My First Telephone 706 - UK Vintage Radio Repair and Restoration Discussion Forum

Lapitude · 5th May 2008, 8:47 pm

I got my very first telephone out of storage and fitted a BT plug cable, couple of 1N4001 diodes, a 3.3kohm resistor, a type 21A microphone (which although expensive is brilliant) and a then a quick polish. Everything worked fine, but on dialling there was a strange metallic ringing (as the dial reached the stop) in the receiver that I can only describe as a sound of an amplified spring? This also happened by tapping the case. After trying a few things I found it was the regulator board. Is it possible the lamp resistor has in some way gone microphonic or is it something else? If it is, does anybody know the spec for the lamps so I can replace them? Can I just leave it out of circuit by reversing the board? I know 706's are probably ten a penny but it was my first (red as well) and remember nostalgically it was so nice not to have to go out to the call box (why did it alway rain?)

Additionally does anyone know of a low cost alternative for the 21A?

russell_w_b · 5th May 2008, 9:46 pm

You can reverse the regulator, although if you live close to the exchange it might overload and cause excessively loud speech. Have you tried it with the original carbon mic in cct? If so, does it make a difference?

Are you sure it's not the dial-off-normal contacts not closing properly, allowing dial-pulses to clatter through the receiver without it being shorted out?

Lapitude · 5th May 2008, 10:11 pm

Hi thanks so much for replying. I tried the original microphone coded DAE, as one of the things to check. Reversing the board stopped the effect so I don't think the dial contacts are the culprit. Tapping the case also caused the same effect prior to reversing the PCB. Hence my question about possible regulator board faults. I would like to repair if this is the problem, as finding a replacement may prove difficult. I will leave reversed at the moment as I do not notice any issues using it

Any ideas regarding 21A alternative, I take it is some form of electret?

AndiiT · 5th May 2008, 10:14 pm

Quote:

Originally Posted by Lapitude

Is it possible the lamp resistor has in some way gone microphonic or is it something else? If it is, does anybody know the spec for the lamps so I can replace them?

I have not known of the lamp resistor going microphonic, although I suppose it is a possibility. The best course of action would be to try and obtain a complete regulator board as I think that the thermistors used on the regulator board had a "special" temperature coefficient.

Quote:

Originally Posted by Lapitude

Can I just leave it out of circuit by reversing the board?

Thats always an option, but as russell mentions if you live close to the exchange you may get too much sidetone, if this is a problem you could try fitting a couple of low wattage 300 ohm resistors either directly in series with the A and B legs of the incoming line, either at the master socket (although not recommended as it would be classed as tampering with the BT installation) or internally to the telephone between the Red and White of the line cord on T9 and T19 respectively.

Quote:

Originally Posted by russell_w_b

Are you sure it's not the dial-off-normal contacts not closing properly, allowing dial-pulses to clatter through the receiver without it being shorted out?

This could also be the cause of your problem so may be worth invsetigating.

Quote:

Originally Posted by Lapitude

Additionally does anyone know of a low cost alternative for the 21A?

I don't think that there is a "low cost" alternative and am not sure where you will have obtained your 21A insert from, but these occasionally crop up in lots of five or six for quite reasonable (around £12 plus p&p) prices on e-bay.

Regards
Andrew

Lapitude · 16th May 2008, 2:09 am

Hi I managed to borrow a regulator board from another 706 and it cured the problem. So it was not the dial contacts. now to find a regulator board, anybody got one?

davidw · 16th May 2008, 8:27 am

It might be worth resoldering all conections on the reg board.Also is the lamp resistor held by a wire strap to stop it moving,it should be! Like AndiiT I rarely found these to be a problem in service.

Lapitude · 16th May 2008, 8:49 am

Thanks very much for the reply. I infact resoldered the lamp joints already and restrapped to the board.I'll do the other joints tonight. Good old thick pcb and tracking, built like the proverbial! Could not help thinking about the woman doing this all day, everyday at the BT factory in Wales all those years ago.

fastmacuk · 26th Jul 2008, 12:52 pm

Quote:

Originally Posted by Lapitude

Hi thanks so much for replying. I tried the original microphone coded DAE, as one of the things to check. Reversing the board stopped the effect so I don't think the dial contacts are the culprit. Tapping the case also caused the same effect prior to reversing the PCB. Hence my question about possible regulator board faults. I would like to repair if this is the problem, as finding a replacement may prove difficult. I will leave reversed at the moment as I do not notice any issues using it

Any ideas regarding 21A alternative, I take it is some form of electret?

I have replacement regulators and also 21A mics. David

5th May 2008, 8:47 pm	#1
Lapitude Diode Join Date: May 2008 Location: Darlington Posts: 9	My First Telephone 706 I got my very first telephone out of storage and fitted a BT plug cable, couple of 1N4001 diodes, a 3.3kohm resistor, a type 21A microphone (which although expensive is brilliant) and a then a quick polish. Everything worked fine, but on dialling there was a strange metallic ringing (as the dial reached the stop) in the receiver that I can only describe as a sound of an amplified spring? This also happened by tapping the case. After trying a few things I found it was the regulator board. Is it possible the lamp resistor has in some way gone microphonic or is it something else? If it is, does anybody know the spec for the lamps so I can replace them? Can I just leave it out of circuit by reversing the board? I know 706's are probably ten a penny but it was my first (red as well) and remember nostalgically it was so nice not to have to go out to the call box (why did it alway rain?) Additionally does anyone know of a low cost alternative for the 21A?

5th May 2008, 9:46 pm	#2
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: My First Telephone You can reverse the regulator, although if you live close to the exchange it might overload and cause excessively loud speech. Have you tried it with the original carbon mic in cct? If so, does it make a difference? Are you sure it's not the dial-off-normal contacts not closing properly, allowing dial-pulses to clatter through the receiver without it being shorted out? __________________ Regds, Russell W. B. G4YLI.

5th May 2008, 10:11 pm	#3
Lapitude Diode Join Date: May 2008 Location: Darlington Posts: 9	Re: My First Telephone Hi thanks so much for replying. I tried the original microphone coded DAE, as one of the things to check. Reversing the board stopped the effect so I don't think the dial contacts are the culprit. Tapping the case also caused the same effect prior to reversing the PCB. Hence my question about possible regulator board faults. I would like to repair if this is the problem, as finding a replacement may prove difficult. I will leave reversed at the moment as I do not notice any issues using it Any ideas regarding 21A alternative, I take it is some form of electret?

16th May 2008, 2:09 am	#5
Lapitude Diode Join Date: May 2008 Location: Darlington Posts: 9	Re: My First Telephone 706 Hi I managed to borrow a regulator board from another 706 and it cured the problem. So it was not the dial contacts. now to find a regulator board, anybody got one?

16th May 2008, 8:27 am	#6
davidw Heptode Join Date: Sep 2006 Location: Woodbridge, Suffolk, UK. Posts: 643	Re: My First Telephone 706 It might be worth resoldering all conections on the reg board.Also is the lamp resistor held by a wire strap to stop it moving,it should be! Like AndiiT I rarely found these to be a problem in service. __________________ Dave G1AGK. My perception is my reality! Last edited by davidw; 16th May 2008 at 8:28 am. Reason: my poor grammar!

16th May 2008, 8:49 am	#7
Lapitude Diode Join Date: May 2008 Location: Darlington Posts: 9	Re: My First Telephone 706 Thanks very much for the reply. I infact resoldered the lamp joints already and restrapped to the board.I'll do the other joints tonight. Good old thick pcb and tracking, built like the proverbial! Could not help thinking about the woman doing this all day, everyday at the BT factory in Wales all those years ago.