wrong, but Q90 is a transmit stage and is isolated from the rx path by C80.  The ring diode mixer (D1 & D2) essentially are reverse diode pair acrosd the input to T2 secondary offering a limiting factor for Q22 in the rx stage.  Actually those diodes are the first stage device (mixer) in the rx."

And agreed Q90 is a transmit stage, but the concern is that just as the corresponding transistor in the BITX40 version, it will NOT be protected by the series capacitor C80, but rather destroyed by it, due to the half wave rectification resulting from Q90's base-emitter junction in the face of a strong incoming waveform.   Agreed that the diode ring mixer are a reverse diode pair across the a transformer secondary, but it is not clear (to me at least) how they offer protection to Q90 which receives PRE Low Pass Filter radio frequency signals.   


Jerry's suggestion for a test,
"Would be interesting to remove C80 (to avoid blowing Q90) and measure the AC voltages into L1
when driving the antenna port with various signals."

implicitly contains the assumption that Q90 is  **NOT** protected in the situation of high AC voltage applied the incoming antenna port, which was the impetus for my suggesting an alternate solution to the perceived problem.


Hope all that verbiage addresses the issues raised,
73,
Gordon Gibby




Albert Morris
 

One last thought that "might" address bot concerns (giving strong signal protection in rx but not introducing possible spurs in tx).  1n914 diodes are cheap.  Put a pair between relay K1 pins 12 & 16.  This will clip/limit strong signsls on the antenna line in rx,  but in tx the relay removes them from Q30 & beyond and actuall sjorts rhe pair out effectively taking them out of any tx components.



Sent from my Verizon, Samsung Galaxy smartphone

Gordon Gibby <ggibby@...>
 

Raj -- thank you for that hilarious bit of experimental data!!!! It adds an honest-to-goodness data point to the discussion!!!! And from that I think further inferences can be drawn:

Again I can't claim to be an expert on transistor failures. Your data of

- enough voltage and enough current to light a "60 mA dial lamp" -- probably a 6 or 12 V device --- gives us some information: the voltage is several volts, possibly as many as 12V RMS and the current is tens of milliamperes or more.

The MMBT version of 2N3904 is rated (at 25 C) for 350mW total device dissipation.

At 0.7 V forward base-emitter that would be 0.5 Amps
At 6 Volt (breakdown into zener effect) -- that would be 58 mA.........


Q90 doesn't get collector voltage when in receive, so there wouldn't be any collector current.

So if as you suggest the failure mechanism is POWER, then I think itis much more likely to occur in the REVERSE current direction where the base-emitter junction V (EBO) max rating has been exceeded and then the device may be destroyed by power dissipation from the current.

So I think a backwards diode across the junction would conduct when the base went 0.7V negative andprevent the power dissipation in the transistor in the reverse current scenario.


________________________________________
From: BITX20@groups.io <BITX20@groups.io> on behalf of Raj vu2zap <@Raj>
Sent: Saturday, December 16, 2017 8:19 AM
To: BITX20@groups.io
Subject: Re: [BITX20] #uBITX Front end overload protection

Hi Gordon,

I have a ham next door- two houses away and I am his elmer. His signals were so strong with SB220s that I got splatter
all the way to 145Mhz.. One day I connect a 60mA dial lamp across my antenna and it was glowing.. thats what is a problem.
There has been reports here that when one transmits on say 20M or 15M from another rig into another antenna then
the signal picked up by the BITX40 40M antenna is strong enough to fry Q13.

If the antenna signal from your own other antenna or a neighbors is really strong then even the mixer will fry IMHO.

The reverse voltage for BE on 3904 is 6V, it is a zener. The current/power that the junction handle is the issue.

If the scenario that I presented does not apply to group members then no need to do the mod.

Raj

At 16/12/2017, you wrote:

Ray --


I was quite aware that Ashhar had stated
"the front end diodes in the mixer will clamp any signal higher than half a volt. this should take care of the very powerful signals.
adding diodes at the open front end can lead to a very large number of spurs in ubitx. unfortunately, unlike the bitx40, ubitx does not have the luxury of a narrow, triple tunec circuit.
- f"

However, I didn't understand how a diode ring mixer, on the other side of a transformer, and further on the other side of a low pass filter, could protect Q90.....

which is precisely why I wrote
"I'm not smart enough to know exactly how the ring modulator would guarantee to protect from this..."

and also why I theorized the actual mode of destruction of Q90 as follows:
"presumably what blows the transistor isn't Forward bse emitter current, but instead a reverse voltage that destroys the junction. "

and as Ashhar had not commented on the existence ALREADY of ONE DIODE i the front end, namely, the base-emitter junction of Q90, I wrote the following:

"the base-emitter junction of this transistor already puts one diode effectively in the input of the rig.

not exactly sure how putting one more would raise the level of spurs tremendously?"


Therefore, based on the above points, I suggested simply adding a REVERSE diode directly across base-emitter of Q90, designed to protect that transistor from being destroyed by a high REVERSE voltage (negative on base) as C80 becomes charged up by the half-wave rectification caused by the base-emitter junction of Q90 in response to a large input RF waveform..... [Q90 essentially performing half-wave rectification and charging up C80.]

The base-emitter of Q90 will conduct when the input waveform is POSITIVE, charging C80 in such a direction that when the input waveform goes NEGATIVE, C80 will add to the voltage and contribute to destroying the base emitter junction of Q90 --- adding a reverse diode across the base-emitter junction would prevent that, and presumably do little to add any additional spurs, if spurs are indeed created by the existence of the base emitter junction of Q90 on the other side of C80 already.

Albert opined:
"I may be wrong, but Q90 is a transmit stage and is isolated from the rx path by C80. The ring diode mixer (D1 & D2) essentially are reverse diode pair acrosd the input to T2 secondary offering a limiting factor for Q22 in the rx stage. Actually those diodes are the first stage device (mixer) in the rx."

And agreed Q90 is a transmit stage, but the concern is that just as the corresponding transistor in the BITX40 version, it will NOT be protected by the series capacitor C80, but rather destroyed by it, due to the half wave rectification resulting from Q90's base-emitter junction in the face of a strong incoming waveform. Agreed that the diode ring mixer are a reverse diode pair across the a transformer secondary, but it is not clear (to me at least) how they offer protection to Q90 which receives PRE Low Pass Filter radio frequency signals.


Jerry's suggestion for a test,
"Would be interesting to remove C80 (to avoid blowing Q90) and measure the AC voltages into L1
when driving the antenna port with various signals."

implicitly contains the assumption that Q90 is **NOT** protected in the situation of high AC voltage applied the incoming antenna port, which was the impetus for my suggesting an alternate solution to the perceived problem.


Hope all that verbiage addresses the issues raised,
73,
Gordon Gibby