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Old 26th Oct 2011, 4:29 pm   #1
bobbyball
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Default 1980s Tyco Big Block "Lego" Phone

Hello fellow forummers!

I don't know whether this is "too modern" but I have one of these "Lego" phones, in very nice condition, but, although it works, there is quite a lound "buzzing" or "humming" noise in the background.

It's not a fault on the line, all the other phones work. You can actually dial out on the phone but the noise in the background is constant. It's obviously not line generated and internal to the circuitry of the phone.

I have tried trawling the 'net in vain for this although one site in the US mentions "if there is a buzzing noise in the handset" they can fix it for US$15, which obviously isn't much so it can't be that big a fault. It would be more then the phone is worth to send it to the US however!!

If anybody out there has any ideas I would be very greateful, before starting to replace components on the (fairly accessible) pcb...

Cheers

Robert Ball.
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Old 26th Oct 2011, 6:41 pm   #2
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Default Re: 1980s Tyco Big Block "Lego" Phone

Would it be mains hum, do you think?
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Old 26th Oct 2011, 7:51 pm   #3
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Default Re: 1980s Tyco Big Block "Lego" Phone

Hi Russell, no I don't think it's mains hum.

It sounds quite like the noise I had on my parents' line a couple of years back when one of the line wires had got trapped by recently installed central heating pipes and developed a leak to earth, can't remember how many ohms it was.

I think it's a fault in the phone itself, there are a couple of IC's and several transistors, diodes and other passive components. The unit was obviously made originally for the US market as there are only two line wires, not three as in most UK intended units which suppress "tinkling" (and I don't mean the watery variety!) and power the ringer via the capacitor in the master socket...

Robert
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Old 26th Oct 2011, 8:17 pm   #4
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Default Re: 1980s Tyco Big Block "Lego" Phone

Ive had this fault on several imported US 'novelty' phones. I put it down to them being american...maybe susceptible to 50Hz noise?
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Old 26th Oct 2011, 8:17 pm   #5
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Default Re: 1980s Tyco Big Block "Lego" Phone

Could you connect to a linesman's test-telephone or a separate 50V d.c. supply away from the line? Just trying to establish if it's hum incoming or hum generated in the telephone itself. Wonder if a few ferrite beads or some chokery somewhere might do the trick?
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Old 26th Oct 2011, 8:20 pm   #6
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Default Re: 1980s Tyco Big Block "Lego" Phone

This might sound silly, but...

Are you musical? Have you a keyboard? Could you compare your hum with a musical note, from which a frequency can be determined? It may assist in ascribing the cause.

Or you could proddle around with a 'scope, if you have one.
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Old 26th Oct 2011, 8:41 pm   #7
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Default Re: 1980s Tyco Big Block "Lego" Phone

Hi again Russell, well i'm in fact staying with a friend in Scotland at the moment, but I often bring "toys" to "play with", and the Lego phone was brought along because my friend's nephew is mad about Lego, however it is a good item to try and fix...

I'm still thinking the noise is generated by the phone, it is like the old "purring" dial tone but without the "purr", not sure of the frequency. I do have a linesmen's test phone but it's in storage at the moment so fairly inaccessible. I also have a 'scope but that is back home.

My own view is that the "fault" is internal to the phone as my friend's other phones work fine and I have tried my phone in all her sockets (that I put in years ago!), it is the same...

Robert
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Old 26th Oct 2011, 9:10 pm   #8
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Default Re: 1980s Tyco Big Block "Lego" Phone

Try reversing the line wires? No promises or anything, but you never know - perhaps the circuit depends on one line wire being "earthier" than the other and acting as a shield.
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Old 26th Oct 2011, 9:27 pm   #9
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Default Re: 1980s Tyco Big Block "Lego" Phone

Hi AJS, thanks for the suggestion.

Had a great deal of trouble just getting a replacement plug on the line cord but can unsoler the wires on the PCB fairly easily so can try that - tomorrow! I 've a feeling it will be the same however...

Cheers again

Robert
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Old 27th Oct 2011, 7:57 pm   #10
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Default Re: 1980s Tyco Big Block "Lego" Phone

American analog telephones use different levels for transmit and receive, than do British phones. If remember correctly the American levels are -16dBM receive and +8dBm transmit. So if you connect an American phone to a British line, you likely pick up more noise from the line as they are more sensitive.

I have had the reverse problem using a British phone on the American system, where it was hard to hear what someone was saying.
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Old 27th Oct 2011, 8:28 pm   #11
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Default Re: 1980s Tyco Big Block "Lego" Phone

Quote:
Originally Posted by MrElectronicman View Post
American analog telephones use different levels for transmit and receive, than do British phones. If remember correctly the American levels are -16dBM receive and +8dBm transmit. So if you connect an American phone to a British line, you likely pick up more noise from the line as they are more sensitive.
Interesting! I have two US telephones: a type 500 and a type 2500 (a la Rockford Files, Kojak, etc...) and I thought it was the other way around! I didn't think there was a problem when I spoke, as I have a loud voice, but Mrs Barnes experienced problems with the other party being unable to hear her properly, having a quieter voice.

I stand to be corrected, however: my telephones may just have dull microphone inserts in them.
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Old 27th Oct 2011, 8:44 pm   #12
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Default Re: 1980s Tyco Big Block "Lego" Phone

Hello again, not actually had chance to reverse the line connections on my phone, however I don't think it is that anyway, although it is worth checking "just in case"

I have converted several "500 series" and "Princess" phones from the US and Canada, and the only problem I have had is making the "ringers" (bells to us!) work - they have to be made much more sensitive if they are to share the line with other phones. I have also very successfully modified an "anti tinkle" circuit from an Irish "NT" phone to stop my phones making the bells on my British units tinkling when I dial out (12v Zener diode and 100 ohm resistor and extra microswitch on the cradle switch), but none of them have been "noisy" and people can hear me fine as well.

I still think the problem with my Lego phone is "self inflicted"....

Robert
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Old 28th Oct 2011, 4:17 pm   #13
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Default Re: 1980s Tyco Big Block "Lego" Phone

UPDATE!

I have tried the phone today and, bizarrely, it works perfectly. This is without reversing the line connections. I will try it again when I get home from my holiday and report back!

Robert
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Old 1st Nov 2011, 8:10 pm   #14
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Default Re: 1980s Tyco Big Block "Lego" Phone

As a final update, I have tried the phone again on my own line at home and apart from a little background "shish", which is there but not quite as much on my trusty "746" phone with electronic transmitter, it works fine. Not quite as loud as the 746 but as MrElectronicman says it probably was always like that.

Altogether a little odd. I had had the thing to bits to clean it but it was thoroughly "warm and dry" when I tested it and got the noise. A good one for the collection then...

Thanks folks for all your suggestions, have gone as as far as needed with this one!

Robert
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