Comments for page: SB-620 Mods

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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

or

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.

Posted by Steve L. February 13, 2010 - 08:48 pm
Hi SteveK,
My objective was to get a scanalyzer which I already had lying around, to work with a newly acquired Heathkit SB-301 receiver. Both items are vacuum tube, vintage stuff. I suppose there are newer such analyzers but I'm not familiar with them. I always thought of the SB-620 as being a strange, unique product. So I was motivated to restore it. BTW, the SB-301 receiver is the same one as used in the movie, Frequency, with Dennis Quaid :)

Posted by Steve K. February 13, 2010 - 08:03 pm
What was your objective with this project? Were there other options, such as buying a newer analyzer?

Posted by Steve L. February 10, 2010 - 10:59 pm
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