|Battery Secondary No. 23 is a Blue plastic cased battery pack that was used
in some 700 type push button telephones. Most of these are now dead or
failing and Peter Bradley has supplied two alternative methods of replacing this
Using a junior hacksaw, carefully cut into the joint near the top of the pack on all four sides (do not cut right into the pack, but just enough into the joint to allow you to lever off the lid with a flat screwdriver blade.
Remove the cells (careful they may have leaked, do not get on hands). The Battery
Secondary No. 23 contains 6 x 100DK Varta 100mAh NiCad cells (total 7.2v) which are
now obsolete. I was recommended by Varta, V150H cells (NiMH 150mAh... much more
To check the charging circuit reconnect the pack to the phone via the connector and then using a multimeter set to 20mA range measure the current across T26 (-ve) and T27 (+ve) located on the side of the metal work of the handset rest/hook (remove the strap while testing). With the handset on the rest, a current of 5-10mA should flow after 30-100 seconds. Replace strap across T26-T27 and measure the charging voltage across the pack terminals which should be in the region of 8v. If all is ok with the charging circuit. replace the cells into the blue enclosure (clean the inside with a tissue if there has been leakage from the old pack, remove the foam liner from the lid and glue the lid back in place using general purpose adhesive, making sure the flying leads exit the enclosure via the groove cut out for the purpose.
These will just fit in the enclosure with some care and patience or failing that, simply throw the enclosure away and use a zip tie to hold the pack in place (Tip; when soldering tagged batteries together use a little wire wool to clean around the weld mark, you will get much better solder wetting and neater result). Use shrink wrap, electricians tape or even several layers of cling film to insulate the pack from the phone metal work, if you use this method.
Allow the batteries to trickle charge by leaving the phone on hook, plugged into the line and after a few hours you should have a fully functional PBU (press button unit). With the Varta cells it will take about 90 hours to fully charge (6 x 150mAh/10mA charge current).
Last revised: November 15, 2020