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