As a frequent user of PACTOR,  I am very excited about the technical ability of this mode.
Unfortunately, I have noted, that many Pactor stations operate with distorted modulation
and produce hereby interference, and due to the bad modulation can't make use of the full
capability of this mode. (The operator does not know that his modulation is bad).
Frequently, a contact is not established even so the signal is strong, or many unnecessary
retransmissions occur. A high percentage of stations is affected by this problem
(home stations as well as mailboxes, especially portable and Boat-stations).

The cause ot this problem is manyfold and here I will give aid, how to master this problem.

Mostly, the bad modulation is caused by RF feedback effects, hum interference of the
modulation path or overdriving the transmitter due to wrong parameter settings.
Sometimes the cause may be a unsuitable power supply of the transceiver.

The following description is based on the PTC-II and PTC-IIe, and PTC-IIpro, but serves
 also for other equipments which produce audio for digital HF transmission.

This applies as well to RTTY and PSK32 mode transmission.

The technical description, circuit diagram with construction guide and a lot of hints
for the prevention of the mentioned problems is available.
Together with a list of providers for the required components.

If you are interested in this article  read this.