1960 Heathkit Amateur Radio Station Restoration -
the famous North American Indian tribe radios!

Welcome to this exciting radio station.  Take a seat while I tell you about it.

Heathkit model RX-1 'Mohawk' receiver, model TX-1 'Apache' transmitter, model KL-1 'Chippewa' amplifier and model SB-10 single sideband adapter are on the operating table.  On a shelf above them, the model MR-1 'Comanche' mobile receiver, model MT-1 'Cheyenne' CW/AM mobile transmitter, and on the top shelf the model UT-1 600-volt power supply to operate the mobile pair.  I also have the model MP-1 600-volt mobile transistor power supply that operates from 12 volts d-c.  At one time I successfully ran these mobile units in a 1976 Chevrolet Camaro.

A Heathkit monitor scope model HO-10 and Heathkit speaker model AK-5 (extreme left) complete the station.  Still to come:  model DX-60 CW/AM transmitter and model HR-10 receiver.

Mohawk - Apache - Chippewa combination

The Mohawk (52 lbs.), Apache (95 lbs.), and Chippewa (61 lbs. excluding external high voltage power supply) are huge and heavy units in a unique light grey and grey-green two tone paint scheme with die-cast aluminum knobs.  The cabinet sizes are each 19-1/2"W x 11-5/8"H x 16"D.  They were the top of the line amateur radios from Heathkit in 1960.  Like all Heathkits from Benton Harbor, Michigan, they were sold in kit form, and you actually had to assemble and build these radios from hundreds of parts by hand, solder the parts together, wire them, etc. using a detailed assembly instruction book.  When you were finished after a few weeks, and if you did a perfect and neat job of it, you ended up with fantastic radios that you could not buy in any radio store.

Mohawk receiver

The Mohawk is a double conversion receiver that covers the 160, 80, 40, 20, 15, 11 and 10 meter bands and 22-26 MHz for external 6 meter and 2 meter converters.  The frequency readout is a slide rule on a lighted rotating drum dial that displays 1 band at a time.  It features variable selectivity:  5, 3, 2, 1, 0.5 kilocycles and a tuneable notch filter with variable Q at a second IF of 50 kilocycles.  While the receiver does not have IF passband tuning, the second LO does have LSB/USB crystal oscillator switching that can be used on AM and CW to choose the side with the least interference.  Other features are a CW/SSB product detector, instant LSB/USB selection, variable BFO to help zero beat an interfering CW signal, fast AVC to follow rapid fading, AM automatic noise limiter, separate RF and IF gain for best signal to noise ratio, antenna trimmer, and a 100 kilocycle crystal calibrator.

The receiver does drift a little bit during the first hour of warm up, and there is some distortion from the product detector and fast AVC (two resistor changes and a capacitor could fix this), but in my opinion these are not as serious as internet rumours convey.  I thoroughly enjoy this receiver as it is and it is a hot performer!

Apache transmitter

The Apache is a CW, AM and SSB (with external phasing exciter) transmitter that covers the 80, 40, 20, 15 and 10 meter bands.  The VFO frequency readout is a slide rule on a lighted rotating drum dial that displays 1 band at a time.  One crystal may also be selected.  It uses two forced air cooled 6146 tubes in the final amplifier that can be operated in 3 ways:

a)  As a class C amplifier for radiotelegraphy (CW):  750 volts plate voltage @ 240 mA plate current:  180 watts d-c input

b)  As a class C amplifier for radiotelephony (AM):  750 volts plate voltage @ 200 mA plate current:  150 watts d-c input, 100% modulated

c)  As a class AB1 linear amplifier for DSB/SSB:  750 volts plate voltage @ 240 mA plate current:  180 watts PEP d-c input.

For DSB/SSB operation, the Heathkit SB-10 single sideband adapter is required, and connects to the Apache transmitter between the RF driver and the 6146 final amplifier using coaxial cables.

The Apache features low level audio clipping adjustable 0-25 dB with a 3 kilocycle splatter filter, and a differentially keyed VFO.  This latter feature and the fact there is no built in RX antenna relay, allows full break in CW operation with the Mohawk receiver when two separate antennas are used.  The VFO is stable and has a spotting switch so the frequency can be set to the receiver frequency, and the VFO tube filament is always on even when the main power switch of the Apache is off!

If you want "high fidelity" audio, the audio clipper can be disabled and larger audio coupling capacitors can be soldered in the modulator.  The modulation is deliberately limited to 90% by the modulation transformer turns ratio for high level clipping, but there is a modification to disconnect the high voltage from the secondary of the modulation transformer and instead connect the 500 ohm tap point of the secondary to one side of the modulation transformer primary to make an in-phase autotransformer, resulting in positive modulation of 125% to the final amplifier and a lower load resistance for the 6CA7 modulator tubes so they are not so prone to flash-over in the tube bases.  In this case the final amplifier plate current should be limited to 175 mA.  Without these modifications, the Apache will sound 'scratchy'...but then again, this was the state of the art sound in 1960 for AM phone and it really did cut through the noise and QRM for some serious AM DX!  It is a wonderful transmitter and I enjoy it very much.

Chippewa amplifier

The rare and unique Chippewa amplifier covers the 80, 40, 20, 15 and 10 meter bands, uses two 4-400A tubes that are cooled by a powerful centrifugal blower motor (that sounds like you are sitting beside a window air conditioner), requires a separate high voltage plate power supply and can be operated in 3 ways:

a)  As an efficient class C amplifier for radiotelegraphy (CW):  3000-4000 volts plate voltage @ 330-700 mA plate current:  for an incredible 1000-2800 watts d-c input!

b)  As an efficient class C plate modulated amplifier for radiotelephony (AM):  2000 volts plate voltage* @ 500-550 mA plate current:  1000-1100 watts d-c input.
     * The voltage is limited only because of the tank capacitors' ratings of 5 kilovolts, and allows 125% positive modulation.

c)  As a class AB1 linear amplifier (DSB, SSB, AM, CW):  3000-4000 volts plate voltage @ 660-500 mA plate current:  2000 watts PEP d-c input.

The above operating ratings are for continuous commercial service.  This was Heathkit's most powerful amplifier offering.  This is one of the few amplifiers out there that you can actually feel comfortable running high power AM phone for long periods of time without fear of a melt down.

This amplifier is grid driven, and has a choice of resistive or tuned grid input (which must be used to get sufficient drive for class C operation).  Class AB1 using the resistive input requires 60 watts peak of drive, using tuned grid input requires only 10 watts peak of drive for full output!  Class C using the tuned grid input requires 40 watts of drive.  The Apache provides this when in the "TUNE" mode and with zero loading, for CW operation.  For AM linear and DSB/SSB operation the Apache should drive the Chippewa using the resistive input so that the Apache operating conditions are correct.

There is a bias control relay that cuts off plate current during VOX operation in class AB1 mode to avoid having to constantly operate the plate relay.  A clamp tube which automatically cuts off the plate current in class C mode during CW key up and the fact there is no built in RX antenna relay, allows full break in CW operation with the Mohawk receiver when two separate antennas are used.

The Chippewa is wired for delayed high voltage plate relay control during tube warm up.  The high voltage cable uses expensive Amphenol HN connectors and requires RG-8/U or RG-213/U solid dielectric coax to withstand 5000 volts d-c.  One warning:  the 0-1000 mA plate current meter on the front panel has the 4000 volts in it, and the plastic meter cover and zero-adjust screw is not sufficient insulation to curious hands.  Hands off!

I am honoured and very pleased to be a Chippewa owner.  This amplifier is rare because it was sold by Heathkit in 1960 only.  Link to my Chippewa amplifier page: http://radios.2y.net/Chippewa/chippewa.html

SB-10 single sideband adapter

By 1960, the sun spot cycle had peaked, amateur radio was a booming hobby attracting thousands of new enthusiasts, radio club stations in high schools abounded, and double sideband (DSB) and single sideband (SSB) suppressed carrier operation were the state of the art communication modes for voice work.  Heathkit jumped in by offering the SB-10 single sideband adapter.

The SB-10 is a classic phasing exciter that can generate AM, DSB and SSB signals on virtually any frequency directly -- no fixed crystal filter, no heterodyne mixer or crystal oscillators required!  The SB-10 requires a source of RF, and this would usually be from the Apache driver stage.  For each band, the SB-10 switches in 2 capacitors to 2 resistors, arranged to provide a 90 degree phase shifted r-f signal pair (2 wires).  The two r-f signals are each fed into their own DSB balanced modulator, and the output of the two balanced modulators are added together and then amplified to 10 watts PEP output level.  In fact, the output of the SB-10 can be put on the air directly!  Normally however, the output would be fed back to the 6146 final ampliers in the Apache, operating class AB1 to generate 100 watts PEP of sideband power.

The real trick in the SB-10 is the audio section.  It contains its own speech amplifier complete with VOX and anti-VOX circuits to interface with the Mohawk receiver and Apache transmitter to provide voice operated transmit-receive switching.  The output of the speech amplifier goes to a balanced audio phase shift bridge and to two audio amplifiers each with balanced transformer output, to provide a 90 degree phase shifted balanced audio signal pair (4 wires total).  A multi-section rotary switch arranges various combinations of connecting these 4 audio wires correctly phased to the two DSB balanced modulators.  Let's label the 4 audio wires 1, 2, 3 and 4:

To generate USB, wires 2 and 1 are connected to DSB balanced modulator 1, and wires 3 and 4 are connected to DSB balanced modulator 2.
To generate LSB, wires 1 and 2 (180 degree shift) are connected to DSB balanced modulator 1, and wires 3 and 4 remain connected to DSB balanced modulator 2.
To generate DSB, wires 1 and 2 remain connected to DSB balanced modulator 1, and wires 3 and 4 are disconnected from DSB balanced modulator 2.

To add optional carrier, either balanced modulator is unbalanced.  A front panel meter serves as a carrier null indicator and shows relative r-f output.

Simple, elegant, classic phasing method of generating DSB, SSB with or without carrier, and AM.  Although not providing the same carrier suppression and opposite sideband supression, and the carrier null drifts and needs frequent adjustment, the audio frequency response of the phasing method is superior to the filter method of generating SSB.