Tech Docs - Sine Systems, Inc.

Important notice: this information applies only to RFC-1/B board revisions where R23 is located as shown in the drawing below. These board revisions will also have an IC labeled CD22202, CD22204 or 75T202 at position U2. This procedure does not apply and should not be necessary for other board revisions.

The DTMF tone detector IC in the RFC-1/B has fixed operating parameters and does not allow microprocessor control. This means that there are no software adjustments in the RFC-1 to modify the behavior of the tone detector. The following modification will increase the incoming analog signal level to compensate for noisy and/or weak signals associated with wireless phones and very long landlines.

Wireless phone service is not very good at passing strong, clean DTMF tones. The worst case, which is unfortunately becoming very common, is to use a wireless phone to call an RFC-1 that is also using wireless service through a telephone line emulation device. While it is great that these devices exist, they attenuate the DTMF tones and make it difficult for the RFC-1 to work effectively.

The following modification is quite simple for someone who is technically knowledgeable and proficient at both soldering and de-soldering. If you do not fit this category then you should contact someone who is more suited to the task. While this modification does not void the factory warranty, damage caused by a poor modification attempt will not be corrected under the factory warranty.

Unplug the ribbon cable to the RFC-1/B control unit (the small box). Cables to the RP-8 can be left in place as it is not necessary to make any changes to the RP-8 for this modification.

Remove the RFC-1/B from the rack if necessary and remove the two screws from the rear panel. Remove the rear panel and plastic bezel from the RFC-1/B. Slide the RFC-1/B circuit board from the chassis. The board should look like the image below.

Resistor R23 should be located near the upper left corner of the RFC-1 circuit board. It is shaded in the image below. There are several versions of the RFC-1 main board. If R23 is not in the same location as shown in this image then this modification is not appropriate for your system.

If the RFC-1/B is connected to a landline:

The modification is a simple resistor substitution and it involves only R23. In most RFC-1/B systems there is a 270K ohm resistor at position R23. Replace this resistor with a 220K ohm, carbon composition, 1/4 watt, 5-10% tolerance resistor. A smaller value resistor is okay and might be needed depending on site conditions. Try to stay above 10K ohms.

If the modification is not successful at solving the problem then substitute a smaller value resistor, about 100K ohms. If that does not work, try the procedure below.

If the RFC-1/B is connected to a wireless phone through a line emulation device:

Most of telephone line emulation devices pass DTMF tones at a significantly reduced level. We do not know if this is a function of the emulation device or an artifact of wireless service but this modification should help resolve the condition.

This modification involves R23, as mentioned in the previous procedure, and R18 which is just to the left of R23 across the two small capacitors. Both R23 and R18 should be removed from the circuit. Do not replace R18 will anything and do not short it pads together! Replace R23 with a low value resistor anywhere from 100 to 1K ohms. If a suitable resistor is not available it can simply be shorted. If you are in the field and do not have a low value resistor available, use the resistor removed from R18. At 33K ohms the value is a little high but with R18 removed from the circuit the value of R23 is less critical.

If the system still does not respond to tones from a remote phone, there is one last modification that can be performed. C1 is the top of the two very small capacitors located between R23 and R18 in the drawing above. Carefully remove it from the circuit.

If all of the modifications described above have been made and the system does not detect tones then there is something else significantly wrong. Keep in mind that if the RFC-1 responds correctly when using the directly attached, local phone then the tone detector does work and the failure is most likely external to the system and possibly environmental.






	How to make the RFC-1/B DTMF detector more sensitive Important notice: this information applies only to RFC-1/B board revisions where R23 is located as shown in the drawing below. These board revisions will also have an IC labeled CD22202, CD22204 or 75T202 at position U2. This procedure does not apply and should not be necessary for other board revisions. The DTMF tone detector IC in the RFC-1/B has fixed operating parameters and does not allow microprocessor control. This means that there are no software adjustments in the RFC-1 to modify the behavior of the tone detector. The following modification will increase the incoming analog signal level to compensate for noisy and/or weak signals associated with wireless phones and very long landlines. Wireless phone service is not very good at passing strong, clean DTMF tones. The worst case, which is unfortunately becoming very common, is to use a wireless phone to call an RFC-1 that is also using wireless service through a telephone line emulation device. While it is great that these devices exist, they attenuate the DTMF tones and make it difficult for the RFC-1 to work effectively. The following modification is quite simple for someone who is technically knowledgeable and proficient at both soldering and de-soldering. If you do not fit this category then you should contact someone who is more suited to the task. While this modification does not void the factory warranty, damage caused by a poor modification attempt will not be corrected under the factory warranty. Unplug the ribbon cable to the RFC-1/B control unit (the small box). Cables to the RP-8 can be left in place as it is not necessary to make any changes to the RP-8 for this modification. Remove the RFC-1/B from the rack if necessary and remove the two screws from the rear panel. Remove the rear panel and plastic bezel from the RFC-1/B. Slide the RFC-1/B circuit board from the chassis. The board should look like the image below. Resistor R23 should be located near the upper left corner of the RFC-1 circuit board. It is shaded in the image below. There are several versions of the RFC-1 main board. If R23 is not in the same location as shown in this image then this modification is not appropriate for your system. If the RFC-1/B is connected to a landline: The modification is a simple resistor substitution and it involves only R23. In most RFC-1/B systems there is a 270K ohm resistor at position R23. Replace this resistor with a 220K ohm, carbon composition, 1/4 watt, 5-10% tolerance resistor. A smaller value resistor is okay and might be needed depending on site conditions. Try to stay above 10K ohms. The modification is complete. Test the system. If the modification is not successful at solving the problem then substitute a smaller value resistor, about 100K ohms. If that does not work, try the procedure below. If the RFC-1/B is connected to a wireless phone through a line emulation device: Most of telephone line emulation devices pass DTMF tones at a significantly reduced level. We do not know if this is a function of the emulation device or an artifact of wireless service but this modification should help resolve the condition. This modification involves R23, as mentioned in the previous procedure, and R18 which is just to the left of R23 across the two small capacitors. Both R23 and R18 should be removed from the circuit. Do not replace R18 will anything and do not short it pads together! Replace R23 with a low value resistor anywhere from 100 to 1K ohms. If a suitable resistor is not available it can simply be shorted. If you are in the field and do not have a low value resistor available, use the resistor removed from R18. At 33K ohms the value is a little high but with R18 removed from the circuit the value of R23 is less critical. The modification is complete. Test the system. If the system still does not respond to tones from a remote phone, there is one last modification that can be performed. C1 is the top of the two very small capacitors located between R23 and R18 in the drawing above. Carefully remove it from the circuit. The modification is complete. Test the system. If all of the modifications described above have been made and the system does not detect tones then there is something else significantly wrong. Keep in mind that if the RFC-1 responds correctly when using the directly attached, local phone then the tone detector does work and the failure is most likely external to the system and possibly environmental.