GPO 332 interference

[Tarzandy]

I have a gpo332 that I have converted. It works fine, in that it dials, rings and you can talk however there is constant interference (crackling) that can be heard through the handset and by the other caller. My knowledge is very limited but I have replaced the transmitter, cleaned the earpiece, replaced the handset wire and the wall cable but nothing has helped. I'm now at a loss - I assume that its shorting somewhere but I have no idea. I'd be grateful for any suggestions

[russell_w_b]

Hello, Tarzandy and welcome!

Can you short the mic out whilst in a call and see if the crackling disappears? If you short it at the terminal block within the telephone it'll eliminate mic and cable, and serve as a starting point for further investigation.

[Tarzandy]

Sorry but I really know very little, how would I short it?

[G6Tanuki]

Do you get the same problem when using a different phone? The problem _could_ be on your phone-line.

If you have a different phone, plug it into the wall-socket then lift the handset and listen for any crackles.

This will at least tell you if the problem is a line- or local-fault.

[Alas the 332 doesn't support DTMF so you can't call 17070 and then select 'quiet line' for a test].

[Tarzandy]

I've tried a different phone- works fine. I've double checked the wiring and everything seems ok

[russell_w_b]

Originally Posted by G6Tanuki

[Alas the 332 doesn't support DTMF so you can't call 17070 and then select 'quiet line' for a test].

You could on another phone on the same line, or you could dial 1 and wait a bit until Number Unobtainable (NU Tone comes up.

[russell_w_b]

Originally Posted by Tarzandy

Sorry but I really know very little, how would I short it?

It would be easiest if you had a lead with a croc clip on either end, but you need to make a solid connection between terminals 4 and 5 on the big terminal block on the base of the telephone.

On the attached photo, it's the terminals on the RHS with the white and red cloth-coloured wires leading to it. You could short them before you dial out but don't expect anyone to hear anything! Just dial 1 or the speaking clock or something and listen for crackling (or not).

[emeritus]

We had a severe problem with crackling more than a decade ago, but ours affected all phones and often made dial-up internet unusable. The cause turned out to be a bad connection at the insulation displacement connector for our drop wire connection at the top of the street pole. It took BT several years to find it! As others have said, you really need to try a second phone (borrow, or buy a cheap one if you don't already have one) to establish whether or not the phone or the exchange line is the problem.

If you are with BT and you call them out to investigate, they will charge you considerably more than the cost of a phone if it turns out your phone was at fault.

[ThePillenwerfer]

Other possibilities are dirty contacts on the dial or gravity switch.

If after trying Russell's suggestion you've still got crackles try moving the red wire in the line cord from T9 to T4. That will bypass the dial and if the crackling stops that's where the trouble must be.

[dagskarlsen]

Crackling noise is not always easy to find, bu you are thinking the right way.

The most common is the transmitter capsule (microphone), and if you have changed it with one that works stable on another phone that should be eliminated. Then you come with bad contacts all over, the pulse switch on the dial (especially if the dial has been oiled with something with silicone).

How to clean those contacts, often made of brass, covered by a thin layer of silver? A bill is made of more solid paper than other things of paper, so it may be a usable tool, just slide it a few times between the close contacts on the dial and even on the hook switch. Interesting to see that that little polishing is good enough for the most. In worst case I have been using strip I got from the dentist, a kind of sand paper. It solved it for me, but It is so tough to the silver that I am not sure I want to recommend it.

[Tarzandy]

So I tried shorting across t4 and t5 - crackling still there. I the moved the red wire from t9 to t4- crackling still there. However, when I put it back on t9 the crackling has gone I'm certain that I had a good connection before but what else could have changed??
Anyway, seems have sorted it - thanks everyone for your help

[russell_w_b]

Originally Posted by dagskarlsen

' Interesting to see that that little polishing is good enough for the most. In worst case I have been using strip I got from the dentist, a kind of sand paper. It solved it for me, but It is so tough to the silver that I am not sure I want to recommend it.'

What we call 'blotting paper' is very good, Dags. I've also used little pink contact-cleaning strips that are non-abrasive to good effect in the past. They were from RS but they only seem to stock the 'Electrolube' versions now - which may be OK.

Don't go near them with a diamond spatula!

[Nickthedentist]

That strip from the dentist will probably be a "Soflex" strip, basically emery paper but on plastic strip (a bit like the leader from a cassette). As you say, too rough if you don't intend to remove the contacts' plating.

[russell_w_b]

Originally Posted by Tarzandy

I'm certain that I had a good connection before but what else could have changed??

You might have a dodgy spade connection on your red lead. They're only tinsel conductors and the crimp might be less than satisfactory.

Or it might be the soldered wire beneath the terminal block that joins T8 to the dial pulse contact.

Using T9? You could remove the link between T9 and T8 and just put the red wire on T8 instead: less faff then and fewer connections to fail. T9 doesn't need to be connected (yes, I know it is on my photo... ).

Pleased you got it sorted,anyway. Do write back if it goes crackly again!

[spamfritter]

I'll just chip in here. Cleaning with an abrasive could damage the contacts. We used to use a washed and dried strip of chamois leather to polish silver or platinum contacts as it just took the tarnish off.

The older 300 series phones use a carbon granule filled chamber inside the microphone and this can also cause crackling although a light tap on side of the handset usually clears it for a while.

[Dave Moll]

Originally Posted by spamfritter

The older 300 series phones use a carbon granule filled chamber inside the microphone and this can also cause crackling although a light tap on side of the handset usually clears it for a while.

See post no. 11 - shorting out the transmitter didn't eliminate the crackle, so I don't think the transmitter was to blame.

[spamfritter]

Missed that. Must get my glasses polished !. When i used to maintain many of the older phones the carbon granule microphones were a pain. They just literally seemed to start crackling after a few years of use. We had boxes and boxes of the inserts and just changed them as a matter of course. Good to see the problem is solved.

[russell_w_b]

Originally Posted by spamfritter

'They just literally seemed to start crackling after a few years of use.'

Sit them on the radiator overnight and give them a few smart taps on a thick cloth afterwards to eke out a bit more life from them (if you're lucky). The smaller inserts in Trimphones are dreadful: far worse than the Inset 13!

[Graham G3ZVT]

I wonder if sealing them into a poly-bag with a desiccant pouch would help?

I used to maintain Panasonic "GX" phone systems that had system phones with carbon microphones. These were current as recently as the '90s and the inserts were just as noisy as any GPO 200 or 300 series phone . What were they thinking?
I always kept a supply of new boxed inserts, but they were not much better than the originals, perhaps they had absorbed moisture in storage.