TMC XFK (C-2749/URT) Restoration Plan

By Russ Miller, WA3FRP

Here are some steps to take in bringing the XFK back to operating condition, especially if the unit has been powered off for a number of years.

Cleaning

The first step is an overall cleaning that entails removing the top and bottom covers. During this cleaning, remove all loose dirt and dust from the front panel and the top and bottom decks. During the cleaning, remove and inspect each tube to make sure that it is the correct type. While each tube is removed, polish it and spray the tube socket with Caig Deoxit D5. (One source for Deoxit is http://store.caig.com/).

Use the Deoxit to clean all of the switching contacts and inside each potentiometer. It may be preferred to wipe each of the switching contacts with Deoxit with a Q tip to avoid over- spraying the phenolic wafer switches.

For those who are concerned about using Caig DeOxit D5 on phenolic rotary switch wafers, please read the following from the Collins listserv:

Attached Message

Hi All,

 For years I've been very careful about spraying Caig DeOxit D5 on phenolic rotary switch wafers, out of fear that the phenolic would absorb the liquid, delaminate, swell, weaken, or otherwise harm the switch.  As a test of this hypothesis, this past week I totally immersed a phenolic switch wafer (12 position) from my junk box in DeOxit and let it sit for 48 hours in the solution. Afterwards, I let it air dry and inspected it closely. To my pleasure and surprise, I found no degradation of the wafer at all. It seems as good as new (not surprisingly, the metal contacts looked great), and the wiper rotates very smoothly.  I can't promise that all phenolic switch wafers will respond as well as this one sample did, but this particular wafer seemed very typical of those appearing in my Collins gear and appears to date from the 1950s or 60s. I conclude from this test that I don't need to be as careful cleaning phenolic rotary switches as I had believed.

Next, I'm going to repeat the experiment, this time immersing the wafer in water for 48 hours.

73,

 Jim Garland W8ZR

Inspection and Initial Power-Up

Tighten all of the screws on the chassis to be sure that the XFK is mechanically solid.  Be careful as some screws attach to non-metal discrete component mounting points under the chassis that may turn and break electrical / mechanical connections.

Once this is done and the XFK chassis is cleaned and tightened, examine each discrete component for obvious signs of previous failure. Check each resistor for evidence of overheating and each capacitor for signs of leakage. Replace all suspect components. Be especially careful of capacitors C55 and C56 and all other electrolytic capacitors as their failure will present high voltage AC to the ungrounded chassis and a potential electrical shock hazard.

 Remove, clean and inspect each fuse. Make sure that each fuse value is correct. If you find a blown fuse, determine the possible causes before going to the next step.  

Once the above steps are completed, it is advisable to put a light bulb, 60W or so, in series in the AC line. This will limit current in case of a fault during the first power up.  Some will use a variac to bring the power up slowly, about 10 volts an hour, but since the XFK does not use a solid state rectifier, I prefer to use the light bulb. And, a danger in bringing the voltage up slowly with a Variac is that if a filter capacitor is shorted, or another fault, the current in the transformer high voltage winding can exceed the safe value long before the primary draws enough current to open the fuse. 

Alternately, while the B+ is applied and tubes removed, you can do a quick check for grid voltage that may be leaking through coupling capacitors from the plate of the previous stage. And by measuring the voltage drop across limiting resistors in the B+ lines, you can identify really leaky bypass capacitors.

While there are no such capacitors in the unit that I restored, a good general rule is to lift one end of all Black Beauty (or "bumblebee") capacitors and check them for leakage with a suitable instrument.  Replace as needed. 

Apply AC power to the XFK and be ready to kill power while observing that all tubes filaments light, the power indicator, 200 KC and XTAL oven indicators all light up and there is an absence of smoke. Wait for fifteen minutes. Note that the oven indicators have started to cycle off and on. Note that each oven is independent of the other and the cycle times will vary.  Check the B+ for proper voltage and for AC ripple using an oscilloscope or digital multimeter.

A complete alignment of the XFK should be done once the XFK is under power and stable.    

Alignment of TMC XFK (C-2749 URT)

Place the XFK on its side with the top and bottom dust covers removed. Note that lethal voltages are present and that every caution must be taken to avoid electrical shock. If you are not familiar with all safety steps necessary to work on this type of equipment, refer this equipment to a competent technician.

1. IF ALIGNMENT OF BAND 2.

1)      Connect a 50 or 70 ohm load to the OUTPUT jack. Either meter current through the load with an RF thermocouple or connect an RF voltmeter across the load.

2)      Set the BAND CHANGE switch to band 2. Rotate the main tuning capacitor (OUTPUT TUNING MC knob) to the full-mesh position (lowest frequency). Note that the hairline on the dial matches the indicator line.  If this is not so, leave the capacitor in the full-mesh position. Loosen the set screws fastening the flexible coupling to the capacitor and rotate the dial until the hairline on the dial matches the indicator line.

3)      Set ceramic trimmers C30, C43 and C46 (on top of the chassis) to minimum capacity.

4)      Use either an external source or the following crystals to supply an input signal source. If an external source is used, connect the signal source to the EXT jack on the rear of the chassis and turn the XTAL selector switch (front panel control) to EXT.

5)      Insert a 2.6 MC crystal in socket No. 1 and a 6.3 MC crystal in socket No. 2.

6)      Turn XTAL selector switch to crystal 1 or supply a 2.6 MC external signal source via the EXT jack on the rear of the chassis and turn the XTAL selector switch (front panel control) to EXT. Turn the OUTPUT TUNING MC dial to 2.8 MC.  Note that the XFK always has an output 200 KC higher than its input source. If you are unclear about this fact, please consult section 4 of the XFK manual.

7)      Turn on the POWER to the XFK and after two minutes turn on the PLATE switch.

8)      Place an RF voltmeter on pin 1 of V4 and tune transformer T1 for maximum voltage by adjusting its slug which is accessible from the top of the chassis. Place the RF voltmeter at pin 5 of V5 and tune transformer T3 for maximum voltage by adjusting the slug accessible on the top chassis. Select the first peak going in from minimum inductance. Tune transformer T5 for maximum output indication in the load by adjusting the slug which is accessible from the top of the chassis. Remove the RF voltmeter and retrim transformers T1, T3 and T5 for the maximum output in load.

9)      Turn XTAL selector switch to crystal 2 or supply a 6.3 MC external via the EXT jack on the rear of the chassis and turn the XTAL selector switch (front panel control) to EXT. Turn the OUTPUT TUNING MC dial to 6.5 MC.  Tune capacitors C30, C43 and C46 for maximum output in load.

10)  Remove tube V1, the 200 KC oscillator tube. Tune the main tuning capacitor (OUTPUT TUNING MC knob) for the maximum output indication in load. This will occur at the crystal / external oscillator frequency. Turn the mixer balance potentiometer R84 (on the bottom of the chassis) for minimum indication in load.

11)  Remove the load. Remove the crystal or external signal source. Set the plate current to 50 ma by turning the POWER knob (front panel) control.  Rotate OUTPUT TUNING MC knob over the entire upper range of the band and note spurious excitation, if any. Parasitics may be evidenced by a dip in the plate current of the final amplifier when rotating OUTPUT TUNING MC knob. If parasitics are evidenced, neutralize same by tuning capacitor C27.

12)  Replace tube V1 and repeat steps 6 and 9.

13)  Check the tuning dial for proper selection. If the above procedure has been performed properly, there will be no indication above 6.5 MC and the lower sideband will appear at 6.1 MC.

2.    IF ALIGNMENT OF BAND 1.

1)      Set the BAND CHANGE switch to band 1. Insert a 1.0 crystal in socket No. 1 and a 2.1 crystal in socket No. 2.

2)      Set ceramic trimmers C19, C24 and C28 (on top of the chassis) to minimum capacity.

3)      Turn XTAL selector switch to crystal 1 or supply a 1.0 MC external signal source via the EXT jack on the rear of the chassis and turn the XTAL selector switch (front panel control) to EXT.

4)      Turn the OUTPUT TUNING MC dial to 1.2 MC. Place an RF voltmeter on pin 1 of V4 and tune transformer T2 for maximum voltage by adjusting its slug which is accessible from the top of the chassis. Place the RF voltmeter at pin 5 of V5 and tune transformer T4 for maximum voltage by adjusting the slug accessible on the top chassis. Select the first peak going in from minimum inductance. Tune transformer T6 for maximum output indication in the load by adjusting the slug which is accessible from the top of the chassis. Remove the RF voltmeter and retrim transformers T2, T4 and T6 for the maximum output in load.

5)      Turn XTAL selector switch to crystal 2 or supply a 2.1 MC external via the EXT jack on the rear of the chassis and turn the XTAL selector switch (front panel control) to EXT. Turn the OUTPUT TUNING MC dial to 2.3 MC.  Tune capacitors C19, C24 and C28 for maximum output in load.

6)      Repeat steps 4 and 5.

7)      Remove tube V1, the 200 KC oscillator tube. Tune the main tuning capacitor (OUTPUT TUNING MC knob) for the maximum output indication in load. This will occur at the crystal / external oscillator frequency. Turn the mixer balance potentiometer R13 (on the top of the chassis) for minimum indication in load.

8)       Remove the load. Remove the crystal or external signal source. Set the plate current to 50 ma by turning the POWER knob (front panel) control.  Rotate OUTPUT TUNING MC knob over the entire upper range of the band and note spurious excitation, if any. Parasitics may be evidenced by a dip in the plate current of the final amplifier when rotating OUTPUT TUNING MC knob. If parasitics are evidenced, neutralize same by tuning capacitor C27.

9)      Check the tuning dial for proper selection. If the above procedure has been performed properly, there will be no indication above 2.3 MC and the lower sideband will appear at 1.9 MC.

3. ADJUSTMENT OF THE  200 KC OSCILLATOR

1)      The proper alignment of these next sections of the XFR requires accurate frequency measurement. The following procedure applies to the older equipments and the same procedure applies to the newer units when allowance is made for the KEY-FAX switch being incorporated into the TEST switch (front panel) control.

2)      Connect a frequency counter to pin 7 of V3.

3)      Set the FREQUENCY knob (front panel control) to zero.

4)      Noting that the 200 KC Oven Indicator (front panel light) is cycling normally, adjust the COARSE FREQ. ADJ. potentiometer to change the frequency to exactly 200.000 KC.

4.  ADJUSTMENT OF THE  KEYER CIRCUIT

1)      Set the XTAL selection switch to 1 and plug into the multiplication rate of 1 on the rear chassis.

2)      Set the FREQUENCY SHIFT CPS dial to 500 cycles.

3)      Using a communications receiver with a BFO, tune to obtain a 2000 KC audio tone.

4)      Vary the TEST switch (front panel control) from M to S and note the tone shift.

5)      Adjust the SHIFT BALANCE potentiometer R38 (Top of chassis) until the shifts are equal.

6)      Adjust the SHIFT AMPLITUDE potentiometer R40 (Top of chassis) until the shift is exactly 500 cycles.

7)      Recheck the shift balance. The total shift should now be 500 cycles, 250 cycles on either side of the carrier.