Comments for page: SB-620 Mods

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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:

Posted by Kelley - WRK 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.

Posted by Steve L. February 25, 2019 - 07:10 am
Hi Anthony, Yes, I think there is a very good chance that the SB620 will work well with the TS520S. It does have a 3395kHz IF, which is the frequency the mods in the article are for. Here are some points of interest from a brief look at the service manual available here: TS520S_service-manual.pdf (6MB) (I will refer to the page numbers of the PDF file, NOT the page numbers printed in the original manual.)
  • Heath recommends connecting to the plate or collector of the mixer stage which is driven by the receiver's variable oscillator. In this case, that stage is Q9 in the "IF unit" PCB.
  • You can see Q9 in the simplified schematic at the top of p.45. In this case, the output of the dual-gate mosfet mixer is its drain. Notice though that it drives a transformer (xfmr) which in turn drives the NB1 connection to the NB unit. That may be a more convenient place to connect. However, the signal voltage probably won't be as high at NB1 as it is on the drain of the transistor. As you can see in my article, signal level was a problem with the SB301 receiver, leading to the need to add an amplifier to the SB620.
  • In any case, you will want to tap off of the point you choose, using a 10pF or so coupling cap.
  • The full schematic on p.54 isn't quite clear enough but you can see Q9 in the right, lower corner of the IF unit. Signal is flowing right-to left. The most visible marker is TP2 in red, which is connected to one of the gates of Q9.
  • The most useful reference is the IF Unit PCB picture on p.20. Q9 is at the bottom of the board. To the left of it is the xfmr, T9. Notice that the output terminals of the xfmr are on the left, one going to gnd and the other to NB1---how convenient! To connect to the Q9 drain, I would solder the 10pF cap to the upper-right T9 pad. The other side of the cap can go to thin and flexible coax (like that used for classic composite video cables). The gnd of the cable can go to the lower left pad of T9 or other nearby gnds.
  • The other end of the coax can go to a chassis mount phono connector or similar. In the rear panel photo on p.13, there are several phono jacks indicated. Don't know if any of those are spares or if one could be repurposed.
I will be happy to help if you have any questions. Good luck with your project!

Posted by Anthony November 14, 2016 - 07:32 am
HELLO, My name is Anthony H. (KG5EES) I'm trying to hook up a 620 to a Kenwood TS-520S do you think it is possible and do you have any advice on how I should go about doing this?

Posted by Steve L. February 03, 2014 - 09:34 pm
Hi Dale, It's best to operate the SB-620 with AGC off, controlling RF gain manually. I don't see a switch on the front panel photo of the HW-101 to defeat the AGC, though. You could add one. From a brief look at the schematic, the auto control seems to go through D101, overriding the manual value supplied through D905. Putting an SPST switch in series with D101 might get you what you need. However, if the AGC is pushing the levels down to invisibility, it could be that your SB-620 simply needs more gain, as mine did. If so, adding the IF booster amp described in the article could help.

Posted by Dale Heidner February 03, 2014 - 08:27 pm
Steve I get a great pep on my HW101 from the calibrator, but when any station comes on with a strong signal, any pips that are on the screen, including the calibrator, bet very tiny or disappear. The AGC also seems to fad down at the same time. I take my feed for the SB-620 from the plate of V12A (pin 6) Any ideas. Otherwise the HW-101 works beautiful. The calibrator pip is about 40dB for a S9+40db signal. Dale W7NAV

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