Deaf Radio Shack DX-390





While visiting my in-laws I worked on a Radio Shack DX-390. My father in law bought it off of eBay to do some shortwave listening and found that the HF receiver was deaf. The DX-390 is known for getting a deaf receiver on HF due to a static on the antenna damaging the Q1 front end FET. Originally there was a 2SK152 as Q1. The DX-390 has little to no antenna static discharge protection and killed the 2SK152. It was not picking up anything. I often go too deep and try hard solutions too early so I wanted to try to see what the sensitivity was before replacing the part. I brought my antenna analyzer with me as I was planning on making some kind of wire antenna to play with the Yaesu FT-817 I also brought. My father in law is also an Amateur radio operator. The nice thing about the antenna analyzer is it can also work as a signal generator for some high level testing. 

I set the antenna analyzer to near 7MHz and just let it run with some wire hooked up to the center pin of the connector. My Ft-817 picked it up very well. The DX-390 not so much. It was barely picking the signal at max RF gain and volume. I don’t know that it had any S units. It could not pick up any SW stations either. 

My father in law had already looked up the issues and bought a replacement FET. A 2SK291. Another ham’s blog, W9JES, it shows a solution using the 2SK291 and 4 1N4148 diodes. Q1 can be replaced by the 2SK291, the only 1 had with it was the pin out of the 2SK291 has the drain and source reversed in reference to the original 2SK152. When I replaced it, I just twisted the leads to end up in the right spot on the PCB. Once it was replaced two 1N4148 diodes were soldered in parallel and opposing directions across the antenna pad and the ground pad. Two more similarly across the the external antenna pad and ground. The diodes are to protect against static on the antenna. 

Once replaced it was able to pick up stations and had a full S meter with the antenna analyzer running next to it with no antenna connected. 

Afterwards we hooked up a long wire external antenna and listened to some 40m and 20m CW and SSB. The DX-390 has a BFO but it’s pretty hard to tune on SSB.




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