TAPR1

From: KC6HPN @ WB6YMH.#SOCA.CA

TAPR FULL-DUPLEX (NEW) 9600 BAUD MODEM MODS BY KC6HPN  (PART 1 OF 4)
                         APRIL 23, 1992

      These modifications correct several problems that can seriously degrade
the performance of the new (not the K9NG) TAPR 9600 baud modem.  They have
significantly improved the usability of my modem.  DCD (Data Carrier Detect)
operation, in particular, went from intermittent to rock solid.  All mods have
been implemented on my (first production run) TAPR modem board, which is
installed in a PK232MBX.
      I welcome any comments and will answer any questions on these
modifications.  Just drop a message in my BBS mailbox, 
KC6HPN @ WB6YMH.#SOCAL.CA.USA.NA.
                                               73, Brian, KC6HPN 

MODULATOR SIDE MODIFICATION:

1.  HOW:    Remove capacitor C5 (0.001 uf).

    WHY:   Op-amps tend to oscillate when driving a non-isolated capacitive
load to ground.  U22B, a TLC274 op-amp is driving C5 to ground through C31.
U22B may oscillate at approximately 400 kHz due to the pole created by C5.
This oscillation will be superimposed on the modulating waveform. To diagnose
this problem, examine the eye pattern at TXA.  The eye should be sharp, if it
is fuzzy, the op-amp is oscillating.

    NOTE:  C5 normally functions as part of an RFI filter to prevent noise
from entering the modem via the TXA line.  I have found that in my situation
C2 and R2 alone are sufficient.  Those with RFI problems should not remove C5
Instead, place a 220 to 470 ohm resistor in series with C31 to provide
resistive isolation from C5.  This resistor will cause a slight loss in output
drive.  If necessary, you may increase output drive by changing R39 to 300 k.

    NOTE:   The first TAPR kits used a TL084 op-amp for U22 which may be
slightly more resistant to oscillation.  However, the TL084 may still
oscillate.
CONTINUED IN TAPR1.1