[Drake] Drake L-4B Solid State Relay Modification

[Dr. James C. Garland]

Hi Gang,

 

The L-4B is one of the best-constructed amplifiers around, but the power
switch is a weak link since it switches the full 240VAC line and is subject
to a large surge current as the HV filter bank charges. Worse still, the
switch is a custom Drake unit that has not been made for decades, so if (or,
more likely, when) it fails, owners are SOL.

 

This afternoon I finally got around to modifying my L-4B to use a solid
state relay to switch the 240VAC and take the burden off the power switch.
The modification took about two hours, with about $20 of parts, most of
which was for the relay. The parts list for the project is:

 

1-- solid state relay, 50A/240VAC, with a 3-32VDC control voltage. (Any
comparable relay with a rating of 25A or greater should work fine. Some
relays use an AC control voltage, which simplifies the circuit slightly. I
just used a relay from my junk box.)

1 - 5.6K two watt resistor

1-47uF/50V electrolytic capacitor (not needed if an AC control relay is
used)

1 - 1N4007 diode (not needed if an AC control relay is used)

1 - five lug terminal strip

1 - 8-32 x 1/2 " flat head machine screw

1 - 8-32x3/8" sheet metal screw.

6 - small ty-wraps

 

The modification took about two hours to complete, and seems to work well.
Mechanically, I had to drill one 1/8in. hole and countersink an existing
hole Visually, the modification came out quite clean, with no kludged or
awkward appearing parts. The power switch now switches only 10mA , which
will give me peace of mind, if nothing else.

 

I uploaded a few snapshots of the modification to Shutterfly, which you can
see at

 

http://share.shutterfly.com/share/received/welcome.sfly?fid=1f77e646d66016b0
<http://share.shutterfly.com/share/received/welcome.sfly?fid=1f77e646d66016b
0&sid=0CbuWjJqyZMWWa> &sid=0CbuWjJqyZMWWa

 

Click on "view album" and then double-click on the first photo to get
started. Note that there is an explanation of each photo on the right side
of your display. Don't click on "slideshow" or you'll miss the explanation.

 

If there is sufficient interest, I'll (eventually) add a webpage for the
project to my website with step-by-step directions and a circuit diagram.
The Shutterfly photos are only intended to give folks a sense of the
project, and not a full description.  Before anyone tries to duplicate the
project, I'd first like to use my modified amplilfier for a month or so to
make sure unexpected problems don't pop up.

 

Tnx and 73,

 

Jim Garland W8ZR

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