Showing posts from May, 2021

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. M

IFR-1000S Restoration, First Mixer

In working on the IFR-1000S Communications Service Monitor, I traced down what I believe to be one of the issues. The first mixer. The first mixer module is made up of a machined aluminum body, a MD-614 mixer, and two teledyne 732-12 RF relays. There are six ports on the module. none of them are labeled. I had to use the coax interconnect diagram in one of the service manual scans, and a frequency counter to figure it out. This lead me to find out my frequency counter actually can go up to 1.3GHz even thought its says 1GHz. which is good surprise.  Below is a labeled diagram of the mixer ports and components. I used a test signal through the antenna port from a signal generator to trace through the modules which lead me to the first mixer. The first mixer has no output on the J4804 on receive. On generate the coax which is normally connected to J4804 measures 1200MHz. The output of the 1200-2200MHz oscillator measure correctly, when the IFR is set to 50MHZ the output is 1250MHz. When s

IFR-1000s restoration, Tantalum Tantrum

 I bought a IFR-1000s "For parts not working" last year. It has been popping fuses ever time power was turned on. I first thought it was the what the IFR-1000s service manual calls the "Duty Cycle Regulator." The duty cycle regulator is a DC to DC flyback transformer. It uses a 20KHz astable multivibrator oscillator to power a transformer with 2 primaries and 3 secondary windings. It takes in around 12-15 volts, what is called "Raw DC" or the output of the 120 VAC to 15VDC transformer and bridge rectifier. The outputs of the Duty Cycle Regulator are 12V, 5V and -39 volts. The Duty cycle regulator is controlled by a comparator on the output side of the 12V. A comparator is connected to a voltage divider on the output side. When the output goes below 12 volts, it sends the 20KHz wave through the transformer to bring the voltage back up.  I made a LT spice simulation of the duty cycle regulator to try to make sense of how it works.  Here is a download link to