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Re: Drake T-4XB Connections to Heathkit SB-220

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  • John
    Thanks for your response, Garey. I like your idea of an external keying circuit. I think I will head that direction. Yes - I do have a scope on the output of
    Message 1 of 3 , Oct 1, 2010
      Thanks for your response, Garey.

      I like your idea of an external keying circuit. I think I will head that direction.

      Yes - I do have a scope on the output of the linear to spot check modulation - usually after a QSY to a different band.

      I subscribe to the theory of not pushing everything to the limit, since a few watts less seldom makes any difference on the other end. I used the linear when I was in Ecuador to help with third-party phone patches, but now-a-days state-side I just rag-chew with other hams. But I would like to have it available in case I want to use it with the Drake twins.

      Thanks again for your help.



      --- In DrakeRadio@yahoogroups.com, Garey Barrell <k4oah@...> wrote:
      > John -
      > I don't have any specifications for the relay contacts in the T-4XB.
      > The plug-in relay in the C Line, similar construction, has contacts that
      > are rated at 2A at 28 VDC or 2A at 155 VAC.
      > Just for reference the L-4B keys with 30 VDC @ about 60 mA. So we can
      > assume that is a 'safe' load for the relays in the T-4X(any) transmitters.
      > All that aside, especially in the T-4XB, I'm sure you've examined the
      > location and wiring of that relay, and perhaps have even considered what
      > would be involved in replacing it, IF you could find a suitable
      > electrical and mechanical replacement. Drake offered a "repair" kit,
      > consisting of a plug-in relay as used in the T-4XC, along with a small
      > PC board and several pages of instructions on converting the 'B'.
      > Now. External, 'soft-key' or other interface units are pretty
      > inexpensive, and if THEY fail it's a rather simple repair, with
      > considerably more modern parts. Personally I would MUCH rather 'baby'
      > those relay contacts so they would hopefully outlast ME.
      > The ALC connection is either a lot simpler, or more difficult, depending
      > upon how you look at it. For one, you really SHOULD have a scope to
      > verify that you're not overdriving your amp into distortion. Actually,
      > it's likely that you wouldn't be able to over drive the SB-220 with a
      > typical T-4X(any). The ALC is negative on the T-4XB, and since it's a
      > 'closed loop' system, e.g., IF the amp is over driven, it puts out a
      > negative voltage that reduces the drive (output) of the T-4XB
      > proportionally.
      > In the L-4B, the RF input is sampled, rectified and used to offset a
      > positive bias set by the front panel "AGC" control. The SB-220 seems to
      > have a similar setup except the threshold is fixed by a resistive
      > voltage divider. It's reasonable to assume that drive for the tubes
      > would be about the same, although again you need a scope to see if the
      > divider sets the threshold properly. Wouldn't be a big deal to adjust
      > those resistors, but most likely not required.
      > 73, Garey - K4OAH
      > Glen Allen, VA
      > Drake 2-B, 2-C/2-NT, 4-B, C-Line&
      > TR-4/C Service Supplement CDs
      > <www.k4oah.com>
      > John wrote:
      > > Greetings.
      > >
      > > It's fascinating that KB1STH asked his question since I just logged in to ask my question about going the other way. I want to be certain about my connections of my beloved Drake T-4XB transmitter to a Heathkit SB-220 linear amplifier (still going strong since I assembled it back in 1976).
      > >
      > > Pins 5 and 8 on the T-4XB connect to the SB-220 antenna relay connector. The SB-220 carries 120 VDC on this line. Are the T-4XB relay contacts capable of handling this voltage? (I have not found any ratings for these in the manuals.)
      > >
      > > Pins 3 and 7 on the T-4XB connect to the SB-220 ALC connector. The SB-220 carries 60 VDC negative threshold voltage on this line. I know that the T-4XB requires negative going transmit AGC voltage on pin-3 to prevent flat-topping of voice peaks due to over-drive. But again, I have not encountered the voltage rating of the T-4xB.
      > >
      > > Does the collective have any insights into this?
      > >
      > > Thank you in advance for your help.
      > >
      > > John
      > > WB0RXL
      > >
      > >
      > >
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