Banner for Heathkit SB-620 frequency conversion article

The Heathkit SB-620 “Scanalyzer” is an IF spectrum analyzer. It graphs the amplitude of signals around the one that your receiver is looking at, so you can see all of the signals around you at a glance. Problem is, it has to be setup for the particular IF frequency of your receiver. The original kit included parts for a wide range of IF frequencies. Only one frequency could be installed though and my unit had the wrong frequency for the classic Heathkit SB-301 receiver I have.

The pdf article describes how to convert it to the IF frequency common to many Heathkit receivers, including: SB-100, SB-101, SB-110, SB-102, SB-104, SB-300, SB-301, SB-303, HW-100, HW-101 and HW-104. The photo above shows it operating with actual 80m signals. Get the article and schematic below:
Converting the Heathkit SB-620 icon

Converting the SB-620 article (1.8MB)
New IF booster amp and coils in the Heathkit SB-620

IF booster amp and  new coils in the SB-620

 

Reader Comments

Posted by Steve L. February 24, 2020 - 03:06 pm
The reply to Kelley's message of February 19, 2020 - 07:02 pm can be seen here:
http://www.tronola.com/misc/Re_ Heathkit_SB-620.pdf
Please note that the posting dates/times for that thread are incorrect because it was an email exchange and was later posted in Reader Comments. The email exchange continued and the rest is available on request.

Posted by Kelley - WØRK February 19, 2020 - 07:02 pm
Hi Steve, Attached are three images of my IC-746. Main is of the main board area, Bottom is the entire bottom of the radio, and Rear is the rear of the radio. The radio is laying on its top.

IC631 is to the left of the three filter canisters in the Main image. It has what I'm assuming are either four silver capacitors or transistors around it.

I have some RG174 coax which is very small 50 ohm coax. I'm not sure how to get the coax out of the radio. I have SMA and RCA jacks that I could mount on the back of the radio. Just not sure where.

If you have any thoughts, I'd be happy to hear them. Thanks

http://www.tronola.com/misc/IC-746_Bottom.jpg
http://www.tronola.com/misc/IC-746_Main.jpg
http://www.tronola.com/misc/IC-746_Rear.jpg

Posted by Steve L. February 19, 2020 - 06:30 pm
Happy to help, Kelley. Fortunately, this is a pretty simple modification, so if you have decent soldering skills, it should be a cinch. The toughest parts are just finding a good physical way to connect the cap for the tap and connecting thin coax to that. I can break it down as much as you need.

By the way, I meant to respond to your comment, "I know there are ways to use SDR and such for these radios, but I miss the glow of the SB-620." I SO understand. Yes, radios today may toss-in spectrum display functions far more advanced than the SB-620, but the feeling of working with some of the old stuff is priceless.

Let me know if you would like to proceed. With a closeup photo of IC-631 and the surrounding components, I could work out a suggested hookup approach.

Posted by Kelley - WØRK February 19, 2020 - 06:26 pm
Hi Steve, Thank you very much for all of the information you have provided. Above and beyond the call I think. I am not great with electronics, but I will carefully review what you've sent and see whether or not I can perform the work needed to make this happen.

Posted by Steve L. February 19, 2020 - 05:16 pm
Hi Kelley, It looks likely that the SB-620 will work with your Icom IC-746. The only caveat is that this is a very complex radio and my understanding of it is rather limited at this point. If you don't already have them, you will want to download the Instruction Manual (IM) and also the Service Manual (SM). Sorry that I didn't keep track of where I found the SM but it is available for free without signing up, etc. [For clarity, I will refer only to PDF page numbers, not the ones which were printed in the manual and will prefix the page number with IM or SM to identify the manual.] The key pages I've found so far are: IM-44, SM-9, SM-10, SM-76 to 83, SM-96 to 99. Also, the schematic is on SM-103 to 126.

As you implied, the place to connect would be at the output of the 3rd mixer, IC-631 pin-6, which is found on the schematic at SM-113. Unfortunately, the print on the PCB layout pages is too blurry for me to read, but you would be looking for a 14-pin SOIC (SMT version of a DIP). There is DC on the output but tapping with a 10pF cap as recommended by Heath eliminates that. Don't know the IF level there but it's likely to be enough for the SB-620, certainly if the 17dB amp covered in the SB-620 mod article is used.

You will want to choose the no-filter option for the 9MHz IF as implied at IM-44, to provide enough bandwidth to make the SB-620 useful. If you go ahead with this, please let us know how it goes. I will be happy to help further if I can be of assistance. Cheers

[Later] Just a few more details to add:
  • IC-631 is a MC1496D balanced modulator. It may be close to transistor Q631.
  • It is pointed-out in a photo at SM-5. (That's service manual PDF page-5.)
  • As mentioned, during operation of the radio, you need the no-filter option for the 9MHz IF. Details on selecting 9MHz IF options are on IM-44.
  • The Icom IC-746 service manual may be downloaded at: http://www.radiomanual.info/schemi/ICOM_HF/IC-746_serv.pdf

Posted by Kelley - WØRK February 19, 2020 - 04:32 pm
[Editor's note: We are posting this thread which was actually an email exchange, to make it available to other readers. There may be minor edits to take care of the difference in media.]

Hi Steve,

I just ran across your article on the SB-620. I've owned one of these units for many years. It's setup with the 455khz IF option. I used it in the past on a Collins radio that I owned. It's been in storage for some time. I know that newer radios have IF's all over the map. I own a Yaesu FTDX-5000 and an Icom IC-746. I'm wondering if you know if the SB-620 can be adapted so it can be used on newer radios (my Icom mentions 455khz at the 3rd IF, but I'm not even sure it would work at that point)? I know there are ways to use SDR and such for these radios, but I miss the glow of the SB-620. Thanks.


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