Field phones Type F - UK Vintage Radio Repair and Restoration Discussion Forum

G4YVM David · 17th Jun 2017, 6:10 pm

I have powered up my field telephones and they almost work! The only thing which doesnt is the microphone circuit in one of them. (Mic inserts swapped and tested satis, ditto coils.)

Anyway, question... is it okay to disconnect EVERYTHING except the bell circuit and the phone, i.e. run the phone as a straight phone, in an effort to find the fault or even just bypass it? By @okay@ I mean, it wont induce further faults?

D

Dave Moll · 17th Jun 2017, 8:33 pm

If you exclude the telephone and bell circuits, what else is there to disconnect than the magneto. And leaving that connected isn't a problem as long as you don't turn the handle.

Although I've never felt the inclination to do it myself, I know others have successfully connected field telephones to "normal" telephone lines.

I'm not sure, however, whether this would be effective as a fault-finding technique.

russell_w_b · 17th Jun 2017, 10:48 pm

Quote:

Originally Posted by G4YVM David

I have powered up my field telephones and they almost work! The only thing which doesnt is the microphone circuit in one of them. (Mic inserts swapped and tested satis, ditto coils.)

Have you measured a d.c. voltage across your mic, starting at the handset plug (for easiness) and heading towards the handset?

G4YVM David · 18th Jun 2017, 7:25 am

Yes. There isn't one. I suspect wiring around the buzzer "relay" so am bypassing that. It's broken, glued and bodged.
D

russell_w_b · 18th Jun 2017, 8:54 am

The mic current from the 3V winding flows through a couple of primary windings of the induction coil / buzzer assembly, but also through contacts on the cradle switch and a set of N/C contacts on the buzzer push-button. Are these switch contacts dodgy?

If you by-pass the induction coil / buzzer by taking the battery positive lead directly to the mic terminal on the handset, you should be able to narrow the fault down. I expect you have a cct diagram but here's one anyway. You have nothing to gain by connecting the instrument across a DEL as the tele would still require a separately-excited mic, and this is where your fault lies.

G4YVM David · 18th Jun 2017, 9:58 am

Thanks Russell. I found the cct on this site in another thead and the system you advocate for narrowing down is exactly what I'm doing, so I guess I'm going right!

Thanks for your info.

D

17th Jun 2017, 6:10 pm	#1
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	Field phones Type F I have powered up my field telephones and they almost work! The only thing which doesnt is the microphone circuit in one of them. (Mic inserts swapped and tested satis, ditto coils.) Anyway, question... is it okay to disconnect EVERYTHING except the bell circuit and the phone, i.e. run the phone as a straight phone, in an effort to find the fault or even just bypass it? By @okay@ I mean, it wont induce further faults? D

17th Jun 2017, 8:33 pm	#2
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: Field phones Type F If you exclude the telephone and bell circuits, what else is there to disconnect than the magneto. And leaving that connected isn't a problem as long as you don't turn the handle. Although I've never felt the inclination to do it myself, I know others have successfully connected field telephones to "normal" telephone lines. I'm not sure, however, whether this would be effective as a fault-finding technique. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

18th Jun 2017, 7:25 am	#4
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	Re: Field phones Type F Yes. There isn't one. I suspect wiring around the buzzer "relay" so am bypassing that. It's broken, glued and bodged. D

18th Jun 2017, 8:54 am	#5
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Field phones Type F The mic current from the 3V winding flows through a couple of primary windings of the induction coil / buzzer assembly, but also through contacts on the cradle switch and a set of N/C contacts on the buzzer push-button. Are these switch contacts dodgy? If you by-pass the induction coil / buzzer by taking the battery positive lead directly to the mic terminal on the handset, you should be able to narrow the fault down. I expect you have a cct diagram but here's one anyway. You have nothing to gain by connecting the instrument across a DEL as the tele would still require a separately-excited mic, and this is where your fault lies. Attached Thumbnails __________________ Regds, Russell W. B. G4YLI.

18th Jun 2017, 9:58 am	#6
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	Re: Field phones Type F Thanks Russell. I found the cct on this site in another thead and the system you advocate for narrowing down is exactly what I'm doing, so I guess I'm going right! Thanks for your info. D