Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.

On all HF bands (from 3.5 to 28Mhz) bands, the alc meter shows drive
variable by mic gain control - this is set up by watching meter
deflection on voice peaks as per instruction manual. In CW mode, power
output is max 100 watts each band.

The EXCEPTION is 80m, where
1.ALC does not register on meter irrespective of voice drive/mic gain
setting.
2.Power output is 180 watts in CW mode.

Something is wrong - I have checked drive level to RF power amp
section with my CRO, same level all bands. I have checked the LPF on
the PA output, the switching relays work OK and every component has
been removed from the board and checked - the caps have, the toroidal
coils were examined for burning/shorted turns/continuity, but not
inductance measured.

Can you suggest what has gone wrong - I am running out of ideas and
knowledge!
My limited theory states that a portion of the RF output is sampled
and fed back to the exciter stage to reduce gain and thus prevent
overdriving and splatter. This ALC detector circuit is common to all
bands, so its unlikely to be faulty. All test results were on a 50R
dummy load, so antenna variations have been eliminated.

Thanking you,

Andrew Blight VK3BFA

"Andrew VK3BFA" wrote in message
om...
Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.

On all HF bands (from 3.5 to 28Mhz) bands, the alc meter shows drive
variable by mic gain control - this is set up by watching meter
deflection on voice peaks as per instruction manual. In CW mode, power
output is max 100 watts each band.

The EXCEPTION is 80m, where
1.ALC does not register on meter irrespective of voice drive/mic gain
setting.
2.Power output is 180 watts in CW mode.

Something is wrong - I have checked drive level to RF power amp
section with my CRO, same level all bands. I have checked the LPF on
the PA output, the switching relays work OK and every component has
been removed from the board and checked - the caps have, the toroidal
coils were examined for burning/shorted turns/continuity, but not
inductance measured.

Can you suggest what has gone wrong - I am running out of ideas and
knowledge!
My limited theory states that a portion of the RF output is sampled
and fed back to the exciter stage to reduce gain and thus prevent
overdriving and splatter. This ALC detector circuit is common to all
bands, so its unlikely to be faulty. All test results were on a 50R
dummy load, so antenna variations have been eliminated.

Thanking you,

Andrew Blight VK3BFA

I believe your theory is correct, but await further detail from the net...

Here's a thought. Because the problem is on the lower frequency band,
perhaps there is a bypass capacitor failure somewhere allowing RF to go
where it shouldn't. Without seeing the circuit I can't suggest more. From
your description above, I'm not sure if you checked cap VALUES or just
looked for burning. Also if there is a choke for blocking RF a short would
also let RF through.

--
Steve N, K,9;d, c. i My email has no u's.

"Andrew VK3BFA" wrote in message
om...
Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.

On all HF bands (from 3.5 to 28Mhz) bands, the alc meter shows drive
variable by mic gain control - this is set up by watching meter
deflection on voice peaks as per instruction manual. In CW mode, power
output is max 100 watts each band.

The EXCEPTION is 80m, where
1.ALC does not register on meter irrespective of voice drive/mic gain
setting.
2.Power output is 180 watts in CW mode.

Something is wrong - I have checked drive level to RF power amp
section with my CRO, same level all bands. I have checked the LPF on
the PA output, the switching relays work OK and every component has
been removed from the board and checked - the caps have, the toroidal
coils were examined for burning/shorted turns/continuity, but not
inductance measured.

Can you suggest what has gone wrong - I am running out of ideas and
knowledge!
My limited theory states that a portion of the RF output is sampled
and fed back to the exciter stage to reduce gain and thus prevent
overdriving and splatter. This ALC detector circuit is common to all
bands, so its unlikely to be faulty. All test results were on a 50R
dummy load, so antenna variations have been eliminated.

Thanking you,

Andrew Blight VK3BFA

I believe your theory is correct, but await further detail from the net...

Here's a thought. Because the problem is on the lower frequency band,
perhaps there is a bypass capacitor failure somewhere allowing RF to go
where it shouldn't. Without seeing the circuit I can't suggest more. From
your description above, I'm not sure if you checked cap VALUES or just
looked for burning. Also if there is a choke for blocking RF a short would
also let RF through.

--
Steve N, K,9;d, c. i My email has no u's.

"Andrew VK3BFA" wrote in message
om...
Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.

On all HF bands (from 3.5 to 28Mhz) bands, the alc meter shows drive
variable by mic gain control - this is set up by watching meter
deflection on voice peaks as per instruction manual. In CW mode, power
output is max 100 watts each band.

The EXCEPTION is 80m, where
1.ALC does not register on meter irrespective of voice drive/mic gain
setting.
2.Power output is 180 watts in CW mode.

Something is wrong - I have checked drive level to RF power amp
section with my CRO, same level all bands. I have checked the LPF on
the PA output, the switching relays work OK and every component has
been removed from the board and checked - the caps have, the toroidal
coils were examined for burning/shorted turns/continuity, but not
inductance measured.

Can you suggest what has gone wrong - I am running out of ideas and
knowledge!
My limited theory states that a portion of the RF output is sampled
and fed back to the exciter stage to reduce gain and thus prevent
overdriving and splatter. This ALC detector circuit is common to all
bands, so its unlikely to be faulty. All test results were on a 50R
dummy load, so antenna variations have been eliminated.

Thanking you,

Andrew Blight VK3BFA

I believe your theory is correct, but await further detail from the net...

Here's a thought. Because the problem is on the lower frequency band,
perhaps there is a bypass capacitor failure somewhere allowing RF to go
where it shouldn't. Without seeing the circuit I can't suggest more. From
your description above, I'm not sure if you checked cap VALUES or just
looked for burning. Also if there is a choke for blocking RF a short would
also let RF through.

--
Steve N, K,9;d, c. i My email has no u's.

"Steve Nosko" wrote in message ...
"Andrew VK3BFA" wrote in message
om...
Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.



I believe your theory is correct, but await further detail from the net...

Here's a thought. Because the problem is on the lower frequency band,
perhaps there is a bypass capacitor failure somewhere allowing RF to go
where it shouldn't. Without seeing the circuit I can't suggest more. From
your description above, I'm not sure if you checked cap VALUES or just
looked for burning. Also if there is a choke for blocking RF a short would
also let RF through.

Well, a bit more information. One is a Yaeusu FT707, the other is a
Yaesu FT747. What is common to both is they have been hacked around to
get them on the 27Mhz CB bands - they both use similar solid state PA
modules. Possibly the CB stuff is a red herring - but there is always
the nagging thought that someone has used the "golden screwdriver" to
do something radical on them. No documentation, of course.

So far have fixed 9 separate faults on the 747, only 2 of them
legitimate "radio" faults (faulty switching diodes on the RF
attenuator, dry joints on the Xtal filter module) - the rest were the
result of "fiddling". The 707 was a bit more straight forward -
oxidised relays, replaced 1 and cleaned the others - plus a big
electro cap on the 12v DC input power line was literally blown like a
shotgun cartridge - very messy, but didnt seem to bother the radio
that much.....

Have checked the caps in the output LPF (both of them) by removing
from circuit, testing with a 500v megger for leakage, and measured
value with cap meter - all ok, within tolerance ie 5% of value (both
from the circuit, and printed on them) - would it be THAT critical for
this application? Havent measured the inductance of the coils - the
solder on them is untouched so they LOOK original, just examined for
burning/shorts/heat distortion.

I know that power output on the lower bands is greater than the higher
bands for tube type PA - would the same apply for modern transistor
devices - maybe a consensus from the group on this
one.......Interestingly enough, the manual for the FT707 only states
"Transmitter power 240w DC input" - no specific figures for each band!

The FT707 has had a 4.something MHz xtal fitted in the aux. position -
ALC works fine on this frequency, but go to 3.5Mhz - nothing, although
the internal power meter (using the same Bruen style coupler, just one
side of it) works fine. My next step is to see if there is DC out from
the coupler on 80M and see where it drops off or why it drops off.

Both are mechanical nightmares with very poor accessibility -the 707
because of tight wiring looms with plugs easily dislodged when trying
to gain access to test points, the 747 because its in a nice
plasticized box with no removable cover plate access to the boards -
and in common with nearly every other piece of equipment I have ever
worked on, the leads are deliberately designed (in both of them) to be
just that half inch too short to allow partly dismantled access and so
live testing.

Am trying to fix my spectrum analyser (dodgy connections on mating
plugs/sockets) to see what they do on air.

And in response to an earlier question - yes, the 80M dummy load was
reactive,(XL=30R at 3.5Mhz) so used a different one that wasnt - still
the same results..... got to thinking, well, two things with the same
fault, then perhaps the test gear itself is faulty ...it wasnt....

If the PA module was unstable due a crook bypass cap (or whatever)
surely the LPF on the output would attenuate the RF watts out to a
sane level - as I said, they both do about 180 watts output on 80m,
and draw about 20 amps on TX - same figure for 100 watts out on the
other bands, dropping off to about 60w on 10 metres. - going round in
circles - is there a TX fault, or is it an ALC fault, or is it both!

Anyway - thanks, people, for ideas and comments, got my brain
thinking about it in a new light from some of the suggestions...

73 de VK3BFA Andrew

"Steve Nosko" wrote in message ...
"Andrew VK3BFA" wrote in message
om...
Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.



I believe your theory is correct, but await further detail from the net...

Here's a thought. Because the problem is on the lower frequency band,
perhaps there is a bypass capacitor failure somewhere allowing RF to go
where it shouldn't. Without seeing the circuit I can't suggest more. From
your description above, I'm not sure if you checked cap VALUES or just
looked for burning. Also if there is a choke for blocking RF a short would
also let RF through.

Well, a bit more information. One is a Yaeusu FT707, the other is a
Yaesu FT747. What is common to both is they have been hacked around to
get them on the 27Mhz CB bands - they both use similar solid state PA
modules. Possibly the CB stuff is a red herring - but there is always
the nagging thought that someone has used the "golden screwdriver" to
do something radical on them. No documentation, of course.

So far have fixed 9 separate faults on the 747, only 2 of them
legitimate "radio" faults (faulty switching diodes on the RF
attenuator, dry joints on the Xtal filter module) - the rest were the
result of "fiddling". The 707 was a bit more straight forward -
oxidised relays, replaced 1 and cleaned the others - plus a big
electro cap on the 12v DC input power line was literally blown like a
shotgun cartridge - very messy, but didnt seem to bother the radio
that much.....

Have checked the caps in the output LPF (both of them) by removing
from circuit, testing with a 500v megger for leakage, and measured
value with cap meter - all ok, within tolerance ie 5% of value (both
from the circuit, and printed on them) - would it be THAT critical for
this application? Havent measured the inductance of the coils - the
solder on them is untouched so they LOOK original, just examined for
burning/shorts/heat distortion.

I know that power output on the lower bands is greater than the higher
bands for tube type PA - would the same apply for modern transistor
devices - maybe a consensus from the group on this
one.......Interestingly enough, the manual for the FT707 only states
"Transmitter power 240w DC input" - no specific figures for each band!

The FT707 has had a 4.something MHz xtal fitted in the aux. position -
ALC works fine on this frequency, but go to 3.5Mhz - nothing, although
the internal power meter (using the same Bruen style coupler, just one
side of it) works fine. My next step is to see if there is DC out from
the coupler on 80M and see where it drops off or why it drops off.

Both are mechanical nightmares with very poor accessibility -the 707
because of tight wiring looms with plugs easily dislodged when trying
to gain access to test points, the 747 because its in a nice
plasticized box with no removable cover plate access to the boards -
and in common with nearly every other piece of equipment I have ever
worked on, the leads are deliberately designed (in both of them) to be
just that half inch too short to allow partly dismantled access and so
live testing.

Am trying to fix my spectrum analyser (dodgy connections on mating
plugs/sockets) to see what they do on air.

And in response to an earlier question - yes, the 80M dummy load was
reactive,(XL=30R at 3.5Mhz) so used a different one that wasnt - still
the same results..... got to thinking, well, two things with the same
fault, then perhaps the test gear itself is faulty ...it wasnt....

If the PA module was unstable due a crook bypass cap (or whatever)
surely the LPF on the output would attenuate the RF watts out to a
sane level - as I said, they both do about 180 watts output on 80m,
and draw about 20 amps on TX - same figure for 100 watts out on the
other bands, dropping off to about 60w on 10 metres. - going round in
circles - is there a TX fault, or is it an ALC fault, or is it both!

Anyway - thanks, people, for ideas and comments, got my brain
thinking about it in a new light from some of the suggestions...

73 de VK3BFA Andrew

"Steve Nosko" wrote in message ...
"Andrew VK3BFA" wrote in message
om...
Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.



I believe your theory is correct, but await further detail from the net...

Here's a thought. Because the problem is on the lower frequency band,
perhaps there is a bypass capacitor failure somewhere allowing RF to go
where it shouldn't. Without seeing the circuit I can't suggest more. From
your description above, I'm not sure if you checked cap VALUES or just
looked for burning. Also if there is a choke for blocking RF a short would
also let RF through.

Well, a bit more information. One is a Yaeusu FT707, the other is a
Yaesu FT747. What is common to both is they have been hacked around to
get them on the 27Mhz CB bands - they both use similar solid state PA
modules. Possibly the CB stuff is a red herring - but there is always
the nagging thought that someone has used the "golden screwdriver" to
do something radical on them. No documentation, of course.

So far have fixed 9 separate faults on the 747, only 2 of them
legitimate "radio" faults (faulty switching diodes on the RF
attenuator, dry joints on the Xtal filter module) - the rest were the
result of "fiddling". The 707 was a bit more straight forward -
oxidised relays, replaced 1 and cleaned the others - plus a big
electro cap on the 12v DC input power line was literally blown like a
shotgun cartridge - very messy, but didnt seem to bother the radio
that much.....

Have checked the caps in the output LPF (both of them) by removing
from circuit, testing with a 500v megger for leakage, and measured
value with cap meter - all ok, within tolerance ie 5% of value (both
from the circuit, and printed on them) - would it be THAT critical for
this application? Havent measured the inductance of the coils - the
solder on them is untouched so they LOOK original, just examined for
burning/shorts/heat distortion.

I know that power output on the lower bands is greater than the higher
bands for tube type PA - would the same apply for modern transistor
devices - maybe a consensus from the group on this
one.......Interestingly enough, the manual for the FT707 only states
"Transmitter power 240w DC input" - no specific figures for each band!

The FT707 has had a 4.something MHz xtal fitted in the aux. position -
ALC works fine on this frequency, but go to 3.5Mhz - nothing, although
the internal power meter (using the same Bruen style coupler, just one
side of it) works fine. My next step is to see if there is DC out from
the coupler on 80M and see where it drops off or why it drops off.

Both are mechanical nightmares with very poor accessibility -the 707
because of tight wiring looms with plugs easily dislodged when trying
to gain access to test points, the 747 because its in a nice
plasticized box with no removable cover plate access to the boards -
and in common with nearly every other piece of equipment I have ever
worked on, the leads are deliberately designed (in both of them) to be
just that half inch too short to allow partly dismantled access and so
live testing.

Am trying to fix my spectrum analyser (dodgy connections on mating
plugs/sockets) to see what they do on air.

And in response to an earlier question - yes, the 80M dummy load was
reactive,(XL=30R at 3.5Mhz) so used a different one that wasnt - still
the same results..... got to thinking, well, two things with the same
fault, then perhaps the test gear itself is faulty ...it wasnt....

If the PA module was unstable due a crook bypass cap (or whatever)
surely the LPF on the output would attenuate the RF watts out to a
sane level - as I said, they both do about 180 watts output on 80m,
and draw about 20 amps on TX - same figure for 100 watts out on the
other bands, dropping off to about 60w on 10 metres. - going round in
circles - is there a TX fault, or is it an ALC fault, or is it both!

Anyway - thanks, people, for ideas and comments, got my brain
thinking about it in a new light from some of the suggestions...

73 de VK3BFA Andrew

On 1 Sep 2004 05:23:46 -0700, (Andrew VK3BFA)
wrote:

It's interesting that your 707 was fiddled with by a CBer. I had a
friend about 20 years ago who made good beer money by "tweaking-up"
countless 707s by maxing-out the preset pot that controls that
circuit. AIR, they did seem to work quite happily with 180W out on 80M
without detriment to the lifetime of the PA., although it's true to
say that even the unmodified ones are pretty gutsy on the lower bands
(140W approx. on CW) I reckon your problem - such as it is - is mostly
the 'golden screwdriver' adjustment and partly just the normal higher
output you get with a factory-spec rig.
I agree there's nothing worse than opening the lid and finding out
some joker has messed about with your radio, though. You won't know
what exactly the extent of his mods - or skills for that matter -
were. :-(

--

"What is now proved was once only imagin'd." - William Blake, 1793.

On 1 Sep 2004 05:23:46 -0700, (Andrew VK3BFA)
wrote:

It's interesting that your 707 was fiddled with by a CBer. I had a
friend about 20 years ago who made good beer money by "tweaking-up"
countless 707s by maxing-out the preset pot that controls that
circuit. AIR, they did seem to work quite happily with 180W out on 80M
without detriment to the lifetime of the PA., although it's true to
say that even the unmodified ones are pretty gutsy on the lower bands
(140W approx. on CW) I reckon your problem - such as it is - is mostly
the 'golden screwdriver' adjustment and partly just the normal higher
output you get with a factory-spec rig.
I agree there's nothing worse than opening the lid and finding out
some joker has messed about with your radio, though. You won't know
what exactly the extent of his mods - or skills for that matter -
were. :-(

--

"What is now proved was once only imagin'd." - William Blake, 1793.

On 1 Sep 2004 05:23:46 -0700, (Andrew VK3BFA)
wrote:

It's interesting that your 707 was fiddled with by a CBer. I had a
friend about 20 years ago who made good beer money by "tweaking-up"
countless 707s by maxing-out the preset pot that controls that
circuit. AIR, they did seem to work quite happily with 180W out on 80M
without detriment to the lifetime of the PA., although it's true to
say that even the unmodified ones are pretty gutsy on the lower bands
(140W approx. on CW) I reckon your problem - such as it is - is mostly
the 'golden screwdriver' adjustment and partly just the normal higher
output you get with a factory-spec rig.
I agree there's nothing worse than opening the lid and finding out
some joker has messed about with your radio, though. You won't know
what exactly the extent of his mods - or skills for that matter -
were. :-(

--

"What is now proved was once only imagin'd." - William Blake, 1793.