300 series low on speech - UK Vintage Radio Repair and Restoration Discussion Forum

kings · 9th Jan 2008, 12:48 pm

Hi All,
I have been given a 300 series telephone which is low on speech quality,all new cords fitted and are all fine also telephone all converted properly, different mics tested on the handset all produce the same low sound if put into my other 300 perfectvolume any help would oblidge.

russell_w_b · 9th Jan 2008, 1:33 pm

A couple of things to be going on with:

How does the DC voltage at the microphone pins compare between good telephone and suspect telephone when the handset is lifted?
What is the state of the 0.1uF RF decoupling capacitor (usually combined in the same can as the ringing capacitor)? Is it S/C?

Out of interest, is the handset cord on your suspect telephone three-core or four-core? How does the wiring compare with your 'worker'? I think comparing obvious visible parts of the wiring, circuitry etc... of your 'worker' with your faulty instrument would be a good start. Then check the following:

The state of the dialling pulse-contacts through which the mic path is routed (strap out terminals 4 and 5 on the dial and see if it makes a difference).
Compare resistances, like-for-like, of the ASTIC (induction coil).

That should keep you going a while!

HTH

Tim · 9th Jan 2008, 6:19 pm

Hi
Are you using original mic inserts or has it been converted to transmitter 21? (red mics as used in tele 746) If not you really need to use the latter. Pop it apart and use the inside and front part inside the handset.

Dave Moll · 9th Jan 2008, 6:28 pm

Except that kings says that the same insert works fine in another telephone.

I wonder whether the fault may lie in the receiver (earpiece) insert as no mention has been made about substituting this. Try swapping these between the two 300s.

kings · 10th Jan 2008, 10:06 am

Hi Guys,
Still doing some of the checks for the first reply but changing the granular mic makes no difference and fitting a new type makes no difference at all as I can't get a thing out of it, I believe they need a required amount of voltage to work them so I feel perhaps here lies the problem. Cheers.

Nickthedentist · 10th Jan 2008, 10:49 am

Quote:

Originally Posted by kings

...fitting a new type makes no difference at all as I can't get a thing out of it, I believe they need a required amount of voltage to work them so I feel perhaps here lies the problem.

No. So long as you're using the type of mic with the red plastic case with a grey bit in the middle, this is not a problem per se. Assuming you are, the front part of the red casing can be pulled off the rear section, after which the inner black/grey module just lifts out.

An even neater alternative which I've discovered for fitting these is to remove the red case, cut a small section out of it with a hacksaw (takes half a minute), then re-assemble. This way, the whole thing can be fitted without having to resort to to bodges using Blu Tac, bubblewrap etc. Neat and easy. Will post a pic if anyone wants.

But as Dave says, do try switching the receiver (i.e. earpiece) first. As you no doubt know, just unscrew the cover, slide off the diaphragm, undo the two screws underneath, then lift out.

The other advice you have received above is also excellent

Nick.

Tim · 10th Jan 2008, 10:55 am

The diaphram plate in the earpeice can get a coating of rust. SLIDE off and clean.
My tele 332 works fine with a modern mic insert.
Check the connections inside the phone also.

Nickthedentist · 10th Jan 2008, 10:59 am

Quote:

Originally Posted by Tim

My tele 332 works fine with a modern mic insert.

Mine too, absolutely excellent (though there seems to be a bit of an excess of sidetone audible in the receiver).

kings · 10th Jan 2008, 11:34 am

Hi Guys,
My thanks to everyone who replied the advice was great, I tried a different chassis just the same, I tried a different handset just the same, I tried a different dial........PROBLEM SOLVED
I then fitted the original back and everthing was still fine so it was something there perhaps something touching or a bad connection but everthing OK now, again many thanks to everyone for being so friendly and helpfull.
Dave.

OldGuyGPO · 14th Jan 2008, 2:18 am

I believe the dial has a contact (off normal contact?) to stop loud clicks in the ear from the back emf of the "A" relays being pulsed. I think it shorts the earpiece, either way you will see this contact operate as soon as the dial is moved and this contact does not pulse. This is all from memory as it was 1971 when I last saw one.

bakelite_gal · 2nd Feb 2008, 10:19 am

I have had this problem before with the dial "off normal" contacts on the type 12 dials. Very often the springset gets lock together by someone accidentally catching the contacts. Tip, while you are examining the dial off normal contacts clean them with Isopropyl alcohol as this often solves the anoying "dial click" you hear when dialling out.

9th Jan 2008, 12:48 pm	#1
kings Tetrode Join Date: Feb 2004 Location: Llandudno, Conwy, UK. Posts: 92	300 series low on speech Hi All, I have been given a 300 series telephone which is low on speech quality,all new cords fitted and are all fine also telephone all converted properly, different mics tested on the handset all produce the same low sound if put into my other 300 perfectvolume any help would oblidge.

9th Jan 2008, 1:33 pm	#2
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: 300 series low on speech A couple of things to be going on with: How does the DC voltage at the microphone pins compare between good telephone and suspect telephone when the handset is lifted? What is the state of the 0.1uF RF decoupling capacitor (usually combined in the same can as the ringing capacitor)? Is it S/C? Out of interest, is the handset cord on your suspect telephone three-core or four-core? How does the wiring compare with your 'worker'? I think comparing obvious visible parts of the wiring, circuitry etc... of your 'worker' with your faulty instrument would be a good start. Then check the following: The state of the dialling pulse-contacts through which the mic path is routed (strap out terminals 4 and 5 on the dial and see if it makes a difference). Compare resistances, like-for-like, of the ASTIC (induction coil). That should keep you going a while! HTH __________________ Regds, Russell W. B. G4YLI.

9th Jan 2008, 6:19 pm	#3
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: 300 series low on speech Hi Are you using original mic inserts or has it been converted to transmitter 21? (red mics as used in tele 746) If not you really need to use the latter. Pop it apart and use the inside and front part inside the handset. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

9th Jan 2008, 6:28 pm	#4
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: 300 series low on speech Except that kings says that the same insert works fine in another telephone. I wonder whether the fault may lie in the receiver (earpiece) insert as no mention has been made about substituting this. Try swapping these between the two 300s. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

10th Jan 2008, 10:06 am	#5
kings Tetrode Join Date: Feb 2004 Location: Llandudno, Conwy, UK. Posts: 92	Re: 300 series low on speech Hi Guys, Still doing some of the checks for the first reply but changing the granular mic makes no difference and fitting a new type makes no difference at all as I can't get a thing out of it, I believe they need a required amount of voltage to work them so I feel perhaps here lies the problem. Cheers.

10th Jan 2008, 10:55 am	#7
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: 300 series low on speech The diaphram plate in the earpeice can get a coating of rust. SLIDE off and clean. My tele 332 works fine with a modern mic insert. Check the connections inside the phone also. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

10th Jan 2008, 11:34 am	#9
kings Tetrode Join Date: Feb 2004 Location: Llandudno, Conwy, UK. Posts: 92	Re: 300 series low on speech Hi Guys, My thanks to everyone who replied the advice was great, I tried a different chassis just the same, I tried a different handset just the same, I tried a different dial........PROBLEM SOLVED I then fitted the original back and everthing was still fine so it was something there perhaps something touching or a bad connection but everthing OK now, again many thanks to everyone for being so friendly and helpfull. Dave.

14th Jan 2008, 2:18 am	#10
OldGuyGPO Retired Dormant Member Join Date: Jan 2008 Location: Ringwood Posts: 5	Re: 300 series low on speech I believe the dial has a contact (off normal contact?) to stop loud clicks in the ear from the back emf of the "A" relays being pulsed. I think it shorts the earpiece, either way you will see this contact operate as soon as the dial is moved and this contact does not pulse. This is all from memory as it was 1971 when I last saw one.

2nd Feb 2008, 10:19 am	#11
bakelite_gal Retired Dormant Member Join Date: Jan 2008 Location: Barnsley Posts: 3	Re: 300 series low on speech I have had this problem before with the dial "off normal" contacts on the type 12 dials. Very often the springset gets lock together by someone accidentally catching the contacts. Tip, while you are examining the dial off normal contacts clean them with Isopropyl alcohol as this often solves the anoying "dial click" you hear when dialling out.