MODIFICATION WORK AT MODULATOR SECTION
(FINAL STAGE) OF HMB 140 TRANSMITTER AT AIR SILGURI
by Abhijit Sengupta Sharma (EA) & Uttam Bhattacharjee (Tech)
per previous record the HMB 140 transmitter at AIR Siliguri runs with single
Modulator Valve since long back. The only Valve BEL 40000 CX which is used in
the Modulator section also pass its stipulated running life as per VSD report.
For previous certain days its gives following problems as per record in the day
to day transmission logging
FACE TO RUN WITH NORMAL MODE
put on/ put off condition of transmitter is not at all smooth. Certain frequent
tripping was observed during few days with huge tripping sound from the
transmitter, suspecting short circuit of HT.
gives intermittent shorting problem between screen grid and
plate (i.e. HT)
distorted modulation with the noise generated at the receiver due to spurious
Last of all on 20th
August 2014 at 14:27 hrs it shows permanent shorting between HT and Screen grid
/ control grid.
are two ways to get
the alternative AM
process for this transmitter
station can run with low level modulation using PWM with MOSFET arrangement as
pre-driver to PA.
•Modify the Modulator
section with the Valve BEL 100000 CX which is actually used in PA section of
this particular transmitter & eventually also a Pentode Valve. This
particular Valve is also available as spare of the other transmitter
(Transmitter - I) which is awaiting for dismantling with respect to installation of
DRM(200KW) transmitter as proposed for this station
resolve the problem quickly and to reduce the breakdown time it is decided to
go with the 2nd option. But there are following problems to achieve this process
BEL 100000CX vacuum tube valve is quiet heavier (about 38 Kgs) than the CQK 25 /
BEL 40000 CX vacuum tube. Therefore the holding structure should be robust
BEL 100000 CX valve is dimensionally bigger than the CQK 25 / BEL 40000 CX
vacuum tube. Therefore to fit it in the final Modulator cubicle is a rigorous
and time consuming job.
particular Valve takes about 200 amps current at the filament instead of 180
amp taken by the modulator valve filament. Therefore filament transformer
should also arrange to fit in the particular cubicle which also is quiet bigger
in dimension (12”X14”X10”) than existing one.
the portion of the holding structure is laying outside the cubicle it may cause
RF shock hazards.
connectivity of screen grid supply, control grid supply should be given from
the existing biasing circuit.
water flow pipe which is ¾ “
in diameter for this vacuum tube should fit with ½” existing water flow pipe
without giving any sort of problem for quantity of water flow as the water
jacket cavity of BEL 100000 CX is more than the existing vacuum tube. So that
‘WF’ indication (water flow safety interlock) may arise.
air flow pipe also
fit to the proper persists as the previous agreement is a rigid ‘T’ section for
supply of Air to both modulator valve, it should be modify with the flexible
air pipe to fit with valve base.
TUBE CQK 50-2
•Frequency 26 MHz
voltage 11 kV
current 10.5 A
grid voltage 800 V
grid current 1.4 A
grid voltage -650 V
grid current 0.7 A
voltage 12.6 V
current 335 A
voltage 15 kV
dissipation 100 kW
grid dissipation 0.7 kW
grid dissipation 2.2 kW
•Diameter 230 mm
•Length 595 mm
To drill the above problem we have done
the following job
observe the dimension of modulator cubicle as follows.
•To make the space
available remove the two numbers of filament transformer of Modulator valve
from the outside.
• Use ½ “/ ¾ “expander with
¾ “to 3” Nipple and a rubber ‘O’ ring for water pipe connection to the valve.
• Remove the one valve and
valve base from modulator cubicle to fit the structure and necessary connections.
• Put the structure inside
the cubicle with the valve base at the top and the filament transformer placed at the base of the
•Connect the filament the
screen gird and control gird connection to the valve base by using proper guage of cable with the
filament verification and the modulator cathode current measurement cable
connection. After that the HT connection has been made from one point.
•Fit the flow pipe at the
bottom of valve by a flexible pipe take from the Txr 1 and fit directly to the LP blower out let after
remaining the pipe connection for existing set up.
connecting all the necessary connection following observation has been made
check the water and air leakage from the respective joints.
the 200 AMP filaments current with Tongue tester.
screen gird voltage used at PA section for this particular valve is 800V but
for the modulator section it is 1070V therefore necessary tap changing of
screen gird supply transformer is made to get about 900V at screen gird. As per
specification of the BEL 100000CX use the screen gird can take up to 1200 VDC,
also the plate voltage up to 15KV.
After all the above
observation the transmitter is put on in normal mode up to HT. As there is no
abnormality in the meter reading and also physical checking the transmitter PA
screen and modulator screen are given and take transmitter on AIR at about 01.00
hrs for testing. Modulations of 20% run for 15 min then give 50% of modulation
for 20 units and then give about 80% of modulation for about 1½ hrs for testing
found no abnormality. Finally the transmitter runs for morning transmission of
August 2014 on right time at 05.53 hrs.