Telephone 312 sound problem

[john777]

Hi all
Am currently trying to get a 312 call exchange telephone up and running.
I can dial out and dial in but have a strange problem when speaking into the mouthpiece
I can only hear the sound through the earpiece on the same handset and not ton the other phone
I thought it must be a wiring problem but have checked the wiring several times and it seems OK
Can anyone give me an inkling of what I may be doing wrong.
Many thanks for reading

[russell_w_b]

Originally Posted by john777

Hi all

Can anyone give me an inkling of what I may be doing wrong.
Many thanks for reading

Hi John,

What are the Call Exchange switch contacts like? You haven't got the diode in cct when you're trying to speak to the other sub, are you? Are terminals 2 and 3 making good contact?

[Dave Moll]

Have you wired it as per the instructions at https://www.britishtelephones.com/pstconv1.htm#312? This should bypass any circuitry specific to shared-service working, though as Russell says, terminals 2 and 3 of the Call Exchange switch need to make good contact to take the diode out of circuit when off-hook.

[russell_w_b]

Are your 'Dial Off-Normal' contacts operating correctly? If D1 and D2 remain closed after dialling, you'll be shunting your AC speech signal to the outside world via the 30 Ohm resistor, part of your ASTIC, and the 2uF bell / spark quench capacitor (or master LJU capacitor) but you'd still have DC on the mic to get induced side-tone in your own receiver. That said, your received audio would be down as well. Is this received audio at a level you expect?

Are you sure what you're hearing in your own receiver is derived from an electrical signal?

Is the 0.1uF radio transmitter filter suppression capacitor across the mic the correct value? You could disconnect this if you haven't the means to test - won't make a difference unless you live near a high power radio transmitter.

[john777]

Hi all
I have linked the terminals 2 and 3 together (the terminals on the block which connect to the dial)
There was an immediate improvement in that I can now hear sound on the other phone.
However I can still hear sound coming thru the earpiece.
Can anyone advise on a next step.
Many thanks

[john777]

Have now tested this by phoning a mate on a mobile and all working well. Can't hear my own voice coming thru earpiece anymore. So looks like all good
Thanks for all your advice

[Dave Moll]

Actually, you should be able to hear your own voice in the receiver (known as sidetone).

[russell_w_b]

Originally Posted by john777

Hi all
'I have linked the terminals 2 and 3 together (the terminals on the block which connect to the dial)
There was an immediate improvement in that I can now hear sound on the other...'

They should've been linked anyway - both sets of 'Dial Off-Normal' contacts and the link to the ASTIC / 30 Ohm resistor - so it sounds like there was a broken connection. Pleased you sorted it!

As Dave said though: you're meant to hear a fraction of your own voice signal in the receiver during normal conversation.