N1EU Corsair Audio Mods

RX Audio Improvement

These mods will significantly lower the hiss level of the Corsair rx audio as well as improving the overall audio quality and extension. To perform these mods, remove the IF/AF board from the Corsair. This is easily accomplished - unsolder the mic bias voltage wire from the mic connector, remove the notch filter knob using allen wrench, unplug all the connectors from the board, remove the 5 screws, lift out the board.

Op Amps

The Corsair II audio output chain suffers, among other things, from noisy RC4136 op-amps, ten of which may be in-line. Unfortunately, there is no direct op-amp replacement due to the non-standard RC4136 pin-outs. However, Cimarron Technology sells a 14-pin DIP carrier designed to replace RC4136 with OPA2134 quality op-amps. Here's the link: http://cimarrontechnology.com/rc4136upgradeadapter.aspx - I purchased the SMT chips separately and soldered them myself with a soldering iron. Check my page dealing with the op-amp replacement: http://corsair.wikidot.com/corsair-opamp

Bandpass Filter and U3C Bypass, Improved Capacitors

I rely on xtal filters for my filtering and have no need for the AF Bandpass Filter, which is outside the AGC loop and adds noise to the rx audio. I remove it from the audio chain along with one more op-amp stage (U3C). I also improve the capacitors in the audio path.

  1. Lift one leg of R65
  2. Unplug connectors 6 and 7
  3. replace C23 (product detector output) with 1uF film capacitor
  4. lift the C36 end of R61 (U1D output) and tack solder 1uF film capacitor to R61. Run a wire from the other side of the 1uF capacitor to the AF Gain control pot.
  5. shunt R42 (47K) with 10K to decrease AGC throttling of RF Gain
  6. short the solder pads of C50 together (redundant dc blocking not needed)
  7. shunt C62 (220uF electrolytic) on AF output with 1uF film capacitor
  8. replace C53 and C56 (1uF electrolytic capacitors) with 1uF film capacitors

I used Epcos B32529C105J polyester film caps, but any film cap will be fine.

Inrad 2.8KHz filter installation
If you want good ssb fidelity, install Inrad 2.8KHz filters in the 9MHz and 6.3MHz i.f.'s after doing the 9MHz filter socket mod described at http://corsair.wikidot.com/corsair-9mhz

After you change the 9MHz filter, perform carrier oscillator alignment and carrier null adjustment described on manual p 3-21.

Listen to a sound clip of W3OZ, K7GKN and friends through my modified Corsair II rx (2.8KHz filters, stock carrier oscillator alignment, PBT centered, AGC off) :
http://corsair.wikidot.com/local--files/corsair-afrx/corsair.mp3

Audio Mod comments
as far as I know, only one other Corsair owner has implemented these audio mods and here are some of his unsolicited comments:

"these mods are simply amazing … my CII audio now sounds like the mp3 sample … the change to the audio quality is really incredible… I have no hiss with the antenna removed. It used to be S3-5 depending upon the band. The receiver sounds full and rich and very quiet. Much more satisfying Rx audio than my K3 or even my Omni VII"

SSB Transmit Audio
see http://corsair.wikidot.com/corsair-aftx for mod to drive the balanced modulator directly