BT 746 - rotary dial does not retract and no dial tone - UK Vintage Radio Repair and Restoration Discussion Forum

lachlan · 31st May 2022, 1:52 pm

Hello all,
Recently bought an old 746 phone, and did the conversion for a modern line. However, there is no dial tone, and the rotary dial does not retract. Can anyone help me, or know of any specialists in Birmingham?
Thanks

Station X · 31st May 2022, 1:57 pm

If you "retract" the dial manually do you get dial tone then?

Dave Moll · 31st May 2022, 3:46 pm

Is it that you get no dial tone even with the dial in its rest position, or is the dial unable to return to this position? If the latter, this would explain the absence of dial tone as the transmitter and receiver are both shorted out while the dial is "off-normal" (i.e. away from its rest position).

Regarding the dial not returning, it would be worth opening up the 'phone and checking that there is nothing obstructing the dial mechanism (which may be enclosed in a plastic "cup" behind the dial).

dagskarlsen · 1st Jun 2022, 2:26 pm

The rotary dial has 5 wires, when the dial is moved out of the rest position 2 of these short out the receiver (loudspeaker) to eliminate noise. 2 of the 3 others shorts out the transmitter (microphone) to protect it from damage, The third is connected to the 2 others until the dial returns, when it returns it sends out small breakes (pulses) on the line to tell the exchange what number it dials, these pulses must be of a pretty accurate timing to work OK. A 0 gives 10 breakes of approx 67 milliseconds with approx 33 milliseconds between during a time of 1 second. when a 5 is only 5 of those pulses etc.

When the dial is in rest position you should have the dial tone.

winston_1 · 4th Jun 2022, 9:48 pm

A non retracting dial is often caused by a broken spring.

31st May 2022, 1:52 pm	#1
lachlan Diode Join Date: May 2022 Location: Birmingham, West Midlands, UK. Posts: 1	BT 746 - rotary dial does not retract and no dial tone Hello all, Recently bought an old 746 phone, and did the conversion for a modern line. However, there is no dial tone, and the rotary dial does not retract. Can anyone help me, or know of any specialists in Birmingham? Thanks

31st May 2022, 1:57 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: BT 746 - rotary dial does not retract and no dial tone If you "retract" the dial manually do you get dial tone then? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

31st May 2022, 3:46 pm	#3
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: BT 746 - rotary dial does not retract and no dial tone Is it that you get no dial tone even with the dial in its rest position, or is the dial unable to return to this position? If the latter, this would explain the absence of dial tone as the transmitter and receiver are both shorted out while the dial is "off-normal" (i.e. away from its rest position). Regarding the dial not returning, it would be worth opening up the 'phone and checking that there is nothing obstructing the dial mechanism (which may be enclosed in a plastic "cup" behind the dial). __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

1st Jun 2022, 2:26 pm	#4
dagskarlsen Heptode Join Date: Jan 2008 Location: Hakadal, Norway Posts: 643	Re: BT 746 - rotary dial does not retract and no dial tone The rotary dial has 5 wires, when the dial is moved out of the rest position 2 of these short out the receiver (loudspeaker) to eliminate noise. 2 of the 3 others shorts out the transmitter (microphone) to protect it from damage, The third is connected to the 2 others until the dial returns, when it returns it sends out small breakes (pulses) on the line to tell the exchange what number it dials, these pulses must be of a pretty accurate timing to work OK. A 0 gives 10 breakes of approx 67 milliseconds with approx 33 milliseconds between during a time of 1 second. when a 5 is only 5 of those pulses etc. When the dial is in rest position you should have the dial tone. Attached Thumbnails

4th Jun 2022, 9:48 pm	#5
winston_1 Hexode Join Date: Oct 2012 Location: London, UK. Posts: 498	Re: BT 746 - rotary dial does not retract and no dial tone A non retracting dial is often caused by a broken spring.