Hallicrafters HT-40


It's been awhile since I had a proper new project; here it is. I decided I wanted to join the AMI-West net, which meets weekly on 3870 KHz. For this I would need a transmitter capable of full-AM on the 80m ham band. After a lot of dithering, I decided to go with a "beginner's transmitter" that I could somewhat afford. This showed up on eBay and I won the auction.



Wednesday, June 25th, 2014: Arrival

The guy who sent it (via eBay) packed it very well. My only quibble was that the air-variable caps should have been closed (they were wide open); but when closed the dial markings read 100, and open they read zero—the shipper may have zeroed them figuring that was the way to do it. Anyway—no harm done.

I haven't done much with it yet other than a physical inspection. I'll need to make or buy a dummy load for it. The Hallicrafters manual tells how to make one with a 40watt light bulb; I may do that in the meantime, but I picked out a 50W, 50Ω non-inductive resistor from Mouser, along with a bunch of other junk that I need to get; so now it's just waiting for my bank account to catch up and I'll put in the order.

Also on the shopping list was the stuff to make the project in this Youtube Video: this guy builds a little circuit that lets him tap the RF signal between his rig and dummy load and feed it into an oscilloscope; from there he can view both the RF waveform and the AF component. It was everything I look for in a video like this: simple, very well explained, and inexpensive. Or it would be inexpensive if I already had most of the stuff it calls for, which I don't.

The crystal showed up the next day, purchased from AF4K.

Not sure yet what I'm going to do about the microphone. The HT-40 uses a funky Amphenol 75-MCIF, which looks an awful lot like the UHF connector that's used for the antenna. Mine looks like the center pin may have broken off in it, but I don't really know. Searches for microphone jacks, meanwhile, dredge up all sorts of things. Right now I'm of a mind to buy a mike and replace the 75-MCIF with whatever the mike's plug turns out to be.

I'm not even going to start bitching about how bewildering the search for a microphone has been.

Wednesday, July 16: Power

The parts arrived Monday and I began to build the dummy load. Tuesday real life interrupted everything, so it wasn't until Wednesday that I was finally got to try it all together. I hooked everything up and began the checklist. Power Off. Band to 80. Drive at center position. Crystal/BFO on Crystal. Crystal inserted. Ammeter selected to Grid Current. Plate Load and Plate Tuning to 100 each. Dummy load attached.

Power on — set to Standby. Go feed the dog while the machine warms up.

Ready to go. Function set to Tune. Adjust drive to max reading on ammeter. At this point I could also see the sine wave on the o'scope, and I could see the amplitude change as I turned the Drive knob. Nice to see that the on-board ammeter and the scope both agree when I max the current.

Back to Standby. Meter switch to RF Output. Function to CW.

Adjust Plate Loading and Plate Tuning to max current on the meter (and again, on the scope). Switch to AM and final tweaks. Then back to Standby.

This is where the wheels fell off. The mic input is an old Amphenol 75-MC1F connector, which turns out to be the same as the more commonly known Switchcraft 2501F, neither of which I have. But that doesn't matter a lot because I don't have a high-z microphone anyway.

Well, actually I do. I took the one off my Webcor Royal tape recorder; it has a 1/4" phone adapter, but by using a 1/4" jack and clip-leads, I managed to jury-rig a connection to the mic input. So does it work? Hell, no.

I ended up plugging the mic into the Webcor recorder itself, which I know works, and of course the mic is dead. Opened up the mike just to make sure there were no easy fixes like an internal broken wire—of course not. The mic is 60 years old; it has retired and gone fishing.

So I'm stuck for the moment until I can come up with another microphone and a better antenna.

Current status: needs microphone wired up; needs antenna