question about modes of transmission

[obrien135]

If a transceiver has am, usb and lsb modes of transmission, does it switch back and forth between being a direct conversion device (or no IF) for AM and a superhet for usb and lsb? The one I am specifically wondering about is a President HR-2600. I am having trouble determining the answer to this question from the schematic. It transmits in AM but not LSB or USB.

George

 

[chuckey]

I would think that it is a superhet receiver which has a BFO (beat frequency oscillator) that is basically a low power IF oscillator and it should have selectable time constants on the AGC circuit. The reason being that if the AGC has a normal time constant (1 second?) and the receiver is receiving SSB, when the person stops talking, there is no carrier, so the AGC winds up the gain until the talker, speaks again. You need a 5-10 second AGC time constant.
Frank

 

[obrien135]

does that mean that I won't see any deflection on the swr meter unless I am talking into the mic? Will an swr meter register an audio signal
?

 

[davenn]

does that mean that I won't see any deflection on the swr meter unless I am talking into the mic?

thats correct, as there's no signal going out till modulation is applied

This is of course different to AM and FM transmissions where there is always a carried present from the time the transmitter is keyed on

Will an swr meter register an audio signal
?

it wont register audio, as an SWR meter is a RF device. BUT you will see the varying RF output as the modulation changes

Oh and I have yet to see a Multimode CB set that uses direct conversion.

cheers
Dave

 

[vfone]

https://www.cbtricks.com/radios/president/hr_2600/graphics/president_hr2600_om_main_sch.pdf

HR2600 it use superheterodine receiver in all modes, FM, AM, SSB, CW.
Most of the part the transmitter side is common for all modes, so if you have problems in SSB mode probably have to check the SSB modulator (IC5) or the carrier oscillator (Q22) and their afferent circuits.