Older 67 card readers and 65 card readers

[James Linder]

Near the top of the card reader control board there are 3 silver cylindrical semiconductor components with axial connectors that have red rings at the tip located near the top of the board, all installed parallel to one another.

These appear to be 3 identical capacitors however they are each marked differently. On the 67 I’m working on now, when looking at the calculator from the backside the center one has 22 uf M which is simple to understand if M is the tolerance rating. The one on the right has 15 K on it, and the one on the left has like a part number on it. Again, Other than the ink markings these 3 components are identical in physical appearance and size.

I’ve been cosmetically restoring HP calculators for years, but I’m a beginner at troubleshooting the individual semiconductor components. I recognize a resister at the very top of the CRC board on the far right, precisely where the later 67 card readers have a capacitor that often needs replacing, but on the older 67 models and the 65’s there is a resistor in that position.

I would tend to assume that the red tips on these silver capacitors are to indicate the anode or more positive end. Does anyone know if all three of these are 22 microfarad capacitors or how to correctly interpret the inked on numbers and letters of each of them?

[teenix]

Looking at the circuit diagrams from Tony Duell, the capacitors are 2 x 22uF, a 6.8uF, and a 3.3uF on the motor. You could replace them with tantalum ones, say 16 - 25V rating.

There looks to be an extra 3.3uF cap on the 97, looks like it is used as a power up reset.

The red mark would indicate the positive end.

cheers

Tony

[James Linder]

Thanks Tony! I might email you a picture.

[James Linder]

Tony, if I increase the resistor on the CRC board will it speed up or slow down the speed at which the cards are pulled through the calculator (motor speed)?

If you recall, the latest versions of the 67 used a variable resistor that could be adjusted using a tiny screw driver, however on the older units this would require replacing the fix sized resistor. I’m thinking HP used something close to a 4.6 K ohm resistor, so if I’d like to speed up the card reader what size resistor would I use? About 20% larger?

Is the key to speeding up the motor increasing the voltage for a constant current circuit as explained below?

A constant current source of 1 Ampere is connected to a 5 Ohm resistor, the voltage produced across the resistor is V = I * R = 1 * 5 = 5 Volts. If the same current source was connected to a 10 Ohm resistor the voltage produced across the resistor is V = I * R =1 * 10 = 10 Volts. Therefore, when a current in an electrical circuit is constant and if the resistance is increased the voltage will increase proportionally to the increase in resistance.

[teenix]

I haven't had a need to adjust a motor speed yet. The 97 service manual seems to suggest increasing resistance will speed up the motor but doesn't specifically say so.

You could remove the resistor and measure it, then with a 10K multiturn trimpot, set its resistance to match, then fit the trimpot to the sense board and adjust the speed as required. Then if you want to, find a 1% or better precision 1/4 W resistor to match the trimpot setting (or as close as possible) and fit it.

The sense amp has a constant voltage source and outputs around 2V to the motor.

See my Classic Notes pages 32.

Ohms law will work with a constant current source feeding a resistor giving the required voltage drop, as long as the source voltage is sufficient and the current source can handle the current.

cheers

Tony

[James Linder]

I replaced the 4270 ohm resistor with a new 6190 ohm and now the motor pulls the cards through much quicker, about the same speed as a brand new 67 did back in 1977.

I ended up having to change out the motor itself because it developed the noise of dry bearings after repeated card reader testing. Although it read and wrote to cards without any errors I will not sell a restored calculator that sounds like that to one of my customers. It would be just a matter of time that the bearings galled and would yield error messages from cards not pulling through at a steady, repeatable speed.

[James Linder]

I’ve always replaced this resistor on the 97 card reader because so many of the 97’s seem to have card readers that barely pull the cards through and the will sometimes stop before making it all the way through.

I have a good question for you. There is a component on the 97 keyboard/display circuit board that will create an error message after a card is pulled through a restored card reader. (Let’s assume the printer works fine and is not an issue.) I know this is true because sometimes I must install a replacement keyboard/display circuit board to get rid of the error message when reading cards. This tells me the card reader components are all working fine as well as the components on the motherboard that impact card reading.

So, if I could figure out what component(s) on the keyboard/display circuit board have failed, I probably have at least 6 of these circuit boards that could be repaired and restored to service. Could I mail one of these to you for you to troubleshoot and return to me? Or could you at least point me to the components on this circuit board that must work properly to avoid card reader errors on the 97?

Thanks

[teenix]

The only thing I can think of is poor power flow between the boards through the connecting pins or a dirty power switch connection. Probably corrosion or dirt on the circuit board holes or poor contact tension, and maybe old or poor quality lubricating paste on the switch contact.

The 97 service manual mentions that some sense ICs did run marginally close to their tolerance limits causing intermittent read errors and I guess slightly reduced supply voltages through the boards may occasionally push the chips of this type into these error conditions.

As far as I can see, the key/display board has nothing else to do with the card reader.

cheers

Tony