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Tecsun R-9700DX Teardown

I've seen a lot of favourable comments on this analogue SW radio via various YouTube channels, along with it's sibling the R-9710. Sold in some (USA?) markets as the Grundig Yacht Boy 250.
Exact specifications and schematics have evaded me so far, however I did find a teardown and comparison at RadioIntel.com from which I've borrowed this photo:

Photo from http://www.radiointel.com/review-review-tecsunr9700dx.htm

It appears to use dual-conversion on shortwave with a fixed crystal local oscillator (see the 10 crystals above the bandswitch). One feature is that the tuning on the short wave bands tune the opposite direction to the AM and FM bands.

Here's my (incorrect) guess of how it works.
  • AM band IF is 450kHz (from the orange block filter) 
  • AM local oscillator tunes 975kHz - 2160kHz.
  • The image of the AM band (1425kHz - 2610kHz) is used as a tuneable first IF for the shortwave bands. Each band is 600kHz wide so it probably uses 2000kHz - 2600kHz.
  • The SW bands tune in the opposite direction to the AM band. To achieve this inversion, the fixed first LO crystal will have to be on the high side of the tuning range. 
  • Taking the tuning range for the 13 meter band as an example, the first LO will be 24000kHz (24MHz)
  • 24000 - 21400 = 2600kHz 
  • 24000 - 22000 = 2000kHz.
This will give a nice bandspread across each of the SW broadcast bands and the use of quartz crystals will ensure it's stability should be close to that on the AM broadcast band.

Update

Well I found a schematic on Tecsun's website and it seems it doesn't work as I first thought.
The first IF for SW is actually 10.3 - 11.0MHz.

And finally after Google Image searching, the schematic from http://www.radioscanner.ru !


I'll amend this article when I have finished measuring the actual oscillator frequencies.

Updated 2nd November 2021 to redirect some dead links to the Wayback Machine.
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Comments

Nick B. said…
Here is the original R-9700 and an English translation from Tecsun's website.
16/2/18 01:54
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