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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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1st Jul 2020, 3:43 pm | #1 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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746 telephone - loud cracking noise in earphone
Hello. I'm part way through converting a 1971 model 746. It came supplied with a 205 rectifier but there is still quite a loud noise in the earphone when the hookswitch is pressed and released. I've tried changing the rectifier and it makes no difference so I've put the original one back in. I also changed the handset but, again, no difference. The phone dials out easily and smoothly with no nasty noises in the earphone and rings well too. This is something I can put up with as I'm not likely to sell the phone afterwards. I'm just concerned it's making more noise than the other 700s I've done.
Thankyou. |
1st Jul 2020, 8:06 pm | #2 |
Heptode
Join Date: Jan 2008
Location: Hakadal, Norway
Posts: 643
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Re: 746 telephone - loud cracking noise in earphone
Just for elimination of possible sources, how does it sound if you short out the transmitter element?
dsk |
1st Jul 2020, 8:49 pm | #3 |
Heptode
Join Date: Oct 2012
Location: London, UK.
Posts: 500
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Re: 746 telephone - loud cracking noise in earphone
This was a common problem with the 746. Dodgy microswitch as hook switch.
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1st Jul 2020, 9:17 pm | #4 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: 746 telephone - loud cracking noise in earphone
I haven't tried that, dag, but the original handset and the temporary replacement I fitted both use the same type of transmitter, i.e. the 21A and the same noise was heard in the earphone.
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2nd Jul 2020, 5:57 am | #5 | |
Heptode
Join Date: Jan 2008
Location: Hakadal, Norway
Posts: 643
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Re: 746 telephone - loud cracking noise in earphone
Quote:
Then you have 2 micro-switches. If you are lucky you may lokate witch of them making the noise. Sometimes it will be better by operate the switch 100 times (When phone is unplugged) Sometimes you may change it. If you find a way to spray in some contact spray, operate the switch 100 times, let it dry.. it may be better. (I have never had this problem myselves, but I have had problem with other micro-switches.) dsk Last edited by dagskarlsen; 2nd Jul 2020 at 6:03 am. |
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2nd Jul 2020, 8:01 am | #6 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: 746 telephone - loud cracking noise in earphone
Thankyou dags. I don't have any contact spray but I can improvise. It could well be the switches as the phone is nearly fifty years old and probably has been unused for many years.
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2nd Jul 2020, 8:15 am | #7 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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Re: 746 telephone - loud cracking noise in earphone
It's an investment worth making. Servisol Super 10 is widely available for about £7.50 a can.
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Mending is better than Ending (cf Brave New World by Aldous Huxley) |
2nd Jul 2020, 10:11 am | #8 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: 746 telephone - loud cracking noise in earphone
I've bounced the switch on and off umpteen times and I believe there is a small improvement. I'm considering using some switch cleaner but the snag is that the microswitches on the 746 are an enclosed unit. It would probably be a case of squirting a small amount where the join is on the enclosure and hope it finds it's way to the switch mechanism!
Thankyou all so far. |
2nd Jul 2020, 10:14 am | #9 |
Heptode
Join Date: Jan 2008
Location: Hakadal, Norway
Posts: 643
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Re: 746 telephone - loud cracking noise in earphone
Its worth the try, just keep all oils containing silicone far away.
dsk |
2nd Jul 2020, 11:19 am | #10 |
Dekatron
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,866
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Re: 746 telephone - loud cracking noise in earphone
You can get some cleaner in around the microswitch’s button.
You can drill out the rivet and dismantle but the bits can fly everywhere and reassembly is fiddly! Avoid! |
2nd Jul 2020, 11:29 am | #11 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: 746 telephone - loud cracking noise in earphone
Thanks Nick and dags. I've had a magnified look at the switch and there is indeed a small gap around the button so that's where I shall aim. No more drilling. I've already had to drill out the screw in the back of the case!
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2nd Jul 2020, 12:54 pm | #12 |
Heptode
Join Date: Jan 2008
Location: Hakadal, Norway
Posts: 643
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Re: 746 telephone - loud cracking noise in earphone
Do not drill out the Tibets. Drilling a Hole om the Walle May be possible... I guess.
dsk |
2nd Jul 2020, 2:24 pm | #13 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,225
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Re: 746 telephone - loud cracking noise in earphone
My experience with the 746 gravity switch is that it does not fly apart if you drill out the rivet. The body slides off revealing the contact assembly. I would be careful operating the contacts with the body off as you can knock things out of position but other than that there is no reason not to drill out the rivet. You can replace with with a 6BA (I think) nut and bolt.
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2nd Jul 2020, 6:23 pm | #14 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: 746 telephone - loud cracking noise in earphone
Thanks Tony but I won't be risking any damage. The noise factor has reduced sufficiently with what I've done so far.
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15th Jul 2020, 6:15 pm | #15 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 598
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Re: 746 telephone - loud cracking noise in earphone
Eventually got some Servisol 10. Several squirts and lots of pressing later and the cracking noise is reduced to what is expected with a 700-type phone. Good stuff.
Last edited by Sparks; 15th Jul 2020 at 6:43 pm. |
15th Jul 2020, 6:42 pm | #16 | |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: 746 telephone - loud cracking noise in earphone
Quote:
I bought a can the other day but haven't used it yet.
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Regds, Russell W. B. G4YLI. |
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15th Jul 2020, 6:51 pm | #17 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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Re: 746 telephone - loud cracking noise in earphone
Assuming this is specifically a contact cleaner as distinct from the standard WD40, maybe I'll give it a try when I need to replace my current can of Servisol - if they're still stocking it by then.
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Mending is better than Ending (cf Brave New World by Aldous Huxley) |
15th Jul 2020, 7:30 pm | #18 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: 746 telephone - loud cracking noise in earphone
It is, Dave. The WD brand has expanded and includes PTFE lubricant too. Maybe it's 'badge-engineered'?
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Regds, Russell W. B. G4YLI. |
16th Jul 2020, 4:59 pm | #19 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,225
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Re: 746 telephone - loud cracking noise in earphone
I don't find the WD40 contact cleaner to be anything like as good as the 'pure' propan-2-ol (isopropyl alcohol) stuff that I normally use.
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16th Jul 2020, 11:09 pm | #20 |
Nonode
Join Date: Sep 2017
Location: Stockport, Cheshire, UK.
Posts: 2,004
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Re: 746 telephone - loud cracking noise in earphone
I've got a can of WD40 contact cleaner that seems to work OK whenever I've used it.
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