Drake twin transmitters

Discussion in '"Boat Anchor" & Classic Equipment' started by KL7CD, Apr 17, 2018.

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  1. KL7CD

    KL7CD Ham Member QRZ Page

    I am trying to get a pair of Drake twins on the air. The receiver is an R-4 I have restored and it works perfectly. The most recent attempt at joining a Xmtr is a T-4XC. Everything works in the Tune, CW and AM modes. Spot works. Problem is with SSB. Max output with a whistle is about 4 watts. On a separate receiver the audio sounds OK, just low (very low) output in that mode. Same mic on AM so that's not the problem. Have cleaned the tranceive and mode switch wafers with no improvement. Same result using either VFO or separate. Any ides?
     
  2. K9STH

    K9STH Ham Member Staff Member QRZ Page

    Check and see if the transmitter has been modified to use with a VHF transverter. If so, then there should be RCA jacks in the positions marked EXC OUT and "A" and "B".

    If so, then there needs to be a jumper between "A" and "B". When that jumper is not present, there is no screen voltage applied to the final amplifier tubes and the power will be very low. Even if there is no jack present in the EXC OUT but there are jacks in the "A" and "B" position, then there needs to be a jumper.

    Although not the exact number of the kit to use the transmitter with transceivers, it is the same as the kit manual on BAMA at

    http://bama.edebris.com/manuals/drake/740modkit/

    The file is in DJVu format. If you need a viewer, then a viewer can be downloaded at

    https://sourceforge.net/projects/windjview/

    Of course, there could be other reasons that there is no screen voltage on the final amplifier tubes or there could be something else wrong. However, lack of screen voltage on the final amplifier tubes is usually the cause of very low output if the final amplifier tubes are good.

    Glen, K9STH
     
    N2EY likes this.
  3. N8YX

    N8YX Ham Member QRZ Page

    Glen,

    From the OP's comments he's seeing full output in CW/AM. Would the lack of screen voltage still be the cause here?
     
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Presumably you are seeing PA standing current OK?

    Given what you've said, my first thought would be a faulty SSB Filter . . .

    What does the low power transmission sound like? Is it proper SSB, not DSB?

    Roger G3YRO
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd agree with Roger above that your SSB filtering should be checked. Do you see the same symptoms when transmitting in either USB or LSB mode on the same band?

    I'd also check everything in the audio path between the mic pre-amp and the balanced modulator. Remember the AM modulation in the T4x series transmitters is carried out in the screen modulator tube which is a completely different signal path than SSB modulation via the balanced modulator. So you could have normal AM modulation levels but if there's something bad in the mic audio path to the balanced modulator (e.g. switch contact, cabling, coupling cap, etc.) then your mic audio levels may be attenuated as they reach the balanced modulator giving you SSB troubles.
     

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