Comments for page: Heathkit IG-18 Mods

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Posted by Dan M. December 22, 2020 - 04:02 am
Good day sir,
I just discover this page, very useful and clear explanations, so I start replicating IG-18...sadly my generator dies several weeks ago(it was an IEMI E0501 produced in Romania, cca 1979) and I was searching for an easy to make and with high quality signal generator. IG-18 seems to be what I'm searchimg.
For now I already built the rotary switches for notch filter(from 3 soviet-make ceramic switches 2x11pos and 4x11 pos for multiplier), the PCB you post above and I'm waiting for IC-s... and the enclosure(the case) wich will be 99% similar to IG-18 original.
I want to ask you - when test time will come - for a little help/advice.
For the moment, I have a single question: is there an equivqlent for U1, actually available in DIP8 casing?

Posted by Steve L. April 10, 2020 - 08:02 am
Hi Shawn, I sincerely apologize that the system which should inform us of a posting, apparently failed for your August, 2018 posting. For you and subsequent readers, I'm posting this belated reply. Getting low-priced, short-run PCBs is covered in our recent article here: http://www.tronola.com/html/tube_analyzer_you_can_build_page3.html#GetPCB.While those vendors typically provide 5 boards with an order, the prices are low enough that it doesn't matter. I don't know whether those vendors literally do single sided boards but you could email them, attaching the ZIP of files from above and ask if they could simply leave the top side blank. We would very much like to know what you find. Thank you for your posting and again, I'm sorry for the delay in replying.

Posted by Shawn S. August 27, 2018 - 06:21 pm
Any recommendations for a board fabrication place that will do a run of one single sided board for a reasonable price. I've used Osh Park but they won't do single sided boards. Shame I don't have the time or patience to figure out how to mod the file. I'd probably add plated through holes with tiny pads to the top layer as well. Any help would be appreciated. Thanks.

Posted by Steve L. September 22, 2017 - 06:13 pm
Hi Shawn, Those turned out to be interesting questions indeed! The short answers are 1N34A diode and a 34VCT transformer, rated for 50-100mA. The main concern about the diode is its forward characteristics. That's because the IG-18 meter scale is corrected for the diode effects at low voltage. Heath's part number for the diode was 56-26 which is shown as a 1N191 in the cross reference table at d8apro.com.

I looked at the NTE replacement but it's not a very good characteristic match. You can actually get original 1N191 diodes on eBay but they sell in a pack of five for $15. The diode I actually used (which worked well) came from Radio Shack more than ten years ago and was an unmarked Germanium, as I recall. Shouldn't matter a great deal though and the 1N34A looks like a good match for the forward spec.

The Heath transformer is given as 34VCT for the wiring configuration I employed. DC current is about 30mA on each rail and the positive rail rises to about 47mA with the square wave converter on. Since the rectifier/filter puts a high load on peaks, it's customary to rate the transformer for a higher AC current, hence the 50-100mA figure. Voltages into the regulators with no load are typically 29VDC.

Let us know how it goes. Will be happy to help if you have further questions.

Posted by Shawn S. September 22, 2017 - 03:25 pm
Thank you, much appreciated.

Do you have a part number for the germanium diode? Also, Being a project nut, I have several small footprint toroidal transformers on hand. What would be the ballpark specs that would work for this? Thanks in advance.

Posted by Steve L. September 22, 2017 - 08:39 am
Hi Shawn, The CD40106B is equivalent to 74C14B and is available at Digikey and other places. (I wish they would indicate that on the "obsolete" page.) The schematic and the parts list in the construction notes have been updated to show the CD40106B as equivalent. Thank you for bringing this up!


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