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 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

Posted by Steve L. November 01, 2013 - 08:47 am
Hi Dale, I'd like to help but don't have enough info on your problem. Did you follow the steps in the "Adjusting the Scanalyzer" section of the article? How did that go? What exactly are you seeing on the Scanalyzer screen? I take it that there is a horizontal line. Is there just one pip in the center?

Be aware that it can be tough to get the controls on the SB-620 set properly to give a good display. Set wrongly, it might very well look like it isn't working. PIP Gain should be max. I recommend that you set the Scanalyzer controls using the steps in the second bullet item on page-10 of the article. There are four paragraphs, beginning with "During operation," which detail the tricky setup.

The tap for the SB-620 should generally come from the plate or collector of the mixer which is driven by the receiver's variable frequency oscillator. What receiver do you have and where did you place the tap?

Posted by DALE HEIDNER November 01, 2013 - 12:31 am
i HAVE A PIP, BUT DOES NOT SCAN. how do I get it to scan the band. I must be hooked up to wrong mixer.


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