Heathkit IG-18 (1973–?)
A build-it-yourself sine/square generator that sold for pocket money, covered 10 Hz to 100 kHz, and has spent the last fifty years quietly earning its keep on repair benches.
Overview
The IG-18 is a squat metal box with terminal strips bolted to the outside, a couple of rotary switches across the front, and a meter to show what's coming out. Heathkit sold it as the "Sine-Square Audio Generator," a solid-state kit intended for laboratory use testing, and it turns up in the catalog from 1973 onward. It runs on 117 VAC. It puts out sine and square waves at adjustable voltage across a 1 Hz - 100 kHz range. That's the whole pitch, and for a working bench it's most of what you need.
What sets the IG-18 apart from a dozen anonymous signal generators of its era is the kit-built construction. It was available both as a kit and factory-built. The kit units were assembled by their owners, so no two are quite alike. Solder quality varies, parts substitutions happen, and the alignment is only as good as whoever followed the manual. An IG-18 that has survived to the present has usually been opened up, adjusted, and kept running by someone who cared about it, and the ones that see regular service work are the ones that still function. The repair culture around these units is well documented: a widely circulated teardown video is titled "Repair, Alignment, and Teardown."
Heathkit's own literature billed the IG-18 as a precision source at a low kit price, and the numbers back that up for audio work. The sine side is flat within -0.5 dB to +0.2 dB relative to 1 kHz across the whole 10 Hz to 100 kHz span, which is the spec you want when you're sweeping a filter or checking a preamp's response. Distortion is documented at less than .1% from 10 Hz to 20 kHz. That's not laboratory-grade by any stretch, but it's honest for the price, and the existence of an aftermarket THD improvement kit suggests owners wanted to push it further. Output impedance is selectable: 600 ohms on the 0-1 V range, up to 1 kilohm on the 3 V and 10 V ranges, with an internal 600-ohm load you can switch in. The sine output will drive high-impedance loads of 10 kilohms or more on every range, or 600-ohm loads up to 1 volt.
Power draw is modest. The manual lists 105-125 VAC or 210-250 VAC, 50/60 Hz, 6 watts, so it runs cool and doesn't add much to the bench's thermal load.
Key Features
1 Hz – 100 kHz Frequency Range
That's three decades of audio in one sweep, which is more than enough for anything short of RF work. The bottom end reaches down to where output transformers and woofer resonance live. The top end covers the harmonics and switching noise that matter when you're chasing a problem. Frequency selection is switch-based rather than a continuous dial, so settings repeat exactly. That gets handy when you're comparing two channels or returning to a test a week later. The flatness spec of -0.5 dB to +0.2 dB relative to 1 kHz across the full range is the number that makes the IG-18 useful as a reference rather than just a tone source.
Simultaneous Sine and Square Output
Both waveforms are available at once. Square waves are the fast way to check rise time, clipping behavior, and the general health of a circuit's high-frequency response. Sine waves are what you use for distortion and level measurements. Having both live means you're not swapping cables or resetting the instrument between tasks. The square wave output is adjustable in voltage, same as the sine side, and the terminal strips on the enclosure make hooking up to a scope or a DUT straightforward.
Selectable 600-Ohm Output Impedance and Internal Load
The IG-18 gives you a choice most budget generators don't: 600 ohms on the low range, up to 1 kilohm on the 3 V and 10 V ranges, plus a switchable internal 600-ohm load. That's useful when you're testing gear designed to see a specific source impedance, since a 600-ohm load lets you simulate a real line-level source rather than an idealized zero-ohm generator. The 600-ohm output impedance is achieved by adding series resistance. That's a simple approach, but it has consequences for distortion into low-impedance loads, and owners who care about that trade-off have written about sacrificing some of the low-impedance capability to improve performance into 600 ohms.
Kit-Built Construction and the THD Improvement Path
Every IG-18 started life as a box of parts and a manual. Heathkit's own documentation notes that parts changes were incorporated over the production run, so two IG-18s from different years may not match component-for-component. The aftermarket has responded. A THD improvement kit exists, along with a meter buffer board component sold separately, and the more ambitious route, documented at length in enthusiast circles, replaces the main board entirely with a design based on the HP-339A. The amount of negative feedback in the IG-18's oscillator has a pronounced effect on the THD spec, which is the lever most of these modifications are pulling.
Metal Enclosure with External Terminal Strips
The case is metal, the connections are terminal strips mounted on the outside, and the whole thing is built to sit on a bench and take knocks. The cabinet shell is Heathkit part 90-401-6, shared with the IP-27, IG-28, and IG-37. Shells turn up separately from the instruments they originally housed, so that's handy to know if you're hunting for a replacement case.
Historical Context
The IG-18 enters the Heathkit catalog in 1973 and is documented as the predecessor to the later IG-5218. The same circuit family also appears under the SG-18, SG-5218, and SG-18A designations, and the THD modification literature covers all of them. An assembly and user manual dated 1968 survives, which complicates the timeline. Either the design predates the catalog introduction or the manual date reflects an earlier revision. The IG-18 had a long production life, and the parts-change note in Heathkit's own documentation confirms it was revised along the way. Owners describe units still working after fifty years.
Audio Express (August 2004, p.57) captured the specific anxiety of powering up a piece of home-built or modified equipment for the first time, watching for smoke and sparks. That applies whether the owner built the thing in 1974 or bought it on eBay last month.
Collectibility & Value
The IG-18 is not a collector's trophy, and the market reflects that. Working units turn up in the $60 range. A recent eBay listing for a "Powers On" example asked $60.00, while the THD improvement kit alone asks $30.00, and the meter buffer board component by itself $25.00. A bare cabinet shell (the 90-401-6) lists at $9.99. These are asking prices, not confirmed sales. The instrument is cheap, the modifications are where the money goes, and the case is worth less than the shipping.
What buyers should expect is maintenance. The documented failure modes are the classic ones for gear of this age: failed capacitors and old solder joints, which manifest as "motor-boating" — a low-frequency oscillation that shows up the moment you power the unit on. A forum account of an eBay purchase describes exactly that: wired to a scope, powered up, motor-boating, traced to sick caps and old solder. That's not a reason to avoid the IG-18; it's the reason a $60 instrument is a $60 instrument. Recapping and reflowing is straightforward work, and restored examples are advertised as "Upgraded Thoroughly Restored." Documentation is plentiful — multiple free PDF manuals are available, including an 84-page assembly manual — so there's no excuse for going in blind.
For tube amp repair specifically, owners report the IG-18 is well suited, and the stock signal quality is described as excellent for the price. It won't replace a lab-grade oscillator.
eBay Listings
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