This was previously published on the Hallicrafters QSL.NET mail reflector sometime
back in the late 90s.
Well, I finally got through my SX28 rebuild: listening to some good ole country
tunes coming out of a voice-of-the-theater bass reflex speaker connected to it,
tropical breezes coming in the window, 807 on the side, and pounding on the keyboard
telling you of my findings with it.
1. General info: This was a previously unsuccessfully repaired receiver I got
with a blown electrolytic can and damaged audio output transformer, S/N H-170655.
This cabinet-less rcvr was sitting around in a storage cabinet for 20-25 years
as one of those future projects to-do. Several months back I dragged it out to
finally do it. The rcvr was definitely a non-A: it had the stand-by switch socket
on the back chassis and the perforated top plate on the RF section cage. A manual
was obtained from HI-Manuals but the circuitry showed many differences from the
SX28 schematic. A SX28A manual showed the circuitry to be almost an 'A' version.
The IF, noise, avc & audio circuits were wired like an 'A' but with non-A resistor
values. The 'A' uses todays standard resistor values like 2.7K & 470K whereas
the non-A uses values like 3K & 500K. The HF sections look like the non-A schematic
and pictures.
2. Dirt & grime: This rcvr was still corrosion free (hard to achieve here in the
semi-tropics). The front panel was stripped and everything cleaned up with Simple
Green, a toothbrush, and LOTS of q-tips. A worn greenie pad was used on the more
tarnished areas of the chassis. That plated chassis took a shine beautifully.
Not wanting to leave the metal unprotected, I sprayed it with silicone lubricant
and wiped off the excess, hoping to leave a light coat of silicone protectant.
This is the first time I've tried this so I'm not sure how it'll work.
3. Front Panel Bezels: All three front panel bezels had loose glass windows in
them. They were all removed, cleaned up, old glue scraped out, and re-glued with
some craft type glue called Bond 527.
4. S-Meter: Is there an S-meter on a 28 that doesn't have the round case splitting?
I've only seen three 28s and all three had their cases splitting and opening up
like flower petals. The cases are a brass/bronze looking material and I wonder
if this condition is metal fatigue from the way they were stamped or drawn. This
meter was disassembled. Mine had some hairline cracks propagating to the front
by the mounting holes already. These areas were wire brushed and soldered to keep
the cases from coming apart completely. After a new paint job and assembly, the
open areas in the case caused by the splitting were just covered with black duct
tape to keep dirt out. I did think about a sleeve over the outside of the case
to push those split ends back down but decided this would just introduce additional
stress in the metal and self destruct faster. The grommet holding the pilot lamp
was hard and standand 3/8 inch replacements were too beefy to fit the hole and
still hold the lamp. A perfect grommet was found in the Eagle Hardware Store electrical
department for those needing one. The parts list says these are 0.5 mA meters;
mine needed 5 mA to go full-scale. Wonder if this is a typo cause it seems to
work fine electrically.
5. Audio Output Transformer: Something had punctured the transformer hard and
went in through several layers of wire. Luckily through a previous request on
this list, I met up with another local BA proponent, Stu WH6H, who offered a replacement.
(Ever wonder why with all those secondary taps they didn't have a 4 ohm speaker
tap, too?)
6. Power Supply Choke: Apparently whatever punctured the audio transformer also
hit the choke. The choke only looked like it was punctured a layer or two. Testing
on a supply showed it 'burning' or 'arcing' between turns. It was disassembled
and 1-1/2 layers of wire were removed. I don't think this affected the inductance
much considering how many more layers were still on it. High temp silicone tape
was used to rewrap the choke. Reassembly and testing showed a good repair.
7. Resistors: The resistors weren't much of a problem. The 30K/2W meter resistor
needed replacement as well as a couple on the 6V6 grids. The plate and grid isolation
resistor values did vary a bit but these are not critical values so were just
left. Cathode resistors were fairly close.
8. Electrolytic Caps: Two non-original cans were looking pretty sad so NOS replacement
were purchased. These were slowly reformed till their leakage went way below 1
mA. The cathode bypass caps in the audio circuits which originally used sections
in these electrolytic cans were replaced with new, small discrete caps.
9. ANL Potientiometer Switch: One section of this DPST switch on the back of the
ANL control wasn't making contact. These controls can be opened up by bending
back the tabs. The switch contacts were looking just fine, just all gummed up.
Cleaning out the switch with Deoxit got it working just fine.
10. Wax caps: Saving the best for the last! The wax caps in this rcvr varied from
looking-good to totally-waxed. Right from the beginning it was my intention to
de-wax this beast. I think there are 39 wax buggers in this anchor, at least I
had 39 in the can when I finished. (Anyone saving these?) Quantities used are:
.01 (3), .02 (21), .05 (14), .25 (1). I decided to use mylar (polyester) film
types with epoxy coating and radial leads. The ones I got were from a local parts
outlet, NTE brand MLR series (www.nteinc.com), virtually same as orange drops
except 1/2 cost, smaller, brown color, and all rated at 630 VDC.
The caps in the IF, audio, & power supply are easy to change, a nice warm-up for
the RF deck ones. Jim's Hallicrafters web page (www.exit109.com/~jimh/radio.shtml)
gives a procedure by A.B.Bonds for removing the sheet metal sides on the underchassis
enclosure to gain access to the RF deck areas. I discovered this page AFTER I
did mine without removing any panels. How you decide to do yours will depend on
your mindset at the time.
By-the-way, if you are planning to change ALL the caps, I don't recommend this
as a first-time project. Go sharpen up your skills first by rebuilding an R390A
or such.
I found the following tools indispensable:
a. 45 watt pencil soldering iron with several tips for custom bending. Those tube
sockets need a bit more heat than transistor stuff. And the solder Hallicrafters
used appears to be higher melting temp stuff.
b. needle nose pliers with cutter at the tip. Imperative for getting in there
and cutting out those wax buggers; see also (g) below.
c. long tip needle nose pliers.
d. hemostats, or scissor-style parts gripper, 5-1/2 inch size, both curved and
straight jaw. Used for holding those caps while you try soldering them in the
RF section. Don't think needle nose pliers alone will do it. These are skinnier
and give you more working room.
e. solder sucker, with extension tube if possible.
f. VERY strong light source (light tube or include fan to blow heat away from
that bright, hot light source. Flashlights won't work unless you have three hands).
g. soldering aid tool with forked tip. This tool turned out to be a great cap
lead cutter (breaker) for those impossible places. By putting the lead in the
forked end and rotating the tool back and forth, you'll eventually break the lead
through fatigue.
h. tiny inspection mirror (dentist type). For trying to see where you can't see.
i. close-in reading glasses if your eyesight is getting like mine.
The RF enclosure has 4 sections from front to rear: oscillator, mixer, 2nd rf,
& 1st rf. Luckily, the oscillator section has no wax caps in it. That would have
been one nasty area to get into. There is one bypass in there but Hallicrafters
put in a mica one (good foresight, huh?).
The mixer and 2nd rf sections each have 4 caps in them. Remove the antenna trimmer
shaft to give you more room. Using the above tools you can change these out without
too much pain. I remember having to crush a couple bigger ones along the rear
walls to get them out. Feel free to remove a ceramic trimmer or two for more room
(You WERE going to do an alignment afterwards, right?).
The 1st rf section: here's where we separate the men from the boys and see if
your soldering skills are what you think they are. First off, there are only three
caps in this section. A.B.Bonds was right in that I didn't even see one till I
was going after the other two. Well, if you don't want to remove those sheet metal
sides you sure as hell have to remove something because you can only SEE what's
way down in there--you can't get to it. I removed the antenna trimmer cap and
the coil between the antenna trimmer cap and the bandswitch. The trimmer cap screws
are easily accessed from the 2nd rf section. The coil's mounting screws are not
too bad to get at; the lower one you may need an offset screwdriver. Replace that
lower screw with a hex head style and you can tighten it up later with only a
small open-end wrench. The coil has 5 terminals. Make a detailed drawing showing
where all the wiring goes before you take it apart. Use whatever disassembly technique
you prefer: desoldering and unwrapping or snip & resolder. The forms will soften
and melt so use minimal heat. Use care around the switch wafers, too. You don't
want a broken wafer here!
With the trimmer cap and coil out, you have SOME access to the tube socket area.
This is where you'll feel like a surgeon or wonder if maybe the rcvr would have
been fine with these 3 wax caps left in. The cap you can't see is under the coil
on the other side of the bandswitch. Use the soldering aid tool to break the leads
and get it out. The line this cap is bypassing goes all across the section ending
up at the coil on the far side of the trimmer cap. Mount the new cap over by that
outboard coil with a ground lug slipped under the removed coil's lower mounting
foot. The original cap location is totally inaccessable without removing another
coil; but the cap doesn't have to be put back there.
11. Misc:
a. I wonder if anyone has ever documented the different changes made. I think
the change to the 'A' version might have been when they went to the module assemblies
in the RF section and redid the cage cover on top - mostly mechanical changes
but few circuit changes. What do others think?
b. Do all non-A tech manuals show the back pictorial view BACKWARDS? Hard to believe
Hallicrafters didn't catch this.
c. I heard the 'A' model doesn't have the -A on the front panel. Is this true?
d. I was impressed that a rcvr this old has an early attempt at a noise blanker,
not just a limiter.
END.