Comments for page: SB-620 Mods

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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.

Posted by Steve L. March 14, 2010 - 09:41 pm
Reply to unnamed poster of 3/2/10: Thank you for the compliment and for the comments but I disagree with a few points. I was aware of the "spectrum analyzer" mode of the SB-620. That isn't a good solution here, because it requires an external oscillator, has no bandpass filter ahead of the V1 mixer to reject image frequencies and the load resistance presented to the receiver tap is much lower, resulting in more attenuation.

The amplifier is not in the wrong place. It is a wideband amp (for simplicity) and should be after the 620's bandpass filter to minimize problems with interfering signals. It also needs a high-Z load to maximize its gain.

It is a good idea to use low capacitance cable for the connection. However, RG62 is listed at 13.6pF/ft versus 20.5pF/ft for RG59. That would increase level only about 3-4dB, which would not be enough to relieve the problem. The amp would still be needed.

Again, I appreciate your comments and the stimulating discussion.

Posted by   March 02, 2010 - 01:29 pm
Nicely done. However, unnecessary. An easier modification involves leaving the SB-620 as-is. Move S2 to the 'Spectrum Analyzer' position. This changes V1 from an amplifier to a mixer. You then supply a heterodyne frequency to J1. This frequency would need to be...

3395 + 455 = 3850 KHz


3395 - 455 = 2940 KHz

This will produce 3395, 3850/2940, and 455 KHz at the plate of V1. Even as simple as it is, the bandpass filter between V1 & V2A has sufficient selectivity to reject the other frequencies.

The amplifier is in the wrong place. It should be placed at the receiver's mixer (ahead of any selectivity), with light coupling to the source. A common mistake is to ignore the instructions in the SB620 manual that state to use RG62 cable to interface the SB620. This cable has less capacitance, forming less of a voltage divider w/ the coupling capacitor on the input end of the cable. Less attenuation equals less gain downstream. Again, nicely done but unnecessary.

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