Banner for Heathkit IG-18 upgrade article

This project takes the popular Heathkit IG-18 (and 5218) audio generator to a whole new level of performance, replacing the main board with a design based-on the HP-339A. The write-up is in the form of what I call construction notes, which is brief.

Heathkit IG-18 Constr. Notes Icon

IG-18SL Construction Notes (1MB)

Finished, modified Heathkit IG-18SL
Bottom view of the oscillator board wiring
Oscillator board
Oscillator board installed. New range switch with cap board mounted.
Assembling the cap board
Addendum flag

Finished IG-18SL

Bottom view of the
oscillator board wiring

Oscillator board

Oscillator board installed. New range switch with
cap board mounted.

Assembling the cap board

Later Developments

PCB Created for the Project!

We are pleased to present contributions from reader Larry Burk, who created an excellent layout for the IG-18SL in Eagle CAE software. After doing the layout, he fabricated his own, single-sided PCB and successfully built the working generator. Performance results seem at least as good as the ones we obtained. In fact, noting that our distortion results were limited by the HP8903A analyzer we used, Larry was able to measure far lower distortion at midband fre­quen­cies, using special techniques. His project, with the PCB, is shown below.

IG-18SL with Burk PCB
Oscboard1 low

The PCB layout, as seen in Eagle is at left. (Click on it for higher resolution.) Note that red topside traces are only to guide topside wiring. They aren’t intended to meet fab rules. Also, everything shown outside of the terminal reference letters is just extra descriptive info, in a reference layer.

Tips on Fabricating the PCB

For readers who want to fabricate the PCB, Larry provides the following tips (edited and augmented):

  • The board is intended for single-sided, DIY fabrication. As such, the topside traces are to guide topside hand wiring and the silkscreen was for guidance, rather than application to the board. While some silkscreen marks may overlap pads, PCB vendors typically mask out the silkscreen over the pads, anyway.
  • The overall dimensions of the board are 5.4 x 4.0-inches.
  • The PCB provides for a few enhancements, over the IG-18SL schematic:
    • Pot R54 (1kohm) is added, in series with R51. If the pot is used, reduce R51 to 2.2K. Adjust the pot for minimum (second) harmonic distortion.
    • Two 0.1uF bypass caps (C49,50) are added at U4.
    • The power supply seems quieter with two 10uF tantalum caps added, across C5 and C6. Those can be tack soldered across C5 and C6, under the PCB.
  • Since Newark no longer has HA2625’s Dick Moore indicates that a LT1468 works very well for U1. It is a direct replacement, except that the compensation components are left out. Those would be C44,45,48, CR7,8, R27,28. Dick also left out C32,46 and R29, though he and Larry think that these last three might be useful. LT1468CN8 is available from Digikey or from Linear Tech directly.
  • Eagle version-6 is required to open the .brd file.
  • Eagle generates errors if an error check is run on the board. That’s because the red topside traces were only intended to indicate where jumper wires are used, not for actual PCB fabrication; as-is, traces overlap. After inspection approve all of them and it will be okay.
  • In this build, the high voltage supplying the neon lamp caused noise/distortion which was hard to locate. It would be worthwhile during checkout to disconnect it at transformer end to see if it is adding anything, especially on the high value range resistances. [Larry’s knob inserts had been modified, requiring that the frequency vernier pot be oriented with its leads near the lamp. Normally, the leads will be positioned on the opposite side from the lamp —ed.]
  • The Electroswitch 6P4T rotary switch for S1, is getting expensive; about $27 now. Mouser has an Alpha rotary switch (SR2921F-0304-19R0B-E9-S-W) for $5.16 that would work but a perf board for most of the larger caps would need to be made and mounted where the old power supply is located, similar to what Dick Moore did. It’s not as nice a switch.
  • For the square wave generator, the CD40106B was substituted for the equivalent 74C14B, since the other chip was handy. R5 needed to be increased to 56K, to obtain a 50% duty cycle. Other specifications were not tested but the chips have similar speed ratings.
  • The Heathkit attenuator resistors were changed to the HP style shown at the bottom of the page on Dick Moore’s site. It was a pretty easy job and is accurate.
  • Gerber files for PCB fabrication are provided below. If instead, you wish to generate your own output files from the .brd Eagle file, the topside metal file should be discarded and not sent to board house, because of the overlaps mentioned above. The silkscreen can still be used, though I didn’t, since the PCB was made at home. If you wish, you can view the Gerber files using a Gerber viewer. Free ones are available at the moment. Note that it may show errors (which are harmless), due to the fact that informational parts of the layout extend past the Eagle zero-reference. That was needed to use free Eagle to create the board.
  • Warning: Larry has kindly generated Gerber files to allow PCBs to be fabricated at commercial board vendors. However, those files have not at this point been used to make a board. Please proceed at your own risk. We would very much like to hear, your results, if you have boards made with the files —ed.

PCB Files for Download

OscBoard silk low
Files pkg pic out

Click on the image at left, for a high resolution pdf file, showing the silk-
screen and topside jumper wires.

Click on the image at left, to download a small zip file, containing the files package for PCB fabrication.

The PCB files package includes:

  • Readme IG-18SL Osc.txt - a list of the files.
  • IG-18SL Osc.brd - Complete Eagle v.6 layout file.
  • IG-18SL Osc.sts - Bottom solder mask
  • IG-18SL Osc.sol - Bottom copper
  • IG-18SL Osc.plc - Silkscreen
  • IG-18SL Osc.drd - Excellon drill data
  • IG-18SL Osc.dri - Readable ASCII summary of drill information.
  • IG-18SL Osc.gpi - Readable ASCII summary of photoplotter information.

Formats: txt, dri, gpi are ASCII. The drd file is excellon drill data. Others are Gerber. Note that this layout is intended for a single sided board. The topside traces shown in the BRD file are only to guide topside jumper installation. They are not suitable for fabricating a topside copper layer on the PCB.

Intriguing Later Developments from Dick Moore*

In addition to creating the PCB for the IG-18SL, Larry Burk also contacted Dick Moore, who built a fascinating website here. Dick has done extensive work on various approaches to enhancing the IG-18. With one of Larry’s PCBs, Dick constructed the circuit and did some advanced measurements which showed that the distortion at lower frequencies could be far lower than what I was able to measure, with the limitations of the HP8903A analyzer. His write up on it is here. I must say that the distortion figures seem almost too good to be true (under 0.0001% at 1kHz, 0.0003% at 10kHz). However, his measurement methods, while indirect, certainly make sense. Note that Dick calls this build the “IG-339A.” As mentioned above, Dick also found that an LT1468 opamp works very well, in place of the hard-to-find HA3-2625.

He also experimented with using the original Heathkit tuning network with the new PCB circuitry, calling that build, the “IG-339B.” His write up on that version is here. The Heathkit tuning network uses a 10:1 ratio of capacitors in the bridged-T, versus HP’s 100:1 ratio. It originally seemed likely that the higher ratio was responsible for the low distortion performance of the HP-339 oscillator. However, he reports measured distortion figures under 0.0005% over 100Hz to 10kHz. While I find this very surprising, Dick obviously has done a lot of excellent work on this, so I would hardly question his results. I really wish that I had time right now to get back into this project but alas, there are so many others that I’m juggling. (Maybe you know the feeling—so many toys, so little time :)

*This article was written in 2012. Sadly, Dick passed away in 2015. However, in 2016, his wife Robin graciously allowed Tronola to preserve his website, Moorepage, where you can find more information.


Reader Comments

Posted by Dan M. [and Ed.] March 02, 2021 - 04:05 pm
Ed. note: Dan M. from Romania was kind enough to supply photos of his remarkable project to re-create a Heathkit IG-18 and then upgrade it to incorporate the redesign originally presented in our Tronola article as the "IG-18SL". Moreover, he has built the "IG-339" board using the PCB layout provided by Larry Burk (covered in the supplement above). Larry's PCB is based on the variation of the IG-18SL which Dick Moore developed as "IG-339". To summarize the versions, we have:
  • IG-18 - Heathkit's original audio generator based on discrete transistors and stabilizer bulb.
  • IG-18SL - Tronola adaptation of IG-18 using the HP-339A oscillator circuit.
  • IG-339A - Dick Moore's version of IG-18SL substituting an LT1468 opamp instead of the rare Harris opamp. Dick also built a version IG-339B (discussed above), which uses Heath's original 10:1 capacitor ratios instead of the 100:1 factor used by HP and the IG-18SL.
Note that Dan not only re-created the IG-18 electronically, but also physically! He actually drew a design for sheet metal pieces to reproduce the IG-18 chassis and had those made. I was amazed and asked why he chose that path instead of, say, using an old IG-18. His reply is below. Here are the photos he sent of the project, along with his comments. I think it's truly impressive:
IG-339 board assembled with Burk layout.JPG
IG-18 Reproduction chassis front view.JPG
IG-18 Reproduction with original circuit half scale.JPG
IG-18 Reproduction with original circuit top side.JPG
IG-18 Reproduction with original circuit bottom side.JPG
IG-18 Reproduction attenuator switch and control.JPG
Bulb for stabilizing level 130V20mA.JPG
IG-18 Reproduction chassis perspective.JPG
IG-18 Reproduction chassis top side.jpg
IG-18 Reproduction chassis side view.JPG
IG-18 Reproduction chassis rear view.JPG

[Before building the redesign] I wanted to see it working, I wanted to play with it... So instead of finishing IG339 as I started earlier, I decided to go for a while with the original design. The dimensions and design of the PCB module placement follow the original Heathkit project. Can I call it a "clone"? Probably not--some dimensions are not identical, some components are changed. (E.g. I used transistors 2N2222 in the Schmitt trigger, 2N2914 and 2N2905 in the oscillator, TIP41 in the power supply. Also used a 230/45V transformer not 230/2x45.) But it's working. Now I'm beginning to calibrate the T-filter, and--of course--waiting for the last components for Mr. Moore's version of the oscillator. That will be the final design. Until then, I'll play with the standard version.

Now...[The reason I'm building the chassis from scratch is that] The only source of the IG-18 [here] is E-bay... and from them we might be talking about $80-100 for the generator plus shipping (about another $90-100) which is more than I can afford for this. Making it from scratch means something like this: Today I have $50 available...hmmm...let's do the chassis. Phone call, send the design, receive the costs: $23. But the fabrication shop wants a minimum order of $50...okay make two... So, in the second chassis I'll mount an Owen L-bridge from Steve Bench's project: After this... I make the list of components... Oh!... in my junk boxes I find lying there, unused, several ceramic rotary switches from 1980, Russian-made, 12pos/4sections...goood... And so on, piece-by-piece, I found all the stuff I need, without involving family money.

I used the files in that [Larry Burk's] zip file, but opened it in Eagle 9 and it works. I used Press&Peel to make just the bottom layer. Press&Peel is covered here: It's just an example of toner transfer to apply an impermeable mask for etching the circuit with ferric-chloride. The toner mask is from a laser printer.

Posted by Steve L. February 19, 2021 - 10:09 am
Hi Dan, If you would like to include pics in a post, just send the post and pics to me by email and I will post it. My email address is shown here. (It's also possible to insert html into a post, linking to a pic hosted on a third-party site.)

The caps in the notch filter in my project have a 100:1 ratio versus 10:1 for the original IG-18. HP used 100:1 in the HP-339A. Analysis shows that the higher ratio achieves a notch depth of about 34dB whereas the Heath ratio of 10:1 has only about 16dB. Greater notch depth should result in better distortion but with a special switch, Heath could reduce the number of capacitors with the lower ratio. However, Dick Moore said that he had good results with the 10:1 ratio.

Posted by Dan M. February 18, 2021 - 04:29 pm Sir,
I hope it's daylight to you...
If I may ask a question... I want to enlight me with a thing that I don't understand:
the original IG18 has capacitors in notch filter in a ratio of 5/0.5 (ex. 5uF with 0,5uF) and in mr.Moore schematics I see 15,8uF/0,158uF...where do that difference comes from?
(BTW... I have a functional IG18 rebuilt with original schema, for testing and it' s said "reglaje"?...adjustements...of frequency switches, notch filter and attenuators. Then I will return to mr.Moore schema for final assembly. I want to add a picture or two with my "creation".. but I can't find the place where to add them...

Posted by Steve L. December 22, 2020 - 04:28 pm
Hi Dan, (First, I think you're fine with the English. The time here appears to be 7 hours earlier than there.)
I compared datasheets of the two JFETs you mentioned to the 2N4092 and I'm sorry to have to report that neither one is suitable for this circuit. The trouble is that their values of Rds(on) are too high. The 2N4092 spec is 50ohms max (35 typical) while LND150N3 is 1Kohm and BF256B may be 220 to 500ohms. Q2 is used as a voltage controlled resistor in the circuit and ideally should be able to vary from well below 100ohms to well above 200ohms. The Rds(on) is the lowest resistance the JFET can do.

The good news is that the 2N4092 is pretty common so most electronics parts suppliers should have it. Wish I could be more encouraging. Will be happy to help if you have other questions.

Posted by Dan M. December 22, 2020 - 03:22 pm
Good day Sir,
(I think it's day at your location, to me is late night, 22:23 EET) :)

Thank you for the quick answer, and for sharing your knowledge with me...
I forgot to ask if the j-fet is critically 2N4092 or I can use something like BF256B or... let's say LND150N3 (they are laying in a box for several months)...
(BTW... sorry for my mistakes in english, I'm not native speaking it.. I'm from Romania...)

Posted by Steve L. December 22, 2020 - 07:02 am
Hi Dan, Thank you for your kind words. The late Dick Moore (whose legacy website is now hosted on suggested that the Linear Technology LT1468CN8 is a good replacement for U1. Note that some auxiliary parts which were needed for the Harris opamp should be left out for the LT1468. Those are C44,45,46,48, CR7,8, and R27,28,29. I will be very happy to help if you have questions and will be interested to hear how your project goes. Best wishes, Steve

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