Heathkit AC-11 Stereo Multiplexer Restore

The Heathkit AC-11 Multiplexer was a kit designed to take the conventional monophonic output of a FM tuner and break it into its left and right channel audio components. Back in the late 1950s, FM tuners were primarily monophonic. It is possible that stereo multiplexers were not built into the FM tuner because at the time several stereo broadcasting formats were being considered by the FCC.

FM Stereo Theory

In the late 1950s, several systems to add stereo to FM radio were considered by the FCC.
The GE and Zenith systems, so similar that they were considered theoretically identical, were formally approved by the FCC in 1961 as the standard stereo FM broadcasting method in the United States.
It is important that stereo broadcasts be compatible with mono receivers. For this reason, the left (L) and right (R) channels are encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker. A stereo receiver or FM Tuner with a Multiplexer will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel.
The (L+R) Main channel signal is transmitted as baseband audio in the range of 30 Hz to 15 kHz. The (L−R) signal is modulated onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal occupying the baseband range of 23 to 53 kHz.
A 19 kHz pilot tone, at exactly half the 38 kHz sub-carrier frequency and with a precise phase relationship to it, is also generated. A 8 to 10 percent modulation level is used by the receiver to regenerate the 38 kHz sub-carrier with the correct phase.
The final multiplex signal from the FM stereo broadcast contain the Main Channel (L+R), the pilot tone, and the sub-channel (L−R). This composite signal, along with any other sub-carriers, modulates the FM transmitter.

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I consult this book often during radio restoration. I grew up in the transistor and diode era and this book taught me a lot about vacuum tube and Selium rectifier technology.

​Heathkit AC-11 Circuit Theory

An FM Receiver, such as the Heathkit AJ-10, recovers the audio and multiplex information from the FM Stereo Broadcast. The multiplex information is used to decode the left and right stereo channels. The audio plus multiplex information is fed into the AC-11 using a shielded cable.
Basically, this input signal to the AC-11 contains the L + R and L - R information, plus the pilot tone. Tube V1A amplifies the signal to compensate for any losses.
Tube V1B lowers the impedance of the signal since it is a cathode follower stage. Coil L3 and capacitor C4 act as a low-pass filter which passes only the L + R signal. This signal can then be adjusted to the proper level by the Separation Control for insertion into the matrix network.
The output of V1A is also applied to the grid of tube V2A, which is another amplifier. From tube V2A, only the L - R signal passes through the bandpass filter, consisting of C5, C6, and L1, to the tuned circuit formed by C16 and L2.
The network consisting of R17, C16, and L2, in conjunction with the bandpass filter, will also attenuate any "storecast" signals which may be transmitted by the radio station. "Storecast" is a commercial music channel, transmitted on the same FM carrier as the stereo program.
The output from V2A is fed also through C8 to a keyed oscillator, which accepts only the pilot tone (19,000 Hz). This tone keys the oscillator so that it produces a 38,000 Hz signal. The output of the oscillator is then coupled through T1 and R18 to the tuned circuit consisting of C16 and L2. The L - R and 38,000 Hz signals are mixed together across this tuned circuit.

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​AC-11 Schematic

Link to Heathkit AC-11 Schematic

​Healhkit AC-11 Restore

The steps below described how I restored my Heathkit AC-11 Multiplexer.

​Step 1 Disassemble AC-11

Remove three sheet metal screws from the back of the AC-11.

Remove the four sheet metal screws from the bottom of the AC-11. Slide the bottom cover towards the back of the unit to remove. Slide the chassis towards the back to remove the top cover.

AC-11 with top and bottom covers removed.

​Step 2 Vacuum Tube Testing/Replacement

Unlike solid state transistors and integrated circuits, vacuum tubes have a finite life span. As they age, fewer electronics are released from the cathode making them less efficient. In addition, the tube filaments burn out much like the filament in a light bulb.

All of the vacuum tubes in the AC-11 Multiplexer were original. I can tell because they are Heathkit branded.
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Test each vacuum tube in the AC-11 Multiplexer. Replace any tubes that are defective or weak.

​Step 3 Electrolytic Capacitor Replacement

As electrolytic capacitors age, their electrolyte dries up causing their electrical capacity to drop and leakage current to increase. It is definitely a good idea to replace 50 year old electrolytic capacitors like the ones in the AC-11 Multiplexer!

​Step 3a - Restoring the Multi-Section Capacitor

Multi-section capacitors, which were aluminum cans containing several discrete capacitors all connected to a common ground, were popular in the 1960s. There were used mostly in the power supply sections of vintage electronic devices. You can purchase replacement multi-section capacitors but they are expensive. I typically rebuild them by replacing their guts with inexpensive discrete capacitors of the same or slightly greater capacitance and working voltage.

C13, C14, and C15, all 20uF 250Volt Capacitors, are all contained within the same multi-section capacitor (underside circled in picture). You must desolder the components connected to it's terminals in order to remove.

Here is a picture of the multi-section capacitor removed.

The inside insulated terminals connect to the positive plates of the internal capacitors. All internal capacitors negative plate share a common ground with are connected to the outside terminals.

I use wire cutters to cut the crimped bottom of the multi-section capacitor. Then I use needle nose pliers to pull the crimped sections away from the base. This allows me to pull the guts of the multi-section capacitor out.

The anatomy of a multi-section capacitor. All three internal capacitors are rolled up into one assembly.

Three replacement 22uF 250Volt electrolyte capacitors will not fit inside the aluminum case of the mulit-section capacitor. I wire one of the capacitors to the base during assembly.

Circled in red is the remaining capacitor that cannot fit inside the aluminum case of the multi-section capacitor.

​The refurbished multi-section capacitor installed back in the chassis. You will never know it was refurbished once the aluminum cover is installed.

I use a Dremel with cutoff tool to remove the area from the base of the multi-section capacitor's aluminum can that I bent with needle-nose pliers. Hot glue holds the aluminum cover in place.

Time to replace C3, a 2uF 150Volt capacitor (circled in red)

​C3 was replace with a 2.2uF 150Volt modern electrolytic capacitor that is much smaller. Make sure you observe the capacitor's polarity when installing.

​Time to replace C12, a 20uF 250Volt electrolytic capacitor (circled in red).

I used a 22uF 250Volt electrolytic capacitor for C12's replacement.

Pictures of the two discrete capacitors replaced.

Step 4 Cleaning and Lubrication 

​It is important to clean mechanical switch contacts and lubricate moving parts. Cleaning the chassis is not required but adds aesthetics to the AC-11 Multiplexer.

I spray contact cleaner into all switches, rheostats, and potentiometers then work the control back and forth to clean the internal contacts.

A toothbrush works great for cleaning up the knobs. Plus the AC-11 Multiplexer will not get cavities ;-)

I use Meguiar's Motorcycle Wax to bring back the luster of the AC-11's case.

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The first step is to check the power supply voltages against the schematic in the manual to make sure they are within spec. I also check the voltages on each pin of the vacuum tubes to make sure they are the same as on the schematic.

Perform the Installation and Operation steps at the end Heathkit AC-11 Multiplexer Manual. This is done with the AC-11 connected to the Multiplex output of an FM Tuner and the right and left outputs connected to the inputs of a stereo amplifier.

​Step 6 Assembly

Follow "Step 1 Disassemble AC-11" section in reverse order to reinstall the top and bottom covers.

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Conclusion

The restoration of this Heathkit AC-11 Multiplexer was most gratifying. This project took my mind off of the stresses of my daily job and reminded me of a simpler time when people had the time to sit around and listen to the stereo for enjoyment.