HP 37E - Battery charging issue

[johnlyles]

Hello. My son works in Business/Finance, and his hobby is finding/purchasing "collectibles". So when I found a HP37E calculator with charger and original pouch in perfect condition for $1.99 at a local thrift shop, I pounced on it (for him). However, when I first connected the charger, nothing happened - the original battery pack had no charge after 16 hours. Ultimately I determined that the original battery pack was bad, so I ordered a new replacement battery pack. The 37E works perfectly with the new battery pack installed (which arrived fully-charged), but after using the calculator for several hours, when I connect the charger to verify that the battery pack will charge normally, nothing seems to happen. Is there visual means of knowing if the battery pack is charging??

I have verified that the charger is good, and the battery pack is good; therefore, if the battery pack is not charging via the charging port on the back-side of the 37E, then there must be an internal component failure, such as a blown capacitor. I am proficient with replacing chips, resistors, capacitors, etc - but I need assistance identifying the component(s) to replace. Does anyone have component-level knowledge of the 37E?

I would like to give my son this FULLY-FUNCTIONING calculator for Christmas. Thanks in advance for any hints or technical assistance! John in Dallas