[Hammarlund] SP-400 restoration, was: "Im recapping a SP600"

[Roy Morgan]

At 06:45 PM 12/2/03 -0500, Dan <danielw@prexar.com> wrote:
>Hello Roy
>
>     I really enjoyed reading your post, I'm starting to do my first
>restoration on a SP-400 SX


Dan,
copy to the list for general interest,


Good for you. I have one older Super Pro.. it's the military BC-779 or 
whatever it's called.  I've had it since 1965 or so, and it has not run for 
a *long* time...

>will the infromation you posted also apply to the SP-400?

To a certain extent.  Here are some points specific to the SP-400:

1) The SP-400 power supply,

  - IF you have an original power supply, is different from the 
SP-600.  The digital manual I have here shows a 115 volt tap on the primary 
with color code BLK-RED and a 125 volt wire with color WH (I think.. it's 
unclear).  So I certainly recommend investigating that if you do in fact 
have an original power supply.. so many of these radios get separated from 
the power supply that its common to find one with a home brewed supply, or 
none.  Move the power connection to the 125 volt primary wire if you have one..

   - Two of the wires from the radio to the power supply (terminals 1 and 
2) carry the full filament current. These leads are separate from the 
ground wire (terminal 3).. the idea is that the filament current would 
cause hum to be introduced in the B- lead if they used a common ground 
wire. And, the filament wires are heavier than the other wires to reduce 
losses.  Bottom line: don't ground the filament supply at the chassis of 
any separate power supply.  It appears that terminal 1 is grounded at the 
receiver terminal strip, along with terminal 3, and that terminal 2 feeds 
the filaments of all tubes with the other side of tube filaments grounded 
at or near the tube sockets.

2) You will almost certainly have to replace every single one of the paper 
caps in your radio, and many of the resistors, too.  Just do it.

3) Don't expect your SP-400 do do well either on the higher frequencies or 
for SSB. The tubes used did not perform well up toward 30 mc, and the 
stability of the local oscillator combined with the lack of a product 
detector make it not so hot for SSB.  You can get the signals, but you'll 
be re-tuning from time to time and listening to less than good detection of 
the sounds.

4) The crystal filter:
  - Be very careful of the crystal phasing control shaft. It is made of 
phenolic and breaks VERY easily.  The cap is insulated from the chassis and 
it needs a phenolic shaft to operate properly.  If yours is broken, arrange 
some substitute shaft made of Plexiglas, phenolic or the like.  If an 
earlier "worker" has soldered a metal shaft on there, the filter will never 
work right.  If I remember correctly, the original phenolic shaft is turned 
down to half diameter and pinned into the drilled end of the phasing 
capacitor shaft.  That, of course, is the weak point.  Getting the broken 
part out without smashing the capacitor body may involve removing the whole 
assembly from the radio so you can support the shaft properly as you drive 
out the little pin.
  - I think the filter crystal is in a housing that can be dis-assembled 
for cleaning. If the filter does not work, you may have to remove the 
filter can from the radio and investigate.  There may be failed mica caps 
in there, too.
  - Align the IF strip according to the actual frequency of the filter 
crystal.  I can't remember if the manual(s) are explicit on this point or 
not, but it's a common method for radios with crystal filters.

5) The audio section:
  - The audio output is a pair of push pull 6F6's.  If you don't have any 
6F6's, and need to find substitutes,  refer to the tube manuals to find a 
proper substitute from among the many similarly based tubes.  6V6's may not 
be good.   Unlike many radios the 6F6's are fed with a driver transformer 
(driven by a single 6F6.)  If you have to replace that transformer, 
rebuilding the stage into a balanced load phase splitter is an 
option.  Beware of the cathode to filament voltage limit of the driver tube 
if you do, though.

  - The cathode resistor, bypassed for audio by an electrolytic cap, 
creates self bias for these two tubes, and should be checked for having 
drifted in value. I suggest replacing the cap for sure, and raising the 
value of that resistor by perhaps 50 percent for two reasons:
     1) Nobody needs "8 watts ... at 10% distortion" in a radio. Nobody.
     2) Reducing the standing plate current in those tubes will make them 
last a LOT longer than they would normally.

  - The normal power supply delivers fifty volts bias to a bias string in 
the radio.. this is used to bias the audio driver tube and other functions 
too.  Home brewed power supplies that don't account for this arrangement 
will not let the radio run right. If you have such a supply, and the bias 
is not there, you can make the bias by installing a little 6 volt filament 
transformer backwards on the filament line in the radio and using a diode 
rectifier and filter setup.

6) The coupling caps between the previous stages of audio are almost 
certainly leaky. This will upset the operating points of the 6F6 driver and 
possibly the 6J5 pre-amp (though there may be no DC on the volume control 
at all).  The cap at the grid of the 6F6 driver will almost certainly have 
to be replaced.

7) If the thing acts badly, investigate poor grounds at terminal strip 
ground points and tube sockets. These radios are old enough for corrosion 
to occur at these metal to metal points and cause all sorts of problems.

8) If the radio has any glass tubes in places where metal tubes were 
originally, be wary of oscillations. The metal tubes had pin 1 grounded 
because it was connected within the tube to the metal shell.   These 
grounded pins do not appear on the schematic I have here at the 
moment.  The glass versions of the same tube may not have an internal 
shield, or the shield may not work as well as the metal enclosed 
tube.  Designers of 1930's radios using glass tubes often built  them with 
metal shields surrounding the tubes.  Sometimes, the phenolic tube sockets 
were missing pin 1 as an economy "feature" and using a metal tube where a 
glass one plus metal shield can or sleeve would cause no end of IF or RF 
oscillations.

9) If you discover any miniature tubes, or additional tubes in your radio, 
it has been modified with any of many mods published over the years to 
improve the sets performance. (I saw one SP-400 with all miniatures cobbled 
into it.)  You may want to dig up articles on the topic to see if you can 
dope out what was done.  (In one SP-400 a friend has, the coils of one band 
have been changed to allow for single-ham-band operation. I assume that the 
military LF band of 200 to 400 kc or similar was changed, but we never did 
dope out what was done to it.)

Good luck with the radio. it will be a fine receiver once you've restored 
it carefully.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan@nist.gov --