Drake L4-B Question....

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

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That "rise in plate current" was caused because your amplifier was keyed the whole time it was on and in the OPERATE position, which allows voltage to be provided to the internal T-R relay. Switching to STANDBY breaks that connection.

    Glad you found the problem, which was simply that you had the active pin grounded on the RELAY line to the amp; that occurred due to it being plugged in backwards -- the "other" wire is ground.
  2. NL7W

    NL7W Ham Member QRZ Page

    That's great!

    So the PTT (vox) plug polarity was reversed? It was keyed with bias applied to the tubes while in "operate" position -- without the radio actually keying the amp. That threw me for a loop... not seeing it or the schematic.

    Best of luck with it. Not following the thread too closely, I take it isn't damaged, being the source RF radio saw the antenna "bypass" when you applied RF to it? Was that the case on transmit? Thanks.

    Last edited: Feb 22, 2011
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The rig would see the amplifier input on transmit, and it should work fine. It was just "hung up" in transmit, not allowing the antenna to be connected to the receiver.

    So the receiver was connected to the tuned cathode input network in the amp, on receive (and the pair of 3-500Zs were working as a lossy preamp:p).

    This wouldn't damage anything unless the 3-500Zs decided to oscillate and blow up the receiver. That would be very exciting.
  4. NL7W

    NL7W Ham Member QRZ Page

    Thanks. A most weird problem! The things you see...

    Yeah, the radio receiver would see the amp's input circuit (a tuned input on the L4-Basher -- didn't know it had one?), because it was continually "keyed". On transmit, it was also keyed... Good grief. ;)

    Good catch, Steve!

    While he's in there, it would be good to clean the relay contacts... while he's at it.

    Of course, before poking any appendages inside, letting it sit for awhile, then shorting out the HV supply after the caps supposedly bleed down -- just to be safe. I assume it has a HV shorting interlock? I don't trust them. :)

    I have an old military HV shorting stick I absconded years ago for just that purpose. ;)

    Last edited: Feb 24, 2011
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