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Old 22nd Nov 2011, 12:06 am   #1
siaynoq
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Default Rectifier 205 and low receive volume.

Hi all. I have a question/issue, I have just restored/rebuilt a c1948 GPO232 telephone. Whilst testing I noticed the receive volume was quite low.

Cut a long story short I notice when I add the 205 rectifier across the receiver terminals it effectively halves the volume. I have rebuilt a number of 300 series and never noticed this before and the volume is considerable lower when its fitted, any ideas what's going on and what I should be looking for.

I have the wiring diagram, check through my wiring it's wired to the older standard ie terms 6-8 are swapped (found that out to my chagrin), doubly cleaned all contacts. The 205's I'm using are made by me, soldering 2 IN4001's back to back but I have also tried using another from a 746, even changed the receiver its self for a later 300 series (NOS).
Without it there can be an awful crack when operating the hook switch or the dial I can see why it's needed. I actually have 2x232's with the same issue that makes me feel I'm doing something wrong.

Your thoughts would be greatly appreciated, Dave.

Last edited by Dave Moll; 22nd Nov 2011 at 11:46 am. Reason: diode nos. corrected
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Old 22nd Nov 2011, 9:25 am   #2
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Default Re: Rectifier 205 and low receive volume.

Have you a measurable d.c. component on the receiver terminals - greater than, say, 10mV? There shouldn't be one, but, if there is, perhaps it's causing a problem. Otherwise I'd have thought that the a.c. signal level would be way below the 600mV or so needed to drive your diodes into conduction. I measure 23mV for N/U tone.

Unless your diodes are nannied...
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Old 22nd Nov 2011, 7:50 pm   #3
siaynoq
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Default Re: Rectifier 205 and low receive volume.

Spot on Russell with your possible diagnosis. (Should have checked for V first) Yes I have 0.96v DC, quite how I'm getting this I don't understand, it is only this phone the other I was wrong it works fine.
Any pointers?
Regardless I am going to strip this down completely as I do not like the existing wiring from the inductor/resistor unit, to many 'repairs', wires to tight and looks a mess so I will rewire it completely and check it whilst I'm doing so.

As an aside I really do not like the old GPO habit of nut/washer/wire/washer/nut/washer/wire/washer....etc if the first nut(s) is not tight or loosens you have a tight connection at the top but a possible bad contact lower down the stack (As I just did) This practice is banned from use on the railways, I used to wire/rewire electromotive trains, in a control panel we may have upto 5-6 on one stud, BR specs specifically state not to wire that (GPO) way and has proved to be causes of intermittent faults and damn hard to locate!
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Old 22nd Nov 2011, 10:15 pm   #4
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Default Re: Rectifier 205 and low receive volume.

Are the 'dial off-normal' contacts clean and completely open when the dial is at rest?
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Old 22nd Nov 2011, 11:59 pm   #5
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Default Re: Rectifier 205 and low receive volume.

Personally I never add a rectifier to 300s or 232's. I have done a few hundred telephones and have never had the problem you describe. Rectifiers were never part of the original design and, IMHO, with correctly adjusted and clean smooth contacts, are unnecessary.

Have you reversed the red and white line in? That is the correct conversion for modern lines. I suspect this will sort your problem out.

100% agree about the screw terminals. I always fully strip them and tighten the bottom nut and build it up from there.
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Old 23rd Nov 2011, 2:23 am   #6
siaynoq
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Default Re: Rectifier 205 and low receive volume.

Seems I may have well located the issue. Upon careful study of the inductor unit I realise it it completely mis-wired! As in lets put a wire where we feel like, never take anything for granted . I'm just about to renew the wiring and connect up to the diagram so should be good to go, I'll post back later just to confirm and wrap this up.
Yes reversed the red/white, thanks for the tip though, confused me at first as used to 300's, ended up drawing a chart listing the various colours used throughout the years including these modem(USA?) cords with Black/Red/Green/Yellow

To 205 or not? The GPO added them at a later date (700's) any idea why? And why is this not an issue with more modern phones, built in perhaps? Certainly with clean contacts you get a clean break but thought we retrofitted them to eliminate the issue as the contacts wear/pit &/or when pressing the gravity hook switch.

Thanks for all your assistance though, the amount of combined knowledge and willingness to share on this site is phenomenal.
My next project is to setup a small PBX (revelation) so you may well hear from me again, although I have all the manuals, literature, diagrams and a system phone so hoping will be an enjoyable experience.
Cheers

Last edited by siaynoq; 23rd Nov 2011 at 2:38 am. Reason: spelling
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Old 23rd Nov 2011, 4:52 am   #7
siaynoq
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Default Re: Rectifier 205 and low receive volume.

Quick update: Yes wiring it correctly has made it into a fully functioning device (and reminded me to take nothing for granted) Thanks again for your insights.
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Old 23rd Nov 2011, 9:31 am   #8
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Default Re: Rectifier 205 and low receive volume.

Quote:
Originally Posted by siaynoq View Post

To 205 or not? The GPO added them at a later date (700's) any idea why?
Pleased you got sorted!

Clipping diodes were fitted to prevent the greater acoustic shock made possible by the increased sensitivity of the (new) balanced armature receivers, as fitted to type 700 telephones. As BB alludes, a properly set up instrument with types 1L or 2P diaphragm receivers - 200, 300 series - shouldn't need clipping, as the dial-off-normal contacts should take care of any nasty clicks by shunting the mic and receiver and quenching the dial pulses.

I guess clipping diodes are a bit of a 'sticking plaster' solution.

Don't get too hung up on wiring colours; they might be anything. Instead, ensure that the common speech cct / bell leg (on a 300, terminals 1, 2; on a 700, terminals 16, 17, 18, 19) is connected to pin 2 on your plug, that the dial pulsing leg (terminals 8, 9 on a 300; terminals 8, 9 on a 700) is connected to pin 5 on your plug, and your bell / anti tink wire (terminals 11, 12 on a 300; terminals 4, 5, 6 on a 700) is connected to pin 4 on your plug.

All these plug locations are referenced to looking at your BT plug with the pins uppermost and away from you. From this point of view, it ought to be possible to convert any telephone to UK plug / socket wiring. Note that plug pins 1 and 6 aren't used normally (some plugs are available with them in place), but are empty slots, so the first metal pin from your LHS is pin 2!
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Old 23rd Nov 2011, 2:53 pm   #9
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Default Re: Rectifier 205 and low receive volume.

Quote:
Originally Posted by siaynoq View Post
To 205 or not? ...why is this not an issue with more modern phones, built in perhaps?
Surely, this only an issue with Loop Disconnect dialling (particularly, using a mechanical dial). If a 'phone is modern enough to use DTMF dialling, there is no acoustic shock to suppress.
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Old 23rd Nov 2011, 4:03 pm   #10
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Default Re: Rectifier 205 and low receive volume.

I guess most modern (i.e post-1990) DTMF telephones are fitted with VOGAD speech ICs, etc, and the elctronickery takes care of the clicks - which may be line clicks as well as dial pulses.

My 8782 DTMF set has a rectifier 205 across the receiver terminals, but it is essentially the same ASTIC hybrid balancing arrangement as a 746.
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Old 25th Nov 2011, 1:51 pm   #11
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Default Re: Rectifier 205 and low receive volume.

I remember when I was a linesman with the GPO I had a very elderly gentleman subscriber (nowadays being politically correct, a "customer") who had consistently complained of low volume. Personally it did not sound that bad to me and I knew that his telephone had been replaced by the guy I was temporarily standing in for.

However I decided to call in the specialist line test engineer and with his signal injector/measuring set and he discovered the 'phone was down about 15 DB (Pond life like myself would not have been trusted with any expensive test gear). The actual fault turned out to be the receiver inset, which was faulty just as the one in his old phone was.

Just shows you can't assume that because something is new/replaced it's definately going to be ok.

regards

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