Cobra 148 GTL-B

Printed Circuit Board PTBM122DOX


Alignment information for the PTBM122 PCB

Preset to channel 40, mode USB, clarifier centered.

The Mode in brackets ( ) indicates which mode the radio should be in.


PLL Circuit


Reading PointAdjustmentDescriptionValue
IC1, pin 3 - Check for 10.240MHz +/- 50Hz 10.240MHz
IC4, pin3 CT6 (USB) AM/USB Carrier oscillator 10.695MHz
IC4, pin3 CT5 (LSB) LSB Carrier oscillator 10.692MHz
TP2 (IC2, pin 4) CT2 (USB) AM/USB offset oscillator 20.105MHz
TP2 CT1 (LSB) LSB offset oscillator 20.1035MHz
TP3T10utput to RF local oscillator37.660-38.100MHz for CEPT
TP1VCO-Coil VCO DC Voltage (DONT ADJUST IF WORKING OK!) 1.2 Volts @ channel 40 (27.405MHz) +/-0.5V

 


Transmitter Circuit


Preset to USB, channel 20, inject 2400Hz, 10mV audio signal.

AdjustmentDescriptionValue
T2 VCO Mixer Output Maximum RF output power / best signal on oscilloscope at TP3
T3 VCO Mixer Output Maximum RF output power / best signal on oscilloscope at TP3 (check even on channels 1 and 40)
T4 Transmitter Output Maximum RF power output
T5 Transmitter Output Maximum RF power output
L16 Transmitter Output Maximum RF power output
RV4 Final Bias (Q7) Connect DC Voltmeter to Q7 base (junction of C55, R34, L13) adjust for 0.70V
RV13 SSB Modulation inject 2 tones, 500Hz and 2400Hz, at 10mV, adjust for 12 Watt pep
RV7 Carrier balance Check / Adjust for NO Carrier on both LSB and USB
RV15 AM Output Power Remove injected audio, Adjust for 6W carrier, a lower power level may be required to attain full modulation.
RV14 AM Modulation Return to single 2400Hz tone @7mV, adjust for 92% modulation (see note above)
RV6 RF Power Meter Adjust to match actual RF power

Receiver Circuit


Preset to channel 20, USB, clarifier centered, NB/ANL turned off, RF gain set to maximum.

Be ready to inject a RF signal with an RF signal generator.

Adjustment Description Value
RV8 SSB AGC ??
T6 RF Input Inject RF signal of 27.205MHz, 1KHz, 30% modulation, (keep the level low to prevent AGC action), adjust for maximum on radios signal meter / audio level at speaker (as measured with a scope or AF-VTVM)
T7 RF Input as above.
T8 RF Input as above.
T9 1.F. 10,7MHz, as above.
T10 1.F. 10,7MHz, as above.
T13 AM IF 455kHz, as above.
T14 AM Detector 455kHz, as above.
T11 SSB IF 455kHz, as above, also set for maximum audio output.
T12 SSB Detector Set for maximum audio output.
T6, T7   turn both approx. 1/4 of a turn clockwise
RV10 AM S-Meter Set RF signal generator to inject a 100uV signal, 1KHz, 30% modulation signal, adjust for "S9" in radios signal meter .
RV9 SSB S-Meter Remove audio from RF signal generator, set for "S9" on radios signal meter.


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