Heathkit SW-717 - User Manual

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Heath of the Month #92 - SW-717 Communications Receiver

Heathkit of the Month #92:

by Bob Eckweiler, AF6C

AMATEUR RADIO - SWL

Heathkit SW-717

General Coverage Communications Receiver.

Introduction:

Over its lifetime Heathkit Company manu-

factured 17 general coverage (GC) communi-

cations receivers. To be considered in this

arbitrary category the receiver must cover

the AM broadcast (BC) band and a signifi-

cant portion of the frequencies above it in the

HF range that ends at 30 MHz. The coverage

should be mostly continuous, though gaps for

technical reasons are acceptable

. Additional

coverage, such as the LW band below the BC

band or frequencies above 30 MHz, is al-

lowed. Some of the included Heathkit GC re-

ceivers only cover to 10 MHz and others only

to 20 MHz. Superheterodyne receivers make

up 14 of the 17, the other three being super-

regenerative receivers. Ham-band only and

even band-oriented SWL receivers are not

considered general coverage, so you won’t see

the SB receivers on the list.

Evidently, general coverage kit receivers

were a profitable product; they seemed to be

a popular beginner’s kit as well as a moder-

ate kit for the more experienced kit builder.

Heath focused on having one or two receivers

on the market for the beginner, usually at

very reasonable prices for modest perfor-

mance. They also manufactured some higher

performance receivers as well as portable re-

ceivers that could run off batteries. Features

varied with cost; crystal filters, dual conver-

sion, S-meter, an RF amplifier stage and a

tracking bandspread control are often miss-

ing on the low-end radios.

The first receivers were the super-regenera-

tive K-1 and K-2 (HotM #s 80 & 81). They were

followed by the superheterodyne AR-1

through AR-3. In 1960 Heath released their

first transistorized GC receiver, the Mohican

GC-1, later updated to the GC-1A (HotM #34).

As part of Heathkit’s early educational series

they produced the EK-2A and EK-2B “Basic

Radio Educational Series” in two parts. In

part one the student builds various circuits

including a crystal receiver and later a re-

generative receiver. However, in part two the

receiver is modified and expanded into a six-

tube general coverage superheterodyne re-

ceiver that covers the broadcast band and 3

to 10 MHz; passing the GC requirements.

Around 1961 Heath started the GR receiver

line which includes general coverage short-

wave radios along with many other general

radios including clock radios and AM/FM

portables. Qualifying GC receivers from this

group are (in chronological order) the GR-81,

GR-91, GR-64, GR-54, GR-43[A] and GR-78

(HotM #62). The solid-state GR-43 and its ‘A’

update are clones of the Zenith Transoceanic

Page ! of !1 9

Here is a link to the index of Heathkit of the

Month (HotM) articles:

http://www.w6ze.org/Heathkit/Heathkit_Index.html

Figure 1: SW-717 General Coverage Receiver!

Note substitute knob to left.

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Summary

Heathkit SW-717 Communications Receiver

Introduction to General Coverage Receivers

Background on Heathkit's general coverage receivers and the SW-717's context.

SW-717 Overview and Specifications

Physical description, dimensions, price history, and key specifications of the SW-717.

SW-717 Circuitry and Block Diagram

Explanation of the receiver's internal circuit design and a block diagram overview.

Detailed Circuit Component Analysis

Analysis of the front end, IF stages, BFO, AGC, detector, audio amplifier, and power supply circuits.

Building and Testing the SW-717 Kit

Steps and considerations for assembling the SW-717 kit, including final testing procedures.

SW-717 Common Problems and Summary

Discussion of typical issues encountered with the SW-717 and a concluding summary of its usability.

Summary

Heathkit SW-717 Communications Receiver

Introduction to General Coverage Receivers

Background on Heathkit's general coverage receivers and the SW-717's context.

SW-717 Overview and Specifications

Physical description, dimensions, price history, and key specifications of the SW-717.

SW-717 Circuitry and Block Diagram

Explanation of the receiver's internal circuit design and a block diagram overview.

Detailed Circuit Component Analysis

Analysis of the front end, IF stages, BFO, AGC, detector, audio amplifier, and power supply circuits.

Building and Testing the SW-717 Kit

Steps and considerations for assembling the SW-717 kit, including final testing procedures.

SW-717 Common Problems and Summary

Discussion of typical issues encountered with the SW-717 and a concluding summary of its usability.

The Heathkit SW-717 is a general coverage communications receiver designed for shortwave listening. Advertised as a "Three Evening Kit," it was intended for beginners to assemble. The receiver covers the broadcast band and up to 30 MHz in four bands, allowing users to tune in to foreign broadcasts, weather reports, ships at sea, amateur and CB bands, and local AM entertainment.

Function Description:

The SW-717 is a superheterodyne receiver, a common design for its time. It features a built-in speaker and a front panel jack for headphones or an external speaker. The receiver operates on either 120 or 240 VAC (60/50 Hz) and draws 8 watts of power. Its primary function is to receive radio signals across a wide frequency range, with features like bandspread tuning for separating adjacent stations and a variable BFO (Beat Frequency Oscillator) for clear code reception. An automatic noise limiter (ANL) can be switched on or off to clip positive noise peaks, improving audio clarity in noisy conditions. The S-meter indicates relative signal strength.

Important Technical Specifications:

Frequency Coverage:
- Band A: 550 kHz - 1500 kHz
- Band B: 1.5 MHz - 4 MHz
- Band C: 4 MHz - 10 MHz
- Band D: 10 MHz - 30 MHz
S-Meter: Indicates relative signal strength.
Headphone Jack: Low impedance headphones or external speaker.
Loudspeaker: Built-in.
Controls: VOLUME (with on/off switch), MODE (AM, Standby, CW), BFO, MAIN TUNING, BANDSPREAD TUNING, ANL (on/off switch on rear panel).
Transistor Complement:
- 40673 (RCA): Mixer
- MPF105 (Motorola): HF Oscillator
- 2N5308: I-F Amplifier
- 2N5232A (Qty: 2): I-F and AGC Amplifiers
- 2N3393: Audio Preamplifier
- X29A829 (GE): Audio Driver
- MPSU05 (Motorola): Audio Output (NPN)
- MPSU55 (Motorola): Audio Output (PNP)
Power Supply: Transformer operated, full-wave bridge rectifier.
Power Requirements: 120 or 240 VAC, 50/60 Hz, 8 watts (0.75 amp fused).
Dimensions: 14-1/2" W x 5-3/4" H x 10" D.
Net Weight: 7 lbs.

Circuit Details:

The SW-717 lacks an RF amplifier. The input from the antenna is coupled by separate LC circuits for bands 'B' through 'D' directly to the mixer (Q2), an RCA 40673 dual-gate MOSFET. Band 'A' (broadcast band) uses a tuned ferrite rod antenna and is not connected to the external antenna connector. The HF oscillator (Q1) uses a Motorola MPF-105 JFET, with power regulated by a zener diode for stability. The IF operates at 455 KHz. The oscillator operates above the received frequency on bands A, B, and C, and below it on band D. Bandspread tuning only adjusts the oscillator frequency, which can lead to minor loss of sensitivity as input coils are not simultaneously tuned.

The IF signal passes through two 455 KHz ceramic filters in series before being amplified by a 2N5308 Darlington transistor (Q3) and a 2N5232A transistor (Q4). The output from the IF section is fed to the detector stage and the AGC amplifier.

The BFO circuit is unconventional, likely a cost-saving measure. Output from the third IF filter is fed back through a potentiometer to the input of the first ceramic filter. In AM mode, the BFO control is set to zero. For CW reception, advancing the BFO control causes the IF to oscillate at 455 KHz, allowing for tone adjustment by main or bandspread tuning. The BFO can also increase AM signal sensitivity through regeneration, but advancing it too far causes oscillation.

The AGC circuit rectifies and amplifies the IF signal via a 2N5232A transistor (Q5), producing a positive voltage dependent on signal strength. This voltage feeds a milliamp meter for relative signal strength. A pi R-C network filters the AGC output, providing a slow decay AGC voltage fed back to the bases of Q3 and Q4 to reduce gain on strong signals.

The MODE switch has three positions:

AM: AGC is fed back to the IF stages.
STBY: Positive bias is removed from IF stages, biasing them to cut-off.
CW: A fixed positive bias is applied to IF stages, causing them to run at high gain regardless of signal strength, set by R24.

The detector is a simple diode circuit using a 1N191 crystal diode (D1). The recovered audio goes to the VOLUME control and audio amplifier. The ANL diode (D3) clips positive noise peaks.

The audio amplifier consists of four transistors (Q6-Q9). Q6 (NPN) drives Q7, which in turn drives complementary power transistors Q8 (NPN MPSU05) and Q9 (PNP MPSU55). Diode D2, a "Stabistor diode," prevents thermal runaway in the output transistors.

The power supply uses a transformer and a full-wave bridge rectifier. It provides 22 volts to the audio amplifier and 10 volts to the rest of the receiver. Two transistors (D9 and D10) act as zener diodes in the low-voltage supply, providing protection against power line spikes.

Usage Features:

Ease of Assembly: Marketed as a "Three Evening Kit," implying quick and straightforward assembly for beginners. The manual guides users through parts inventory and circuit board stuffing.
Tuning: Features slide-rule tuning with a LOG scale (0-100) and bandspread tuning (5-0-5) for precise station separation.
Mode Selection: AM, Standby, and CW modes are available via a front panel switch.
Portability: Tabletop sized, measuring 14-1/2" W x 5-3/4" H x 10" D, making it relatively compact.
Antenna: Includes a built-in ferrite rod antenna for the broadcast band and connections for an external SWL antenna.
Indicator Lights: Two pilot lamps illuminate the main and bandspread slide rule dials and the S-meter. The left bulb (#53) is brighter than the right (#1813).

Maintenance Features:

Kit Construction: As a kit, it requires assembly, which familiarizes the user with its internal components, potentially simplifying future repairs.
Component Accessibility: Much of the circuitry is on a single circuit board, making it relatively easy to access for inspection or repair.
Documentation: The instruction manual provides necessary information, including how to change the transformer primary wiring for 120/240 VAC operation.
Alignment: Final testing includes resistance and voltage checks, with alignment possible using test instruments (signal generator, VTVM) or without, though instruments yield better results.
Design Considerations: The use of ceramic filters instead of IF transformers simplifies alignment.
Known Issues/Modifications: Some units may exhibit issues like a main tuning knob sticking out too far, leaning tuning coils, or improperly soldered ceramic filters, which may require user intervention to correct. The front panel's thin paint and plastic construction can make disassembly problematic and allow light leakage from pilot lamps. Some builders might modify the AGC circuit or add an additional RC filter to improve audio.