An Inexpensive Approach to Increasing the FLEX 1500 to 100 Watts

One of the unfortunate shortcomings of the marvelous little FLEX 1500 is its very low output RF Power. It only puts out 5 watts of PEP power, or 5W of SSB, CW and 1.5W of AM.
There are a few solutions of varying complexity and cost, of course.

1. Just use it the way it is, QQRP.
2. Incorporate several amplifier solutions on the market to boost to 50W-150W. That makes it a very respectable sounding rig with the receive capabilites of FLEX and the power of a FLEX 3000.
3. I have also used a small TenTec 505 amplier left over from the early TenTec days but it really wasn't designed for this type of use. (I describe this on another page under Flex Experience)
4. Obtain a Hi Power RF amplifier that will drive to full legal limit with just a few watts. Homemade ones are not for the beginner or even mid-level ham and storebought ones are very expensive, unless you are fortunate to find an old Johnson Thunderbolt or Heathkit Chippewa.

A Very Easy and Affordable Solution

I was speaking with a friend of mine, Fred, K1NVY about an old TenTec Triton lV, solid state 100W tranceiver he had had since the 70's. He asked me if I'd like to take it and play with it as it was of no longer any use to him. He had done a lot of modification to it for FSK, it didn't work quite up to snuff BUT, the PA still put out 100W.


Fred and I started talking about the possibility of inserting the meager output of the FLEX directly into the input of the Triton lV Power Amplier. What follows is the journey to that end. I can tell you it works outstanding and there is no reason that this method can't be used by anyone owning a FLEX 1500 who is yearning for more power.

The first thing I did when I got the Triton from Fred was bring it home and pop the bottom, which gave access to the input of the power amp.


There was a small coax jumper between the lo-pass filter, just after the low level driver, that went to the RF Amplier (PA) Driver Block.
You almost have to pinch yourself when you look at this and say, "you mean all I have to do is plug the FLEX output into the Triton RCA jack on the back of the PA/Driver block?" In theory that is exactly all that is needed. In reality, however, a lot has to go into making this happen.

1. You can't just plug it in because on receive the FLEX would be listening thru the PA/Driver block. Few signals thru that!
2. All that is really necessary to do this is the addition of a 12vdc SPDT relay to act as an antenna changeover relay.
3. Make sure you cut the antenna permanently from the receive circuit of the Triton. Just cut the wire that goes to the receive side.
4. Power the relay from any 12vdc source in the Triton and switch it with the FLEX auxillary N/C contacts so the receive antenna is available to the FLEX.

A suggestion is in order at this point. I made a relay box for the FLEX to expand by 3X
the FLEX auxillary relay contacts. I  made 3 N/O and 3 N/C sets of contacts using a 12vdc 3P2T relay and closed it using the 12vdc FLEX source and switching it with the single set of FLEX auxillary contacts. Having these sets of contacts made this whole exercise a lot easier. I also switch my amp with a set.
5. Use one of the empty RCA jacks on the back of the Triton to insert the FLEX RF output. 
6. Connect this jack to the SPDT relay armature of the new relay mentioned above.
7. The normally closed contact (the up relay position) would connect to the antenna jack.
6. The normally open contact would connect to the PA/Driver module's RCA input.
O'kay that is done. Make sure it closes when you key the FLEX.

Next, you have to have a way to "key" the PA/Driver circuit "ON" so the RF from the FLEX can be amplified

 1. Take one of those N/O relay contacts from your handy dandy new relay box and connect it to the "KEY" jack on the back of the Triton. Make sure you have the mode switch on the Triton in CW position.

You are almost done. The place where I "Broke into" the RF line coming from the low level driver should have a load placed back on it to keep things from self destructing. I put a 47 ohm 2W resistor across it.

There are a few things that you need to do to make sure the rf out of the new "Amplifier" is clean.
Since the ALC is no longer active, make sure you keep the output at 100W by lowering the drive on the FLEX. It WILL go much higher and very likely burn out the finals with continued use at that level or at the very least cause your signal to go into distortion and splater all over the band.

AM Operation - I use my setup a lot with AM. Since AM is 100% duty cycle, I noticed that the PA heat sink was getting VERY hot. So, I installed a fan on the back to keep things nice and cool.

As you can see, it is powered continuously by plugging it into the 12vdc auxillary jack. I drilled holes in the heat sink and fastened the fan on with small wire ties.

COMMENT: Most solid state rigs utilize this PA/Driver modular system. If you've got one that is excess to your needs or one that has problems in ANY part of the rig other than the PA/Driver Section, try this scheme out for yourself. Also, theoretically you could do the same thing with any transceiver provided you put the FLEX output THRU a 0.01uf capacitor into the grid of the driver tube. You would have to experiment to see how much drive is necessary. It MAY be too much but an attenuator could be used to crank it down to an acceptable level.