The problem is with the MMIC (Miniature Microwave Integrated Circuit) amplifiers used. These amplifiers look like regular metal can transistors, but usually have four or more leads. The makers of the receivers designed the high gain IF amplifiers to work at 70 MHz, but some of these devices have high "gain-bandwidths", and are capable of working (and oscillating) well beyond their rated frequencies. As an example, the ICs used in the AGC stages, usually oscillate between 800 to 1200 MHz, while one of the output transistors in the 70 MHz IF Filter Module of WLOX-TV's MRC receiver, was clocked at 1.8 Ghz.
How do you know if you have a receiver problem? Unplug the input to your 70 MHz IF. Plug in a spectrum analyzer at the 70 MHz output on the rear panel of your MRC or other radio. Look at all frequencies up to about 3 GHz, and especially between 1 and 2 GHz.
You can check the output of each stage by using the 70 MHz output
line to the rear panel, and plugging it into the "output" of each stage.
If you find a signal present hundreds of MHz above the IF frequncy (excluding harmonics), you have a problem! You can usually find the bad stage by puting your finger on or near trasnsistor/MMIC cases in various stages. When you find one that makes the oscillation change frequency or go away, you've found the offending stage. Here's what you need to do to solve a typical problem:
MRC's 70 MHz IF Filter Module.
Output stages are in the upper right corner.
Q13, on the right may be oscillating. Q12 on the left, is usually OK.
Notice that the "tabs" on the transistors (7 o'clock position) are not soldered
Solder the tab of Q12 & Q13 to ground. Use a 100 watt soldering gun,
for as short a time as possible. Using a low wattage soldering pencil
will cause excessive heating of the transistor, and possible failure.
Add a tin or brass "ground strap" to the chasis. You need to provide a solid
ground path. #22 wire does not work well at microwave frequencies.
If you have further questions, call us at 1-. If you want us to check a receiver for proper operation, or do troubleshooting, the charge is $250. In addition, many other problems can be easily detected with our extensive equipment.