[AMRadio] Gates BC-1T


D. Chester k4kyv at charter.net
Sun May 6 23:57:22 EDT 2007


"Now I'd be
interested in what folks think, pro and con about the 1T.  I happen to
think it's one of the neatest looking Gates in that power class."

There is where you and I would have to disagree.  I think the BC1-T has to 
be the ugliest excuse for a broadcast transmitter ever made!  The epitome of 
its ugliness is that hump that pertrudes out, down at the bottom of the 
cabinet, under the door, over the grille for the air intake filter.  Also 
the ridiculous little triangular shaped viewing window.  If they had made it 
adequate in size and rectangular in shape, you would actually be able to see 
the 833A's clearly without having to move just to the exact correct position 
in front of the cabinet.  But mine didn't cost me a penny except for the 
trouble of picking it up and converting it, so I can't look a gift horse in 
the mouth.  Once I got it going, mine works very well.



"I'd also like to hear from anyone that has tuned it to 160.  Will it go
there without mods, other than re-tapping coils, etc."

I changed out the fixed capacitors in the output circuit.  Scaled them all 
down proportionally from 1230  kc to 160m, and reduced the number of active 
turns in the fixed coil.   Whether or not you can just retune it depends on 
the original broadcast frequency.  I suspect you could just retune it if the 
original frequency was at the top end of the BC band.



"What sort of drive do the drivers require from an external VFO, exciter?"

I modified the oscillator unit in mine.  Changed the xtal oscillator stage 
into a grounded grid VFO buffer.  Grounded the control grid, disconnected 
the feedback capacitor from the cathode, and ran a coupling capacitor to a 
BNC  connector on the back of the osc unit to feed the VFO output to the 
cathode of the 12BY7.  It drives well with 50 ohms output from the VFO @ 
about 200-250 milliwatts.

I removed the frequency adjust capacitors from the xtal oscillator, and 
mounted a variable capacitor with a real shaft  for a knob in the place of 
one of the xtal frequency adjust trimmers, and used that capacitor to tune 
the resonant circuit between the 12BY7 stages from the front  panel. 
Otherwise, you have to open the transmitter, remove the cover  from the osc 
unit, and take an insulated tuning wand to adjust the slug tuned coil every 
time you change frequency.  I had to remove about 1/3 of the turns  from the 
slug tuned coil to make it resonate all across 160m.

Other mods: I modified the control circuit to run the relays on DC.  The 
a.c. contactor relays sound like a chain saw running, and nothing I did 
would quieten them.  Also, I replaced the mod  reactor with a potted unit. 
The original one talked back so badly that I could just barely crack the mic 
gain before it started accoustical feedback.  I modified the cooling fan 
circuit to run them at half voltage.  They still run, and adequately cool, 
but make much less noise.  Originally they sounded like a vacuum cleaner 
running.  Basically, the transmitter was designed to be remotely controlled 
from the studio.  With the noisy a.c. relays and fans, and loud mod reactor, 
it was totally unsuitable to be located anywhere in the same room as the 
operating position.

I added CW capability to mine, but that's too long a story to tell here.

Another change - I couldn't get enough grid drive for the 833A's in the 
final, even on the orginal broadcast frequency.  For 160, I replaced the 
original final amp grid /driver plate coil with one of similar size, made 
from air core (miniductor) stock, and that brought up the grid drive about 
40%.  The original one is wound on a bakelite coil, using close spaced Litz 
wire.  It's probably OK for the lower frequencies, but I suspect 160m is a 
little too high for Litz wire to operate efficiently, plus the phenolic coil 
form is not exactly low-loss.

Also, the driver/final stages with the 807's were squirrelly on 160.  I 
re-arranged the wiring in the final amp stage so that,  except for the 
output network components in the top of the transmitter, everything was 
grounded to one point on the chassis.  In the original circuit, they just 
grounded each component directly to the chassis at the point closest to that 
component.  Using the common grounding point made the final and driver 
stages much more stable.

Another bit of flaky wiring that I corrected was that the rf output lead 
running from the oscillator unit to the 807 grids was completely unshielded, 
and integrated into the wiring harness along with the filament and d.c. 
leads!  I replaced that with a piece of coax, adding another BNC connector 
to the oscillator unit and soldering the other end of the coax directly to 
the 807 grid pins.  Someone told me that Gates eventually made an identical 
change in later transmitters.

73,

Don k4kyv





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