Subject: Carrier Null - 32S-1/3 Method 1 : Derived from the Collins Archives -- September 1997 (#112) The procedure you need to use to null the carrier in your 32S-3 is as follows. To properly do this you need either a receiver or RF VTVM and a Dummy Load. The best way is the receiver method using a receiver with a short pick up antenna tuned to the test frequency of 3.6 mhz. Do the following: 1. Set receiver to 3.6 mhz using an antenna of 6-8 feet. 2. Setup your 32S-3 to 3.6 mhz and load the unit into a dummy load. 3. Turn mic gain fully CCW. 4. In either LSB or USB with PTT on transceiver energized. 5. Tune receiver for this signal at 3.6 mhz. You should hear a carrier. 6. Now, adjust Carrier Balance (R14) and Trimmer (C11) for minimum signal. These adjustments interact. 7. Alternate between LSB and USB for best null on both sidebands. 8. Using your plate meter as a device to determine carrier suppression is not accurate. If you use the method described above you will meet the -50db down figure on the output signal for carrier suppression. Once this is done you can forget about carrier suppression for a long time. In addition, the inherent mechanical filter is also providing additional suppression. Remember, you must seek a null that is the same on both LSB and USB. A compromise is necessary sometimes. Bob - K4NV. Method No. 2: The best way to do the carrier null adjustment is to use a receiver like a 51J4 tuned on 3600. Now you can connect the 51J4 antenna thru a 50 ohms pad(like a 20 -30-or 40 db fix attenuator or even better a 0 to 100 db step attenator )to a special adjustable pick up sensor made by BIRD model no.4273-100 assembly that screw to one side or the other of a BIRD model 43 Directional wattmeter in line to a Dummy load.. Those pickups are very popular to connect a frequency counter ,spectum analyser or a scope to the 50 ohms transmitter coax. Now the beauty of it is : you can adjust the amount of signal to the 51J4 in order to use the S-meterin his most sensitive parts of the scale ( let say S2 ) between the receiver and the pickup sensor. Also you can keep track of the results in DB's of attenuation for S2 for the next job ...specialy if you use one of those 0 to 100 db step attenuator. In fact your 51J4 with a narrow filter become a manually controled spectrum analyser . Just one more idea. Yves Wn4i Method No.3: I null the carrier by looking at the signal on a scope. First I tune the radio into a dummy load at 3.6 MHz. Then I turn the mic. gain all the way CCW and connect a scope lead to the RF output replacing the DL. Then closing the PTT switch, I can see the suppressed carrier. I can switch between USB and LSB and get a balance between the two for the null. Best of all I can see the individual cycles in the wave form and look for any distortion. As for how much the remaining carrier measures, I know from much experience when a carrier is sufficiently suppressed. On the older rigs, I may have to test change components in the balanced modulator circuit to achieve the required suppression, or to get both the USB and LSB to balance out with equal suppression. BTW, I do this at 3.6 MHz, because my inexpensive scope only goes to 6 MHz! Method No. 4: With reference to the KWM-2/2A Here's a service hint I gleaned from fixing a KWM-2 last night. SYMPTOM: Carrier wouldn't null adequately. Only got about 20db of carrier suppression, balance pot was turned all the way CCW, and balance trimmer cap had no effect. FIX: Replace C6, a 0.47 uF/25V ceramic capacitor. C6 is the coupling cap from the audio stage to the balanced modulator. HINT: Before replacing C6, unplug V3 and see if the carrier suppression balances normally. If it does,then you can be certain that C6 is faulty. EXPLANATION: Poor carrier suppression is usually associated with a problem in the balanced modulator. Normally the culprit is a defective diode in the diode ring (a small rectangular black package with 4 leads coming from it), an open 1K balance adjustment pot, or a dirty 5-25 pF balance trimmer capacitor. This particular problem is unusual, because none of the balanced modulator components were at fault. The balanced modulator works by coupling RF from the 455 kHz xtal oscillator through the diode ring to the primary of an interstage transformer. In the absence of audio, the RF is shunted equally by the diodes to both sides of the primary winding. There is thus no RF potential difference across the winding, so no RF is induced in the secondary of the transformer and the carrier is suppressed. When audio from the mic preamplifier is coupled into the diode ring, it unbalances the diodes, causing RF to be coupled into the secondary of the transformer. The RF amplitude fluctuates at an audio rate, producing a DSB suppressed carrier signal. In the KWM-2, coupling from the mic preamplifier to the diode ring is via C6, a 0.47uF ceramic capacitor. In my KWM-2, C6 was leaky, showing a resistance of about 4K ohms. Because of this leakage, it was allowing some DC voltage to appear on the diode ring. This DC voltage biased two of the diodes in the ring at all times, causing them to conduct and making balance impossible to achieve Replacing C6 cured the problem completely. Jim Garland W8ZR I used to use an RF voltmeter, but I found that by actually viewing the waveform on the scope ocassionally showed distortion due to defective circuits that I would have missed on the voltmeter. 73, Jerry W7BUN Back to 32S-1 data page.
