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Drake TR4C oddity. Any ideas.

Discussion in '"Boat Anchor" & Classic Equipment' started by KB9VY, Sep 26, 2018.

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  1. KB9VY

    KB9VY Ham Member QRZ Page

    Just got a used TR4C today. Overall, fairly decent condition, but have noticed something really odd. There is a 6AU6 tube plugged into the noise blanker socket. Anyone have any idea why this would be? Thanks!

  2. KK6IYM

    KK6IYM Ham Member QRZ Page

    What does the underside look like. Either it is a mod using that socket or someone found a new way to use a 6AU6. If the noise blanker switch works then that would be the cheapest 34NB ever.
  3. KB9VY

    KB9VY Ham Member QRZ Page

    I uploaded 3 picture files. Hopefully those are visible! There are some additional capacitors added to the noise blanker jack (now a 6AU6 tube). There is wiring added that connects over to the audio output board. Working through the schematic trying to figure out what this could possibly accomplish.

    Attached Files:

  4. KB9VY

    KB9VY Ham Member QRZ Page

    Spent some time tracing the modifications against the schematic. It appears this mod was done to replace the transistor (Q5) with a 6AU6 tube based audio preamp using the socket for the noise blanker. Heck of a lot of work for a transistor replacement...

    Now I will need to see if I can undo this mod and return the circuit to it's factory design state. I could really use a good photo of the component side of the small vertical audio preamp PCB with resistor R187 on it. It appears all components are still in place with the exception of the transistor.
  5. W2AAT

    W2AAT XML Subscriber QRZ Page

    R L Drake still exists. Unfortunately, they don't make ham radio stuff any more. I had a full C-Line setup and Drake support was always responsive to requests. You might give them a call. They may be able to provide you with visual documentation of the component(s) you are interested in....
    Let us know as to how you make out...
    KB9VY likes this.
  6. W8CF

    W8CF Ham Member QRZ Page

    K4OAH produced some excellent documentation for servicing Drake equipment including the TR4C. Plenty of pictures and component locations. He is now an SK but his wife is continuing to fill orders from his web site.
    WA7PRC likes this.
  7. KB9VY

    KB9VY Ham Member QRZ Page

    I have been trying to figure out how to order. It appears you cannot order directly from the site, only via mail?
  8. W2XS

    W2XS Ham Member QRZ Page

    I recently ordered the information for my T-4X and R-4A twins. It is really clear and detailed, and there is a ton of information and pictures. Here is the link:

    Send an email to:

    and you should get an answer. 73, John W2XS
    KB9VY likes this.
  9. KB9VY

    KB9VY Ham Member QRZ Page

  10. KB9VY

    KB9VY Ham Member QRZ Page

    I received the service information from k4oah. It helped a great deal. I believe I have returned the circuit back to factory, except for the noise blanker socket. It is completely bypassed. I replaced the transistor for the audio preamp circuit. Once I get the power supply rebuilt, we will find out what works, and what doesn’t.

    Thanks for the suggestions.

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