[Hammarlund] SP-600 RF Deck

[k2cby]

There is one ABSOLUTE ESSENTIAL in removing the RF deck. Be sure that the
bandswitch is mid-way between any two detents (i.e. completely disengaged)
before actually taking the strip out. If not, you are going to mangle the
leaf contacts which are brittle and impossible to replace. (Been there done
that. Tears will surely follow.) 

 

As to your other question, it certainly isn't necessary to use a solder
sucker. Just be sure to apply plenty of heat.

 

There are four things you have to disconnect:

 

1.	The little wire that runs from the little plastic feedthrough at the
front of the RF strip to the crystal switch. Use the solder sucker here. 
2.	The insulated wires that go to the little enclosure on the side
marked "L32     T1      L31."  The terminals are all on the top and easy to
get to once you remove the two acorn nuts and the cover plate. The problem
is that the wires don't have very much slack. Don't clip them, and try not
to mangle them. Here is where a solder sucker is useful, and there is plenty
of elbow room to use it.
3.	The bare #20 wires that connect the stator sections of the tuning
capacitor. There isn't room to use a solder sucker, but these are fairly
easy to get to and shouldn't cause a problem. The only trick is bending them
(or leaving them bent) so they can be easily reconnected. If you get into
terrible trouble you can clip them and replace them once the unit is apart,
but it's a pain to get them just the right length.
4.	The only really hard job is undoing the five or six ground tabs that
connect to the frame of the tuning capacitor. Here your best friend is a
manly soldering iron. (I'm willing to bet that the Hammarlund production
workers used a 150-watt American Beauty with a 3/8 inch tip). Fortunately,
you can't overheat anything, so just let it sit and cook until the solder
really flows. The parts are so massive that the joint will stay hot long
enough to give you adequate working time. Use a pair of thin (for access and
to avoid sinking off too much heat) long-nose pliers and pry the tabs loose
from their moorings or just pry with a thin screwdriver. Neatness doesn't
count - just try not to break anything. Once you get the RF strip out it is
easy to clean off the extra solder and to straighten the mounting tabs with
a pair of duckbill pliers. One trick - don't forget the ground connection at
the very end (farthest to the rear) of the tuning capacitor. It isn't all
that obvious, and it's the only one that's really hard to reach. If you
should break one of the grounds, you can substitute a solder lug or a piece
of coax shielding.

 

Recapping, etc.

1.	By all means, use ceramic capacitors - that's what Hammarlund wound
up using on the later production runs.
2.	There are two methods of recapping: (a) replacing one at a time; or
(b) chopping the old caps out all at once. The first method more or less
insures that you will connect the replacements to the right places, but it
gives you 
a lot less elbow room. The second (which I prefer) requires you to
double-check the schematic and the wiring diagram. (Note here that all
SP-600 variations are similar but not identical. Read Andy Moorer's
excellent article (www:jamminpower.com)and make sure you have the right
schematic in front of you before you start.) 
3.	Don't be slavish. You don't have to use the same ground connections
as the manufacturer. Disc ceramics are a lot more compact than paper
capacitors. Just keep both leads short and use the nearest ground tab.
4.	You do not want to pry open the RF section twice. Therefore, measure
all of the resistors before you close up and replace any questionable ones.
Don't go wild - the standard was +/- 10% -- that's plenty good enough.

Reassembly:

 

1.	With the RF strip separated, clean all the solder tabs and get rid
of the excess solder.
2.	Line up all of the connections mechanically before soldering any of
them.
3.	Solder the grounds first, then the tuning capacitor connections,
then the insulated wires, then the crystal switch.
4.	Don't cheat. Solder all the grounds. The SP-600 is a great receiver
primarily because it has a really selective front end. The last thing you
want is to have RF floating around and past the tuned circuits because you
left one of the tuning capacitor ground straps floating in mid air.
5.	Forget everything Heathkit told you about taking four turns around
every bollard (tie point). Just tack solder, but make sure you have a good
joint. You are making an electrical connection, not mooring the Queen Mary.
(You will be grateful for this advice if you every have to remove the RF
strip again.)

 

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net 

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