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Old 1st Jul 2020, 3:43 pm   #1
Sparks
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Default 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.
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Old 1st Jul 2020, 8:06 pm   #2
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Default 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?
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Old 1st Jul 2020, 8:49 pm   #3
winston_1
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Default 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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Old 1st Jul 2020, 9:17 pm   #4
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Default 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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Old 2nd Jul 2020, 5:57 am   #5
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Default Re: 746 telephone - loud cracking noise in earphone

Quote:
Originally Posted by Sparks View Post
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.
That should eliminate the source to be the handset.

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.)

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Old 2nd Jul 2020, 8:01 am   #6
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Default 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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Old 2nd Jul 2020, 8:15 am   #7
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Default Re: 746 telephone - loud cracking noise in earphone

Quote:
Originally Posted by Sparks View Post
I don't have any contact spray...
It's an investment worth making. Servisol Super 10 is widely available for about £7.50 a can.
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Old 2nd Jul 2020, 10:11 am   #8
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Default 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.
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Old 2nd Jul 2020, 10:14 am   #9
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Default Re: 746 telephone - loud cracking noise in earphone

Its worth the try, just keep all oils containing silicone far away.
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Old 2nd Jul 2020, 11:19 am   #10
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Default 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!
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Old 2nd Jul 2020, 11:29 am   #11
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Default 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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Old 2nd Jul 2020, 12:54 pm   #12
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Default 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.
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Old 2nd Jul 2020, 2:24 pm   #13
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Default 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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Old 2nd Jul 2020, 6:23 pm   #14
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Default 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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Old 15th Jul 2020, 6:15 pm   #15
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Default 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.

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Old 15th Jul 2020, 6:42 pm   #16
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Default Re: 746 telephone - loud cracking noise in earphone

Quote:
Originally Posted by Dave Moll View Post
It's an investment worth making. Servisol Super 10 is widely available for about £7.50 a can.
I see 'The Lakes' in Cockermouth now have in stock the new WD40 contact cleaning spray at a similar price. Available elsewhere, no doubt.

I bought a can the other day but haven't used it yet.
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Old 15th Jul 2020, 6:51 pm   #17
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Default 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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Old 15th Jul 2020, 7:30 pm   #18
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Default 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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Old 16th Jul 2020, 4:59 pm   #19
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Default 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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Old 16th Jul 2020, 11:09 pm   #20
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Default 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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