Aug 2, 2009

Getting the most out of your rig; another mod for the TS570SG on 50MHz

One is never satisfied ..
Quite regularly I'm trying to raise my 'Rig-Fun-Factor'. Finding a 'nicer rig' so to speak. That means that besides an inspiring user interface and great ergonomics, it must offer exceptional RX performance too. In short; it must look nice, be fun to use but also capable of digging deep in the layers of noise to catch that weak dx on 50MHz.
I have had the pleasure to use an IC746 for several weeks during a 50Mhz dx-pedition. But that rig had to be returned to its owner. Again I'm not giving up on the 570 yet.

My TS570SG does a great job but still can use some help RX-wise.
I have made some changes in the past (see my blog on 30 may 2008) being:
  • Added Inrad 400Hz CW filter
  • Exchanged std Kenwood SSB filter for Inrad 2100Hz
  • Added temperature controlled crystal heater (K├╝hne Electronics/ DB6NT
  • Tried external 50MHz pre-amp SP6 (SSB Electronics)
Still not being satisfied when comparing it to my 2nd 50Mhz set-up (Elecraft K2+XV50 transverter), I decided to take this one step further.

Modify the input circuitry of the TS-570SG
Looking at the schematics one recognizes lots of circuitry before the first mixer stage; low pass filtering, attenuator, bandpass filtering, 2 stage pre-amp, more filtering and zillions of pin diodes in the complex RX path(s). Using pin-diodes as RF switch is the cheap and easy way for any manufacturer.
There's been a lot of hype on the subject and I do not want to get into the discussion of how much noise and/or IMD distortion a pin-diode might create. I do not have enough experience in this area nor any equipment to measure the difference. But the fact remains that there are some 32 pin-diodes connected in or to the RF signal path at all times.
Why not get rid of all of the above and try something rudimentary?

Plan: Use external 50MHz pre-amp and inject its output directly into the 1st mixer, bypassing al front-end stages.

The SSB SP6 is a robust 2-stage pre-amp which already has a sharp helical bandpass filter at its output (See blog of 2 feb 2008). That covers the front-end +bandpass filtering. Next we need to determine where to inject the amplified 50MHz signal.

The block schematic shows the RX stages of the receiver input. Let's cut the RX path at the red marked spots.
If we forget about the attenuator and first 60Mhz low pas filter stage, we can take a short and simple route by using the 570's internal cable routing.

Just move the coaxial cable from CN1(RF Unit input) to CN7 (test point 1st mixer).
Now there's still some circuitry connected to that 1st mixer input. You need to disconnect that by removing just 2 pin-diodes.
See next picture for more details on how and what:

What steps to take:
  1. Remove coax cable from CN1 and insert at CN7
  2. Remove pin-diode D30
  3. Remove pin-diode D43
  4. Bypass R35 (5k6 at CN7 test point input)
  5. Create proper RX/TX switching for the pre-amp, using the 570 REMOTE connector and a separate PTT foot-switch (the SP6 already has vox operation built-in, but it can't hurt)
Needless to say that the TS-570SG will not proper operate on any other band but 50MHz.
Don't try this at home if you have no experience in electronics. I can not help when Murphy decides to assist in your personal attempt.

Test and operate the new set-up
What do we have now?
A 50MHz pre-amp and a TS-570SG with an RX direct 1st mixer input.
Switch-on the rig and GO!
Interesting to see that it all works without quirks and/or additional smoke..... :)

On weak signal comparison the modified 570 seems now on par with the Elecraft K2+XV50 combo. It sure is more sensitive and also sounding different. But that is all subjective, It will take weeks of comparison before I can make a conclusion. But more important let's see how much fun I will have with this new dedicated 6m rig.

There's plenty of Es openings on 6m this season, see my 6m twitter blog. So let's enjoy this!