Heathkit SB-101 Rebuild and Restoration Notes

Luckily, there are no completely white wires in the harness, so I was able to deduce which wires were supposed to be yellow.  The blue and gray wires were beginning to look somewhat alike too – the blue ones had all turned dark gray and the gray ones were still gray but a lighter shade.  All of the wires with color stripes were easy to distinguish since the color stripes themselves had not faded at all.  The harness wires went back into place without much trouble. 

Having been careful about unsoldering the harness wires during removal, there were very few with ends I needed to strip, but many wire ends needed a bit of cleaning to ensure that the solder would wet them.  I used very fine finishing sandpaper, pinching the wire end between two sides of a small folded piece, with two fingers applying pressure and a rotating motion axially around the wire end.

I only had one case in which a PC board foil pad came loose.  It was a small island used only for a binding spot between two wires, connecting to nothing else on the board, and thus there were no board-mounted components to hold it in place.  I was able to keep it in place by feeding its connecting wires through to the component side and bending them over to keep it from pulling loose.  With two wires thus inserted it was stable.  If I had it to do over again I’d probably put a dab of super glue under the pad too.

When it came time to put discrete, point-to-point components back in, I discovered that it was relatively easy to figure out which components of the same value went where, judging by the length of their leads and sometimes how they were bent.  I had not disassembled the capacitor-resistor combinations used for bypass on the final amplifier tube sockets, so they went back in as assembled units.

In the process of reassembling the front panel I became an expert on the SB-series dial mechanism out of necessity.  Although the assembly instructions are detailed, mostly clear and well written, the resulting assembly won’t necessarily work very well if you start with a non-working mechanism as I did.  To figure out which pieces were out of spec, I had to understand how it worked.  Damaged or dirty parts can also cause problems.  Mine were very grimy so I started out by cleaning them.  Even the “new” kHz dial I had bought needed cleaning.

The mechanism is surprisingly simple.  The main kHz dial is mounted directly on the LMO shaft, which can make five complete revolutions covering 500 kHz.  There is a spiral track on the back of this dial that progresses radially outwards as the dial rotates clockwise (as viewed from the front).  In this spiral track rides a nylon pin, like a turntable needle riding in a vinyl record groove, but moving outwards from the center rather than inwards as the dial rotates from 0 towards 100.  The pin, in turn, causes a lever to which it’s attached to move outwards and towards the right as the dial rotates, rather like the tonearm of a turntable (but, again, backwards).  This lever, in turn, causes the slide rule dial pointer at the top to move from left to right and mark off 100s of kHz, one for each complete rotation of the kHz dial.  That mechanism alone completely maps one-kHz calibration onto the LMO shaft position.  However, 100kHz per revolution is too fast a tuning rate for normal operation so it must be geared down. 

Instead of using a set of gears to slow down the tuning rate at the knob, Heathkit designed a much simpler arrangement, based to a large degree on the Collins S/Line dial mechanism (see my other article that discusses the comparison).  The knob shaft, at its opposite end inside the front panel, has a pair of thin spring-metal disks separated by about 1/8 inch on the shaft.  This pair of disks straddles the inner edge of a metal ring mounted on the front of the main dial.  Since they touch the inner edge of the ring, the dial turns in the same direction as the knob (whereas if they were touching the outer edge, the two would turn in opposite directions).  Even though there are no gear teeth involved, and the pair of metal disks merely touches the dial ring, it’s enough to smoothly rotate the dial.  The knob shaft is secured to the front panel with a brass bushing.  The pressure the metal disks exert on the dial ring is adjusted by moving this bushing up or down very slightly before tightening it in place.  Moving it downward makes the tuning stiffer.  If you move it down too much, the pair of metal disks bend apart and cause extra wear on the whole assembly.  If you move it up too much, they lose contact with the dial and begin to slip. 

The kHz dial hairline, or fiducial mark, is engraved in a clear plastic window attached to the center of the main dial that rotates around its bushing which mounts on the LMO shaft.  It’s held in place with a snap ring so it can rotate freely.  When the assembly is mounted to the LMO, the hairline window is held in position by an assembly similar to the one for the main tuning knob.  A pair of smaller metal disks is positioned to contact the curved upper edge of the hairline window and is attached to a small shaft that extends through the escutcheon to a small metal knob.  When you turn this knob, it causes the hairline window to rotate, moving its position relative to the kHz dial.  The trick to making this work right is to tighten the knob’s setscrew on the shaft such that it tends to pull the hairline window away from the main dial.  That way it doesn’t rub up against the dial as it turns.

Now that you know how it works, I can describe what was wrong with mine (aside from being dirty and cracked).  I imagine these problems are common in all SB-series units.  All of the problems with the main dial itself arose from misplacement of the various parts and were solved by closely following the assembly instructions.  But sometime in the past, the metal disk at the end of the hairline adjustment shaft had become disconnected and someone had tried to solder it back on.  But they had not properly pushed it down onto the shaft until it hit the “shoulder” that keeps it spaced properly from the other disk.  The spacing was so large that the disks didn’t grip the hairline window tight enough to keep it from rotating on its own and so it would not stay where you set it.  A simple fix was to push it down on the shaft with a hot soldering iron tip.  It took a couple of tries to get it fixed parallel to the other disk.

Finally, I arrived at the bottom of page 78 of the 151 page manual and was greeted by the words “This completes the wiring of the Transceiver.”  Cool!  The assembly was not quite done yet but the long process of disassembly and reassembly was almost complete.

The resistance checks uncovered a mistake.  The bandswitch boards must be assembled with their rotors oriented properly.  Their center holes, through which the shaft passes, are not round – each one has two “flats” that match the cross section of the shaft and a little notch on one of the two flats.  Thus they can be oriented in two ways, with the notch either up or down.  Three of the four boards go in with the notch up and one goes in with it down.  I had not read the instructions carefully enough and put them all in pointing up, even though the boards have the proper orientation printed right next to the switch wafer.  It was easy to correct.

Next step was to do the initial power-up checks.  I immediately knew something else was amiss since upon turning on the power neither the pilot lamps nor the tubes lit up.  Also, after just a few seconds of power, the power supply cable was a bit warm.  The resistance checks had failed to turn up a short in the filament supply wiring.  Since the filament wires go all over the place, it takes some time to locate such a fault and since the schematic only indicates connectivity, not the routing of actual wires, it’s of limited help.  The 12.6 V filament supply is split into two tiers of 6.3 V each.  Some filaments are connected from tier 1 to tier 2, some from tier 2 to ground, and the 12.6 V ones go across the whole thing.  Resistance checks indicated that the top tier – the brown-white wires – was shorted to ground, whereas the second tier – the brown wires – was just fine.  The next step was to unsolder the brown-white wires one at a time to try to isolate the short.  This one was caused by one of the wires in the harness rubbing too tightly against one of the PC board mounting screws and shorting to it.  Again – it was a simple fix.

Next try, everything lit up fine.  In fact, all the basic supply voltages seemed fine.  I even could hear a hiss from the audio section, and a faint signal from the calibrator.  RF and AF gains, mode switch, even preselector, all operated as expected.  This meant that most sections of the receiver were confirmed as operating.  However, I noticed two new problems before even following more check-out instructions.

The LMO was unstable.  I traced this to the output coax lead which made the frequency jump around wildly when I moved it.  This was simply a contact problem  I cleaned the phono socket collar and the ground shell of the plug with Deoxit and everything was fine.

Next problem: the relays did not energize – neither the PTT line nor the Tune position of the mode switch caused them to operate.  I traced the problem to the grey wire power lead going to RL1, which had become disconnected.

Now for alignment.  Following the procedure in the manual went fairly smoothly – everything peaked up close to where it was originally set.  But I noticed two problems.  First, the calibrator signal fell off in strength dramatically on bands above 7 MHz.  By the time I got to the 29.5 band it was barely moving the s-meter.  To check whether this was a receiver problem or a calibrator problem, I used a signal generator at a fixed level to check sensitivity on all the bands.  This confirmed that I was just getting an ever weaker signal out of the calibrator.  I’ve not yet tried to figure out why, and will have to go back to it later.

Getting the calibrator set correctly is important because if it’s not, the VFO will not line up well from band to band.  Get it right and you can change bands without having to move the main dial pointer very much at all.  In fact, I’m very impressed with the SB-101 LMO.  It’s very linear – calibration hardly changes from one end of the band to the other – and after warm up it’s surprisingly stable.  Heathkit did a good job of design and the manufacturer, TRW for this particular unit, did a great job of picking the right temperature compensating or tracking components.

There was only one hitch in aligning the transmitter.  I found that while the manual instructs that you should get full scale grid current after tuning up, on some bands it was significantly lower – even though the output power was normal.  I discovered that the position of the driver neutralizing wire had a big effect on this.  By changing its position around I was able to get equally reasonable levels of grid current on all bands.  By the way, this wasn’t an issue on the HW-100 since there’s no built-in way to measure grid current on that rig.

To finish things off, I installed new feet, ordered on the Internet.  I also cleaned, then oversprayed the cabinet with a matching SB color paint I bought on eBay.  Heathkit used various shades over the run of the SB-series, so it was a close match (since there was no specific target to match).

It was a fun project that brought back memories of building Heathkits years ago.  Below is a picture of my "new" SB-101.