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Strange receive problem

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Joseph Reed-2

Strange receive problem

I have been pulling my hair out for about a month with this problem, and perhaps someone has seen it before and can point me on the right path.

I have an issue on 30m receive only. When I transmit on 30m I get full power, and it receives normal, but after a while it loses most of its sensitivity. Transmit again and it will hear the other side of the earth. I have noticed that once it goes partially deaf I can get a High Curr indication on the first dash I send, but after that it puts out full power with no complaints and of course after every transmission it is that beautiful K2 receive.

Here is the kicker - 20m is great. The only difference in the LPF is K3 switches out some capacitors. I have resoldered all the connections, and replaced K3 (bought a Hakko 808 just to do it).

I think we can eliminate the BPF as nothing changes in it for either 30 or 20. And since I replaced K3, I am beginning to give up hope that it is something in the LPF.

Guys, I'm running out of ideas. These are latching relays so it isn't like they can partially break contact. Help!

Thanks,
Joe N9JR
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Vic K2VCO

Re: Strange receive problem

Joseph Reed wrote:

> I have an issue on 30m receive only. When I transmit on 30m I get
> full power, and it receives normal, but after a while it loses most
> of its sensitivity. Transmit again and it will hear the other side
> of the earth. I have noticed that once it goes partially deaf I can
> get a High Curr indication on the first dash I send, but after that
> it puts out full power with no complaints and of course after every
> transmission it is that beautiful K2 receive.

Check the 30m feed line, connectors, etc. Also test it into a dummy
load. Although you might have some difficulty determining receive
sensitivity, you'll notice if you get the Hi Curr indication. Maybe the
problem isn't actually in the K2! Poor contact in coax connectors often
looks like this.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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Joseph Reed-2

Re: Strange receive problem

Thanks Vic,

But its not the antenna or coax. I had already tested it, besides being a multiband vertical it shouldn't happen on only one band. According to the 259B my SWR is 1.2:1, and brand new coax from the antenna to the switch.

Thanks for taking time to give me your advice,

Regards,
Joe N9JR

Vic K2VCO <[hidden email]> wrote:
Joseph Reed wrote:

> I have an issue on 30m receive only. When I transmit on 30m I get
> full power, and it receives normal, but after a while it loses most
> of its sensitivity. Transmit again and it will hear the other side
> of the earth. I have noticed that once it goes partially deaf I can
> get a High Curr indication on the first dash I send, but after that
> it puts out full power with no complaints and of course after every
> transmission it is that beautiful K2 receive.

Check the 30m feed line, connectors, etc. Also test it into a dummy
load. Although you might have some difficulty determining receive
sensitivity, you'll notice if you get the Hi Curr indication. Maybe the
problem isn't actually in the K2! Poor contact in coax connectors often
looks like this.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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Don Wilhelm-3

RE: Strange receive problem

In reply to this post by Joseph Reed-2

Joe,

Try checking the RF voltage out of the VCO (at TP1) - compare 30 meters to
40 and 20. If the 30 meter output is low, that could be the major cause of
your problem. If this is the situation, look first at the voltage at R30
for clues. Normally the 30 meter VCO alignment falls quite within range,
but if the capacitors in the VFO selection area are not quite the right
value, it could make any one band go out of range.

Other than the BPF and the VFO range, there is not much that is band
specific in the receiver - all it takes is things tuned to the correct
frequencies.

73,
Don W3FPR

> -----Original Message-----
>
> I have been pulling my hair out for about a month with this
> problem, and perhaps someone has seen it before and can point me
> on the right path.
>
> I have an issue on 30m receive only. When I transmit on 30m I
> get full power, and it receives normal, but after a while it
> loses most of its sensitivity. Transmit again and it will hear
> the other side of the earth. I have noticed that once it goes
> partially deaf I can get a High Curr indication on the first dash
> I send, but after that it puts out full power with no complaints
> and of course after every transmission it is that beautiful K2 receive.
>
> Here is the kicker - 20m is great. The only difference in the
> LPF is K3 switches out some capacitors. I have resoldered all
> the connections, and replaced K3 (bought a Hakko 808 just to do it).
>
> I think we can eliminate the BPF as nothing changes in it for
> either 30 or 20. And since I replaced K3, I am beginning to give
> up hope that it is something in the LPF.
>
>

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Joseph Reed-2

RE: Strange receive problem

Thanks Don,

I'll move the probe to TP1 and see if it changes when receiver sensitivity falls off.

I thought that if VCO voltage was low PLL lock would be lost. When this happens, the PLL stays locked, as verified by listening to stronger stations. Also the VCO/PLL chain is before any RF amplification of a transmit signal. Hence my perplexing question. A single transmitted dash brings it back. (As I was typing it went quiet, and a single dash made things right.)

To pose another hypothesis, when it goes insensitive (I can still copy very strong signals) it is almost as if the 20m components were being switched back in. There is no relay chatter, and I set sidetone level to zero, and AF to zero after it happens, and when I transmit no relay noise. Could the I/O controller be the problem? Sending a small voltage to the relay, but not enough to latch it back causing a high resistance connection at the wipers to partially enable the 20m components in the BPF. And transmitting a signal forces it closed?

After I added the KX60V I went through and did the VCO voltage measurements and 30 was fine.

You are correct that there are very few portions of the radio that are band specific, namely the VCO circuit which switches the diodes, the LPF and BPF, and the I/O controller which affects the first three.

I'm feeling pretty stupid as I can't find the problem. I knew once I posted you would be kind enough to give me your experienced opinion. I am just suprised that as the Dean of Support you have not seen this before.

Again thanks for taking time to consider my plight.

Regards,
Joe N9JR

Don Wilhelm <[hidden email]> wrote:
Joe,

Try checking the RF voltage out of the VCO (at TP1) - compare 30 meters to
40 and 20. If the 30 meter output is low, that could be the major cause of
your problem. If this is the situation, look first at the voltage at R30
for clues. Normally the 30 meter VCO alignment falls quite within range,
but if the capacitors in the VFO selection area are not quite the right
value, it could make any one band go out of range.

Other than the BPF and the VFO range, there is not much that is band
specific in the receiver - all it takes is things tuned to the correct
frequencies.

73,
Don W3FPR

John-483

Re: Strange receive problem

In reply to this post by Vic K2VCO

I have the same problem on one of my 40M dipoles, hears good for a while
and then the signals go way down. One tap of the key and receive is
restored. I know the problem is at the remote coax switch, most likely some
corrosion at the coax connector. I've had the problem in the past with
another coax connector and agree that Vic's suggestion has merit. When I
always ran QRO in the past I never had the problem, don't know if the KW
kept it clean or what?. Since I've been total QRP for the past 5 years I've
seen this problem 3 times. I clean all the connectors and the problem
clears. Worth a shot?? RF does some strange things.

John
k7up

At 04:33 PM 22/04/06, you wrote:

>Joseph Reed wrote:
>
>>I have an issue on 30m receive only. When I transmit on 30m I get
>>full power, and it receives normal, but after a while it loses most
>>of its sensitivity. Transmit again and it will hear the other side
>>of the earth. I have noticed that once it goes partially deaf I can
>>get a High Curr indication on the first dash I send, but after that
>>it puts out full power with no complaints and of course after every
>>transmission it is that beautiful K2 receive.
>
>Check the 30m feed line, connectors, etc. Also test it into a dummy
>load. Although you might have some difficulty determining receive
>sensitivity, you'll notice if you get the Hi Curr indication. Maybe the
>problem isn't actually in the K2! Poor contact in coax connectors often
>looks like this.
>--
>73,
>Vic, K2VCO
>Fresno CA
>http://www.qsl.net/k2vco
>_______________________________________________
>Elecraft mailing list
>Post to: [hidden email]
>You must be a subscriber to post to the list.
>Subscriber Info (Addr. Change, sub, unsub etc.):
>http://mailman.qth.net/mailman/listinfo/elecraft
>
>Help: http://mailman.qth.net/subscribers.htm
>Elecraft web page: http://www.elecraft.com

Don Wilhelm-3

RE: Strange receive problem

In reply to this post by Joseph Reed-2

Joe,

Not the counter probe - use an RF Probe or a 'scope - the significant factor
is the RF voltage, the counter probe will only provide the frequency and I
expect that will be correct.

What you may be encountering is low injection to the mixer - the level
should be +7dBm for the TUF-1 mixer - if that level is reduced, the mixer IF
output will also be less than it should be - and decreased receiver
sensitivity will be the obvious result. This is separate from a VCO lock
condition, but the VCO lock condition becomes more unstable at lower (R30)
voltages.

The other possibility that I can think of is a leaky coupling capacitor -
particularly those associated with the D6/D7 switch - in combination with
the RF Chokes, there could be some strange condition that only occurs on 30
meters - A short burst of transmit would change the DC conditions on the
capacitor and restore things to 'normal' until it charged up again - as far
as I know, the only real way to check for that is to substitute capacitors
in hopes that it will be found - not the best way to de-bug, but sometimes
that is the only way to go. Remember that Murphy rules in instances like
this - if it can't happen, it WILL!!

73,
Don W3FPR

> -----Original Message-----
>
> Thanks Don,
>
> I'll move the probe to TP1 and see if it changes when receiver
> sensitivity falls off.
>
> I thought that if VCO voltage was low PLL lock would be lost.
> When this happens, the PLL stays locked, as verified by listening
> to stronger stations. Also the VCO/PLL chain is before any RF
> amplification of a transmit signal. Hence my perplexing
> question. A single transmitted dash brings it back. (As I was
> typing it went quiet, and a single dash made things right.)
>
> To pose another hypothesis, when it goes insensitive (I can
> still copy very strong signals) it is almost as if the 20m
> components were being switched back in. There is no relay
> chatter, and I set sidetone level to zero, and AF to zero after
> it happens, and when I transmit no relay noise. Could the I/O
> controller be the problem? Sending a small voltage to the relay,
> but not enough to latch it back causing a high resistance
> connection at the wipers to partially enable the 20m components
> in the BPF. And transmitting a signal forces it closed?
>
> After I added the KX60V I went through and did the VCO voltage
> measurements and 30 was fine.
>
> You are correct that there are very few portions of the radio
> that are band specific, namely the VCO circuit which switches
> the diodes, the LPF and BPF, and the I/O controller which affects
> the first three.
>
> I'm feeling pretty stupid as I can't find the problem. I knew
> once I posted you would be kind enough to give me your
> experienced opinion. I am just suprised that as the Dean of
> Support you have not seen this before.
>
> Again thanks for taking time to consider my plight.
>
> Regards,
> Joe N9JR
>
> Don Wilhelm <[hidden email]> wrote:
> Joe,
>
> Try checking the RF voltage out of the VCO (at TP1) - compare 30 meters to
> 40 and 20. If the 30 meter output is low, that could be the major cause of
> your problem. If this is the situation, look first at the voltage at R30
> for clues. Normally the 30 meter VCO alignment falls quite within range,
> but if the capacitors in the VFO selection area are not quite the right
> value, it could make any one band go out of range.
>
> Other than the BPF and the VFO range, there is not much that is band
> specific in the receiver - all it takes is things tuned to the correct
> frequencies.
>
> 73,
> Don W3FPR
>
> > -----Original Message-----
> >
> > I have been pulling my hair out for about a month with this
> > problem, and perhaps someone has seen it before and can point me
> > on the right path.
> >
> > I have an issue on 30m receive only. When I transmit on 30m I
> > get full power, and it receives normal, but after a while it
> > loses most of its sensitivity. Transmit again and it will hear
> > the other side of the earth. I have noticed that once it goes
> > partially deaf I can get a High Curr indication on the first dash
> > I send, but after that it puts out full power with no complaints
> > and of course after every transmission it is that beautiful K2 receive.
> >
>

Joseph Reed-2

RE: Strange receive problem

Dude,

You are indeed the King. Let me press forward with your suggestions. I'll let you know what I find.

Joe N9JR

Don Wilhelm <[hidden email]> wrote:
Joe,

Not the counter probe - use an RF Probe or a 'scope - the significant factor
is the RF voltage, the counter probe will only provide the frequency and I
expect that will be correct.

What you may be encountering is low injection to the mixer - the level
should be +7dBm for the TUF-1 mixer - if that level is reduced, the mixer IF
output will also be less than it should be - and decreased receiver
sensitivity will be the obvious result. This is separate from a VCO lock
condition, but the VCO lock condition becomes more unstable at lower (R30)
voltages.

The other possibility that I can think of is a leaky coupling capacitor -
particularly those associated with the D6/D7 switch - in combination with
the RF Chokes, there could be some strange condition that only occurs on 30
meters - A short burst of transmit would change the DC conditions on the
capacitor and restore things to 'normal' until it charged up again - as far
as I know, the only real way to check for that is to substitute capacitors
in hopes that it will be found - not the best way to de-bug, but sometimes
that is the only way to go. Remember that Murphy rules in instances like
this - if it can't happen, it WILL!!

73,
Don W3FPR

> -----Original Message-----
>
> Thanks Don,
>
> I'll move the probe to TP1 and see if it changes when receiver
> sensitivity falls off.
>
> I thought that if VCO voltage was low PLL lock would be lost.
> When this happens, the PLL stays locked, as verified by listening
> to stronger stations. Also the VCO/PLL chain is before any RF
> amplification of a transmit signal. Hence my perplexing
> question. A single transmitted dash brings it back. (As I was
> typing it went quiet, and a single dash made things right.)
>
> To pose another hypothesis, when it goes insensitive (I can
> still copy very strong signals) it is almost as if the 20m
> components were being switched back in. There is no relay
> chatter, and I set sidetone level to zero, and AF to zero after
> it happens, and when I transmit no relay noise. Could the I/O
> controller be the problem? Sending a small voltage to the relay,
> but not enough to latch it back causing a high resistance
> connection at the wipers to partially enable the 20m components
> in the BPF. And transmitting a signal forces it closed?
>
> After I added the KX60V I went through and did the VCO voltage
> measurements and 30 was fine.
>
> You are correct that there are very few portions of the radio
> that are band specific, namely the VCO circuit which switches
> the diodes, the LPF and BPF, and the I/O controller which affects
> the first three.
>
> I'm feeling pretty stupid as I can't find the problem. I knew
> once I posted you would be kind enough to give me your
> experienced opinion. I am just suprised that as the Dean of
> Support you have not seen this before.
>
> Again thanks for taking time to consider my plight.
>
> Regards,
> Joe N9JR
>
> Don Wilhelm wrote:
> Joe,
>
> Try checking the RF voltage out of the VCO (at TP1) - compare 30 meters to
> 40 and 20. If the 30 meter output is low, that could be the major cause of
> your problem. If this is the situation, look first at the voltage at R30
> for clues. Normally the 30 meter VCO alignment falls quite within range,
> but if the capacitors in the VFO selection area are not quite the right
> value, it could make any one band go out of range.
>
> Other than the BPF and the VFO range, there is not much that is band
> specific in the receiver - all it takes is things tuned to the correct
> frequencies.
>
> 73,
> Don W3FPR
>
> > -----Original Message-----
> >
> > I have been pulling my hair out for about a month with this
> > problem, and perhaps someone has seen it before and can point me
> > on the right path.
> >
> > I have an issue on 30m receive only. When I transmit on 30m I
> > get full power, and it receives normal, but after a while it
> > loses most of its sensitivity. Transmit again and it will hear
> > the other side of the earth. I have noticed that once it goes
> > partially deaf I can get a High Curr indication on the first dash
> > I send, but after that it puts out full power with no complaints
> > and of course after every transmission it is that beautiful K2 receive.
> >
>

Geoffrey Mackenzie-Kennedy-2

Re: Strange receive problem

In reply to this post by Joseph Reed-2

Joe N9JR wrote on Saturday, April 22, 2006 11:10 PM

> I have an issue on 30m receive only. When I transmit on 30m I get full
> power, and it receives normal, but after a while it loses most of its
> sensitivity. Transmit again and it will hear the other side of the earth.
> I have noticed that once it goes partially deaf I can get a High Curr
> indication on the first dash I send, but after that it puts out full power
> with no complaints and of course after every transmission it is that
> beautiful K2 receive.

I had the same problem with 40m 18 months or so ago and posted comment on 20
November 2005 when Mark, KJ7BS, experienced a similar problem. In my case
the problem was caused by a 'dirty' contact of K1 in the BPF. Although
problems with bifurcated 'wipe clean' contacts in sealed relays very rarely
occur, they can arise.

> I think we can eliminate the BPF as nothing changes in it for either 30
or 20.

Perhaps. When your receiver goes partially deaf on 30m, have you tried
switching to 40m (not 20m) and back to 30m to see if the receiver recovers -
without transmitting? This would exercise K4 in the 20m / 30m BPF. If the
receiver recovers then K4's contacts become possible suspects. If it does
not recover, then the BPF assembly is likely to be OK.

73,
Geoff
GM4ESD

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