HP 67 Circuit Board

[teenix]

The card reader seems a reasonably simple setup but there is a lot of complexity there and sometimes tracing a problem can be challenging.

A card read for example, the HP software continually asks the Card Reader Controller IC (CRC), largest on the CPU board, if there is a card inserted. After the card is detected, the HP software will instruct the CRC to set the sense IC in read mode (WE pin = LO), then another CRC command to turn the motor on. The CRC does this be pulling the MS (Motor Start) pin on the sense IC = LO. The sense IC then energizes the motor and reads and conditions the card signals and sends them to the CRC which collects them in buffered groups of 28 bits. When the buffer is full, the CRC lets the software know and it will then ask the CRC for this data so it can be stored in memory. The process continues for 32 times to collect the 896 bits on the card. The software then asks the CRC if the card is out and then tells it to turn the motor off (MS pin = HI). This is all subject to timeouts and a checksum error which displays as "Error" on the display.

The MS input on the sense chip is pin #17, the 4th down from the top right corner. This should be around 4-6V when HI and close to zero volts when activated relative to GND.

Possible causes for it not going LO may be corrosion related,

the MS connection to the sense chip board, 2nd pin from bottom is not contacting properly
the sense board traces going to the sense IC
the CPU board traces

check the motor wiring connections, especially if they are the plug in type

worse and harder to trace problems

CRC IC
sense IC
software in the ROM

If you suspect one of the connecting pins between circuit boards is faulty, check the hole that it fits into for corrosion and clean it. If it is still faulty, maybe the tension on the pin to hole has reduced over time. However, those small pins are brittle, so if you attempt to expand them to get a better contact you have to be careful or they will snap.

You could check the motor voltage when it is supposed to be turning on. If there is none, or very low, disconnect the motor and check it again. If voltage is there now, the sense IC could be struggling to drive the motor. If this is the case, it may get warm or even hot. If you still have the lab power supply connected, you should see a current change if the motor is attempting to start - running normally with a card about 180mA.

Check the battery connections and wiring to sense board, especially frayed connections at the solder joint.

cheers

Tony

[metawops]

Thank you so so much, Tony, for taking the time and answering so detailed and trying to help!!! So much appreciated!!
Please don't mind if I'm being silent for a couple of days. I don't find the time very often to work on this project. I hope I'll have the chance to do so soon and will certainly report back here how it goes.
Cheers for now,
Stefan.

[metawops]

Today I had a little time and made a few tests. Turns out that the motor sometimes starts but sometimes not. More precisely: it starts only the first time after applying power to the calculator. I was able to take a video of the current „state of the union“, please find it here (~176MB).

Two obvious things:

1. The sound is irregular so I assume the force the motor tries to apply is irregular, too. Maybe the reason could be a not perfectly round replacement tubing?
2. When trying a 2nd, 3rd, 4th, … time we see that the motor *tries* to move but it can‘t. Why did it work on the 1st try then? And what keeps it from spinning up after the first time?
   

Any advice from the experts?

[teenix]

Unscrew the motor and remove it from the plastic Mount.

Try the turn the drive roller manually. It should spin very easily. If the replacement "rubber" part is too wide or the diameter is too large it can rub on the plastic surrounds. It should be relatively easy to check for roundness too.

A larger than normal diameter can also cause excess pressure to be placed between it, the card and the little nylon roller that supports the card under the drive wheel even when the drive pin is adjusted for maximum clearance. All these effects can slow the mechanism and greatly load the motor and the sense chip motor driver. Some of this is explained in the Classic Notes starting at page 34.

If the sense chip is getting hot during the motor testing then there is a problem somewhere and will most likely lead to some (possibly expensive) failure.

If the sense chip is heating up, maybe it goes into "hot tempered and cranky mode" and won't work until it has cooled a bit. I don't know if it has overload protection built in like more modern drivers.

Have you replaced the missing screw. If not, maybe the motor is offset a bit from its normal center line.

cheers

Tony

[[kby]]

While you’ve got that motor dismounted via Tony’s previous post, does it start reliably when you insert a card? Obviously it won’t pull a card through but try it a few times and it might help you figure out whether it’s an electrical or mechanical fault.

You might also want to check the tantalum start cap on the motor.