Collins 21E Adventure

Paul K2ORC                                                                                                                     Click on thumbnails to enlarge

Chapter 8 

            Update November 2003

    Necessity is the mother of invention.

What do a windshield washer hose and an old RCA public address system cabinet have in common?   Both are now part of WB2SYQ's Collins 21E .

There's been lots of tinkering, wiring, testing and
substituting since we last checked on the progress Norm's been making with the 21E.  Rewiring is nearing completion and some marginal and non-functioning components have been identified and replaced.  Various decks, including the 300J
exciter (shown in Chapter 5) are cleaned and back in their cabinets.  Assisting for the past few months has been John Flinn, N2NOL, an EMI engineer who works with Norm.


The problem of how to use the transmitter's original three phase final stage power transformer, discussed in Chapter7, was resolved when the W2ZM sale (memorialized in the AMFone.net Photo Album pages)  yielded a single phase, 3.3
kva multi-tap transformer. 

It will be recalled that the three phase power transformer was originally meant to be housed in a metal cabinet that resembles a steam radiator enclosure.  Opting for accessibility, Norm decided to place the "new" piece of final iron in a large, black crinkle cabinet that began life as home to a RCA public address system.  The accompanying photo also shows a large, 240 volt, 28 amp
variac mounted on the back wall of the cabinet above the transformer. 

The transformer cabinet, resting on a wooden platform similar to the one that keeps the 21E cabinets off the concrete floor, sits about three feet to the rear of transmitter power supply cabinet.  A length of floor level conduit containing the high voltage wiring will run from the transformer cabinet to the base of the 21E's power supply cabinet. 

 Around 40 inches in height, the old RCA cabinet is an ideal height for holding a  manual and other
material, lectern style, during work. 


Prior to mounting the power transformer and the variac in the RCA cabinet, testing had to take place.  The transformer primary is 240 volts and the secondary, although not center tapped, offers a choice of three taps: 1,450, 2,080 and 2,900 volts.  As designed, to produce 1 kw out the transmitter final needs 2,500 volts at 750 ma of plate current.  This is a good starting point for calculating the power requirements for amateur use.

The way the variac was tested to make sure it was capable of handling a sustained current level was ingenious.  Connecting the variac to the household service, Norm dialed it for about 120 volts and connected a toaster oven and a broiler to the variac's wiper and common end.  Turning on
the power and the appliances, he let things sit for a
while.  The combined load of the two current hungry appliances showed that the variac was up to the task of handling the load the plate iron will be subject to during transmissions.

A departure from the original design of the 21E was the decision to replace the mercury vapor rectifiers with a sturdy  homebrew bridge rectifier to be mounted in thepower transformer cabinet. While aesthetically lesspleasing than the purple glow of hollow state rectifiers, the solid state bridge will provide a level ofdependability and stability that argues for its use.  The rectifier tube sockets remain and the tubes, though no longer functional, can still be placed in their sockets so there will not be a void when one looks through the window in the 21E's power supply cabinet. 


The four diode stacks in the bridge rectifier are mounted on two boards.  The two stacks on the top board came from a 3 kv power supply for a linear amplifier that Norm was building with the late Eliot Taylor, K4EJT.  These are rated at 1,000 PIV, 6 amps.  Because Norm did not have enough of the 6 amp diodes to construct the other two legs of the bridge, he put together the bottom stack with 1,000
PIV, 3 amp diodes wired in parallel.  Beneath the bridge you can see the filter capacitors, ten 390 uF, 350 volt caps wired in series to produce a value of 39 uF at 3.5 kv.






The blower, cleaned and working, has been bolted to the bottom of the 21E PA/modulator cabinet. It has yet to be mated with the flange that funnels forced air to the deck that houses the pair of 3CX3000 modulators and the single 3CX2500 final.


When the transmitter was in service at WEBO air from the blower was directed to the deck through a short section of sheet metal ductwork.  The metal duct, however, was damaged during disassembly and a replacement must still be fabricated. 

 Interestingly, a photograph in the 21E manual
shows the original duct between the blower and the
modulator/power amplifier deck was made of what appears to have been some kind of cloth.

The modulators and final sit in sockets recessed beneath the deck. Air from the blower flows past the base of each tube and up through the holes in the deck in which the tubes are positioned.  To maximize cooling, the ceramic tubes are enclosed in glass chimneys that sit flush with the deck.  Rubber gaskets fitted to the base of the chimneys prevent forced air from leaking out between the chimney and deck thereby assuring that as much cooling air
flows around the tubes as possible.

The original chimney gaskets had deteriorated so Norm decided to make new ones.  He cut lengths of rubber windshield washer hose to match the circumference of each chimney.  A slit was then made along the length of each hose and the base of the chimney was seated in the slit. Voila, replacement gaskets!


As a protective measure, Collins attached an air switch to the blower, visible in the photo at the upper left corner of the blower.  If the blower malfunctions while the transmitter is operating, the air switch shuts down the transmitter. This prevents further overheating and potentially some very expensive damage.  Remember that several years ago each of the 21E's rebuilt ceramic tubes
cost $400.  A spot check of the present market shows prices for new Chinese-made replacements nearly double that figure.


After firing up the blower and hearing the noise it made, it was clear that having the operating position anywhere near the transmitter would not be a good idea.  Norm has decided that after troubleshooting and initial on air testing, the 21E's operating position will be in his second floor shack.

At the spring 2003 Timonium hamfest, Norm picked up a nice Harris Stereo 80 solid state broadcast console to use with the 21E.   The Stereo 80 awaits its new task while the Gates "Producer", shown in Chapter 6, has gone to another owner.

Remote operation details are not completely worked out at this time, but among other things use of a closed circuit TV system to monitor the transmitter meters is planned. 

 An example of how CCTV is used for this purpose is depicted on Jay, N3WWL's website.
Jay uses a simple CCTV setup to keep tabs on the Raytheon RA-250 which is located on the first floor of his house, while operating from his second floor shack.  

Although the 21E will not be ready for this year's Heavy Metal Rally (December 27-28),  a tentative target of spring 2004 has been mentioned for its debut on 160 meters. 

Stay tuned!

Paul, K2ORC

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