TR-7 Truble Shooting Tips
Trouble Shooting the TR-7 is not for the faint of heart and the advice offered by R L Drake is actually not helpful. Their advice simply states identify the problem area and then replace with know good circuit boards. That approach will soon leave you with your undershorts at your ankles and everything is swinging in the breeze!
Hindsight suggests this is where to start.
1. Given the complexity of the TR-7 a lot of research is required to understand how the radio functions and this involves involved internet snooping and watching you tube videos. Your mind cannot hold all of this info so create a folder on your computer to collect the info you have researched.
2. Create a problem statement that defines what you are observing. In my case there were two problems, with the first being it does not receive and the second that as you engaged the bandswitch on the bands 160-20M the digital display read 95 MHz and for the 15-10M bands the display read 10MHz. Of note there are two VCO's in the TR-7 with one covering the spectrum from 1 to 15 Mhz and the 2nd covers the spectrum 15 to 30 MHz. Thus BOTH VCOs were off frequency. To cover 1 to 30 MHz the VCO use up conversion and thus the LO outputs must be in the range 48 to 78 MHz (48 to 63 for the lower frequencies and 63 to 78 MHz fr the higher frequencies. In looking at the frequency injection to the up convertor the VCO frequencies were all less than 50 MHz. Along the way understand the frequency scheme. All received signals are translated to a range of 48 to 78 MHz and thus the 1st IF stage is at 48.05 MHz.
3. Think about how you could test the radio using alternate means. My evaluation was to pull the VCO board and inject a local LO (from my RF generator which tops out at 60 MHz and good enough for testing through 30M). That one little test enabled me to determine the TR-7 beyond the Up-Converter was fully operational.
4. Step 3 enabled me to identify 4 boards that were involved with the LO Generation and were out of the loop for the Step 3 ests. I purchased three known good replacement boards and for the 4th board I built a Dummy Board as shown in the manual. The Board swap as per the Drake instructions did not resolve he matter. Truth be known I think all of the four original boards are good.
5. The failure of step 4 caused me to look elsewhere and the first look was at the VCO Board itself. The first question was --is it being powered and initially I said yes as I validated 13.8 VDC to the VCO. But now upon closer inspection I spotted that the board was also fed 24 VDC from a small switching supply board internal to the TR-7. That voltage was 0, both at the VCO iteslf and at the output of the power supply feeding the VCO. A bit of jury rigging a 24 VDC supply to the VCO and we now have a working receiver. Not withstanding a blown switching transformer, the culprit may be a 75 cent LM78L24 TO-92 three-terminal regulator.