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Old 11th Nov 2017, 7:59 pm   #1
bigus01
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Default GPO 332 transmitter issue

I have a 332 phone fitted with a 184 handset. The phone works fine with the original carbon transmitter. To improve outgoing sound quality I have tried to upgrade to an electret microphone. For some reason, despite connecting to the same contacts as the carbon mic it will not work. I have tried a genuine 21A GPO transmitter and also the cheaper alternatives with no success, refit the old carbon and all's fine again. Any idea's gratefully welcomed.
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Old 11th Nov 2017, 9:29 pm   #2
Dave Moll
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Default Re: GPO 332 transmitter issue

Do you have any other 'phones in which you can try the 21A? It may be faulty.
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Old 12th Nov 2017, 9:43 am   #3
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Default Re: GPO 332 transmitter issue

Unfortunately no, however the 21A is now the third transmitter I have tried so I suspect it is an issue with the phone or handset.
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Old 12th Nov 2017, 8:53 pm   #4
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Default Re: GPO 332 transmitter issue

Can you measure the d.c. voltage dropped across your 21A mic with it in circuit and the handset off-hook? I believe you need at least 4.5V to energise it, whereas a carbon inset will work with less.
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Old 12th Nov 2017, 9:33 pm   #5
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Default Re: GPO 332 transmitter issue

It is 6 volts on dial tone & then drops to 3.5.
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Old 12th Nov 2017, 10:47 pm   #6
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Default Re: GPO 332 transmitter issue

I measure 6.8V on dial-tone, dropping to 5.81 when dial-tone is replaced by N/U tone. This is on my Ericsson N1071 which has the same gizzards as a 332, but has a handset type 164 (3-terminal) not the 184 like yours. Not that the handset type should make a difference, of course.

So when you blow into yours on receipt of dial tone, and with 6V across it, can you hear your blowing in the receiver?
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Old 13th Nov 2017, 4:23 pm   #7
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Default Re: GPO 332 transmitter issue

No, can hear absolutely nothing.
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Old 13th Nov 2017, 8:24 pm   #8
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Default Re: GPO 332 transmitter issue

Could you put up a photo of the connecting block inside your telephone showing all the connections, especially how the four-wire Handset 184 has been connected to what is normally a 3-wire handset 164 arrangement?
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Old 13th Nov 2017, 8:49 pm   #9
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Default Re: GPO 332 transmitter issue

Hope these are OK.
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Old 13th Nov 2017, 10:11 pm   #10
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Default Re: GPO 332 transmitter issue

Your wiring is all wrong. Looking at your mic, it looks like you're using the green wire as a common mic / rec wire, the white wire as the other rec wire and the red wire as the other mic wire. Leave this end as is.

At the telephone connector block, you need to connect your red wire (receiver) to T4, your green wire (common transmitter/receiver) to T5 and your white wire to T6.

Give that a go and see what happens.
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Old 13th Nov 2017, 10:20 pm   #11
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Default Re: GPO 332 transmitter issue

Take no notice of the lettering on that mic pin connection in your handset that says M / R2. It's out of a Handset 164. Your common connection is the green wire linking M1 and R2 as indicated on the bakelite. Or so it looks...
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Old 13th Nov 2017, 10:40 pm   #12
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Default Re: GPO 332 transmitter issue

No joy unfortunately. Dial tone seems louder but no mic and carbon inset does not work either with this configuration.
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Old 13th Nov 2017, 10:51 pm   #13
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Default Re: GPO 332 transmitter issue

Hmmm... Wonder if the RF suppression cap is OK or S/C? This is a 0.1uF cap that shunts the mic and prevents it being used as a demodulator of strong RF from nearby transmitters, and is contained in the same can as the bell capacitor.

With the mic removed and disconnected, what sort of resistance do you measure across terminals T4 and T5 on the telephone connecting block?
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Old 13th Nov 2017, 11:07 pm   #14
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Default Re: GPO 332 transmitter issue

I have 150 ohms.
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Old 13th Nov 2017, 11:15 pm   #15
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Default Re: GPO 332 transmitter issue

You should be seeing an open circuit! With the mic out, there should be no path round. Can you remove the chassis and get to the dial? It looks like one of your 'dial off-normal' contacts is closed and you're measuring the d.c. resistance of the ASTIC via the DON contact. This contact shunts the mic and receiver when dialling so you don't get clicks.

You could prove this by disconnecting the dial entirely at the five-terminal dial terminal block (on the chassis behind the dial), but remember to connect dial-strip terminals 4 and 5 together. These are the dial pulse contacts that should be closed to make the loop.
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Old 13th Nov 2017, 11:18 pm   #16
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Default Re: GPO 332 transmitter issue

Will check this out and let you know.
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Old 13th Nov 2017, 11:49 pm   #17
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Default Re: GPO 332 transmitter issue

Here're a couple wiring diagrams of a 332. You might find them useful. The un-doctored one shows the dial strip terminals.
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Old 14th Nov 2017, 4:29 pm   #18
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Default Re: GPO 332 transmitter issue

Dial contacts are all fine, looks like I'll have to accept defeat or let an "expert" take a look at it. Thanks for the assistance though.
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Old 14th Nov 2017, 5:01 pm   #19
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Default Re: GPO 332 transmitter issue

Did you actually disconnect the dial from the dial tagstrip and repeat the resistance check? If so, are you still reading 150 Ohm?
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Old 14th Nov 2017, 6:14 pm   #20
bigus01
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Default Re: GPO 332 transmitter issue

To be honest,no. I gave the contacts a visual inspection and checked their operation manually with a meter across. Checked continuity of all cables. Get o/c across T4 &T5 if I disconnect green wire from T6.
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