GPO 706L crackling - UK Vintage Radio Repair and Restoration Discussion Forum

Scott Hatley · 8th Jan 2011, 12:56 pm

Hi everyone,
I bought a GPO 706L telephone a few months ago and all has been working perfectly until last week when a crackling seems to appear on a call. It gets worse as the call goes on until you cant hear the other person at all. Both people can hear the crackling.
I think it may be the mic as when i blow in to it it makes the crackling worse.
Thanks for any help.

Station X · 8th Jan 2011, 1:01 pm

Short-circuit the microphone, most easily done by putting both spades under one terminal. Dial a couple of digits. If all is quiet the mic is at fault and should be replaced.

Another possibility is dirty contacts on the regulator (small plug in circuit board). Remove and reinsert it a few times

Scott Hatley · 8th Jan 2011, 1:21 pm

Hi thanks for the reply,
I shorted the mic and there was no crackling so must be that, do you know how I can get a replacement?

davidw · 8th Jan 2011, 1:33 pm

Quote:

Originally Posted by Scott Hatley

It gets worse as the call goes on

Classic symptoms of a dud carbon transmitter inset.
Another possibility is the hand set cord. Whilst doing as Graham suggests, shake the cord about.

Station X · 8th Jan 2011, 1:35 pm

Use an electronic replacement No. 21A. They crop up fairly regularly on the likes of eBay.

http://www.britishtelephones.com/trans.htm#t16

MartinUKL · 9th Jan 2011, 8:23 pm

I seem to remember that the early 706s used an older (all metal) mic that actually plugged in to the back of the handset which had a protruding pin (a hangover from the 300 bakelite range). If that is the case you may have to modify the connections to change it which will require a small amount of vandalism. On the upside, if it is the all metal type, simply baking it in the oven will 'unstick' the carbon granules, apparently that does work but I have no idea what gas-mark.

russell_w_b · 9th Jan 2011, 9:39 pm

Quote:

Originally Posted by MartinUKL

'If that is the case you may have to modify the connections to change it which will require a small amount of vandalism.'

Not really. The metal mic insert to which you refer (Transmitter, Inset, No:13) was attached to the 706 handset by flying leads in the same manner as the more modern transmitters were attached. One lead was attached to a removable pin, which can be withdrawn from the mic and unscrewed from the lead, leaving a spade, and the other pin was attached by a spade to a captive bolt and nut on the spring-ring at the back of the mic.

The more modern plastic and metal carbon mic (Transmitter, Inset, No:16) shares the same dimensions as both types of T21 electronic insert, and they will fit right in.

8th Jan 2011, 12:56 pm	#1
Scott Hatley Retired Dormant Member Join Date: Jan 2011 Location: Knebworth Posts: 2	GPO 706L crackling Hi everyone, I bought a GPO 706L telephone a few months ago and all has been working perfectly until last week when a crackling seems to appear on a call. It gets worse as the call goes on until you cant hear the other person at all. Both people can hear the crackling. I think it may be the mic as when i blow in to it it makes the crackling worse. Thanks for any help.

8th Jan 2011, 1:01 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: GPO 706L crackling Short-circuit the microphone, most easily done by putting both spades under one terminal. Dial a couple of digits. If all is quiet the mic is at fault and should be replaced. Another possibility is dirty contacts on the regulator (small plug in circuit board). Remove and reinsert it a few times __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

8th Jan 2011, 1:21 pm	#3
Scott Hatley Retired Dormant Member Join Date: Jan 2011 Location: Knebworth Posts: 2	Re: GPO 706L crackling Hi thanks for the reply, I shorted the mic and there was no crackling so must be that, do you know how I can get a replacement?

8th Jan 2011, 1:35 pm	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: GPO 706L crackling Use an electronic replacement No. 21A. They crop up fairly regularly on the likes of eBay. http://www.britishtelephones.com/trans.htm#t16 __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

9th Jan 2011, 8:23 pm	#6
MartinUKL Retired Dormant Member Join Date: Sep 2010 Location: North London, UK. Posts: 15	Re: GPO 706L crackling I seem to remember that the early 706s used an older (all metal) mic that actually plugged in to the back of the handset which had a protruding pin (a hangover from the 300 bakelite range). If that is the case you may have to modify the connections to change it which will require a small amount of vandalism. On the upside, if it is the all metal type, simply baking it in the oven will 'unstick' the carbon granules, apparently that does work but I have no idea what gas-mark.