[R-390] SP-600 IF output buffer mod and cap replacement

[2002tii]

After receiving a number of requests, herewith the information to 
determine whether an SP-600 needs its 455 kHz IF output buffer 
amplifier modified, and if so, how to do it.

First, I may not have made it entirely clear in my previous post what 
the problem is -- on units that have the problem, it isn't in the IF 
itself, it is in the buffer stage that drives the 455 kHz output jack 
on the rear panel.  The harmonics of the 455 kHz IF signal that are 
generated in the overloaded buffer amp are then received by the radio 
as spurious signals.

There were (at least) three IF output schemes used by 
Hammarlund.  The older ones give very high output (10V or more p-p) 
with lots of harmonics (clipping on the negative swing), and the 
newest one gives a very clean 1 or 2V p-p.

In the oldest version the buffer ampifier -- a cathode follower 
(V16a) -- is fed straight (through C145) from the tied plates of the 
IF Driver (V11) and BFO Buffer (V12).

The newest version uses an additional connection inside T5 to feed 
the follower (through C145 and some shielded cable) from the top of 
the secondary.  This has two advantages -- it reduces the feed level 
by the gain of the IF Driver (V11), and it picks off the IF output 
feed before the BFO injection.  The newest version also uses a tapped 
coil (L53) as an autoformer on the output (the cathode of V16a feeds 
the top of L53 through C147, and the IF output jack connects to a tap 
a ways down on L53).

The middle version has the old-style feed plus L53.  I believe this 
is the most comon configuration.

If one has an old-style or middle-style unit I highly recommend 
changing it, because the distortion in the follower due to the high 
signal level radiates harmonics of 455 and causes spurious signals.

To change it, one can go into T5 and add the connection for C145.  In 
this case (assuming one already has L53, or adds it) you end up with 
exactly what Hammarlund built later.  Or, you can take the feed 
without going into T5 by putting C145 on the junction of R55/R56 (the 
output of T5).  I prefer the second method, because it comes out at a 
bit lower level and does not unbalance the load on the secondary of 
T5.  In either case you need to feed the signal from T5/V11 through 
shielded cable, grounded at both ends, to the grid of the follower 
(V16a).  C145 goes at the T5/V11 end of the coax to minimize the 
capacitive loading on T5 (the capacitance of the shielded cable ends 
up in series with C145).  You don't need to try to chase down an 
appropriate coil to use for L53 -- the follower can just drive the IF 
output directly.  If you want, you can use a resistive voltage 
divider in place of L53 to lower the signal level some more.

Alternatively, if you don't use the 455 kHz output, you can reduce 
spurious responses even a bit further by disconnecting C145 from the 
tied plates of V11 and V12 and connecting it to ground instead.

About replacing the paper caps in an SP-600:

I've used .022 uF/600V Sprague type 715 Orange Drops, and I've used 
02 uF/1000V ceramics.  The ceramics work better (the IF leakage into 
the RF at 910 and 1365 Kc is definitely better with the ceramics, 
particularly if you keep the leads as short as possible).  Hammarlund 
specified ceramics for many of these caps in 1955, then apparently 
went on building radios for years with all paper caps.  They must 
have had a big inventory of the paper caps they needed to use up. . . .

Note: these are all bypass caps on the DC supplies -- none are audio 
coupling caps, where ceramic would be a poor choice.  There are only 
two audio coupling caps -- C143 & C149, both 5100 pF mica.  You can 
change them to .005 or .01 uF Orange Drops if you want, and will 
realize some improvement in the audio.

One thing to make sure of is that you get EVERY SINGLE paper cap out 
of the radio.  Some are hiding in the RF box, some in the IF cans, 
some in the turret assembly, and some beneath wiring and switches 
under the chassis.  There are about 51 in all.

You don't neeed to worry about the values (some of the original caps 
are .01, some are .022, and it varies from radio to radio).  Use all 
02 or .022 and you'll be fine.

Best regards,

Don