It uses 1 triode tube 3cx800a7 with all the circuits needed for a reliable "KEY_DOWN" work even if the project is more than 20 years old.
Here I will describe some improvementes made by me on this still fine RF amplifier.
The first feature has been added due to the blower's noise.
The continous presence of the rotating fan did not let me concentrate at the band listening,
specially when the PA is in st.by for long times.
Its job is simple: during TX the blower runs at maximum: when the PA goes to St.by the timer let it run at full rate about 1 minute ,
before reducing the speed via a regulated dimmer.
Should be needed a longer delay increase the 1Mohm resistor.
The lower speed will be found by reducing the dimmer to the point the blower makes no much noise, but still rotates
The dimmer is not critical :any filtered brand with external potentiometer is OK.
I have used an item found on www.conrad.de, its code is 130004 - 62 .
Its insertion in the circuit is simple: located the two wires running to the blower cut them and insert the IN-OUT wires of the dimmer.
This mod. can be added to any amplifier.
The second feature is a simple add on:
the grid protection circuit , in the Rake800, activates itself in a sudden and very silent mode and has to be resetted manually.
The only indication of the active protection are the TX lamps remaining in red-tx condition.
This mode of keeping safe the PA is a little confusing specially when the operator does wear the headphones.
I had the need to know in any moment what the state of the amplifier was\is: active or blocked.
The simplest solution was to change the K5 protection relay with a 2 pole model. One pole remains on the grid protection.
The second pole drives a jumbo flashing led.
In this way I have a very strong visual indication of the activated grid proptection.
© nov.03, mar.06, upd giu.09