R-C High Z Phone Line Interface (Caveat: contains ye olde ASCII schematics)

[[email protected]]

Hi, everyone!

I saw this circuit in the Tech FAQ
(http://www.tech-faq.com/make-linemans-handset.shtml)

OFFICIAL Agent 04 Generic Test Set Modification (tm)

Ring >---------------------------------> to "test set" phone
Tip >------! SPST Switch !-------->
!-----/ ----------!

from !-------/!/!/!/!--! C = 0.22 uF 200 WVDC Mylar

cable pair ! C R ! R = 10 kOhm 1/2 W
(alligators) !--! (------------! SPST = Talk / Monitor

Has my somewhat outdated electronics experience failed me, or is there
still a path for DC current even with the switch open?

With that in mind, I changed the schematic to:

Ring >---------------------------------> to "test set" phone
Tip >------! SPST Switch !-------->
!-----/ ----------!

from ! ! C = 0.22 uF 200 VDC Mylar

cable pair ! C R ! R = 10 kOhm 1/2 W
(alligators) !--! (--/!/!/!/!--! SPST = Talk / Monitor

Would the few mA of AC passed to the hybrid in the phone, as the author
claims, be sufficient to produce a signal at the earpiece of an average
phone? In my (admittedly lazy and highly unscientific) test, I got no
output at all at the earpiece with 22nF and 10kR, although I didn't get
the opportunity to try a higher value capacitor or proper fault
diagnosis.

I was hoping someone out there in Usenet-land had tried this and might
be able to advise before I commit to an afternoon of soldering and
cussing

Thanks,

Neil

 

[Andrew Holme]

Hi, everyone!

I saw this circuit in the Tech FAQ
(http://www.tech-faq.com/make-linemans-handset.shtml)

OFFICIAL Agent 04 Generic Test Set Modification (tm)

Ring >---------------------------------> to "test set" phone
Tip >------! SPST Switch !-------->
!-----/ ----------!
cable pair ! C R ! R = 10 kOhm 1/2 W
(alligators) !--! (------------! SPST = Talk / Monitor

Has my somewhat outdated electronics experience failed me, or is there
still a path for DC current even with the switch open?

With that in mind, I changed the schematic to:

Ring >---------------------------------> to "test set" phone
Tip >------! SPST Switch !-------->
!-----/ ----------!
cable pair ! C R ! R = 10 kOhm 1/2 W
(alligators) !--! (--/!/!/!/!--! SPST = Talk / Monitor

Would the few mA of AC passed to the hybrid in the phone, as the author
claims, be sufficient to produce a signal at the earpiece of an average
phone? In my (admittedly lazy and highly unscientific) test, I got no
output at all at the earpiece with 22nF and 10kR, although I didn't get
the opportunity to try a higher value capacitor or proper fault
diagnosis.

I was hoping someone out there in Usenet-land had tried this and might
be able to advise before I commit to an afternoon of soldering and
cussing

You should get output at the earpiece if you stick to the original circuit
with the cap and resistor in parallel. Yes, there is a DC path, but as the
author points out, the resistor is too high to cause loop detection at the
exchange.