Wednesday, May 1, 2013

W7DTG's lil adventurer

 Finally got to dig into the Johnson adventurer that was given to me by another ham radio operator for the price of shipping. Not only was that a great deal on this little 1950's transmitter, but I have learned a lot as I dig into it.

 The Johnson adventurer was a novice type low power CW transmitter manufactured in the late 50's and early 60's. It could be purchased as either a kit or pre-assembled at a bit higher price. The total power output was right around 30-35W. It could also do AM with the add-on modulator kit.

 The Johnson adventurer uses a modified peirce crystal oscillator using a 6AG7 tube. The CW key is coupled to pin#5 (Cathode) of the 6AG7 and pin #4 (Cathode) to produce the signal. It is a very basic transmitter and the schematics show how simple it truly is. As you can see from the schematic, the transmitter is either crystal or VFO controlled.

 I was able to reconnect the 2 700VDC 8 ohm capacitors C15 & C16 to ground after checking them out on my Oscilloscope as well as my VTVM, but for some reason after bringing the adventurer up on my Variac the radio tends to cook them after being on after about 20m. Something in the circuit is COOKING them. IT is strange that the caps are rated at 700 VDC and yet they are cooking and smoking just a bit.


 The 5U4G tube is rated at about 500 VDC so the 2 700 VDC caps should be able to handle that voltage. The 5U4G works as a voltage rectifier converting the incoming AC voltage into DC for the transmitter.

They were leaking wax and bulging on the ends after normal load up time. I was able to get about 35 watts output before shutting the transmitter down.

 When I was testing the output I also listened to the transmitter on my Hallicrafters S-40A receiver and the tone sounded great. Just a hint of a pitch change when you first keyed the Adventurer but very subtle. Either the johnson and Hallicrafters were drifting at the same rate or i'm doing something right.

 At any rate I haven't electrocuted myself with this transmitter so all in all i'm doing great.