Gates BC-1T and BC-500T:

160m Conversion Tips

Phil Galasso, K2PG

The Gates BC-1T and its smaller counterpart, the BC-500T, are excellent-sounding transmitters. Because of the lead lengths in these transmitters, I strongly recommend against attempting to convert them to any band higher than
75 meters.

To save yourself a lot of aggravation during the conversion, be sure to check your transmitter on its original frequency by running it into a dummy load. The Gates BC-500 and BC-1 series transmitters contain a built-in dummy load. To use it, put the J-plug into the bottom position. This also pushes down a plunger which switches on a warning light. The warning light on the dummy load takes a 230 volt, 50 watt bulb. Bulbs may be obtained from:

Griffith Electric Supply Co.

5 Second, Trenton, NJ 08540

(609) 695-6121

(609) 921-8482

Griffith Electric may also have the 10S6 230 volt indicator lamps used in this transmitter. The warning light on the dummy load subchassis is not an overtemperature indicator. It warns the transmitter operator that the transmitter is working into the dummy load, rather than the antenna.

For the initial tune-up, remove the high voltage rectifiers. Locate the box containing the 12BY7 oscillator and buffer stages and remove the cover. The crystals are of the vacuum type. They look like large octal-based tubes. No crystal ovens are used. The crystal pins are 3 and 7. Plug in a 160 meter crystal. Bypass the door interlocks at the bottom of the transmitter and turn on the transmitter. The exciter stages should come up about 20 seconds after you turn on the filaments. Using a nonconductive tuning tool, carefully adjust the coil slug and screwdriver slot (capacitor) near the buffer tube.

Be careful to stay away from the plate caps of the 807s, as they will be energized with potentially lethal voltage! Tune these slugs and the RF DVR TUNING control for maximum PA grid drive on the multimeter. You should get about 150 mA of drive. If you do not get sufficient drive, you may have to change a capacitor in the buffer tank circuit. You may also have to change a tap on the driver tank coil.

Once you achieve proper RF drive, turn off the transmitter and disconnect the primary power. In the PA tank compartment, you will probably have to disconnect one of the fixed loading capacitors to the right (as you are facing the front of the transmitter). For operation on 1885 kHz, you will have to change a coil tap. Facing the rear of the transmitter, the tapped coil should be tapped 11 turns from the left. To get the PA tuning and PA loading coils into the "ballpark", the PA TUNE coil should be set so that the roller is 15 turns from the front of the transmitter and at the top of the winding. The PA LOAD coil should be set for the third turn from the front of the transmitter and at the bottom of the winding. To neutralize the transmitter, adjust the neutralization capacitor so that a slight "rocking" of the PA TUNE adjustment does not cause a change in the PA grid current. If you cannot achieve complete neutralization, try changing the tap on the driver tank coil, retune the driver, then adjust the neutralization capacitor.

While the feature of having the exciter come on automatically while warming up this transmitter is fine for normal broadcast operation, it produces interference when you are trying to hear the other station in a normal amateur radio contact.
To defeat this, connect a contactor with a 230 volt coil so that the coil is in parallel with the primary of the HV plate transformer and the contacts switch the AC to the LV plate transformer. That way, the exciter comes on when you turn on the high voltage.

Be sure to use relays for turning the plate voltage on and off by remote control. The control ladder in this transmitter uses a very dangerous 230 VAC. To use the remote feature, locate TB-1. Facing the front of the transmitter, this is the long terminal strip below the driver PC board. To turn the plate on, connect a set of normally open contacts between terminals 8 and 9. To turn the plate off, disconnect Wire #110 from terminals 10 and 22, then substitute a set of normally closed contacts.

In normal use, never defeat the door interlocks and never operate the transmitter without the protective grille in the front. Keep the door closed. It will ensure the proper flow of cooling air across the 833 tubes.