Heathkit Mohican GC-1A Shortwave Communications Receiver Restore

My dad was an avid Heathkit builder. He built this Mohican Shortwave Receiver in 1965, several years before he married my mom and I was born. My father only had a basic knowledge of electronics which was enough to successfully build this receiver. Unfortunately, he did not have access to an RF Generator required for the proper alignment of the RF (Radio Frequency) and IF (Intermediate Frequency) stages of this receiver, so reception was marginal at best.

My dad's Heathkit Mohican Receiver before restoration work began.

Still, he proudly displayed his Heathkit Mohican Receiver on his dresser in my parents bedroom. This receiver was perched on my dad's dresser from the time of my early childhood until his death several years ago, when it came into my possession.
The last time I remember the Heathkit Mohican Receiver worked was when Ronald Reagan was still president!
As a tribute to my father, I spent a month restoring his Heathkit Mohican GC-1A Receiver to a condition close to when it was originally built in 1965.

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​Heathkit Mohican Manual Download

Heathkit Mohican GC-1A Communications Receiver Manual Download

​You will need a copy of the original "Assembly and Operation of the Heathkit Mohican Communications Receiver" manual in order to do a proper restoration. Click on the above link to download.

​****Polarity Alert****

The Heathkit Mohican receiver requires 12 Volts DC and draws a maximum of 35 milliamps. Power is provided by either eight D batteries or the optional XP-2 AC Power Supply.
Due to the extensive use of PNP transistors in the Mohican Receiver. Heathkit designers chose to make the chassis positive ground. This means if you choose to power the unit with an external power supply the metal chassis should be at 12 Volts DC positive potential. A good place to connect the 12 Volts DC negative potential is to the Muting terminals at the back of the chassis, which should be jumper-ed during normal operation.

The Muting terminals where you connect negative 12 Volts DC is circled in the picture.

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XP-2 AC Power Supply Recap

"Recap" is the replacement of old electrolytic capacitors in vintage equipment. Electrolytic capacitors have a finite lifetime. As they age, the electrolyte dries up causing their capacitance tends to fall in value. This is most evident in power supply filter capacitors. You will hear "AC hum" from the speaker of the equipment as the filter capacitor's capacitance drops in value.

XP-2 AC Power Supply with Manual

The XP-2 AC Power Supply is installed in the back of the Mohican Receiver. It is held in place by two snap fittings. Use the pull ring and pull it straight back. Disconnect the internal power supply cable. Remove the rectangular aluminum cover.

The old nasty electrolytic capacitors.

The replacement electrolytic capacitors installed. Notice they are much smaller in physical size than the originals.

The nasty old yellowed original AC cord.

The new replacement AC cord installed.

Look at the cool way I attached the .1Mfd capacitors between the Input and Ground and Output and Ground pins. These capacitors will prevent the 7812 from oscillating during high loads.

​After the electrolytic capacitor replacement, the output voltage shot up to 15 Volts DC. I installed a 7812 Voltage Regulator to keep the Voltage at around 12 Volts. Do NOT secure the regulator tab to the metal base. Remember, positive ground.

Below is a schematic containing my modification to the XP-2 Power Supply.

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​Disassembly

Time to remove the metal cabinet from the chassis so that you can get at the electronics!

Remove the antenna by unscrewing and pulling up and out.

Remove the two screws from the back for the receiver on the lower right-hand side.

Remove the two screws from the back for the receiver on the lower left-hand side then slide the chassis forward until you can access the speaker.

Remove the four nuts that hold the internal speaker to the top of the cabinet. The chassis is now free and clear of the cabinet.

Bottom view of the chassis.

​Cleaning and Lubrication

The Mohican Receiver's chassis has not seen the light of day since the 1980s when my dad tried to re-align the RF and IF amplifier sections. It had many layers of dust and dirt inside.

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Most radio restorations require the use of contact cleaner to remove years of oxidation from switch contacts. 

Look at all the dust and dirt on the IF Board!

The rest of the chassis had a layer of dust and dirt on it as well.

I used an air compressor to blow all of the dust and dirt off of the chassis. Damp Q-Tips helped clean the less accessible areas of the chassis and IF Board.

I used electrical contact cleaner to clean all the potentiometers connected to the front panel. I would spray the cleaner into the small cutout by the terminals then work the potentiometer back and forth through its full motion.

The dial lights did not work even though my ohmmeter indicated that the bulb filaments were still intact.

Corrosion in the dial light bulb sockets prevented the light bulbs from making good contact.

The dial lights worked once again after I cleaned the bulb sockets with a Q-Tip dipped in denatured alcohol.

I used Labelle 107 Oil to lubricate all rotating parts including the shafts of potentiometers and variable capacitors. This oil will not harm plastic parts.

I used Labelle 106 Grease to lubricate the gearing connected to the Main Tuning and Band Spread capacitors. This grease will not harm plastic parts.

I used a touch of Super Glue to secure the Main tuning dial pointer to the dial string. The pointer was loose and would not track properly along the dial.

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I use Labelle oil and grease for my radio restoration projects. I already have these products around as I am also a model railroad enthusiast. These products will not harm plastic.

Recap

As mentioned, "recap" is the replacement of old electrolytic capacitors in vintage equipment. Electrolytic capacitors have a finite lifetime. As they age the electrolyte dries up causing their capacitance to fall in value. This section describes the replacement of electrolytic capacitors in the Mohican receiver.

C56, C57, and C58 are all housed in the same case. Each has a value of 100Mfd with 15Volt Max Rating.

Triple capacitors are hard to find these days. I replaced C56, C57, and C58 with three discrete electrolytic capacitors of the same rating.

The next capacitor to be replaced was C59. It has a value of 50Mfd and a 15Volt Max Rating.

I replaced C59 with a 47Mfd electrolytic capacitor with a 15Volt Max Rating. Most capacitors have capacitance tolerance of 20%, so this value is close enough.

C55 is a wax covered capacitor rated 150Mfd at 15Volt Max Voltage Rating.

I replaced C55 with a 100Mfd and a 47Mfd electrolytic capacitors in parallel, which combines for a total of 147Mfd capacitance. This is close enough to the original value.

C27 is a 10Mfd capacitor with a 10Volt Max Rating.

C27 was replaced with a new electrolytic capacitor of spec similar to the original one.

C54 is located on the underside of the printed circuit board and is connected to a terminal strip. It has a value of 50Mfd with a 15Volt Max Rating.

I replaced C54 with a 47Mfd electrolytic capacitor with the same voltage rating.

​Volume Control Repair

For some reason the 5K Potentiometer used as the volume control suddenly froze. I could not turn it.

Access to the volume potentiometer can be achieved from the underside of the chassis. Remove the knob then the nut that attaches it to the front panel. It takes some finessing to get it out so that you can unsolder it.

Turns out the spring that contacts the resistive material somehow got folded over causing the extreme increase in turning resistance. Once repaired, I did a continuity check to make sure the potentiometer was functioning properly.

The process of soldering all of the wires back on the volume control potentiometer was a daunting task as the wires are very short. With some patience, you will be able to solder all wires to the proper terminals of the potentiometer.

​Receiver Alignment

It is important to go through the Receiver Alignment procedures in the Heath Kit Assembly manual in order to get peak performance out of your Mohican Receiver.
I would definitely recommend going through the Receiver Alignment procedures if you were not the original builder of the Mohican Receiver as kit builders range in all skill levels. A lot of kit builders may have never went through the Receiver Alignment procedures, as they may not have had access to an rf Generator, which is required for the alignment process.
I would recommend going through the Receiver Alignment process even if you were the builder of the Mohican Receiver. It is possible that the receiver could have gotten out of alignment after 50 years as components slightly change value due to age.
​

​IF Amplifier Alignment

One of the alignment steps is to adjust the slugs in the Input and Output IF (Intermediate Frequency) Transformers for maximum unmodulated audio output. This will require an RF Generator, VOM (Volt-Ohm-Meter), and TV Alignment tools to accomplish the task. While the Mohican Receiver is on and adjusted for maximum volume, you inject a 455KHz signal, modulated 30% by superimposed audio signal, into the Input IF Transformer and adjust first the slugs of the Input IF Transformer then the slugs of the Output IF Transformer for maximum AC voltage. See page 61 of the "Assembly and Operation Heathkit Mohican Communications Receiver" for details.

IF Transformers are circled in the picture. The right one in the picture is the Input IF Transformer and the left one is the Output IF Transformer.

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Believe it or not, you can still find TV Alignment tools to purchase. These tools are rarely needed these days with Digital TV receivers and SDR (Software Defined Radio) but were once a must for any serious electronic technician.

Dial Pointer Adjustment

It is important to properly adjust the pointers for both the Main and Bandspread tuning dials. For Main tuning dial calibration, you move the pointer as far to the right as it will go and make sure the Main tuning capacitor's plates are fully open. If not, you loosen the set screw on the gear driving of the Main tuning capacitor and adjust to fully open then tighten the set screw.
For the Bandspread dial calibration, you move the pointer to the vertical line on the left side of the dial, loosen the set screw on the gear driving the Bandspread capacitor, and adjust the capacitor's plates until fully meshed. Then you move the Bandspread pointer to the Set mark on the right side of the Bandspread dial and adjust the rotational stop at the back for the Bandspread capacitor so that you can't rotate the dial any farther to the right of the Set mark.
See page 62 of the "Assembly and Operation Heathkit Mohican Communications Receiver" for details.

The set screw for the gear connected to Bandspread Capacitor is circled in the picture.
​

Band Alignment

The EICO Model 324 RF Generator I used for this Band Alignment procedure.

Band Alignment is the process of adjusting the tuned circuits in the RF, Mixer, and Oscillator sections so that the dial readings for a given band are accurate. When you turn the Main dial to 1200Khz you should be receiving stations at that frequency, right? The Band Alignment procedure requires an RF Generator, VOM (Volt-Ohm-Meter), and TV Alignment Tools at the minimum. I question the accuracy of the analog dial of my old EICO Model 324 RF Generator. To ensure accuracy, I tune my Yaesu FRG-7700 receiver to the desired frequency, then turn the VFO knob on the EICO RF Generator until I hear the unmodulated audio signal on the Yaesu receiver coming from the RF Generator.
See page 63 of the "Assembly and Operation Heathkit Mohican Communications Receiver" for details about the Band Alignment process.

Band Alignment - Initial Setup

​My alignment process varies slightly from the manual as I use my Yaesu FRG-7700 Receiver to check the accuracy of the RF Generator output and use it to check the Mohican Receiver's oscillator frequency.

Setup and ready to do the Band Alignment!

1. Connect the RF Outputs of the RF Generator to the Antenna and Ground connections on the back of the Mohican Receiver.
2. Connect a VOM to the connection terminals on the back of the Mohican's speaker. Set the meter for the 2 Volt AC Range.
3. Turn on RF Generator and let warm up so that its frequency output stabilizes.
4. Arrange the Mohican Receiver so that you have access to the top of the chassis, where the RF, Oscillator, and Mixer trimmer capacitors and transformer slugs can be adjusted.
5. Power on the Mohican Receiver and set the front controls as follows:

• Volume: Midpoint
• AVC Switch: On
• ANL Switch: Off
• RF Gain: Fully Clockwise
• Bandspead: Pointer on "Set" Position on dial
• Bandswitch: Set to the desired band

6. Power on the Yaesu FRG-7700 (or any Shortwave) receiver, use a test lead as an antenna and set it in close proximity to the Mohican Receiver.

​Band Alignment - Process

​The alignment process is pretty much the same for each band. The steps are as follows:

The trimmer capacitor screws and coil slug screws for each band are clearly marked on the top of the metal chassis.

1. Set Band Switch to the desired position.
2. Set the RF Generator to an RF Frequency (with a 30% modulated Audio Frequency) that is at the high end (right side) of the Mohican Reciever's dial. Verify the RF Frequency using a second receiver that can receive the AM signal at that frequency.
3. Set the Mohican Receiver's Main tuning dial to the same frequency as the RF Generator output.
4. Adjust the oscillator trimmer capacitor screw for the specific band until you hear the demodulated audio signal from the RF Generator coming from the Mohican's speaker. Set screw for maximum AC Voltage on the VOM connected to the speaker terminals.
5. On A, B, C, and D bands, the Mohican uses high-side injection. If set correctly in step 4, you should hear the Mohican's oscillator carrier signal 455Khz above the frequency that is set on Main tuning dial if you set the antenna of a second AM receiver close to the chassis. On the E band the Mohican uses low-side injection and you will hear the Mohican's oscillator signal 455Khz below the frequency set on the Main tuning dial.
6. Adjust the mixer trimmer capacitor screw for maximum loudness of the demodulated audio signal. Set for maximum AC Voltage on the VOM connected to the speaker terminals.
7. Adjust the RF trimmer capacitor screw for maximum loudness of the demodulated audio signal. Set for maximum AC Voltage on the VOM connected to the speaker terminals.
8. Set the RF Generator to an RF Frequency (with a 30% modulated Audio Frequency) that is at the low end (left side) of the Mohican Reciever's dial. Verify the RF Frequency using a second receiver that can receive the AM signal at that frequency.
9. Adjust the oscillator coil slug screw for the specific band until you hear the demodulated audio signal from Mohican's speaker. Set for maximum AC Voltage on the VOM connected to the speaker terminals.
10. Adjust the mixer coil slug screw for maximum loudness of the demodulated audio signal. Set for maximum AC Voltage on the VOM connected to the speaker terminals.
11. Adjust the RF coil slug screw for maximum loudness of the demodulated audio signal. Set for maximum AC Voltage on the VOM connected to the speaker terminals.
12. There is some interaction between the trimmer capacitors and the coil slugs. Repeat steps 2 through 11 until the modulated RF signals are correctly aligned on the dial.
13. Repeats steps 1 through 12 for Band Switch settings A,B,C,D and E.

​Testing

Time to see the fruits of my "Band Alignment" labor and connect the Mohican receiver to my G5RV external antenna. I was amazed at all the stations I was picking up of each band!

​Cabinet Restoration

Years of tobacco smoke have taken their toll on the Mohican's enclosure. You can't just wipe the tar off, it is embedded in the paint.

Even the aluminum carry handle has a brown hue due to tobacco tar.

Be very careful what you use to clean the Mohican's enclosure! I used a mild polish that was no more abrasive than toothepaste and it left these strange watermarks. Still the watermarks beat the brown tar coating that originally coated the enclosure.

Another picture of the strange watermarks that appeared after I lightly polished the enclosure.

I used motorcycle chrome polish and an old toothbrush to clean the Mohican receiver's knobs, antenna, and handle.

Look how pretty the knobs and antenna look after polishing!

​Mohican Receiver Assembly

Just follow the Mohican Receiver Disassembly process, in reverse order, and you will have it back together in no time flat!

​My dad's newly restored Mohican Receiver in action!

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