The AM Forum

We never had the AM Window Drake AM mods drawn out in a schematic.

Bob, WA2POW is modifying his T4X and was nice enough to send me his CLEAN, redrawn before and after modification schematics. We only had the description in the past, so these mod schematics will be helpful to those folks in the future.

I wanted to get it into the our archives and the Google, etc.,  search engines.

Description:
https://www.amwindow.org/tech/htm/t4.htm


T4-X and T4, etc., schematic mods before and after below:

(click on the BLUE .jpg LINKS below for proper up/down orientation)

Tom, K1JJ

Two observations if I may:

If the 500k is there to control the Final's power, one would need at least a 15 W resistor to do so since one needs to control at least 5 mA of screen current.

Secondly, when the resistor's value is changed, this changes the time constant of the Controlled Carrier circuit which changes the frequency response of the audio sent to the screen.

Suggestion: control the modulator's current (and hence the final's output power) via a low value 5W variable Cathode resistor with a large electrolytic bypass capacitor, and dispense with the 500k resistor, substituting it with a 12k, 5W resistor in order to maintain a constant audio response. This may require changing the value of one of the plate load resistors in order to establish the proper quiescent screen voltage.

Here is a similar "Clamp" or Shunt modulator we developed for a DX-60:


Phil

Thanks for the suggestions on the circuit, Phil -

I'll pass them along to WA2POW since he will be doing the mods.

These original mods were from Chris/ W2JBL and a number of guys successfully did them including me... but I always welcome improvements. I have long since sold my T4X so cannot try the new ones.

I've always been interested in trying the screen modulated DX- 60 mods so will hold onto yours.

Later -

T

here is another circuit from Georgio IN3IEX:

http://www.ing.unitn.it/~fontana/TrueAMT4XB.pdf

More stuff here, look for TR4: (some are also here on the AM Window)

http://www.ing.unitn.it/~fontana/dida.html

I like it!   A simple MOSFET modulator is a great idea.

T

Please note that this mod is only for the T4 series of transmitters. The transceivers TR3, 4 etc use a different modulator altogether. The best I've found for them is the modified DX60 circuit like the one from WA1QIX. I'm using one similar to it in my TR3 and TR4CW. The stock circuit in them uses a 13DE7 so the DX60 mod is a natural.

OK, thanks Chris.

I had added in the TR-4 by error -   and have corrected the headings on my own posts for just T4 series.

Later -

T

Going on 10 years with the JJ/JBL mods in my T4X and all is well! You won't regret it.