772 not charging - UK Vintage Radio Repair and Restoration Discussion Forum

onewatt · 18th Jul 2010, 1:22 pm

Being lazy I posted this under the 772 battery thread but without success so will try with the correct title.

I have the 772 working on pickup but batt charge is not behaving. I've used a couple of 3.6v nicad cells on a piece of veroboard and loaded them in the battery carrier, the dial out worked until the batteries went flat.
I connected the charging circuit as per N872 table 3 - Strap T26/27, Yell to T9 & green to T19. I thought I did see around 9vdc across the battery connections (batt disconnected) on a couple of occasions before pickup and then off at pickup which I expected, but now I'm only seeing about 0.6 of a volt before pickup and half that after.

No smoke appeared!

Any idea's Gents

AndiiT · 21st Jul 2010, 10:32 pm

Hi,
It may be that the exchange that your telephone is connected to is not able to provide enough current to charge the battery in the phone, this may be due to a combination of factors such as line length or even the type of equipment used to provide your exchange line.

You must remember that these phones were desgined to work on older exchanges where the line current may have been higher (and consequently have a reserve of current for charging) than that provided by todays "modern" electronic exchanges.

An alternative method of charging your phone batteries would be to press a 10 Volt "wall wart" power pack in to service and wire the telephone for Power pack charging as per note 3-5 on the N diagram.

Regards
Andrew

onewatt · 21st Jul 2010, 11:00 pm

Hi Andrew
Thanks for info & suggestions. I'm only getting a fraction of a volt across the battery connection with the battery disconnected so I suspect it's something in the charging circuit not behaving. I did see about the right volts once but now it sits at about 0.6v and drops to .3 on pickup. A bit more info on the charging circuit would help me to see whats happening but I've not been able to find anything.
I was trying to reduce the number of wall warts and let my service provider pay for charging

Regards
Chris

AndiiT · 22nd Jul 2010, 10:59 am

Hi,
As the idle (on hook) line voltage is between 42 and 48 volts, nominally, there must be some means of reducing the Battery charging current between the Yellow and Green, and the Red and Black, leads on the Push Button Assembly.

Try making some DC resistance measurments between these leads, I suspect that the Yellow is associated with the Red lead and the Green with the Black, and see what sort of readings you obtain.
Alternatively, if you can open up the Push button assembly enough to physically trace the charging circuit take look around there for open circuit/high value resistors or, if fitted an open circuit transistor (if there is one fitted)

Unfortuantley there aren't many (if any at all) details floating around of the internal circuitry of these push button assemblies as they were intended to be repalced as a complete item if found to be faulty.

Regards
Andrew

18th Jul 2010, 1:22 pm	#1
onewatt Hexode Join Date: Jun 2005 Location: Buntingford, Hertfordshire, UK. Posts: 362	772 not charging Being lazy I posted this under the 772 battery thread but without success so will try with the correct title. I have the 772 working on pickup but batt charge is not behaving. I've used a couple of 3.6v nicad cells on a piece of veroboard and loaded them in the battery carrier, the dial out worked until the batteries went flat. I connected the charging circuit as per N872 table 3 - Strap T26/27, Yell to T9 & green to T19. I thought I did see around 9vdc across the battery connections (batt disconnected) on a couple of occasions before pickup and then off at pickup which I expected, but now I'm only seeing about 0.6 of a volt before pickup and half that after. No smoke appeared! Any idea's Gents

21st Jul 2010, 10:32 pm	#2
AndiiT Octode Join Date: Aug 2005 Location: Saltburn-East, Cleveland, UK. Posts: 1,786	Re: 772 not charging Hi, It may be that the exchange that your telephone is connected to is not able to provide enough current to charge the battery in the phone, this may be due to a combination of factors such as line length or even the type of equipment used to provide your exchange line. You must remember that these phones were desgined to work on older exchanges where the line current may have been higher (and consequently have a reserve of current for charging) than that provided by todays "modern" electronic exchanges. An alternative method of charging your phone batteries would be to press a 10 Volt "wall wart" power pack in to service and wire the telephone for Power pack charging as per note 3-5 on the N diagram. Regards Andrew

21st Jul 2010, 11:00 pm	#3
onewatt Hexode Join Date: Jun 2005 Location: Buntingford, Hertfordshire, UK. Posts: 362	Re: 772 not charging Hi Andrew Thanks for info & suggestions. I'm only getting a fraction of a volt across the battery connection with the battery disconnected so I suspect it's something in the charging circuit not behaving. I did see about the right volts once but now it sits at about 0.6v and drops to .3 on pickup. A bit more info on the charging circuit would help me to see whats happening but I've not been able to find anything. I was trying to reduce the number of wall warts and let my service provider pay for charging Regards Chris

22nd Jul 2010, 10:59 am	#4
AndiiT Octode Join Date: Aug 2005 Location: Saltburn-East, Cleveland, UK. Posts: 1,786	Re: 772 not charging Hi, As the idle (on hook) line voltage is between 42 and 48 volts, nominally, there must be some means of reducing the Battery charging current between the Yellow and Green, and the Red and Black, leads on the Push Button Assembly. Try making some DC resistance measurments between these leads, I suspect that the Yellow is associated with the Red lead and the Green with the Black, and see what sort of readings you obtain. Alternatively, if you can open up the Push button assembly enough to physically trace the charging circuit take look around there for open circuit/high value resistors or, if fitted an open circuit transistor (if there is one fitted) Unfortuantley there aren't many (if any at all) details floating around of the internal circuitry of these push button assemblies as they were intended to be repalced as a complete item if found to be faulty. Regards Andrew