BC 1T Conversion
Gates BC1T AM Broadcast Transmitter
Conversion to 75M AM Phone
By Mack McCormick, W4AX August 2023
The purpose of this guide is to provide a list of the steps I took to convert a Gates BC1T AM Broadcast Transmitter to the 75M ham radio band. This was a big project and required three weeks or about 80 hours. Before starting the project I read every article I could find on the Internet and Electric Radio. The most helpful article had notes taken from Timtron’s, WA1HLR, conversion of a similar transmitter at the ARRL HQ. I only used some of his data for the crystal oscillator conversion.
I owe a huge thank you to Jimmy, K4KHV, and John, K4VZS, for making this project possible by sourcing and sharing the Gates.
All my pictures and videos are located here but I’ve only included the key pictures in this article. Gates BC1T Videos and Photos . The videos in the link explain a lot of things.
Manual (so you can follow along in the schematics)
CAUTION: THIS TRANSMITTER USES INSTANTLY LETHAL VOLTAGES AND CURRENTS. DO NOT ATTEMPT THIS MODIFICATION iF YOU DON’T HAVE EXPERIENCE WITH GOOD HIGH VOLTAGE SAFETY PRACTICES.
Assembly and Cleaning
The transmitter is likely to be very dirty after years of use and storage. I recommend taking your time to clean it thoroughly. I used a small engine pressure washing wand (Harbor Freight) with a Simple Green solution and lots of scrubbing with rags, paint brushes, etc. I rinsed with distilled water and blew everything dry with compressed air. Of course typical precautions apply to not spray in meters, coils, transformers, etc. I recommend completing this step outside. It will be messy!
I recommend you build a wooden base with wheels to move the transmitter during work and afterward. It weighs over 800 lbs. I used 20 one inch captive ball bearings (Harbor Freight) on the wooden base I constructed using the plans in the Gates manual. The iron is very heavy so get a helper if you have a bad back. Carefully inspect all wiring, especially the high voltage wiring during this step. 30 KV silicone HV wire is available on Amazon. Inspect the entire transmitter carefully for any burned or missing parts (mine had several). Download and print a manual at UPS or Fedex that is spiral bound so you can lay it flat.
By far the most troublesome and difficult parts to clean in my transmitter were the two roller inductors in the final tank coil. If I were ever to attempt this again I would completely remove them from the transmitter (big hassle to remove) and clean them thoroughly outside the transmitter. These need to be very clean or the roller will not make good contact with the coil. I also found it necessary to put a light coat of Noalox on the shaft the roller slides on. You don’t want any poor connections because there will be 4.2 amps of RF current in this circuit during tuning and operation.
I recommend cleaning the inside of the 833a filament connectors with an appropriately sized rifle bore brush.
Pay careful attention to reconnecting the iron. The schematic and an ohm meter are your friend. Hopefully all the wires were labeled before they were disconnected.
Iron installed, HV wiring replaced, solid state rectifiers, and filament transformers removed from the circuit
Solid Stating the Power Supplies
The Gates BC1T uses mercury vapor rectifier tubes which are fine but can cause catastrophic issues in case of a short. I decided to solid state all three power supplies; High Voltage, Mid High Voltage, and Bias. I used drop in solid state replacements but your favorite diodes (appropriately rated) will do fine. Since the filament transformers are no longer required I completely disconnected the primary and secondary windings (one less point of failure). This requires moving the positive wire from the filament transformer to the sockets on the solid state rectifier replacements. (it's assumed you are comfortable with these sorts of changes but I’ll be glad to provide more detail if needed). As an example of missing components my transmitter was missing the 100K 100W HV bleeder resistor.
The Tuned Circuits Conversion
This is the heart of the conversion. The BC1T uses three tuned RF circuits; oscillator, intermediate power amplifier, and final tank circuit. It is helpful to review the theory. To move a tuned circuit from 1 MHz (the broadcast band) to 4 MHz the top of 75M you must remove one-half the inductance and one-quarter the capacitance. This gets you in the ballpark. I always use a capacitive and inductive impedance calculator and can get dead on with my values. The ballpark calculation provides a sanity check.
The Oscillator
All these changes were made with the module on the bench. Testing was done with a bench power supply and test equipment.
C3 - 10 pf
C4 - 100 pf (note C3/C4 provide the voltage divider for the cathode and crystal. If C4 is much larger the oscillator will likely be unreliable)
R3 - change to 37K 2W by removing and adding R5 in series
R5 - jumper the pads (no resistor)
R7 - bypass with a .01 50V or higher capacitor
R9 - 33K 2W
C11 - remove if present
L3 - surgery required. Remove from the oscillator module completely. Remove L3 completely. Carefully remove 50% of the turns. This will be all of the top winding and a bit of the bottom winding. To resonate with C4, a 140 pf variable capacitor, set to approximately 30% meshed (42 pf) (-j975 ohms) you will need L3 to measure 40uh (+j975 ohms). This wire is very easy to solder back without stripping. Take about 40% of the winding, measure and then take more. You can’t undo your mistake easily. Once you’re successful the difficult part is over. I recommend an LC meter for this step.
Rewire the XY1 crystal socket so the wires connect to a standard FT-243 crystal (if that’s the style you’re using).
The Intermediate Power Amplifier
L8 is the only component change needed. Others recommend removing the coil and then removing turns. That is not necessary and all changes can be made with new taps. Perhaps a bit of theory first. It is important that C16 (the RF ground) remain in the center of whatever tap end points are used.
- Move the tap that goes to the 833 grid via the capacitor to coil to tap 5 (from the end closest to you).
- Make a new strap from copper and move the top end of the coil that comes from C17 to tap 4.
- Move the RF ground (C16) to tap 6.
- Make a shorting strap from the cold end of the coil (C10) to tap 8.
Here the math for how I calculated the inductor value. C7 is a .00025uf (250 pf) capacitor which has an capacitive resistance at 3885kc (30% meshed) of -j546 ohm. Therefore we need an inductor of +j546 to resonate at 3885kc (22uh). You only need to be close since we’re dealing with a variable capacitor.
Now is a good time to neutralize the 833a triodes. Disconnect the AC and discharge the capacitors. Connect a signal generator or spectrum analyzer 3885 Kc output to the plate of the 833a’s. Connect an oscilloscope or spectrum analyzer to tap 5 of L8. Loosely couple these via 10pf capacitors. Tune C10, the neutralizing capacitor, for minimum signal. Everything is now neutralized.
The 833a final tank tuned circuit
C11 and C12 - stack one on top of the other in series
L13 - short out or bypass
C13 and C14 - rewire (using straps) in series
Build a 40uh DC safety choke (20 AWG enameled wire) from RF output to ground. Without this choke, if C9 fails, you would have 3000 VDC on the antenna terminal and antenna. I used 2 inch PVC and forgot the number of turns but it is easily calculated.
I strongly recommend cold tuning the tank circuit. Disconnect the AC and discharge the capacitors. Connect a 2700 ohm resistor from the 833a plate terminal to ground (this simulates the plate resistance). Inject a 3885 Kc signal at the antenna terminal via a signal generator or spectrum analyzer. Look at the signal using an oscilloscope or spectrum analyzer receiving at the plate terminal of the 833a’s. Adjust the Tuning Inductor for maximum signal. You will find that the Loading inductor will be fully counter clockwise (not in the circuit) into a 50 ohm load. By cold tuning the circuit before applying transmitter RF it will be very close to the correct tuning spot. This will prevent the HV overload relay from tripping when you first apply high voltage. Don’t forget to remove the 2700 ohm resistor on the plate of the 833a’s.
Here is the math on the DC Safety Choke
40uh at 3885kc is an impedance of 976 ohms which has negligible impact in parallel with the 50 ohm circuit impedance.
Here is the final circuit (minus the DC safety choke)
Other Recommended Items to Replace
Contactors
Both contactors were VERY noisy as you’ll hear in the initial videos. Purchase four pole 240VAC coil replacements (Amazon). Very quiet now.
Vacuum Tubes
I measured and decided to replace all vacuum tubes (my normal process during restorations). I have several videos in the link at the top of the doc on how I tested and quantified every 833a. Most were very substandard and were obviously pulled from other hardware.
Variac
I decided to add a 240VAC variac to the HV transformers primary. I did this for two reasons; allow tuning at low power and also to stay within the 375W carrier amateur legal limit. Important note: Gates recommends removing one anode cap from the 8008 rectifier for tuning. This is a very very bad practice as the stored inductive voltage in the power supply choke is kicked back into the single connected rectifier and transformer when the power supply is operating as a half-wave rectifier. If you don’t add a variac then please make a circuit to switch the HV transformer primary to 120VAC during tuneup or amateur operation.
Antennas and Switches
I recommend you NOT use any antenna tuner or switches in the RF output path. I also put up a dedicated 75M antenna.
Audio Feedback Ladder (out of phase)
An audio feedback ladder is used in the modulator to reduce distortion. In my case every 2.2M 2W resistor was way out of tolerance. I suggest you replace them all. You’ll find the modulator bias cannot be set correctly if there are issues here.
Cooling Fans
Both fans were defective and replaced with 8 inch muffin fans in parallel. I rewired the fans to use 120VAC and they run whenever the filament supply is ON. There is a tap on the 10V filament transformer primary to provide 120 VAC.
LV Time Delay
I bypassed XK5 to time delay circuit for the low voltage power supply since there are no longer mercury vapor rectifiers installed. Keep this relay if you decide to use mercury vapor rectifiers.
XV5 5R4 Rectifier
I also solid-stated the bias rectifier 5R4.
Initial Power Up and Testing
OK, you’re feeling brave and ready to test if any magic smoke escapes. Power on the filament switch and all the tubes should light. After the LV delay timer cycles (60 seconds I think) you should note RF drive when the multimeter switch is in the grid position. Follow the procedure in the Gates manual. You’ll want at least 125ma. Peak L3 and C9 on the oscillator. Peak C7 on the front panel IPA. If your grid current is way off this value then stop and investigate. Note: the 650VDC power supply and therefore the oscillator and IPA are operating without the HV switch being pressed. (we’ll fix this later for ham radio use). You should be able to easily hear the signal in a receiver. CAUTION: HIGH VOLTAGE IS PRESET AT THIS POINT (650VDC)
If you’ve reached this point then make sure a dummy load is connected. Apply approximately 120VAC to the primary of the HV transformer using a variac or your own circuit. Go ahead and press the HV switch. Since you’ve pre-tuned the tank circuit you should see considerable power on the RF ammeter(a couple of amps). Rotate the tuning control until you have a sharp dip in the plate current. Congratulations, your transmitter is working. Now apply the full 240VAC to the primary of the HV transformer. Quickly redip the tuning control and you should have approximately 4.2 amps of RF current. Adjust the modulator bias using the procedure in the Gates manual. Test the modulator by applying audio and looking at an oscilloscope connected to the RF sample line. Power the transmitter off, disconnect the AC cord and make sure all caps are discharged. Now let's make it push to talk.
Making it Part of a Transmitter/Receiver Combination
We need to make a transmit-receive relay, mute the receiver, and add a push to talk circuit. I’m an advocate of using inexpensive solid state timing circuits. Solid State Timing Relays . Using this relay board we can set the timing for the firing of relay 1 to 8 in order to about 250 milliseconds (or longer if needed). Here is how I’m using the relays:
Relay 1 - closes to provide 26 VDC to the vacuum relay. This is the most important device to trigger first since we don’t want to hot switch 4.4 amps of RF. The vacuum relay is VERY fast and will close in under 20 ms.
Relay 2 - I’m using a Collins 75A-4 for a receiver so you may wire this relay differently. Relay 2 applies 26 VDC to the muting circuit.
Relay’s 3-7 - not used.
Relay 8 - Closes to provide 240 VAC to the coil of the plate contactor. Terminals 8 and 9 of the TB1 terminal strip on the Gates.
Note: I strongly reconnect keeping this timing circuit OUTSIDE the Gates (I placed it on top). I burned up (RF overload) the Timing Relay board when I tried placing it inside the Gates high RF environment. This was even though it was shielded and the case grounded. You will note that I generously used ferrite beads on every leading entering or leaving the relay board.
Now we need to rewire the Gates for PTT. Disconnect and cap off the primary wires on T2. Make new wires and run them over to the primary connections on T1 the high voltage transformer. This ties the 650 VDC low voltage to turn on at the same time as the 2700 VDC high voltage. Now the oscillator and IPA will not operate until the transmitter is keyed.
One more change. Remove the jumper wires on K2 (plate contactor) terminals 1 and 3 and also terminals 2 and 7. This prevents the plate contactor from latching. Note: Do not use the plate ON switch (S7) on the front of the transmitter. In fact you should disconnect it. Pressing this switch would cause the full RF to be transmitted into an open vacuum relay.
Put it on the air and enjoy. You’ll need to provide +12 to +15 dbV over 1 volt to the input of the Gates. Here is a photo of the Gates on the air at Art, N4JK’s, QTH in Florida.
My plan is to remote the transmitter into the shack (50 yards away) but for now I have a temporary operation setup in my woodworking shop. I will remote turn the transmitter on/off plus key using a 24 VAC circuit. The RX RF will be returned via coax. The audio will be sent to the Gates over TCPIP using Cat 6 cable and A/D and D/A converters.
Notes on 833a Tube Testing
In case you haven’t noticed I’m big on measuring circuits/components and doing the math in designs. I was frustrated that I didn’t know how “good” my 833a tubes were and therefore decided to build a tube tester. Watch the videos in the link at the top of the page for details. Needless to say I was surprised (shocked) by how little emissivity my 833a’s exhibited. I therefore decided to replace them all with new ones ($200 each).
