"J" wrote in message ...
"Andrew VK3BFA" wrote in message
om...

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

May be the sensitivity of the ALC detector is different at various
frequencies in the 3.5 to 30 MHz range. This circuit usually uses a
toroidal core transformer. May be they made a batch using the wrong core
type. May be the transformer has nulls in sensitivity around this
frequency. Or If your problem is at 3.5 MHz, may be is not designed
optimally and could do with a few more turns. Not too many more or it may
lose sensitivity at the 30 MHz end. Try to figure a way to check the ALC
detector sensitivity over the frequency range.

A good suggestion - unlikely, as it would have shown up from new and
been known by now. However, will check the DC volts out of the alc
detector cct - possibly the glue used to hold it together has gone
funny and had some strange effect. Need to trace the wiring thru - in
the FT707 its a real pig due interlacing but will persist over the
next few days when I have time from more commercial (pay the utility
bills) jobs.
de VK3BFA Andrew

"J" wrote in message ...
"Andrew VK3BFA" wrote in message
om...

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

May be the sensitivity of the ALC detector is different at various
frequencies in the 3.5 to 30 MHz range. This circuit usually uses a
toroidal core transformer. May be they made a batch using the wrong core
type. May be the transformer has nulls in sensitivity around this
frequency. Or If your problem is at 3.5 MHz, may be is not designed
optimally and could do with a few more turns. Not too many more or it may
lose sensitivity at the 30 MHz end. Try to figure a way to check the ALC
detector sensitivity over the frequency range.

A good suggestion - unlikely, as it would have shown up from new and
been known by now. However, will check the DC volts out of the alc
detector cct - possibly the glue used to hold it together has gone
funny and had some strange effect. Need to trace the wiring thru - in
the FT707 its a real pig due interlacing but will persist over the
next few days when I have time from more commercial (pay the utility
bills) jobs.
de VK3BFA Andrew

"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...

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

May be the sensitivity of the ALC detector is different at various
frequencies in the 3.5 to 30 MHz range. This circuit usually uses a
toroidal core transformer. May be they made a batch using the wrong core
type. May be the transformer has nulls in sensitivity around this
frequency. Or If your problem is at 3.5 MHz, may be is not designed
optimally and could do with a few more turns. Not too many more or it may
lose sensitivity at the 30 MHz end. Try to figure a way to check the ALC
detector sensitivity over the frequency range.

"Andrew VK3BFA" wrote in message
om...

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

May be the sensitivity of the ALC detector is different at various
frequencies in the 3.5 to 30 MHz range. This circuit usually uses a
toroidal core transformer. May be they made a batch using the wrong core
type. May be the transformer has nulls in sensitivity around this
frequency. Or If your problem is at 3.5 MHz, may be is not designed
optimally and could do with a few more turns. Not too many more or it may
lose sensitivity at the 30 MHz end. Try to figure a way to check the ALC
detector sensitivity over the frequency range.