[Hallicrafters] SX 115 Refurb (long)

[Rocco Lardiere]

Gents:

I am nearing the end of refurbishing an SX 115 and thought some notes
might help other poor souls out there who have encountered some of these
challenges.  Some of these topics are applicable to SX 101's and 100's.

First, a note of thanks to Ron, W6QM, who came up with four missing
knobs for this radio - not an easy thing to do, but it sure looks better
with the correct knobs!

I bought this radio on the Internet in a moment of foolishness.  The
seller indicated that it lacked the pointer, and the picture showed a
very dirty chassis and the missing knobs on the front.  But 115's are
hard to find in any condition, and I like challenges, so I won the
thing.  Kind of like winning a St Bernard puppy.

Problems noted and fixes:

Missing pointer - it was there, just needed to restrung properly - the
manual is good.  BTW - there are at least two versions of the manual out
there - some slight differences.

Dirt - cleaned off easily with a damp sponge.  I foolishly tried alcohol
and simple green first - water cut through the southern Calif attic dirt
much better.  Lots of elbow work required, however - removed faceplate
to get at things.  Lesson learned - try water first.  The chassis held
up very well - some pots and switch shafts had some minor rust -
Hallicrafters did well with the chassis corrosion coatings.  Even the
tube shields were treated and cleaned up nicely.  The front panel is
perfect, but, as usual, there are chips out of the black wrinkle
panel).  (Any easy fixes?)

Tubes, crystals, and switches - found two 6DE6's in place of original
6DC6's.  Used Deoxit D5 liberally, including tube pins.  Crystals were
rusty but intact, except for low 10-M, which had been replaced to allow
27 MHz reception (which explains a lot about the condition of this
radio).  

Power supply - brought it up slowly with variac - not too much ripple,
and VR tube ok.

Alignment notes (only for the advanced or terminally brave).  This is my
second 115, so I have included some general notes here - yours may
suffer from some of these common problems.  As found, I could hear weak
signals on most bands, but only LSB and LSB/am.  I took this as a very
good sign - a lot better than dead.  (My first 115, which had been
claimed to work "well," was found with the rear wafer of the bandswitch
unengaged due to misplacement of the shaft - which did NOT help the
gain).  Watch out for previous bodges.  

Step 5-4, number 2 (third conversion crystal oscillators) - the USB slug
of T8 CAN be accessed through the hole in the chassis, but watch out for
HV in the vicinity.  I finally got the crystal to oscillate and set the
level 6 dB down on the "gentle" slope, according to the S meter
(instructions say to use the output meter across the meter terminals -
not the same, but it worked for me).  Note - I had to "oil can" the
crystal to get it to oscillate - I borrowed the USB crystal from my
other 115 to tune the circuit - then worked on the balky crystal.  LSB
slug of T8 was easy.  Most IF cans way, way off as found, but easy to
adjust to my "precision" signal generator (do use a counter - this
frequency is critical).

Step 5-4, number 5 - found C33 trimmers out of alignment.

Step 5-6 - bandpass antenna and first mixer coils.  Easy except for
bottom 20-M antenna slug, which stuck and has probably fractured in
place.  Left it alone, because close enough - but needs to be replaced
some day.

Step 5-9 (factory gain meter control).  Watch out.  This is a good
measure of the open loop gain of the radio, and any faults will show up
in how you need to adjust this meter sensitivity control.  I found a
gain problem in the VFO tracking in both 115's.  See fine print note 2
under "Injection Volvates" on Figure 17.  This should read, "VFO
injection at 2nd mixer will measure -1.6 volts at pin 2 of V6."  That
-1.6 volts means RMS volts, or roughly 4.5 v peak to peak on your high Z
scope.  The engineers who wrote this did not want to put this into the
general alignment instructions, probably because it involves a bandpass
response from T3.  Note - I had to replace the T3 tuning cap C47 on the
first unit to tune this transformer.  On this 115, I used this
procedure: adjust bottom core with AGC off at 14.00 for max gain at
injection test point.  Adjust top core at 14.500 for max gain.  Monitor
at V6 Pin 6.  S/B 4.5-8 V p-p with very good flatness across the VFO
range (try another 6CB6 if you have low gain).  If you can't get this
right, the receiver gain will never be as designed, and the AGC
adjustment may make no sense.  I suspect that this general problem may
be common to the HT 32 transmitters - I may get into my HT 32B and try a
similar adjustment for T101 - see step 7-12, which does NOT talk about
T101 and overall flat response) (has anyone tried this?).

Step 5-10. -11 (BFO frequency adjustment, notch adjustment).  BFO was
found to be WAY off on both 115's.  I measured the center BFO freq at 47
kHz on the second.  The manual procedure is not sufficient to adjust the
frquency.  This is probably due to the cap's moving around over the
years, but I did not disturb them (one is a temp compensation cap, the
other is a precision value).  Easy fix - determine which direction of
core rotation is needed to bring the correct freq to center of the shaft
rotation - loosen the setscrew on the grooved shaft, accessible through
the side holes on the chassis - adjust so that the setscrew limits are
correct.  This works for the notch also, and 50 kHz mark should be at
exactly 50 kHz.  You may want to set the notch a long way away from the
passband - it is always there, and it affects the shape of the
passbands, even in the off position.  I left mine in the factory
position - but the receiver tends to be too sharp in the 5 kHz AM
position, and part of this is the notch.  BTW - OFF is at the low
(clockwise) limit, below 50 kHz, I THINK - SOMEONE CORRECT ME IF YOU
THINK THE OPPOSITE).  This places the notch on the opposite sideband, to
improve opposite sideband suppression - but it does affect the gain over
the selected sideband.  Placing OFF at the high end will cut the high
freq audio response in AM at 5 kHz.  Choose your poison.  I am tempted
to move the notch way out of the passband and ignore it forever.

Step 5-15 (AVC amp adjustment).  IF you have adequate open loop gain,
this adjustment will follow the manual exactly - worked perfectly on
both my receivers, after fixing all the other problems.

I did not mention bad tubes, but you know how to substitute to find
those, right?

Next steps - repaint the cabinet.  Any hints on what kind of airbrush or
other sprayer to buy? I am tired of spray cans.

Performance so far - absolutely great - 50 microvolts is S9 on all bands
with the "factory" gain pot at midrange. AVC is wonderful - bands sound
transparent, with weak signals easy to pick out.  I prefer my 115's to
all my other receivers, including a nice 75A-4 (heresy).  When properly
aligned, the 115 is an absolute pleasure to use - right at the top of
60's tube receivers.  This one will be a companion to my B&W 6100
(another saga which I will write if there is any interest).  

If anyone has a problem with a 115, drop me a note - there are a few
garden variety things I did not write up. 

73,

Rocco Lardiere N6KN