Lafayette Telsat 140 no SSB XMIT

[Lkaskel]

Hi Gang,
Interesting issue here. I have a Telsat 140 that will not transmit on side band. It does transmit with modulation on AM. It has receive audio and when you are transmitting in side band it does light the on air light. The driver and final have 13 volts on the collector in side band transmit. For some reason it is not passing audio from the mic jack.

Any thoughts?

Thanks as always!!!

 

[Lkaskel]

Hi Everyone,
While I am waiting for parts on for the Washington base I decided to open this radio back up and look at it again. The issue is that it does not transmit on SSB. It does transmit on AM and the transmit light does come on in SSB transmit mode. I went through the alignment again to remind me of where I left off. For the synthesizer alignment it went find until the last step. That is where you adjust T1 and T2 for maximum as measured at the antenna jack during USB and LSB xmit with a 2400Hz tone. My scope does not pick up anything during transmit. It does show the carrier and modulation in AM although the modulation looks horrible. Also, the power is driving backwards during modulation in AM. I did check the bias voltage per the alignment manual which is supposed to be 35ma between the emitter and ground of the final. I lifted the emitter and could only get 128uA during transmit but it did vary as I turned the bias pot.

Any thoughts here?

Thanks as always!!

 

[nomadradio]

Okay, so this radio is a bit of a whacked-out design. The audio amplifier used to drive the speaker and to modulate the driver/final in AM is also the source of sideband transmit audio. Comes through RV11 and from there to the SSB modulator chip IC4.

Making sure you have just more than 8 Volts DC on pin 6 of IC4 while transmitting would be important, if it turns out that mike audio isn't getting stopped on its way to IC4.

73

 

[Lkaskel]

Well, this one is fixed. I was able to use a Lafayette Telsat 120 (very rare) mobile that has the same board as a reference. After tracing the audio through the radio from the audio amp and found the signal stopped at the balance modulator. A tantalum cap C78 was bad. I replaced it and it is now working.

Thanks for the direction on this!!!

 

[nomadradio]

Rock on! Congratulations for running it down.

73