Gates BC-1G Modification Notes
(W4QCU, March-July 2005)
Revised February 2007

 

PLEASE NOTE:

1. I've attempted to provide accurate information in the following notes and drawings, but accuracy is not guaranteed.  Do your own checking before making equipment changes.
2. Most of the component numbers cited below are from the Gates BC-1G schematic diagram.  Unfortunately, I don't think this schematic is posted on the web.  The  BC-1T schematic is available, but there are differences.   Components that I added are highlighted in bold where they are referred to by number.  Numbering of new components is my own creation. 

 

Correction of AS-Found Abnormalities: 

The wire to one side of the coil of K2 had been disconnected.  Reconnected but later disconnected (see below).

 

Added the missing wire 180 from TB2-4 to TB2-5.  Don’t know why it was missing.

 

Removed found jumper from TB2-3 to TB2-4.  The jumper had the effect of continuously closing the reset switch.  Don't know why this was done. 

Removed found jumper from TB2-1 to TB2-2.  The jumper appears to have been used to get around a problem where the main power switch S40 would not make contact.  With this jumper in place, the main power relay K1 is energized when the local-remote switch is placed in remote.  Repaired S40 with contact cleaner.

 

Wire 30 had been disconnected at TB1-23.  The wire was connected to one wire in an external cable and the other wire was connected to TB1-23.  Returned to normal wiring, i.e. connected wire 30 to TB1-23.

 

The front door interlock switches were found taped closed (bypassed); tape was removed.

HV transformer T40 in my transmitter has a different primary arrangement than shown on the schematic, i.e., there is no tap.  Terminal  A on K3 connects to terminal  C on K2, and wire 87 connects these points to one side of T40.  The other side of T40 connects to terminal  A on K2 and terminal C on K3 (as shown on the schematic).  Cabinet wire 37 (to ground) had been disconnected from TB2-26 and a jumper was installed from TB2-26 to TB2-18.  This had the effect of applying 230 vac to the primary of T40 in both Hi and Lo power.  The jumper was removed and cabinet wire 37 was reconnected to TB2-26, restoring the low power mode (115 v to the primary of T40). 

Removed an as-found external cable (shielded) that was connected to TB2-9, 12, and 13 (ground).

Repairs:

 

Replaced defective K8 (Amperite 120NO30) with a solid state adjustable relay (120 vac coil, 0-30 sec.).  Rewired socket for coil on pins  2 (120 vac) and 7 (grounded) and two N.O. contacts (pins 6 and 8 and pins 1 and 3).  Lifted the wire from pin 3 and taped. 

Removed defective (open) resistor R13 and connected wire 163 to wire 164.  The effect is to defeat automatic modulator bias adjustment when switching between Hi and Lo power.

 

Rear door interlock switch was defective and bypassed by shorting wires 57 and 29. Replaced the switch with an Omron  No. A20GVZ-B7-K (available form Mouser) and returned the circuit to normal. 

 

One of the quick connect studs was missing from the bottom of the front screen door.  A source of supply is Aviation International Corp. in Miami, FL.  Part No. 85-12-180-16 ($2 each). 

Over-coated much of the cabinet to repair surface rust and grime.  Touch-up paints (from computer color match):  Benjamin Moore Iron-Clad Alkyd, low lustre metal and wood enamel.  Front door (light color):  Medium base C163-2? tinted with YW-4.75, OY-7, BK-9, GY- 2.  Cabinet (darker color):   Deep base C163-3B tinted with OY-9, BK-11, WH-12. 

MODIFICATIONS – POWER AND CONTROL:

Modifications are illustrated on following drawings:  control (ladder) diagram, wiring diagram for plate switch S45, and wiring diagram for the bias and 600v transformers. 

 

One of the HV rectifier boards was damaged, although probably operable.  Replaced the boards with an aluminum plate that acts as a chassis for additional control components as well as a pair of  K2AW half-wave rectifier blocks.  Components mounted on the aluminum plate are the following.

K2AW rectifier blocks rated at 14 KV, 1 amp.C50, C51, R50, and R51, which were originally mounted on a small board adjacent to the HV rect. boards.

· T50 supplying 24 vac and 40 vac

· Rectifiers, filters, etc. for 24 vdc  power supply and 50 vdc power supply

· 24 vdc relay K50 (interfaces T-R switch at operating position with other control components)

· 24 vac relay K51, provides time delay on release

· 120 vac relay K53, provides time delay on energize

· Resistors for HV soft start

· 16-position terminal board, TB5

 

The wire numbered 185 on the Gates schematic was added from XF2-2 to terminal G of K1.  This wire is not normally installed in BC-1s with solid state rectifiers.  The addition conveniently allowed existing wires 19 and 54 to provide switched 120 vac to new transformer T50 when the main AC power is energized.  This also energizes wire 56 which remains unused with the end taped. 

Removed 500 ohm 225w resistors and motor-driven 500 ohm rheostat from left side of the cabinet.  According to documents that accompanied the transmitter, these resistors were used to drop plate voltage for 500 watt operation.  In place of the resistors, a Powerstat (auto transformer) was added in series with the primary of T40.  The rating is 9 amps at 240 vac.  The line is connected to terminals 1 and 2 and the load is connected to terminals 2 and 4.  The auto transformer allows continuous adjustment of transmitter power.  Plate current (loading) is adjusted such that Ep/Ip (RF amplifier load resistance) remains constant.

 

The 50 vdc power supply provides protective bias for the 833 RF amplifier tubes.  The supply is connected across bias resistors R15 and R16.  The power supply floats with respect to ground.  The negative side is connected to the grid end of R15/R16, and the positive side is connected to the cathode end.  A blocking diode protects the filter cap and diodes in the 50 vdc supply when the voltage across R15 and R16 rises to over 300 v with application of grid drive.

 

Transmit-receive control scheme.  Basically, the bias, 600v, and HV supplies are all switched.  A time delay relay (K51 and new relay K52) are used to ensure proper sequencing (antenna transfer, bias on, and 600v on before HV on and HV off before bias off, 600v off, and antenna transfer).  More description below.

 

Lifted wires 39 and 61 from K2 (high power contactor), left them connected, and taped.  This prevents K2 from latching the Hi Power switch (S45) closed.   In the normal control scheme: S45 is momentarily pressed, K2 energizes (assuming all interlocks are satisfied), and S45 is latched closed.  S43 is pressed de-energize K2 and remove the latch.  As modified, the high voltage supply is energized only while S45 remains pressed or the external transmit switch remains closed. 

Lifted the wires at TB42, terminals 17, 18, and 20 that connect the normal control circuitry to S45 and taped the end of each wire.  New wires connect the switch in parallel with the remote transmit-receive switch at the operating position.   As-modified, either S45 or the T-R switch at the operating position energizes new relay K50 to initiate switchover from receive to transmit. 

Moved wire 60 from the K2 coil to a new terminal strip.  This allows a K52 contact to be placed in series with power to the coil.  

Lifted wires 64 and 67 from T1 terminal 1, kept the wires connected together, and taped the end.  This allows a K51 contact to be placed in series with power to T1 while continuing to supply power from the F2 side to other loads.

Lifted wire 46 from the “230” terminal of T42 and taped the end.  This allows a K51 contact to be placed in series with power to T42.

K8 (30-sec. time delay relay):  One pole is used to switch power to the 600 v transformer.   The second pole is in the line to interlock relay K9.  Originally, both loads were connected to a single pole. 

As-Modified Transmit -  Receive   Switchover Sequences

 

Receive to Transmit: 
Switch thrown to “transmit” at the operating position :
      One switch contact energizes K50 (completes the circuit from 24 vdc supply in the BC-1 to the K50 relay coil).
      One switch contact grounds the station control bus*, which mutes receivers and shifts the antenna to the transmitter.
One  K50 contact closes “turning on” time delay relay K51. (K51 provides delay on release.)
One K50 contact closes partially completing the circuit to the K2 contactor coil.
One  K51 contact closes energizing T1 (Bias), T42 (600v), and K52.
One K51 contact closes providing a second path from the station control bus to ground (relay contact is in parallel with the transmit-Receive switch contact).
The K52 contact closes completing the circuit to the K2 contactor coil.
K2 contacts close applying 240 vac to HV transformer (through an auto-transformer) and slow start network consisting of current limiting resistors and time delay relay K53 (on delay).  K53 is energized when the K2 contacts close.

 

Receive to Transmit:

Switch thrown to “receive” at the operating position:
      One switch contact de-energizes K50.
      One switch contact removes one of the two ground inputs to station control bus*. 
One K50 contact opens initiating the off-delay function of K51.
One K50 contact opens.  This de-energizes K2 even though the K52 contact remains closed.
K2 contacts open de-energizing the HV power supply.
One K51 contact opens after 0.5 seconds, de-energizing T1, T42, and K52.
One K51 contact opens after 0.5 seconds removing the remaining ground input from station control bus.  This removes receiver muting and transfers the antenna to receivers.
The K52 contact opens – no control function; merely returns to pre-transmit condition

 *  Station control system uses multiple 12 vdc relays to mute receivers and a 24 vdc vacuum relay for antenna transfer.  One side of each relay coil is connected to a voltage source, and the other to a common "station control bus".  A blocking diode prevents current flow between the 24 v and 12 v power supplies.  The relays are activated by grounding the station control bus thereby completing the circuit from the voltage source through each relay coil to ground. 

 

MODIFICATIONS – FILAMENT SOFT START:

 

Added soft start for the 833 filaments.  New solid state time delay relay K54 (120 vac coil, 0-30 sec.), along with three parallel 220 ohm, 25 watt dropping resistors in series with the primary of T3, provides soft start for the 833 filaments.  The relay and resistors are mounted on the shelf assembly adjacent to T3.   

MODIFICATIONS – RF CONVERSION TO 75 METERS: 

Modified connections to the two crystal sockets so that either octal base, broadcast crystals or FT-243 style crystals can be used. 

Modified oscillator per recommendations of Steve, KJ8CQ.  Removed C3 and changed C4 from 800 pf to 220 pf.  Instead of removing turns from buffer tank coil (L3), I replaced it with a 1 amp choke from Radio Shack (less than $1).  The choke has an inductance of 100 uH, so I removed turns to reduce the inductance to approximately 39 uH. 

 

Additional oscillator modifications:  Disconnected one side of C12 at J1, to reduce loading on the oscillator assembly.   Extended the ground connection at pin 3 of each crystal socket to pin 5.  This allows a FT-243 style crystal to be inserted in either socket between pins 5 and 7.  At crystal socket No. 1, also grounded pin 6 and connected a shielded wire from pin 6 to the grid of the buffer stage.  This provides a VFO connection to crystal socket No. 1 using a FT-243 style plug (in lieu of a crystal).  I found that a direct connection to the buffer grid provides much better drive that connecting a VFO to the oscillator stage in place of a crystal.

 

Modified 807 output  coil L4 as follows (per KJ8CQ).   Connected a shorting strap from the cold end (neutralization capacitor connection) to the fifth tap from the cold end (or the fourth tap from the hot end).  I achieved neutralization with the bias resistors connected to the second tap from the hot (807 plate) end.  I did NOT move the strap that connects the 833 grids to first tap from the hot end. 

 

Notes re: 833 drive and neutralization:  (1) I found that soldering or re-soldering ALL the connections and joints points between the 807 plate coil and the grid caps of the 833s made it much easier to neutralize the 833s.  I am experiencing low drive to the 833s, only 100-110 ma grid current.  Have yet to figure out why.  Also, the manual says that the screen voltage at the 807 RF drives should be 400V.  I measure a bit under 300V.  The manual may be incorrect, because 400V exceeds the maximum screen voltage listed in the RCA tube manual.

 

Modifications in the output tank circuit (per KJ8CQ):

· C42 (250 pf) placed in series with C43 (250 pf) (instead of in parallel)

· C44 removed

· L41 shorted out (by moving the tap)

· C45 replaced with three capacitors totaling approx. 750 pf.  I used a 620 pf G-2 mica and two 50 pf door knobs (15 kv). 

MODIFICATIONS – AUDIO:

 

Disconnected attenuator board AT1 and connected audio input directly to T2.  Input is run from a stereo phone jack (mounted on the rear of the transmitter) to T2. 

 

Screen dropping resistors (R13, R14, R15, R16, R24, and R25) for the cathode follower 807s were moved from the audio board to a terminal strip mounted behind and adjacent to the board.  The 150 K resistors were increased to 5w and the 47K resistors to 3w.

 

On the audio board, added 4.7uf/450 screen bypass capacitors in parallel with the existing .47 uf caps. 

Replaced R4 with a 1K, 50W adjustable resistor.  Allows adjustment of B+ to the audio board.

 

Resistor bank R20 through R25 was modified to provide a sample of modulated HV to an external scope.  (The original use is to allow external monitoring of plate HV).  A blocking capacitor (.01 uf, 6KV disc ceramic) was added is series at the connection of wire 84 to R20.  The scope (horizontal input) is connected to the bottom of the resistor bank.

 

MODIFICATIONS – MISCELLANEOUS:

Mounted outlet box for auxiliary equipment (scope, audio chain, VFO, etc.)  on the back of the transmitter.  Switched 120 vac is supplied from TB1-6 and TB1-8. 

NOTE CONCERNING 833 SOCKETS 

The sockets MUST be a bit loose to accommodate expansion of the glass envelope as the tubes heat up.  If the sockets are tightened securely to the mounting bar, the seals will crack, and goodbye tubes!