Hallicrafters HT-40

[DMOD]

Updated: 9/2021 With Text, Schematics, and Theory of Operation:



Phil - AC0OB

[KK4RF]

Phil,
     Thanks for the message on the HT-40. I'm running mine stock and for the most part have been getting fairly good audio reports. Your mods do look pretty interesting, however, and I may look into them again at a later date.
     Working on another ScreenModulated/controlled Carrier rig, the KnightKit T150-A. It does not get very good audio reports, however. I plan to read up on as many mods for that radio as possible.I'd like it to sound as good as the HT-40. Thanks again, Phil.---Marty, KK4RF---

[DMOD]

Thanks for the message on the HT-40. I'm running mine stock and for the most part have been getting fairly good audio reports. Your mods do look pretty interesting, however, and I may look into them again at a later date.

I am glad to hear your audio is acceptable, but I would bet if you looked at the modulation with a scope, it probably has no more than 75% modulation unless it has been modified.

And having the Red Led modulation indicator is a lot of fun. 

If you are working on the T-150, check out these mods here as well:

http://amfone.net/Amforum/index.php?topic=34680.0

I have another update coming with actual operating parameters noted during operation.

Phil - AC0OB

[n4joy]

This is a good write-up.  I firmly believe the HT-40 is perhaps one of the best carrier controlled transmitters for AM beginners.  I use mine on 10 meters and always receive good audio reports with its stock configuration.

[DMOD]

Thanks.

I also get good audio reports as well.

With a few component changes and additions the audio and modulation can be improved.

The R-C feedback circuit from the Modulator cathode back to the speech amp really helps to reduce distortion.

And the 6DQ5 final is both low cost ($7.50 to $10.00) each and pretty darn rugged.

Phil - AC0OB