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Old 6th Nov 2017, 3:43 am   #1
DrBagPhD
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Default Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Hi guys! I've recently come into possession of a GPO 332 telephone, and have attempted to convert it to work on a modern system. It should be noted that the telephone was in less than ideal state when I received it, missing the cable used to connect it to the phone line, and having been kept in a loft for some time.

Using the information available here and the wiring diagram available here I removed the relevant straps, installed a 3.3kΩ resistor & a 205 rectifier, and double checked the handset was wired correctly (it appears to be).

The telephone dials out fine and can receive calls, with a beautifully loud ring might I add!

However, the receiver's volume is so low I initially thought it wasn't working at all and spent a solid 90 minutes pouring over the wiring to make sure I hadn't made a mistake somewhere. I discovered that making tones by pressing buttons on my mobile were getting through, but barely. I can hear the pulses when dialling just as clear as I can on my GPO 746, but the dial tone is so inaudible it may as well not be there, outside of when I press down on the hook and I can hear it for a half second or so. I've spent a good 4 hours trying to get this telephone working and I'm ready to accept defeat and ask you wonderful people for help. I've attached images of my (temporary, I promise!) wiring below. I've also tried wiring up my 746's handset but got the exact same problem, but it could simply be the handsets are not interchangeable. In case it helps, the telephone has a Receiver Inset No 1L installed. I'm a complete layman when it comes to vintage telephones so please excuse any incorrect terminology or faux pas I may have made.


To summarise:
Call audio is quiet almost to the point of not existing, no sound appears to be transmitted either.
The telephone dials out & receives calls fine.
Pulses can be heard clearly when dialling out.
Dial tone becomes audible for a moment when hook is pressed down.


(Somewhat unrelated: Could anyone tell me what the pictured rubber gasket is? It fell out while I was disassembling the telephone to take pictures and I've no idea where it came from, thank you)
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Old 6th Nov 2017, 6:31 pm   #2
russell_w_b
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Hi Dr Bag, and welcome to the forum.

Before proceeding and looking at that first picture of your mic chamber, I would reposition that white wire and spade terminal away from the red one! It looks like they're touching, and, if so, your mic will be shorted out. Not that that would necessarily result in a muted receiver, but just saying...
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Last edited by russell_w_b; 6th Nov 2017 at 6:43 pm. Reason: Extra meaningful 'necessary' added!
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Old 6th Nov 2017, 6:42 pm   #3
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Sort the aforementioned white wire out by moving it away from metallic contact with the red wire and see if the receiver improves before doing anything else (as mentioned earlier).

If not, try your 332 handset (receiver only, red and green wires) on T1 and T2 of your 746 to see if the receiver works on there at normal volume.

Then report back.
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Old 7th Nov 2017, 10:19 am   #4
DrBagPhD
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Hi Russell, thanks so much for the warm welcome and advice!

I positioned the white wire as suggested to no avail (though it does look far neater, now), however the receiver does indeed work fine when wired up to my 746, so I suppose that means there's something wrong with the 332 internals?
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Old 7th Nov 2017, 1:48 pm   #5
els1967
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

I've had a similar problem like this before but I can't remember what fixed it. Personally I would check the induction coil with a multi-meter to see if the ohms through it's various connections are the same as the wiring diagram as I've had a few that have been broken.
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Old 7th Nov 2017, 2:37 pm   #6
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Quote:
Originally Posted by DrBagPhD View Post
'.. so I suppose that means there's something wrong with the 332 internals?'
You need to look at the 'dial off-normal' contacts on the dial. Looking at yours, both sets of contacts look to be closed, and they should be open when the dial is at rest. You see that little cam on the dial centre behind the big central nut? It should be pushing those contacts open and letting them close when the dial is turned. It needs adjusted slightly so that the insulated bit on the point of the cam is holding the contacts open.

The function of these contacts is to short out the receiver when dialling so you don't get loud clicks (one set of contacts), and to make a CR circuit across the dial pulse contacts to quench arcing and to prevent distortion of the dialling pulse (the other set). If these contacts are closed when the dial is at rest, your receiver will be shorted out.

I see your instrument has a 'slipping cam' dial on it (Dial No:10). If you look here, there's a good bit about setting it up. But if you wish to prove this, insert a sliver of insulating material between the 'off normal' contacts (not the pulse contacts!) and see if the volume increases.
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Old 7th Nov 2017, 4:21 pm   #7
DrBagPhD
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Quote:
Originally Posted by els1967 View Post
I've had a similar problem like this before but I can't remember what fixed it. Personally I would check the induction coil with a multi-meter to see if the ohms through it's various connections are the same as the wiring diagram as I've had a few that have been broken.
Please do let me know if you remember the solution! I'll certainly give your suggestion a try if all else fails, I'm hesitant to break out my multi-meter as electronics have never been my strong suite. Thanks so much for the idea, though!

Quote:
Originally Posted by russell_w_b View Post
You need to look at the 'dial off-normal' contacts on the dial. Looking at yours, both sets of contacts look to be closed, and they should be open when the dial is at rest. You see that little cam on the dial centre behind the big central nut? It should be pushing those contacts open and letting them close when the dial is turned. It needs adjusted slightly so that the insulated bit on the point of the cam is holding the contacts open.

The function of these contacts is to short out the receiver when dialling so you don't get loud clicks (one set of contacts), and to make a CR circuit across the dial pulse contacts to quench arcing and to prevent distortion of the dialling pulse (the other set). If these contacts are closed when the dial is at rest, your receiver will be shorted out.

I see your instrument has a 'slipping cam' dial on it (Dial No:10). If you look here, there's a good bit about setting it up. But if you wish to prove this, insert a sliver of insulating material between the 'off normal' contacts (not the pulse contacts!) and see if the volume increases.
Hi Russell, thanks again for the advice. I won't have access to the phone for a few days (I'll be out of town) but as soon as I do I'll try what you've suggested and let you know. Really appreciate your detailed help!
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Old 7th Nov 2017, 6:03 pm   #8
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Apologies for double posting but I had a bit of spare time so was able to try adjusting the off-normal contacts. They were indeed both closed. Adjusting has, however, brought an odd issue to light.

I've attached an image for reference, as I'll be calling the outer contact A, and the inner contact B. It's worth noting that the microphone doesn't appear to work with any configuration of the contacts.


When both A & B are closed, the telephone acts as described above.

When only A is open, sound is heard loud and clear through the receiver.

When only B is open, the telephone will not answer a call when the handset is taken off the hook. Instead, a sort of buzz-buzz (as opposed to ring-ring) is heard coming from the receiver.

When both A & B are open, the result is the same as when only B is open, but there is also a faint white noise coming from the receiver instead of no dial tone at all.
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Old 7th Nov 2017, 6:40 pm   #9
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

You could always disconnect terminals 1, 2 and 3 at the dial strip tag-block: that will remove the D.O.N contacts from cct. It might be easier and less damaging than trying to poke sweetie wrappers, bits of paper or what-have-you between the contacts.

When you adjust the cam, you'll need to remove the dial from the telephone. To do this, remove the small screw at the six o'clock position inside the dial aperture and twist the dial anti-clockwise about an eighth of a turn, looking from the front of the telephone.
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Old 7th Nov 2017, 11:46 pm   #10
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Try operating without a dial (ie, like a CB telephone) as described above and see what happens. Then report back.
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Old 8th Nov 2017, 1:26 am   #11
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

If you disconnect the dial, don't forget to short out the pulsing contacts, as there will be no loop with these open. The symptoms you describe sound as though the pulsing contacts are open when "B" is open, so that this is also failing to make a loop. I assume that your "faint white noise" is the sound of monitoring a line when the exchange hasn't detected a loop to insert dial tone.
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Old 8th Nov 2017, 8:04 am   #12
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Or temporarily swap the dial with that from your 746.
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Old 10th Nov 2017, 3:00 pm   #13
DrBagPhD
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Hi all! I'm back and ready to continue learning and repairing!

I tried removing the dial as per Russell & Dave's suggestions. I gave the contacts a wee clean and used a spare strap to short terminals 4&5 on the dial tag-strip.

Unfortunately, the result is the same as when both off-normal contacts are open. That is, if you take the handset off the hook there is a faint white noise coming from the receiver instead of no dial tone at all, and the telephone will not answer a call when the handset is taken off the hook. Instead, a sort of buzz-buzz (as opposed to ring-ring) is heard coming from the receiver.

What's the next port-of-call?

Quote:
Originally Posted by Nickthedentist View Post
Or temporarily swap the dial with that from your 746.
This is a good idea, though I'm guessing if removing the dial completely hasn't resolved the issue then the problem lies elsewhere?
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Old 10th Nov 2017, 5:38 pm   #14
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Have you a multimeter? Can you check that both sets of gravity-switch contacts are sound and clean and working as they should?
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Old 10th Nov 2017, 7:53 pm   #15
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Have you tried removing the 205 rectifier?

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Old 10th Nov 2017, 8:09 pm   #16
DrBagPhD
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Quote:
Originally Posted by russell_w_b View Post
Have you a multimeter? Can you check that both sets of gravity-switch contacts are sound and clean and working as they should?
I do! Though to be honest I've only ever used it as a glorified continuity tester. From what I can tell, the both the left & right switches are closed when the handset is lifted, and open when it's down. Though, just to confirm, the attached image is what I'm supposed to be checking, right?

Quote:
Originally Posted by TimLiggins View Post
Have you tried removing the 205 rectifier?
I have indeed, the only difference is the acoustic shock becoming far less of a problem when it's installed, so it seems to be working correctly.
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Old 10th Nov 2017, 8:41 pm   #17
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Have you checked the carbon-granule transmitter inset. If this is open-circuit, that would cause the symptoms described, including the sound level returning to normal when its DON contacts (labelled "B" in post no. 8) are closed.
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Old 10th Nov 2017, 8:54 pm   #18
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Quote:
Originally Posted by Dave Moll View Post
'Have you checked the carbon-granule transmitter inset. If this is open-circuit'.
You could short out the mic temporarily to prove this.
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Old 11th Nov 2017, 10:25 pm   #19
DrBagPhD
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

Quote:
Originally Posted by Dave Moll View Post
Have you checked the carbon-granule transmitter inset. If this is open-circuit, that would cause the symptoms described, including the sound level returning to normal when its DON contacts (labelled "B" in post no. 8) are closed.
Quote:
Originally Posted by russell_w_b View Post
You could short out the mic temporarily to prove this.
We have a winner! Shorting the mic contacts produced a dialtone. It turns out the contacts for the transmitter were pushed quite far into the handset, and the transmitter itself was rather grimy. I cleaned it up with some isopropyl alcohol and gently bent the pins outwards. That seems to have fixed everything, especially thanks to Russell noting the cam on the dial needed adjusting.

Thanks to you wonderful lot I now have a fully functioning telephone.

Thank you all so, so much! I can't express how grateful I am that the phone is now working.
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Old 12th Nov 2017, 5:40 am   #20
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Default Re: Converted GPO 332 (mk1) receiver EXTREMELY quiet, help!

I'm glad you're now sorted!
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