Drake L4-B Question....

Discussion in 'Amateur Radio Amplifiers' started by KS5L, Feb 21, 2011.

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  1. KS5L

    KS5L Ham Member QRZ Page

    I have been using an L4-B for the past several years. It works great, easy kilowatt out with about 60 watts in from IC746. My question is: Why are my received signals attenuated about 1 1/2 S units when the amp is in operate mode? Does anyone know how to prevent or rectify this situation? Unfortunately, I am not an electronics expert. Thanks ....

    Bob, KS5L
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Two possibilities:

    1. Your amplifier is really not switching back to "receive," and the internal T-R relay is hanging in the "transmit" condition. When you're receiving, is the transmit lamp still illuminated?

    2. The internal T-R relay contacts are pitted, corroded, dirty, oxidized, or misaligned so they don't allow signals to make it through the relay without being attenuated. This wouldn't be unusual after "several years," relay contacts don't last forever. This could be an easy fix (like just wiping a dollar bill or similar through the receive relay contacts to clean them), or a slightly more difficult fix (replacing the T-R relay entirely). If you want to clean the relay contacts, never use a file or an emory cloth or sandpaper or anything like that. I use a dollar bill (or if you're rich, use a $100 bill), which is abrasive enough to do the job fine. A piece of regular paper wetted with WD40 also works. Never use anything abrasive to clean relay contacts!
  3. KS5L

    KS5L Ham Member QRZ Page

    Thanks for the suggestions. I just recently had some work done on the amp and neglected to mention this little problem. Until now, I have just dealt with it by putting the amp on standby while I am receiving.
  4. WW3QB

    WW3QB Ham Member QRZ Page

    This tells me that the amp is not being keyed correctly. How are you keying the amp with the 746?
  5. KS5L

    KS5L Ham Member QRZ Page

    Using an Ameritron ARB-704 relay that came with the amp. It appears to be functioning fine.
  6. NL7W

    NL7W Ham Member QRZ Page

    I agree with Steve... the relay contacts are likely dirty. Clean with paper, or a relay burnishing tool, which can be had for 4 or 5 dollars, or less.

    Coating contacts with DeoxIT Gold wouldn't be a bad idea either. If you soak a stiff paper strip or burnishing tool with the DeoxIT Gold, the paper / tool combo would clean and lube the contacts without leaving too much behind.

    It works for me... :)

    Steve -- NL7W

  7. WW3QB

    WW3QB Ham Member QRZ Page

    He can receive well when he puts the amp in standby. That's switch S4 which opens the circuit to relay RY1. The VOX jack J3 for keying also opens that same circuit (they are in series). So something is keeping the amp keyed that only switching S4 open unkeys it.
  8. NL7W

    NL7W Ham Member QRZ Page

    Could very well be... But with signals being down 1 to 1.5 S-units (6 to 10 dB), it seems unlikely. One never knows though; we're TS'ing from afar. :)

  9. WW3QB

    WW3QB Ham Member QRZ Page

    The think to try is to remove the plug from J3 while receiving and the amp is not in standby. If the signal improves when it is out, we know the problem.
  10. KS5L

    KS5L Ham Member QRZ Page

    Thank you all for your feedback. WW3QB hit the nail on the head. A little further research on my part informed me that the polarity on the vox jack was likely reversed and would show a rise in plate current while in operate mode. I reversed the plug, and voila, no plate current and no more received signal attenuation. Problem solved.
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