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

[Dieter]

(11-12-2017 07:10 AM)johnlyles Wrote:  ...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 cannot say much about the calculator's internals, but here is a first test that may help: Simply run the 37E until the low battery annunciator comes on. That's a red dot in the upper left corner of the display, in the leftmost segment, right above the minus sign of a negative number (which then appears like the upper half of a "÷" sign). You may run a program to discharge the batteries faster. After the red dot has appeared, plug in the AC adapter. Within a few seconds the dot should disappear, indicating that the voltage has come back to normal. This way you can at least detect if the adapter correctly connects to the calculator. Which does not neccessarily mean that it also charges the battery pack. ;-)

In order to check if the adapter's output actually reaches the battery you may simply connect a multimeter to the two battery contacts and see if the voltage rises if the adapter is plugged in. You might insert two thin metal strips (e.g. folded aluminum foil) between the calculator's contacts and the battery for better access with the probe tips.

Dieter

[PANAMATIK]

There is more hope. I don't think that electronic components are blown. It is more likely only a contact problem at the two pin charger connector. Test the connectivity with a multi meter. Perhaps you can reestablish the connectivity inside. When this is successful or in any case I wish you a merry christmas.

Bernhard

[johnlyles]

I did check the "simple" things that were suggested. The test results are very confusing. The charger output is 10.73 VAC. The 37E requires approximately 2.5VDC. When I insert the charger with no battery pack installed, with my multimeter connected to the battery pack leads, I read 2.35VAC, and 0 VDC. The calculator will NOT power on when only the charger is plugged in (no battery pack installed), and the LCD display flickers violently. Should the calculator power on with only the charger connected (no battery pack installed)?

When the 37E is running on the battery pack, inserting the charger causes one of two things to happen: either NOTHING happens, or the calculator's display goes dead.

If I jiggle the charger leads while the charger is connected to the 37E, then I do see voltage fluctuations on my multimeter. I am pretty sure the problem is either a flaky connection where the charger plugs in, or a bad component.

The good news is that the calculator runs fine on alkaline batteries (with tin foil connecting the two leads). I would prefer to restore it to its original working status. Anyone have a fully working 37E for sale??!! Now that I have 37E "fever", I would really like to give my son one that he can depend on. This one seems to be very flaky.

Thanks... John in Dallas

[jebem]

Hi, John,

The Spice series power supply schematics are published at the end of this Kees page.

If the machine works fine using charged batteries or alkaline batteries, then the switching DC-DC power supply is fully operational.

These machines are NOT designed to work from the charger adapter alone (having the battery pack removed).
It is the battery that works as the regulator device for the input DC voltage to the switching power supply.

The charger adapter just supplies AC voltage that is rectified by diode D1 and:
- Applied thru resistor R1 current limiter (8.2 ohm) to the battery when the machine is powered off.
- Applied directly to the battery when the machine is powered on.

The charger nominal output voltage is 9VAC, so reading 10VAC is fine.
These chargers presents a high internal impedance, so it is normal to see this kind of 10VAC high voltage dropping to 2 or 3 Volts under load.

So, I would investigate the obvious suspects here:
- Check all the power supply PCB traces and switches for good continuity.
- Check all the charger adapter wiring and connector for good continuity.
- Check the rectifier diode D1 and Resistor R1.

Applying AC voltage from the charger to the machine while having the machine in powered off state, the DC voltage at the battery terminals should be around 2.4VDC for a good working battery that is not completely depleted.
You should be able to see a DC voltage increase when connecting the charger.

I would never power on these Spice series while charging the battery, to minimize the chances of destroying the internal IC's.

[johnlyles]

Awesome!!!!! Thank you!! I am off next week and will follow those recommendations. Thanks again!

[johnlyles]

Good news!!! FIXED!!! There was no glaring problem, but several small problems. The good news is that after disassembling the 37E, thoroughly testing and cleaning the flex ribbon path to the DC-to-DC power module, and the charger connections, the unit is working great!!! Even the original battery pack has charged a bit, although I do not expect it to be usable. Here is what I found: the R1 resistor and D1 diode tested good. The connection points between that power module and the power leads on the back-side of the unit's motherboard appeared to be "hazy", as in oxidation. I used De-Oxit to clean those contacts. Also, the small board that provides the "+" and "-" contacts for the battery, and interfaces to the flex ribbon cable was loose, causing it to slightly cocked when the charger was plugged in, and therefore not properly aligned - perhaps a millimeter off. The two charger-connection leads on that module also looked hazy, so I also cleaned them thoroughly with Deoxit. One of the small rivets that secures the battery terminals to the flex ribbon cable was loose. I squeezed it as tight as possible. After tightening and cleaning everything, I noticed that my voltage readings were no longer jumping from one reading to another every second. I was surprised when a small chip fell off of the motherboard, but I used the schematic provided by the last poster, that included a "repair the 32E" link. In that document there was a picture showing the orientation of the TI chip(s). I read where HP received a patent for their method of clamping the chips to the motherboard. The most challenging part was removing the outer back cover from the calculator! I now need to polish up that seal at the base of the calculator. Thanks SO very much for the suggestions! John in Dallas

[jebem]

Hi again, John,
Well done! Congratulations.
I'm glad another machine was rescued from oblivion.