The Resurrection of the Drake TR-7

[Short Synopsis: Pete is gifted for Christmas a Tech Special, Drake TR-7 from his #2 son and daughter-in-law. The radio does not transmit or receive and as you tune through the bands the display reads 95 MHz on bands between 1-15 MHz and then reads 10 MHz on the bands 15 to 30 MHz. This should have been a flashing red sign to me. But my resolution of the issue did take a course that forced me to learn how the TR-7 was designed. By elimination the answer (after 5 weeks of plodding) was a 24 VDC Power Source to one of the devices on the VCO board. The device a MC78L24 cost $0.38 cents from Digi-Key. I found the issue because I saw no voltage on the 24 VDC rail and substituting an external 24 VDC --Boom the Receiver is working and the frequencies displayed are correct -- Thank You FT-8!

Parts have been ordered from Digikey but are lost somewhere in an ice storm near Grand Forks, ND. Thank You Christy! So once the new device is installed we can check out fully the receive functions and look into any transmit issues. But since there is extensive commonality between the Receive and Transmit circuitry the problems, if any, would be in the exclusive transmit circuitry. Dumb S--t Luck often plays a huge role is making things work!

The 78L24 parts arrived and installed and now the TR-7 is working on Receive. I began the initial transmitter tests and the TR-7 produces full output on 40/20/80M but significantly less on 10/15/160M. Still tracking down that issue. I believe the Pre-Driver, Driver and Final RF stages are OK (20/40/.80M data) . Thus there are some earlier problem areas such as the HP Filter and other boards involved with the TX functions. The output is less on 15M than 10M --so a clue about filters. The TR Relay was inspected and that appears OK. I have made 4 contacts on 20/40M but am reluctant to use the TR-7 for regular QSO's until such time I find the issues with the earlier and later band switch positions using the transnmitter. All four QSO were positive and no reports of Wonky signals. I have cut the one motherboard trace and so the TR-7 transmits on all frequencies. I want to test out 60 and 17 Meters and observe their outputs. TBD.]

In 2022, my son Tim and his wife Amy as a Christmas present gave me a Tech Special Drake TR-7. The radio itself is in fairly decent physical shape only it didn't receive or transmit. I guess they felt I needed to twist my brain cells in knots trying to get this radio to work. They were not wrong -- my brain is twisted!

The usual clean the contacts and reinsert the circuit boards did not accomplish much. It did appear that many of the panel controls were working such as the AF and RF gain controls and the PBT as well as the mode select. The band switch on the other hand on 160-20M showed a display of 95 MHz and on 10/15M showed 10 MHz. [This was a clue that I missed.]

A review of the block diagram (WB4HFN's handiwork) led me to a path of 4 possible boards.

This path was affirmed when I pulled the VCO Board and simply injected an LO Signal from 48 to 78 MHz into the UP Converter Board. With this LO the TR-7 receives on the air signals and all of the back end circuits work! I was very encouraged that the TR-7 could be resurrected.

The 4 suspect boards are the DR-7 Digital Display, the Digital Control Board, the Translator Board and the VCO board. A word here about the DR-7 as it not only controls the Digital Display but also has logic on the board that is a part of the 48-78 MHz signal generation. The Drake engineers provided a switch on the back apron that turns the TR-7 into a 150 MHz General Purpose Frequency Counter. I tested that functionality -- and Boom that part works. But Drake cautions that capability could work but the additional digital logic may not.

I purchased three of the 4 boards as the Drake malfunction mitigation process is to try and isolate the problem area and swap in known good boards. I bought replacement VCO, Digital Control and the Translator Boards and when installed, the same problem remained. So we are back to the Digital Display Board.

Early TR-7's (mine is S/N 1394 so early) could be purchased with out the DR-7 and in its place is a Dummy Board, the schematic of which is provided in the service manual. This led me to the homebrew fabrication of a dummy board.

Thus I had to design a plug in board and fabricate a replacement using something I could readily make without resorting to a formal PCB. I managed to find all the parts although the two IC's, MC14050B's are in the SOIC form factor and literally unavailable in the 16 Pin DIP. I bought 10 pieces but now had to find some carrier boards (W1REX) and a purchase was made but these are hung up in a Maine snowstorm. By accident I stumbled upon a substitute IC, the CD4050B which I had a stock of in the PDIP. Should have checked the substitutes before doing anything. A true Rookie Mistake!

The Header Pins and Vector board have a 0.156-inch pitch and the spacing of the two boards to be jumpered with the Dummy Board are not exactly on that spacing which prompted drilling a set of holes in the Vector Board which can be seen on the left side Top photo) and Bottom side (lower photo) of the perforated board. The IC PCB was fabricated on my CNC Mill. The header pins came in 9 Pin stock so I had to cut one of the header pin stock to get 3 Pins and when added to he 9 Pin price gave me 12 Pins that are used on the Digital Control Board. Drake cautions that the Header Pins DO NOT wire straight through and so I had to create two wire lists so that the right wires go to the right pins. The lower photo above is the completed Dummy Board installed in the TR-7. Houston we have ignition!

It was hoped that the Dummy Board would enable two things to happen. Firstly if the radio operates with the board installed then indeed we know that the DR-7 Board was the failure point. Thus the next task is to find the issue on the DR-7 board. When I was testing the radio with the externally provided LO -- Damn it sounded great! This test also proved the LPF/HPF/Band Switch functions worked. So the odd men out were the 4 Boards and now we think it is but one board. [This was not necessarily a good assumption about the Band Switch as there as an ancillary function of the Band Switch ... read on.]

I have a suspicion why the radio initially may have not worked. There is the AUX-7 Board which turns the TR-7 into a General Coverage Transceiver or one that may be operated on Crystal Controlled frequencies. This board was installed backwards in the card cage assembly. The radio may be operated without the board but a reversal may have caused a signal input to the DR-7 that smoked a piece of the circuitry. The output of the AUX-7 feeds the DR-7 when operated other than in the normal mode. [This was just a suggested theory and reversal may have no impact.]

The TR-7 Resurrection Project is much like the task of getting Mary Jo into the backseat of the 57 VW Beetle. I perhaps did not mention she weighed > 200 lBs -- it was indeed a struggle but much fun along the way!

So, with the Display Dummy Board installed the TR-7 still did not operate on receive. I pulled the VCO Board and injected an LO signal of the proper range 48-78 MHz and the radio receives on the air signals. This led me to conclude after checking the VCO output frequency on all of the band ranges that any signal being sent to the Up Converter was less than 50 MHz. The TR-7 will not operate unless the VCO is in the range 48-78 MHz.

I now turned my attention back to the band switch as I had thought it was OK as I could receive signals with the external LO for the Band Switch setting. But there is an additional set of wafers on the Band Switch that produces a BCD code for each band that is read by the Digital Control Board to set the ranges on the two VCO's. The TR-7 Manual has a Truth Table of the Band Settings and the BCD Code for that band. I thought perhaps if the code was not correct the proper VCO BCD address would not be set I measured he status of Pins 1-4 on the Digital Control Board as you engaged the Band Switch and it matched the Truth Table 1 for 1. So, the BCD code was eliminated as suspect. Thus you could now proclaim indeed the Band Switch is OK!

This caused me to ponder that the VCO board may actually be OK but the Voltage being supplied to the Varactors may not be of the correct value. On the VCO schematic there are 4 examples of voltages to set the Varactors for 4 Bands. I now needed to measure those voltages. The VCO has two Voltage inputs one at 11 - 16 VDC to operate the electronics and then a second voltage input of 24 VDC that provides the source voltage to an Op-Amp that actually generates the Varactor control voltages. I found that there was no 24 VDC to the Pin (5/20) on the VCO Board. The source for that 24 VDC comes from an internal power supply board that has a DC to DC Inverter built into the Board. There was no 24 VDC at the output from the power supply board. Just like in the Chinese Laundry No Tickee No Washie. No Juicy (voltage) no Workie!

There are three possible failure points on the power supply board including the DC to DC Inverter, the diode ring rectifier and the 24 VDC regulator which outputs on Pin 20/2. I have some tests to determine which one or several of the components are at issue. It turns out that there is a secondary winding on the DC to DC Inverter transformer that is used to produces a negative 5 volts to Pin #1 (20/1). That voltage is present so that gives good cause to think that the Inverter section is OK! So, that leaves the diode ring rectifier and the 24 VDC three-terminal regulator for evaluation.

There is a quick test to verify the current assumptions as to the problem. That test would be to power up the TR-7 with the PS-7 power supply. A prior step would be to remove the connection to pin 21/2 on the power supply card and feed that removed wire with a regulated 24 VDC. If the receiver works then we know the culprit indeed is the 24 VDC and it may get down to a failed 75 cent three terminal 24VDC regulator. Assuming this corrects the problem then the next step would be to repair the power supply board.

I followed thru with the above idea only I used two supplies with one at 13.8 VDC to power the receive section and the second with a regulated 24 VDC for the VCO. The video at the mast head shows that result.

This cured the issue of no receive (and also impacting transmit) but regardless there still has to be 24 VDC on the output pin of the power supply and 24 VDC input to the VCO. The power supply board must be repaired to resolve this. In hindsight I should have checked for 24 VDC earlier in the process, But the fact that I didn't check it early on did cause me to learn a lot more about the innards of the TR-7. So in a sense a very positive outcome in terms of learning more about the TR-7.

Specific testing of the internal power supply board after removal from the radio resulted in replacing C2108, a 220 Microfarad Filter Cap on the 10 VDC rail the feeds the DC-to-DC Inverter. The removed part looked discolored and so the voltage rating was up sized from 25 VDC to 50 VDC. Pulling and testing the on-board 78L24 device verified that it was toasted! We also saw about 50VDC (unloaded from the Bridge Rectifier) to the Input side of the regulator. Thus a conclusion the DC to DC Inverter is working. But as a caution, once the new 78L24 is installed on the board and the board re-installed in the radio and the TR-7 powered on -- I will check the voltage at Pin #2 on the Power Supply board to assure we have a regulated 24 VDC. Once that is verified then I will connect that 24 VDC to the VCO -- no sense blowing up a VCO board!