British GPO 786 Trimphone DTMF generator

[Paul]

Hello,

I'm trying to get a model 786 'trimphone' working. I got the 'phonbe through a car boot sale yesterday.
I *thought* it was working OK but having taken it apart, cleaned it, and put it together again, I've found
the DTMF generator isn't doing anything. When off-hook, I get a dial tone. When I press a key, the dial
tone cuts out for a fixed interval and then returns. I notice there's a 4094 on the circuit board and I have
my suspicions I've ESD'd it.

Does anyone have a circuit diagram or pointers for the DTMF side of this 'phone please?

Thanks,
Paul

 

[Mike Berger]

I don't know if this applies in the UK, but early touch tone phones in the
U.S. were polarity sensitive.

Paul wrote:

Hello,

I'm trying to get a model 786 'trimphone' working. I got the 'phonbe through a car boot sale yesterday.
I *thought* it was working OK but having taken it apart, cleaned it, and put it together again, I've found
the DTMF generator isn't doing anything. When off-hook, I get a dial tone. When I press a key, the dial
tone cuts out for a fixed interval and then returns. I notice there's a 4094 on the circuit board and I have
my suspicions I've ESD'd it.

Does anyone have a circuit diagram or pointers for the DTMF side of this 'phone please?

Thanks,
Paul