Improvements to this simple AM transmitter?

Thread Starter

Yabba Rays

Joined Oct 5, 2016
21


This AM amateur radio transmitter for 1.8 to 2.0 MHz was published about 40 years ago. It's my era and for nostalgic reasons I'd like to build it. Apart from adding some supply decoupling in the modulator, can anyone see anything obviously wrong? I suspect IRF510 FETs would be preferable now - they're good up to about 10 MHz.
 

DickCappels

Joined Aug 21, 2008
7,432
Off the top of my head the VN66AF was good for much less current than the IRF510 so there is a good chance the IRF510 will require more drive current to keep up with the 160 meter signal. Notice that the gate driver is an HEF469UB (UB!) which is rather Wimpy.

There are other new MOSFETs that have lower current ratings and lower input capacitance.
 

Thread Starter

Yabba Rays

Joined Oct 5, 2016
21
Off the top of my head the VN66AF was good for much less current than the IRF510 so there is a good chance the IRF510 will require more drive current to keep up with the 160 meter signal. Notice that the gate driver is an HEF469UB (UB!) which is rather Wimpy.

There are other new MOSFETs that have lower current ratings and lower input capacitance.
Thank you for your reply. Can you suggest any gate drivers that are a bit more robust for this project?
 

DickCappels

Joined Aug 21, 2008
7,432
If you change the MOSFETs to a different type you will need a new driver circuit and a new output network. Unfortunately the VN66AF and VN67AF are out of production. Not so unfortunately, they are available on the surplus market.

40 years? That sounds about right. I had forgotten how long it has been.
 

sparky 1

Joined Nov 3, 2018
543
That's a modified junior Wyvern 1972 (49 years ago) JW Rimmer used fixed crystal oscillator divider chain to parallel buffer 2 gates 4069 into IRF510
The published output suggests ( the Driver stage missing) probably 2N3553 in the UK there were a few of those one called "little joe".
The Wyvern junior was not variable. Don't need the wide band driver, must have been like the "twofer" but not so compact.
Not completely sure but this is authentic Green design.


See link below look for Wyvern part 1
http://www.g4kfk.co.uk/html/top_band.html
The modulator is in part 3. I am fairly certain JW Rimmer would have studied all three parts and he did get published so the trail and history is getting a little closer. Wonder if Harry's homebrew has one of those.

By the later 70's you can see the typical US upgrades to the bigger radio some contests could be monitored easily using 6 meter converter option. Those QSO not around much Anyhow. When this radio is set for the best reception of the JW Rimmer then that transmitter would match.
If you do not match the bigger radios of that era it is not going to be the best replication. The Kenwood radio shows the design and style fit and finish that was popular in the US during that period.
 
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