38S Mods for 17M band

Most Noteworthy changes:

• I.F. frequency changed to 4.0 Mhz, uses three crystals in ladder filter.
• Super VXO uses two 22.11 Mhz crystals in parallel
• Audio preamp reduces common-mode noise
• 2^nd mixer uses pin 5 for audio and pin 4 for (18 MHz) transmit output
• Extra tuned circuits in transmit chain to select 18Mhz.
• One 4053 switch eliminated, another re-placed between audio stages

 

Many traces must be cut, and some added. These are not documented, but you can likely see some of them on the bottom-side photo of the printed circuit board. Other cuts are on the top-side that youll have to figure out yourself.

 

The 1N4004 varactor diode was replaced with a vari-cap of greater range, a 1N1404. Four turns extra were wound onto a molded 3.9uH commercial inductor to give a tuning range that includes the bottom of the 17M band.

            I used 4.000MHz parallel resonant crystals in the I.F. and in the NE602 oscillators. This choice makes the VXO range stretch rather far at the lower end of 17M. Id suggest using 4.000 MHz. series-resonant crystals instead youll lose a bit at the top of the band, but wont have to stretch as far at the bottom. I found the top end extended into SSB territory a waste of bandwidth. With this super VXO, a ten-turn tuning resistor was needed. Youd find that a one-turn pot gives much too quick tuning.

 

I have the notorious 38S thump pretty well beat, by moving one of the CMOS switches past much of the audio gain, between the two op-amps. Moving this switch here requires a good bit of hacking PCB traces, but is worth the effort in cleaner transmit/receive switching. A PNP audio preamp  reduces audio noise coming from the 7808 regulator.

            One CMOS switch was eliminated, at the output of the 2^nd NE602. Two outputs are required one for the transmit signal at 18MHz, the other is receive audio. Since the NE602 has two similar outputs anyway, switching isnt required.

            I found that some of the small ceramic capacitors in the early audio stages caused microphonics. Any coupling capacitors should be mylar, polyester, or polyproplyene.