When using a good preamp and/or a high power PA you need to ensure that the relays are switched to the right position at the right time. The device that does this is usually called a transmit sequencer, as it manages the sequence of turning power on and off to relays, PA-bias, preamp, and keyer lines.
There are many reasons for using a sequencer. This is a list of some of them.
For our 70cm PA/Preamp combo we had been using a simple set up using two manually operated switches configured like this:
Some might ask how this could work but it does since it is virtually impossible to switch both switches at exactly the same time. These were of differnt types and required quite some force to switch. When either one has been switched there is no power available to the relay and preamp or keying gate. When both are in the same direction there will be power to one and only one of the ends.
This was a pretty secure but extremely irritating method. When we got the 100W PA + DB6NT preamp for 23cm this solution was not acceptable anymore.
One thing that had irritated me about other hams sequencers was that when they send CW they much too often loose parts of the first character they send. Some have the hang-time so short that they loose the first dit of every word, while others just loose things when they shift from rx to tx. So I did not want IM4RPP to be heard on the band instead of SM4RPP.
The basic elements in my sequencer is a shift-register and a counter. This could be realised in a CPU but since I am a professional software developer I know how easy it is to get a small bug into the software that in obscure situations cause outputs to go into the wrong state I prefer using normal logic devices.
Since I only found a suitable shift-register in CMOS the whole construction is based on CMOS devices. The shift-register I use is a 4517 and the counter is a 4040. I have connected these devices as shown here:
When the key is pressed three things happen:
Sooner or later the counter reaches it's "end", where the output
goes low again. The count should be selected to a value that
enables all information to have passed through the shift register,
which mean that it only contains key-up information.
As the output of the counter goes low the clock pulses to it stops
coming and it will not overflow.
CMOS devices are more sensitive to ESD than normal TTL. A break-down of something in the shift register or counter could cause the outputs to go to states that cause damage. I did not want this to happen. Therefore there is an additional circuit using two relays that guard the first circuit. Under normal operation this circuit should be passive but if an error condition occurrs it will be there to protect the preamp from damage:
Explanation:
The Kenwood rigs we use have as I see it one big problem. There
are keys on the front panel that cause the transmitter to start
operating. So no matter how many blocks you put on the keying and
PTT lines you might accidentally push a key on the rig that will
still ruin the preamp.
There is however a solution to this aswell and it spelled ALC.
On every single Kenwood rig I have seen/used there is an
external ALC line on the back which can be used to limit the power
output from the rig. For some reason the voltage that will cause
total blocking of output power is different. For the TR-751 it's
specified as -6V, for the TS-711 it's -?V and for the TS-2000 it's
-?V.
I do not know why they have done like this but it really hurts. I
have not been able to get any information about what happen if you
apply for instance -9V to all of them so I would have to recommend
using the voltage adapted for the specific rig.
The fact that the ALC votage is negative is something that is problematic when you construct electronics that is fed with +12V only. In my case I had to include a voltage inverter ICL7660 to get the negative feed.
Explanation:
The circuits shown above have been combined into a single construction that provide good protection and switching of both CW-keying and PTT line. What should be added is an oscillator for an apollo-beep and circuitry for switching the secondary PTT line used with digital modes. That is however a simple mod that should be possible for anyone with knowledge enough to build the circuit from my description.
The construction above was squeezed into a 100*160mm strip-board. It was quite a job to make it fit and one big mistake done was not to include the power supply for each IC into the board. There are a lot of loose wires on the board for just supplying voltage to the ICs.
Since I did want some additional functionality and it wont fit onto the strip-board I will sometime have to make a proper circuit board layout for this, but that is a later thing to do.