Factory Installation View


The APO Cancel Module (taken from the installation instructions)


The APO Cancel module is a retrofit modification to the FF-800 repeater controller that improves control autopatch performance. Audio from the controller that is fed to the telephone line (tones, speech, or repeater traffic) can interfere with the DTMF decoder during the control autopatch monitor function (cntl406). Under certain circumstances, this can cause the FF-800 to miss DTMF codes entered by a control operator over the phone line. The APO Cancel module reduces the possibility of interference to the DTMF decoder by subtracting the controller audio from the patch audio that is fed to the DTMF receiver.

Theory of Operation

The APO Cancel module uses an inverting op-amp to "subtract" the controller audio from the control patch audio that is fed to the DTMF decoder. Refer to the FF-800 schematic included in the FF-800 Operators Manual. The repeater audio for output on the phone line is derived from U17 pin 14. This is the "APO" signal that drives the phone line interface transformer via R62 and C10. The APO Cancel module taps this signal using the U17 socket-header that is provided with the APO Cancel module (note: on factory installed modules, the U17 tap point is at R62). The APO signal is then fed through a POT on the APO Cancel module to an inverting op-amp with a gain of 2.5. The POT is used to balance the canceling effect which requires user adjustment upon installation. The output of the inverting op-amp is fed through a 100K resistor to U18 pin 6 to form a summing network at the input of the U18 phone line RX op-amp. The inverted repeater signal is thus added to the signal from the phone line interface transformer.

The input to the phone line transformer will have incoming and outgoing audio superimposed. By adjusting the POT on the cancel module, the repeater audio that is present at the phone line transformer secondary is effectively removed from the phone line audio (API) that is sent to the rest of the controller. Of course, the canceling effect is not precise. This is partially due to the fact that the impeadance of the phone line is reflected onto the secondary of the phone line interface transformer. This impeadance is not simply resistive (it will generally appear to be inductive due to the long cable length to the central office equipment) and forms part of the divider network with R62 that is used to generate the cancel effect. However, this canceling circuit is able to achieve enough attenuation of the repeater audio to prevent DTMF decoder interference.


Refer to Figure 1 to identify the module components. This circuit modification is accomplished by removing the IC at U18 and installing the APO module in its place. U18 is then installed in the module. Additionally, the IC at U17 is removed and the socket-header assembly is installed at U17. This IC is then replaced at the U17 socket-header assembly.

Figure 1. APO Cancel module layout, field installed version.


The cancel module must be adjusted to achieve optimum performance. Furthermore, the adjustment procedure must be carried out with the FF-800 plugged into the actual phone line that it will normally use. This is because variations in the phone line impeadance can have a significant impact on the adjustment.

A noise source is required to perform the adjustment. The best source is to simply open the squelch on the repeater receiver. However, if you can not open the squelch (i.e., if your receiver has built-in CTCSS decoder) you may have to obtain another receiver or noise source to perform this adjustment.

You must also be able to monitor the DTMF valid signal. This signal is visible on the FF-8010 display. If your system does not have a display installed, you will have to monitor the DTMF valid signal at the DTMF RX I.C. (U21 pin 15). DTMF is valid when this signal is approximately 5V.

Using a separate phone line, call the FF-800. You may need to get someone to assist with this step. Note that you can not use a local telephone on the same line as the controller because any other telephone sets that are off hook on the FF-800 line during this adjustment will alter the phone line impeadance and the adjustment will not be correct. Thus, all of the control commands must be entered from the calling telephone set (this could be a cell phone, or a local telephone on a separate phone line).

If you have difficulty locating a null with the APO pot, you may have to reduce the control patch monitor level. This adjustment is fixed at 1/4 amplitude for firmware versions 3.07 and earlier. V3.08 introduced a command to allow adjustment of the monitor level: "cnfg855*". Note that the v3.08 configuration defaults to full monitor level. To reduce the level (to 1/2 amplitude), enter "cnfg855*1", re-enter the CAP Monitor command, and repeat the pot adjustment. If this does not result in a sufficient null, try the 1/4 amplitude setting, "cnfg855*2". The monitor amplitude is stored in EEPROM and is not a part of the state settings. Once modified, the change will be retained automatically.

APO Frequently Asked Questions

Why do I need this adapter?

The APO module is specifically targeted at users who use the Control Patch Monitor or the Receiver via Patch features. If you don't use or need either of these features, you don't need the module.

APO Module Pricing