Hallicrafters SX-28 and SX-28A For Sale

New Fixes on SX-28A 4/17/1998!

Contents

• What's the difference between the SX-28 and SX-28A
• Companion SX-25 Panadaptor
• SX-28 or SX-28A Physical and Electrical Specifications
• SX-28A Restoration Log
• SX-28 Restoration Log
• Bandpointer threading version 1
• Bandpointer threading version 2
• SX-28 Restoration Tips from Contributors
  • A. B. Bonds on Original Capacitors and Resistors
  • A. B. Bonds on access of Capacitors in RF Deck
• Chuck Dachis Book on Hallicrafters Radios

What's the difference between the SX-28 and SX-28A?

I have read a few accounts of the differences between the SX-28 and the later SX-28A. I own both. Here is what I have found. In an advertisement for the SX-28A in the 1946 edition of the Radio Amateurs Handbook I have extracted the following text out of context. "The SX-28A is a further refinement of the famous Model SX-28 that achieved such popularity with amateur and professional operators prior to Pearl Harbor."....... "The traditional sensitivity and selectivity of the Model SX-28 have been further improved in this new Super Sky-Rider [SX-28A] by use of "micro-set" permeability-tuned inductances in the r.f. section. The inductances, trimmer capacitors, and associated components for each r.f. stage are mounted on small individual sub-chassis [phenolic boards] and may be removed as a unit for easy servicing."

I have taken photos of the r.f. sections from the pre-WWII SX-28 (1940) and late WWII SX-28A as shown below. The inductors in the SX-28 are mounted directly to the main chassis and are adjusted using the protruding screws. Whereas, in the SX-28A, the inductors and capacitors for each section are mounted on boards to form modular units. The inductors are tuned by directly accessing the notched tuning slugs. I have the manuals to both the SX-28 and SX-28A. After examining the schematics carefuly, I have detected no differences in circuitry nor tube types. Both panels are marked as an SX-28, however one other notable difference is that the cover over the tuning capacitors and RF tubes on the SX-28A is fastened by snap-on fasteners holding the entire cover, whereas on the SX-28, a small cover over the tubes is removable with thumb screws.

Panadaptor: A companion S-35 Panoramic Adaptor could be ordered with the receiver. A photo and additional information can be seen in Chuck Dachis book on Hallicrafters receivers.

This just in:

The most obvious way to tell an SX-28 from an SX-28A without looking inside is simply by noticing that the older SX-28 model had true "steering wheel" type tuning knobs with no material between the spoke. This was pointed out to me by Warren D. Anderson in an e-mail I received from him on 11/5/1998.

SX-28 or SX-28A SPECS

Bands

Tubes

Electrical

SX-28A Condition when received

Physcial:

Receiver chassis and components were all in shiny clean mint condition. Case is dirty and badly dented in several places. Front panel in excellent condition with some chipping around screws. Antenna terminal post pair was replaced with modern plastic type - not original. Almost all tubular capacitors had been replaced with mylars.

Functional:

Reception works well on all bands and frequency calibration is fairly close. Tone quality is excellent. BFO did not work, no signal with selectivity switch in crystal positions. S meter frozen in high reading position. Bandspread band pointer string is broken. Everything else seems to work OK.

SX-28A Restoration Log

• Ordered and received reproductions of manuals for both the SX-28 and SX-28A from W7FG Vintage Manuals.
• I started by simply removing and replacing the BFO tube to make it work - just a bit of dirt, I guess.
• Removed s meter, needle was physically frozen. Disassembled s meter and found needle pivot out of pivot screw. I reset it with no problem. Upon replacing s meter functioned, but no sensitivity. I measured the 27K 2W resistor in the S-meter tube anode circuit, (R30). It measured around 50K. Went to Radio Shack and all I could find were 2, 100K 1/2 watt resistors which I slapped in parallel with the bad resistor to give me a value of about 24K. This made the S-meter work properly, but the resistors heat up too much and the meter often has to be re-adjusted. Bought 27K 10W resistor at Lashen Electronics in Denville, NJ, but haven't used it yet.
• Eyeballed crystal filter area and compared it with the other SX-28 and found a wire broken. On resoldering the wire there is now a signal when when the selectivity switch is in the crystal positions, but the filter hardly works very well. No noticable difference when crystal phasing is adjusted.
• Ordered dial cord for bandswitch pointer, still waiting.
• Ordered gray wrinkle paint for cabinet, still waiting.
• Asked one autobody shop for estimate on smoothing out the cabinet. The guy said he was really swamped and it would probably cost me a couple hundred. Then at a hamfest, I asked a collector/restorer and he recommended that I get the cabinet dipped to strip the paint and buy some Bondo & hardiner and puddy the thing myself. I will get the cabinet paint stripped and ask a couple more autobody shops before trying this myself.
• 4/6/98 Received gray wrinkle spray paint for cabinets and new dial drive cable for band switch pointers from Antique Electronic Supply part number S-M75A-25, 0.028" (Standard Thin).
• 4/6/98 Replaced S-meter tube anode resistor (R30) with 2.7K 10W.
• 4/6/98 Installed new dial cord making bandspread band pointer operate with bandswitch. Only problem with receiver at this point seems to be the selectivity control and crystal filter don't seem to be adjustable. S-meter is somewhat stingy. SX-28 S-meter (other receiver) more sensitive once I got it working on 4/11/98. Need to check the circuit out. Maybe a weak 6B8 tube (V7). Put that on my list.
• 4/11/98 Got Crystal filter to operate. The outside crystal phasing capacitor terminal was toucing the metal sheild around it. I simply bent the pin from the shield. So, that combined with the previously reconnected wire underneath the chassis put the crystal filter into operation. The "Broad", "Medium" and "Sharp" don't seem to behave in that order, though.
• Stripped old paint from case, got dents knocked out and filled by autobody man. Got case repainted.
• Bought good old fashioned rack screws and washers. Waldom 10-32 X 3/4" Oval Head machine screws, Steel Nickel plated #KF-510. Waldom #8 screw size Steel Cup Washers Nickel Plated #KCW-8. --- at Lashen Electronics.

BANDSWITCH POINTER THREADING

(Two versions)

Someone sent me some E-mail asking me how to thread the dial cord for an SX-28. The manuals for the SX-28 and SX-28A do not contain instructions or a picture, so I had to figure it out for myself. After doing so, I created the figure below using 3D Studio MAX 2 and Photoshop. For those interested I will explain a bit of that process. I suppose there are are other, cheaper tools, but it was easier for me to actually construct a rough 3D model of the panel, chassis and other parts with the panel material set to transparent. The panel picture was obtained by taking a picture of it with my digital camera, loading it into Photoshop, adjusting the brightness to maximum and the contrast to almost minimum so that a faded image would be created. After placing the parts, in 3D Studio MAX 2 I simply created a line following the path and lofted a circle onto the line. If you don't understand what that means, it doesn't matter for this discussion. Since there weren't any instructions for this, I had no idea what the parts were called, so I made up my own names for the parts. The "bandspread" dial is really a hamband dial which is calibrated only for 10, 20, 40 and 80 meters. I converted the picture to grayscale to make sure that the parts contrasted properly for the "color challenged."

First Version

1. One string is going to be used for both switches so start by cutting a piece of dial drive cable about 3 feet long and dip the ends in or paint the ends with nail polish or some other lacquer. Set aside to dry. The nail polish will harden the ends making it possible to thread the cable through the small holes in the pointer tabs.
2. First, remove and clean any old remaining dial cord from each pointer tab.
3. Place the bandswitch in the .55-1.6 MC position (for least tension)
4. Loosen both the main and bandspread tension levers so that they move with some friction (not flopping around) and position them to the upright position (for least tension).
5. Now, take the dial drive cable and cut one of the ends in within the nail polished area and thread it through the hole in the main band pointer tab then tie it. The pointer tabs are attached to front of the spring loaded assembly which can be pulled up and down to slide the band pointers.
6. Wrap the cord around the top pulley, the pulley on the end of the main bandspread tension lever and then the third pulley attached to the chassis. It helps to have a pair of long slim needle-nose pliers to grab the cable while feeding it through tight spaces.
7. Continue down to the bandswitch and wrap the cord around the screw in the side of the bandswitch shaft.
8. Do NOT go directly to the next pulley, but wrap once clockwise around the bandswitch shaft, feeding it UNDER the loop to prevent slippage, then continue to the pulley just below the bandspread tuning shaft.
9. Continue up to the pulley fastened to the bandspread tension lever and then to the topmost pulley on the bandspread band pointer assembly.
10. Continue around that pulley down to the tab on the spring-loaded bandspread pointer, cutting this end within the nail polished area. Feed it through the hole in the tab and tie it slightly tight.
11. Once the entire cord is in place, with the bandswitch in the .55 - 1.6 MC position position the main tension lever so that the pointers pount to the bottom most band calibration (standard broadcast).
12. Now move the bandswitch to the 3.0 - 5.8 MC position for adjustment of the bandspread pointer.
13. Now adjust the bandspread tension lever, tightening the cord till it points to the bottom most band calibration line. I will help if you lift the spring-loaded pointer assembly with your finger.
14. That's it! Now, hopefully, when you turn the bandswitch it will pull the string for both pointers, moving them to the appropriate band. Note that when the bandswitch is in either the .55 - 1.6 MC and 1.6 - 3.0 MC positions, the bandspread pointer is pointing below the lowest band on the bandspread dial since there is no corresponding band for those positions. If the pointers are not correct try adjusting the tension levers. If that doesn't work, you may have to untie an end, tightening it up and retying it. Unfortunately, on my SX-28, the bandswitch had no mechanical limits to prevent continuous rotation. The switch should not be capable of moving directly from the .55-1.6 MC position to the 21-42 MC position. If it is and you do that it will either break the string, make it slip or fall out of the pulleys. My SX-28A has such limits, but I haven't yet figured out where it's implemented. If anyone has any ideas, please drop me a note.

Second Version

Ken Kinderman - WB9OZR saw this page and sent me a threading diagram and instructions that he obtained. Neither one of my receivers match this diagram identically so I will keep my diagram up, although I do need to add the spring on the bandspread side. I have been told that these receivers vary somewhat even within the same model. Below are the text copied directly from the instructions with the diagram.

1. Make sure both metal indicator arrows are in the down position and there is no tension on the springs (F) and (G).
2. Set the receiver on end with bottom of chassis facing the operator.
3. Set the band switch to position 1.6-3.0 MC. (Machine screw on band switch shaft bushing is now facing operator.)
4. Cut the dial cord at leas 26 inches long.
5. Make a slip knot 10 inches from one end of cord; place over machine screw (A), and draw tight.
6. Set band switch to position .55-1.6 MC.
7. Wind short end of cord once around bushing (X), then position cord over pulley (1), under pulley (2), and over pulley (3).
8. Open slot (B) with pliers; run cord through brass blocking washer, tie a double knot, and insert through slot (C). (Do not allow any slack in cord.)
9. Close (B) with pliers.
10. Position the longer length of cord under pulley (4) and over pulley (5) and pulley (6) respectively.
11. Insert and pull cord through brass sleeve (D) and blocking washer, and tie cord to end loop (E) of tension spring (G), allowing only a slight tension on spring. Make sure there is no slack in this length of cord.
12. Turn band switch to each of the six positions, noting the position of the metal indicator arrows.
13. Tension and position of the knots with respect to the brass blocking washers are critical, making careful stringing of the cord necessary. Adjust the tension brackets if necessary. (Cord may break if not correct.)

SX-28 Condition when received

Physical:

Receiver in good physical condition with no damage, but very dirty. Both main tuning and bandspread pointer cords missing, but threading will not be difficult. Unlike the SX-28A most caps and resistors are the original. Cabinet is badly dented and is not an SX-28 cabinet. The power cord was completely missing. Some controls were loose.

Functional:

Receiver works well on all bands and calibrations is close. Crystal filter works better than on the SX-28A, but not very well. S meter somewhat. Front panel in mint condition! Selectivity doesn't seem to work as expected.

SX-28 Restoration Log

• Found a spare PC power cord and cut the female end off. Standing the unit on its side with power transformer side down, I fed it through the hole in back of the chassis, next to the power transformer and stripped it, connecting the green wire (for the ground prong) to the nearby ground lug next to the octal socket on the back of the chassis. The other two wires were connected to pin 5 of the tranformer and to the end lug of the fuseholder.
• Not knowing what would happen upon applying power, I tipped the unit back to its normal position. One electrolytic capacitor looked a bit crushed between the clamps holding it and having experienced exploding electrolytic capacitors before, I wanted to protect myself before applying power. It's wise to sit the chassis on a non flammable surface that you don't care about. Fortunately the receiver worked with no sparks or smoke.
• Ordered dial cord for band switch pointers.
• Cleaned RF section covers to look mint using Clorox brand "Soft Scrub" cleanser using a toothbrush (see photo above).
• Cleaned the dirty tubes.
• Replaced S-meter bulb. Noticed 1/2" Grommet around S-meter bulb dried up, needs replacement.
• 4/11/98 (R30) S-meter resistor way out of range. Replaced with 27K, 10W. S-meter now operates fine. It seems to be much more sensitive than the one on the SX-28A. Found meter zero adjust pot (R29) intermittent. Will need to replace that.
• 4/11/98 Found nothing wrong with selectivity control, except that the knob was on wrong leading me to believe it wasn't working properly. Repositioned knob. Selectivity positions all seem to be working perfectly!
• 4/11/98 Noticed some arcing inside one of the 6V6GT audio output tubes. Put that on my list! Works OK for now.
• 4/11/98 Noted that most of the paper tubular caps are .01, .02 and .05 uF. Will probably pick up a half a dozen of each all rated at 600V. More on the list!
• 4/11/98 At the suggestion of Hank Arney - KN6DI, I got some 3M (green) Stripping Pad Cat. No. 7413 at the local hardware store and started to rub off the chassis dirt using no liquids. Works like a champ! For very narrow spaces I pushed it with the end of screwdriver. I blew the loose dirt out with GC Electronics "Airjet". There are other brands you can get at electronics stores. It's probably a good idea to wear safety glasses and hold your breath while the dirt flies! (you wonder what kind of organisms are in it!)

SX-28 Restoration Tips From Contributors

A. B. Bonds on Original Capacitors and Resistors

This seems to be a fairly common activity these days, but then there is a lot to restore.

Observations:

The unit in question is an SX-28 of fairly early vintage, has uninsulated power resistors. Cosmetically clean, functioned only weakly. I noted that the power transformer had been replaced, as well as several wax caps. My guess is that leakage killed the transformer.

Every wax cap in this unit tested suspicious. Even the blue kinda plastic Aerovox replacements were not reading well. I use an old Heath magic eye cap checker, which does HV leakage tests. Most caps were a little leaky, some terribly so. All of them would not read strongly, i.e., even at the best bridge balance the eye did not open very far, if at all. Replacement caps checked fine. Many of the waxies were melted to the point that the innards were loose in the tube.

Bottom line: Replace ALL of the wax caps, even if they look OK. They will fry sooner or later. See my post on getting to the RF deck. My primary amendment to that is that three of the four modules must be removed completely for reasonable access, the total job takes 8-10 hours.

About half of the resistors were not in spec, given the rated tolerances. Most read too high, a few read too low (by a factor often in some cases). About a third (of the total) were sufficiently far off to justify replacement, a non-trivial pursuit.

General rules: Any resistor used for screen dropping (with a bypass cap) will be bad, due to excessive current through the bad bypass cap. Any 1 meg resistor will read at least 1.5 megs. The plate resistor for the S-meter amplifier will be way off. It is 27K, rated at 2 w. Mine read 73 k. The load on this resistor varies with rf strength, but it can be as high as 3.5 w. Miraculously, none of the other power resistors were bad.

Bottom line: Check all resistors. In nearly all cases, you will not have to remove an end to do so.

General observation: Halli construction of the era was very robust. Leads are very tightly wrapped, and often crowded on a lug. The chassis is deep and crowded, so there is a tendency to blow off a replacement because it looks impossible. It is not impossible, and if you don't do it, that component will be sure to fail next. The situation reminds me of the OB-GYN doctor who retired and took a course in auto mechanics. For his final project, he showed the instructor a car in which he had done and entire engine rebuild. He was especially proud of the fact that he had done the whole job
through the tailpipe....

73, and stick with it A. B. Bonds

A. B. Bonds on access of Caps in RF Deck

There are 11 wax capacitors in the RF deck of a Hallicrafters SX-28. While it is possible to replace a few of them with hemostats and a long thin soldering iron, some are so buried that they are not even visible. There follows a description of how to open up this deck. It is not for the faint of heart. I am limiting the description to access to the RF amplifiers, since they are the most difficult to get to otherwise. The theme may be continued on to the oscillator etc. at your own pleasure.

1. Remove the top of the RF/variable cap box and pull the four tubes.
2. Tag and then desolder the wires connected to the last gang of both the main tuning cap (two wires, one terminal) and the bandspread cap (3 wires, two terminals). I found the solder in this BA to be _very_ high melting point, so you will need a hot iron.
3. Turn the receiver over and rest it on its top with the rear apron facing you. All descriptions of position will be with respect to this orientation.
4. Go to your chiropracter for an adjustment.
5. Use a 1/4" nutdriver to remove all of the screws on the upper edges of the RF side shields (these screws hold the internal shields in place).
6. Remove the two screws holding the back of the bandswitch to the rear panel.
7. Turn the bandswitch to band # 2 (this is for access in the next step). Remove the bandswitch knob. If the shaft has any burrs from the knob setscrews, use a fine file to smooth them down.
8. Loosen the setscrews on the dial indicator drum (this is the brass cylinder that is on the bandswitch shaft, located in the gearbox).
9. Gently pull the bandswitch shaft all the way out. DO NOT lose the little tension spring (looks like a bent washer) that is between the dial indicator drum and the front panel of the gearbox. Let the indicator drum hang on its string.
10. Loosen the setscrews on the antenna tuning shaft (at the coupler to the cap) and pull the plastic tuning shaft all the way out.
11. Find a GOOD Philips screwdriver, not a Chinese knock-off. Torn-up Philips screws are impossible to remove, and the ones you have to hit next are in impossible places.
12. Remove the four Philips screws that attach each side-shield to the chassis. They are directly below the upper screws that you removed in step 5. This is easy on the right side. To remove the ones on the left side, you must unclip the filter cap attached to that shield and unscrew (with a 3/8" nutdriver) the attachments of the shielded pair that are grounded through the two inner mounting lugs of the power transformer. By lifting the shielded pair (which is covered by some kind of brownish tubing) you can (barely) get to the Philips screw heads. Also, remove the grounding lug for the filament circuit from the left shield.
13. Desolder the wire going through the right shield to the rightmost coil in the rearmost compartment.
14. Tag and desolder the pair of wires that go to a switch wafer and run through the top middle of the divider ("front") for the rearmost compartment.
15. Unscrew the nuts holding the antenna input posts, thereby freeing the antenna leads internally.
16. Remove the screw and nut holding the braid connection that runs to the rear chassis wall.
17. Remove the Philips screws holding the dividers down. There are four for each divider. A long grabber (hemostat) is essential to get these suckers out.
18. Gently pry the sides of the RF compartment apart. Then, pull the rearmost divider toward the rear to unhook it from underneath the baseplate in the next forward compartment.
19. At this point the rearmost divider can be swiveled up to the left. There are some more wires holding it in on the left side, but they do not have to be removed to get reasonable access to the tube socket. This also affords good access to the next compartment.
20. Replace all the caps and most of the Rs while you're there. I found that all three caps in the rear compartment were marginally leaky (caused the eye on my Heath cap checker to close about halfway) at 150 volts. Note that one of the caps is under a coil and can't even be seen from above. I also found about half of the Rs to be out of spec (e.g., 135k for 100k).
21. Reassemble, then go find a cold 807.

This whole process takes about 4-5 hours if done carefully.

Good luck! A. B. Bonds

BACK TO "JIM'S RADIO ROOM" MAIN PAGE

Other Related Sites

• Hallicrafters Inc., Skyrider Diversity model DD-1
• Roger Radio Communication Electronics
  Repairs and Restoration
• Want to join a mailint list like Hallicrafers or Boatanchors? Well, Brent Chapman's "Majordomo" mailing list manager at QTH.NET makes it easy.

Vacuum Tube Links

• Vacuum Tube Valley
• Electron Valve
• Audio Amateur Corporation
• Firebottle Audio