KWM-2/2A Service information

KWM-2/2A Service information ... KWM-2/2A Service information The following items are intended to be used as aids in troubleshooting. NOTE: A number of these suggestions and changes are ...
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The following items are intended to be used as aids in troubleshooting.

NOTE: A number of these suggestions and changes are incorporated in the KWM-2/2A of late serial numbers. It is suggested that these changes not be made unless trouble is encountered in that particular area. Items marked by an asterisks (*) are recommended for all KWM2/2A equipments.

Capacitor C25 is 3pf in some transceivers and results in poor coupling between R2 and L4. Changing C25 to 6pf (CPN 961-0122-000) will improve coupling.

VIF alignments: Transformer T2 is properly aligned when the top slug is out, or closest to the top of the can. Inductor L4 is correctly peaked when the slug is in or closest to the centre of the coil. L4 should be peaked with the dial at 55.

Low output on 10 meters is sometimes caused by the parasitic suppressor coils L15 and L16 having their turns too closely spaced. Seperating them too far can result in parasitic oscillations.

Low crystal injection voltage on 10 meters can be helped with a new crystal or 6U8A tube.

Low P.A. output can be caused by a too low screen voltage. Check resistors in the screen circuit.

The 6CL6 plate trimmer capacitor C132 used on 20 meters should be set at one-third capacitance and L14 adjusted to resonance with C132. It is necessary to then realign the 15 meter trimmer C138 and the 10 meter trimmer C134.

Poor grounding of the RF ccoil shield cans may cause oscillations, particulary on 80 meters. This can be remidied by loosening the two nuts holding the shield cans and retightening them.

If the receiver lacks sensitivity, check the coax leads near the relay cover on the RF coil shield cans. Dressing the leads away from these cans will help.

A low S-meter reading might be caused by R150 being less than 180 ohms. Check and replace if necessary.

Resistor R140 is a value selected for transmitter gain. It usually is 12K to 18K ohms and is connected across T1.

There are some 6AZ8 tubes which develop positive voltage at the grid causing AVC and ALC trouble. They may also cause excessive S-meter and ALC meter zero drift. If the voltage (no signal condition) at V1 or V3 grid differs from AVC or ALC line by more than 0.1 volt, the offending 6AZ8 tube should be replaced. The nominal AVC line voltage is in the vicinity of 1.0 volts. The ALC line will be approximately -1.3 to -1.8 volts.

Resistor R175 (47 ohms) and capacitor C262 (0.01 uf) is added between tube socket XV3, pin 3 (6AZ8 cathode) and ground to provide bias voltage and improve operation of the IF amplifier.

VOX instability can be reduced by adding 0.1 uf capacitor across contacts 12 and 13 of relay K2.

Insufficient carrier suppression is sometimes caused by power supply ripple. It may be reduced by adding a 4.0 uf capacitor C264 (CPN 183-1763-000) between tube socket XV3, pin 8 and ground.

A machine gun effect of the VOX relay can be remidied by adding a 0.01 uf capacitor (CPN 913-3013-000) from tube socket XV1, pin 7 to ground. This effect is caused by relay transients fed through the ground lead of R3 which is connected to emission switch S9C.

If the ALC meter will not zero, resistor R170 (220 ohms) across ALC zero potentiometer can be removed. This will extend the range of the adjustment.

The value of capacitor C14 at the input of the mechanical filter has been selected in some cases to resonate with the mechanical filter and will not always agree with the schematic diagram.

* Because of the variation in tube and circuit tolerances, The transmit second mixer,V6, 12AT7, sometimes runs above the rated plate dissipation. This may result in reduced tube life. To promote longer tube life, the plate dissipation has been reduced by changing the plate resistor, R143, from 1500 ohms 1/2 watt, to 2200 ohm, 1 watt. (CPN 745-3366-000)

* To provide improved ALC zeroing, resistor R38, 150 ohm 1/2 watt, has been changed to 220 ohm 1/2 watt. (CPN 745-1324-000)

* To eliminate a parasitic oscillation tendency in the tone oscillator circuit, capacitor C261 has been added from tube socket XV2 pin 2 to ground. This is a 100 pf capacitor. (CPN 912-2817-000)

* To eliminate the possibility of ALC coupling capacitor C157 breaking down, a capacitor rated at 500 vdc should be used. (CPN 913-3152-000)

The BFO instability in some transceivers has been eliminated by changing the value of two capacitors.
A. Capacitor C53 was changed from 22 pf to 15 pf. (CPN 912-2760-000) It is located at tube socket XV11 pin2 to ground.
B. Capacitor C55 was changed from 47 pf to 100 pf (CPN 912-2816-000). It is located on tube socket XV11 pin 3 to ground.

Audio squeal in transmit when AF gain control is advanced. This problem is caused by a positive feedback path through R164 and the primary to secondary capacitance of audio transformer T6.
Correction procedure: Change C104 from 0.0047 uf capacitor to a 0.01 uf (CPN 913-3013-000). C104 is located between XV16, pin 9 and a tie point. Change R164 from 220k ohm to 470k ohm (CPN 745-1464-000) R164 is located on the tie turret near XV16. Add R182, 68 ohm resistor, (CPN 745-1303-000) from the 4 ohm tap of the audio output transformer T6 to ground. The 4 ohm tap is a yellow lead from the transformer to an ajacent tie turret.

A few KWM-2 transceivers have shown signs of an audio squeal at the instant the transceiver returns to receive from the transmit function. Reducing the microphone gain momentarily will stop the squeal but it may start again at the end of the next transmission. There is a feedback loop being created from the microphone through the balanced modulator and into the receiver first IF implifier and then to the speaker. A check of this condition may be made by operating the transceiver in the receive mode, disconnecting the antenna, set the MIC and AF GAIN controls at 12 o'clock and speak into the microphone. If the audio heard from the speaker is quite loud, the following change will eliminate the effect. Install R122, a 47k ohm 2 watt resistor (CPN 745-5722-000), from vector turret E40 terminal D, to tube socket XV3 pin 7. This will put positive voltage on the cathode of the microphone amplifier cthode follower tube when in the recieve mode, thus cutting off the audio feedthrough. This change will have no effect on the audio voltage to the VOX circuitry. (refer ASAB No. 1006)

To reduce the 3.5 Mhz transmitted spurious, move the ground end of C19, 0.01 uf capacitor, located at socket XV5 pin 8, to the same ground lug as C18, a 0.01 uf capacitor, the other end of which is connected to socket XV5 pin 3

A transient occurs in the RF output of some KWM-2 transceivers when the VOX drops out. This may be eliminated by installing a 100k ohm 1/4 watt resistor from relay K@ terminal 4,-70V line, to relay K2 terminal 5, Receive -70V / transmit-ground line.

It has been found that in some units thecrystal calibrator trimmer capacitor does not have sufficent range to allow the crystal to be properly calibrated. For transceivers exhibiting this problem it is suggested that the 5pf capacitor, C267, be replaced with a 10pf capacitor.

In some cases a "yelp" is emitted from the speaker immediately after the transceiver returns from transmit to receive. This undesireable sound usually occurs when the RF gain is at maximum and the audio gain is set high. The mike gain has no apparent effect on the problem. For those units experiencing this problem, it is suggested that the following changes be made: (refer to schematic section F)
A. Disconnect the end of the 4.7k ohm resistor R176, connected to L9-4, and reconnect it to L9-3.
B. Install a IN1490 diode, CR10, with the anode to L9-4 and cathode to L9-3.

A low frequency spurious oscillation caused by the mutual inductance of coils L3 and L33 can be reduced by relocating capacitor C238, 0.01uf, to the junction of coil L3 and resistor R169.

To keep any 60 hz ripples from affecting the zero on the ALC meter, add a 20uf electrolytic across the -70 volt bias line at the input of R189 to ground. R189 is located close to the back of the emission switch. (Yves,WN4I)

If you have a drive or idle current problem, give the band switch a workout, running it backwards and forwards a few times. If the unit sits for a while the switch contacts oxodize and get a bit resistive. This is something that you may want to do every once and a while, as these kind of problems will drive you nuts if you have to go look for them.

Having problems with drift? This may be the solution.

For a complete listing of modifications, you will find them here.

Collins Meter Repair: Charlie Jobes, 125 Denmar Lane, Hendersonville, NC 28791, 828-891-4355 He has been working on meters for 40 years and is 91 years old. He does a heck of a job.
(William J. Diamond WR�T)

The above information is derived from Collins Amateur Service information letters and Agency Bulletins. For further information refer to your KWM2/2A service manual or instructin book, Section 7, Illustrations, Schematic changes and equipment differences, identifiers 1 through 39 and their descriptions.

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