Read?Write HP 67 card reader

[teenix]

The 47uF is directly across the battery when the unit is switched on. All of the LED models had this cap, although the Classics and Woodstocks had a 60uF type.

It is likely used for power supply decoupling and smooth out small load changes placed on the power supply. The sense IC is also connected directly to the battery when switched on, and when the CRC tells the sense IC to start the motor, a brief short is placed across the supply and draws a lot of battery current. As the motor speed ramps up, this load will drop and at max speed it will be a fairly constant as determined by the speed setting resistor which tells the sense IC how much voltage to feed the motor - around 2.4V. DC motors can also generate a nasty negative spike when switched off, but this might be handled by the sense IC.

The 47uF cap could act to continue to supply voltage to the power circuit while this motor spike occurs, but it is not large enough to absorb a long drop in supply voltage. The LEDs are also connected to the battery, (Classics via driving coils) and this will create supply noise as well, so the cap can help reduce this. The 22uF is acting similar for the 6V Vss supply for the ICs.

Low ESR tantalums might be of use here, I don't know if the originals were.

The battery itself is the biggest absorber for the motor spikes, so all connections should be kept clean, including the brass connector on the charger terminals and the battery should be charged and in good order.

There are 1 or 2 smaller caps on the power supply (2.2uF each), depending on the model. These are used to smooth the Vgg rail (-12V), and for the Classics, a supply for the cathode driver IC (I think around 8V). One was used for the ROM0 led driver in the Woodstocks.

There is another small 2.2uF cap that is used to time a power on reset signal for the ACT.

For the sense IC, there is a 6.8uF capacitor connected to the power input pin and 2 x 22uF caps used for the head circuit, plus a 3.3uF on the motor terminals to reduce motor brush noise. These might be due for replacement as well. The 97 has an extra 6.8uF cap which I think was used to stop the motor from briefly running when the calculator was turned on/off.

Any of this circuit noise that finds its way into the inputs to the sense IC (basically a high gain amplifier) could affect its ability to read the head signals properly. It was reported that dirt/corrosion around the head input pins could also affect the reading ability.

cheers

Tony