1/4 Harris Platinum Amp Build Bias Question...

Hi All,
I'm starting my first amp build utilizing a Harris Platinum module for 6 meters.
I need to come up with a circuit to sequence the RF input and bias voltage (RF first then bias voltage on transmit).
Question: When switching from transmit to receive, does the bias voltage need to be removed before the RF? If not, a simple on-delay circuit using a 555 timer with a 200ms delay should do the trick I'm thinking.
I just want to make sure as to not toast this module as they are getting hard to come by and replacement parts are impossible to get.
Thanks in advance and 73

Doug N0UAE

 

I'm unfamiliar with that module and didn't research it to see what it is; however sequencing is usually based entirely on T-R switching speed of whatever it is that switches the amp in and out of the RF path between exciter and load. If it's a relay or more than one relay, it pays to know what the relay switching and settling time is. With P-I-N diodes that are properly selected for the application, it will be microseconds and you normally don't need to sequence anything.

But if it's relay switched, whatever sequence timing is used should normally be used both going from RX to TX and also going back from TX to RX. Many modern transceivers have this capability built-in: You just select the time delay and it will close the AMPLIFIER KEYING connection to key the amp, switch relays and set bias before it outputs any RF energy, and then do the same in reverse (kill the RF before switching the relay) when going back to receive. Even my little Ten Tec Jupiter has that adjustable function via its menu.

For those rigs that don't have such a function, I'd certainly suggest whatever you add work on both ends (switching to TX and also back to RX) to protect both the amplifier and any relays it contains.

 

You take more risk by trying to do switching in the amplifier circuit.
It does not have any advantage.
Once the amplifier is powered up it stays in that condition ready for RF drive
Set the amplifier bias for correct idle value by measuring the idle current with a meter.
Do the sequencing outside with the radio relay delay setting or relay timing outside if needed.
Good luck.

 

The W6PQL control board does T/R sequential switching in three events. Bias, Vcc, ALC holdoff, fans, antenna relay can all be controlled with Jim's control board
http://www.w6pql.com/
Jan N8CBX

 

I have 1 of W6PQL's boards. Works great & has more inputs/outputs than I needed. Getting 3 more for different amps. Great quality & support.

Ed

 

If you don't want the complexity of the control board W6PQL makes a low cost stand alone 4 port sequencer.

 

Each Harris Platinum module consisted of 16 Philips (now NXP/Freescale) ON4402H LDMOS transistors (50 VDC devices) connected in 4 pallets of 2 push-pull pairs each and then combined.
Having seen this TV design transmitter, when introduced in 1990s,
the Fans produces significant acoustic dB.

Harris Platinum I — Solid State Channel 2 and 3 TV Amplifier Modules
for use on amateur radio 50 or 70 MHz bands
by Lance Collister, W7GJ (Six-meter EME enthusiast)
http://www.bigskyspaces.com/w7gj/HarrisAmps.htm

http://bigskyspaces.com/w7gj/Platinumi-1.pdf

John Sortor, KB3XG published a three part series, on converting the Harris TV modules, in the the May, June and August 2010 issues of "Cheese Bits", the official newsletter of the Mt. Airy VHF Radio Club in Southampton, PA.
http://www.bigskyspaces.com/w7gj/HarrisCheeseBits.pdf

 

You want the sequence to be:

When you press the ptt switch,

Switch tr relays to tx position.

Turn on bias voltage (depletion mode finals, that is a final that draws full current with no bias voltage, for enhancement mode devices, turn on b+ before bias)

Turn on b+ to amp

Key transmitter.

You don't want relays or pin diodes switching with RF applied, and you don't want RF applied to your amp before b+ and bias are applied to your amp.

If you have a Rx preamp, you want it's relays to be open, and the preamp de-energized before the tx relays close.

Also, if possible, use relays that provide a 50 ohm resistive load on the inputs and outputs of both the final amp and preamp when they are switched out of circuit.

Rege

 

No need to remove power from a preamp! In fact under bias it will withstand more abuse! I do terminate into a 50ohm load when I build my own.

 

Do not bother switching the bias. Leave it on as long as the amp is powered. It's less than some tube amps draw via the filament.