[Hammarlund] SP-600-JX-6 fixed!

[David Wise]

>From my records:

July 19,2007

In the archive I saw a few complaints of distortion
at high signal.  After bringing up my first and only
SP-600 (recap, check resistors, test/sub tubes, align),
I noticed this problem too.  With AGC working normally,
several local stations were distorted unless I backed off
the RF gain.

By injecting signal at each stage to get the same
final output, I traced this problem to V11, the
Driver, and cured it completely with a simple change.

PROCEDURE

1. Look at V11 (6BA6) pin 1 (G1).  There is a
low-value resistor to an IF transformer terminal.
(R55; the manual says 10 ohms, mine was 22.)

2. On this same IF transformer terminal,
there is a 100K resistor (R56).  Disconnect it.

3. If there is also a 330K resistor (R113, Issue 3
and up), perform step 4 with a 390-ohm resistor.
If not, connect a 330K resistor between here and
ground, and do step 4 with a 470-ohm resistor.

4. Disconnect V11 pin 7 (cathode; pins 2 and 4
must remain grounded), and wire the resistor of
step 3 from pin 7 to ground.

The following parts become unused:
R56 (100k), R57 (10k), C127 (.01uF), C128 (10uF)

DISCUSSION

My SP-600 matches Issue 2 of the manual.  They
reduced V11 bias (from -10V to -7.5V) in Issue 3.
I tried this, and it helped slightly, so I kept going.

As I went from -7.5 to -5, it cleared up.  At -2
it was distorted again.  I guess the large grid
signal on this stage drives V11 near cutoff on
the negative swings, which clips the negative
peaks of the audio coming out of the detector.

Reducing the bias increases the gain, of course, so
I attenuated it (with a shunt resistor on the grid)
to get a fair comparison.  It was still clean when
I had the gain distribution back to normal.

I decided to try an unbypassed cathode resistor,
like V7 (Gate) in Issue 3.  I found a lucky
"sweet spot"; with G1 returned to ground instead
of -10, 470 ohms gave me the desired gain and the
correct bias (about 4V) in a single stroke.  Audio
is clean, even on the mile-away blowtorch that takes
my meter to 100 and used to be unintelligible.
I tried several different 6BA6's; all were fine.

My V11 socket had a ground wire running from pin 2
(G3) through the center shield and pin 7 (K), to
one of the socket's ground terminals.  I cut pin 7
loose, wired the rest to the socket's other ground
terminal, and ran my new resistor from pin 7 to the
original ground.  I disconnected R56 from V11 pin 1,
and connected a new 330K resistor from pin 1 to
ground.  (This imitates R113 from Issue 3,
and keeps the mod consistent and reversible.)

That's for Issues 1 and 2.  For Issue 3 and up, use
390 instead of 470 because V11's gain is initially
higher, and since R113 returns G1 to ground, simply
disconnect R56.

I can't figure this out.  Why did Hammarlund modify
V7, which wasn't the problem, and not V11, which was?
Contrariwise, why don't you others have trouble?

Regards,
Dave Wise
SWL in Portland Oregon

________________________________________
From: hammarlund-bounces at mailman.qth.net <hammarlund-bounces at mailman.qth.net> on behalf of wb3fau55 at neo.rr.com <wb3fau55 at neo.rr.com>
Sent: Monday, August 21, 2017 5:58 AM
To: hammarlund at mailman.qth.net
Subject: [Hammarlund] SP-600-JX-6  fixed!

So I came across a CD containing a lot of SP-600 info.  There I found an answer  to the mod done with the meter adj pot.  As wired,  it controlled IF gain.  Rewired to stock,  problem now gone.  A bit of overload on local signals,  but lowering RF gain does  the job there.  wb3fau
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