Adding an AM Filter to a Drake TR-4C

By: Mark Gilger, WB0IQK

(Reference Jeff Covelli’s, August 2001 issue of ER)

This is a follow up to the article written by Jeff Covelli, WA8SAJ in the August 2001 ER.  Jeff’s article covered the modifications to his Drake TR-4CW, to be able to operate on AM. Because the TR-4CW’s are getting difficult to acquire, I wanted to make the modifications to a straight TR-4C, which is still in plentiful supply through various sources.

The TR-4C differs from the CW version in that it does not have the built in CW filter. To perform Jeff’s modification on the ‘C’ version it would require adding a small relay and fabrication of mounting hardware for the new additional 6KC AM filter. It also requires adding a small half wave bridge rectifier, to supply the needed +12 VDC to the relay. The 12 VAC supply voltage can be obtained from two sources. The first is to tie it into the nearest 12-volt filament supply. The second, and the one that I always use, is to tie it into the 12 VAC supply, that is located on the bottom of the remote VFO jack. This is located right next to the USB/LSB filters. Pin 7 is the 12.6 VAC and Pin #8 is ground. This 12 VDC can also be used to power a cooling fan.

The relay I used was Radio Shack part number 275-249A and sells for less than $5. It’s mounted in location ‘B’ on my PCB board layout, Figure #4. I chose to tie wrap mine to the bracket. Super Glue or RTV would also work. The relay was pulling about 60ma when I tested it and I wanted to reduce the current flow as much as possible, but still maintain solid keying action. I ended up with a value of 120 ohms, in series with the coil, which got me down to about 20ma draw.

The new 6KC filter, International Radio ( 541) 459-5623, part number 2311 is mounted to location ‘A’ on the PCB board layout.

This modification will take several hours, the most of which is the fabrication of the mounting board. I used double sided, copper glad PC board as the construction material. All panels are soldered to each other on both sides of the board. This construction turned out to be very sturdy, but yet somewhat flexible. Figure #1 shows the design that I finally settled on.  This was the 3^rd revision. Each of the predecessors developed problems with component clearance once I attempted to mount them inside of the case. When tying in the new filter, use RG-174 coax, or a substitute. Make sure all shields are hooked up. I tied the shields, on the mounting bracket side, and soldered them right to the board.

Step-by-step procedures:

1. Fabricate the filter and relay support PCB using dimensions in Figure #3.

2. Support ‘C’. The slot should be cut to fit the filter stud indicated in Figure #2 of about ¼ x ¼ inches.

3. Mount filter to area ‘A’ on board.

4. Disconnect T-6 and T-13 coax from the center of the top and bottom of filter select switch S7.

5. Hook wires disconnected in step #4 to the center pins on the relay. These pins are the common relay input.

6. Hook up new coax (RG-174) to the center pins of S7 switch position that were disconnected in step #4.

7. Hook up the other end of this coax to either the NC (normal closed) or NO (normal open) pins. In my case, I used the NC pins since I’ll be using the radio primarily for AM and wanted the relay deactivated during this operation. If you use the NB position, it disconnects the ground when switch is in the ‘ON’ position.  So when moved to the OFF position, the relay is pulled in and active during SSB operation. I would suggest wiring your remote switch, if not using the NB switch, so the relay is deactivated (NC relay contacts)  in your favorite mode of operation. In my case, AM.

8. Use new coax (RG-174) to hook up to the NC position (if noise blanker is used). Hook the other end to the new filter.

9. Solder a 30pf capacitor to each pin of the filter. See Jeff’s write up.

10. Look up +12VDC leads to the relay.

11. Optimize current draw through relay (see above explanation).

12. Mount relay into position ‘B’ on mounting PCB.    

13. Follow procedures laid out in original article.

14. Solder a 30pf capacitor to each pin of the filter. See Jeff’s write up.

15. Hook up +12VDC leads to the relay.

16. Optimize current draw through relay (see above explanation).

17. Mount relay into position ‘B’ on mounting PCB.    

18. Follow procedures laid out in original article.

As pointed out in Jeff’s article. You should always have a fan mounted in the general area of the final amplifier section. In AM mode this becomes even more important due to the heavier draw that AM puts on the 6JB6 finals.

I’ve gotten many excellent audio reports and I hope you enjoy this project as much as I did