Heathkit IG-112 FM-Stereo Generator
A hundred-dollar kit that let a serious hobbyist align a multiplex tuner without a five-hundred-dollar instrument. A magazine found it measured up against gear costing ten times as much.
Overview
When Audio introduced this generator to readers, it started with the price problem. FM stereo generators were expensive, with several of the lower-priced ones quoted at $500, and few service agencies even owned one. Heathkit's answer was a kit that Audio (February 1966, p.50) priced at $100. That put stereo-multiplex alignment within reach of the buff who took FM seriously but couldn't justify lab-grade money.
The instrument is three generators sharing one compact chassis. Per Audio (February 1966, p.50), it contains a multi-frequency audio generator, a multiplex signal generator, and a 100-mHz FM signal generator. The FM output centers at 100 mHz with an adjustment for ±2 mHz. Deviation is continuously adjustable to a maximum of 75 kHz at a 60 Hz sweep rate. For scope work, sweep width is adjustable to 750 kHz. The magazine called the output "moderately high," enough in its words for the most insensitive tuner, and three step-switches each give the FM signal 20 dB of attenuation.
Audio modulation is switch-selectable at 400, 1000, or 5000 Hz. The user can also pick 19 kHz, 38 kHz, and SCA (65 or 67 kHz). A composite-signal output offers left- or right-channel selection, in or out of phase, and any of the available audio frequencies can be taken directly. For a complete tuner alignment, the IG-112 also supplies crystal-generated FM markers at 10.7 mHz, 90.95 mHz, 96.3 mHz, 101.65 mHz, and 107 mHz, built around a 5.35-mHz crystal. Both the 19-kHz and 5.35-mHz oscillators are Pierce crystal-controlled circuits. Audio (February 1966, p.50) singled them out for stability and accuracy at those critical points.
The front panel carries five knobs. A central function switch sets the mode among R.F. Sweep/I.F. Marker, Audio and Mono FM, Phase Test (left plus right in phase), Left Channel, and Right Channel. To its left is the 19-kHz pilot-level control, which also holds the power switch. To its right is a multi-purpose control labeled Deviation - Sweep Width - Composite Level - Audio Level, and what it does depends on the selected mode. Along the lower edge are the frequency selector, the composite-signal/audio-output connector, the I.F. marker output connector, the three R.F. attenuator slide switches, and the center R.F. frequency control. The only rear-panel operating control is a slide switch selecting the 65 or 67 kHz SCA signal.
Construction is pure Heathkit paper-bag: all resistors in one bag, all capacitors in another, sorted before assembly. Audio (February 1966, p.50) reported good-quality parts throughout and no stinting on hardware, with leftovers in every screw size. It put the build at a bit over ten hours. The manual drew praise as "magnificent," with excellent, detailed illustrations. No addendum shipped, so the reviewer expected errors, and found two. One was a page-23 instruction to thread a wire through a center post on a ceramic socket that has no center post. The other was alignment instructions referring to an L0 that turned out to be screened Ls on the chassis. Alignment itself takes another hour or more and requires both a scope and an a.c. VTVM.
Audio (February 1966, p.50) used the finished unit to check a tuner that had just been gone over with equipment listing at ten times the cost, and got the same results. The scope traces were as good a signal as one could want, at a fraction of the price.
One caution for anyone planning to use the IG-112 to service multiplex decoders: a later service note on vintage stereo gear observed that the IG-112 doesn't have enough separation to align a multiplex section for best operation. It remains a fine general FM alignment tool.
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