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Procedure to adjust card reader speed in HP65/67/97



#13

Dear fellow HP aficionados:

Some people owning old HP65/67/97 have reported difficulty
with card reader speed adjustment, especially after mounting
a new friction wheel. As my previous post on the topic did
not yield any useful information on how to do it, I worked
out a procedure that does not need a priori knowledge of
any factory adjustment parameters:

The proposition is that HP orginal cards were written with
in-spec bit density and did not change over the ~30 years.
Just as you would expect from a top-quality manufacturer.
(Hopefully so !)

So if we have a means to compare bit density on an original
card with its copy written on a HP65/67/97, then the motor
speed of the latter can be adjusted to yield the same bit
density as was on the original HP card.

As a means to compare bit density we use another HP65/67/97
(not the one to be adjusted) with good card drive (not worn)
but the speed of this one does not matter (may be a bit off)
as long as it does not change much between reads, which can be
verified by applying simple statistics on the measurements.

Bernies' procedure: put a digital sampling oscilloscope or
a 30ms retriggerable monoflop followed by a digital timer /
counter (I used a HP 5327A, has nice Nixie Tubes) on one of
the two read lines coming from the card reader amplifier chip
of the HP65/67/97 to be used as a measurement tool.

Set up to measure the total time interval from the first read
pulse to the last read pulse. Run original HP card through for
a few times (i.e. 20 times). Note the time intervals. Use a HP
calculator to get mean and standard deviation. In my case, I got
mean = 1110,6 milliseconds with a standard deviation sd = 4.4 ms.
Beware: other cards / calculators may give different results.

3 * sd / mean is a indication of the accuracy you can achieve
with this improvised measurement setup - in this case, about 1.2%
accuracy with nearly full confidence, enough for the purpose.

The mean is the value you want to get from a card written on
the HP to be adjusted. Then you have identical bit density on
the original HP card and its copy.

Now the repeat loop for speed check / adjustment:

Copy the original HP card on the HP to be adjusted. Run the
copy through the measurement HP. Compare the time interval
of the copy with the mean "reference" time interval taken
from the original HP card.

If the copy gives higher readings, bit density is too low,
speed must be increased, and vice versa. After each speed
adjustment, make a new copy to get the new bit density and
repeat the measurement.

Replacing the original speed setting resistor of the HP to
be adjusted with a trimpot / resistor combination is helpful,
but only needed if speed needs be corrected, so don't solder
before you have the first result. Some well - known tricks
as marking the previous settings on the pot or measuring
and noting the resistance before and during adjustment will
help to quickly zero in to the final setting, on which both
cards (original and copy) will yield the same time interval,
and hence, will have the same bit density.

The beauty of this scheme is that no reference other than the
original HP card (standard pac) is needed. The measurement HP
can be adjusted afterwards by interchangig roles. One final
hint: checking the measurement HP with the original card now
and then will give confidence during the adjustment procedure.
If the "reference" time interval starts to deviate, the motor
speed of the measurement HP is not very stable (weak battery?)
but the adjustment procedure can continue with targeting the
new "reference" time interval. Always do the statistics to be
sure to have reliable results.

I have tested the above on three HP67s, and all now work fine.
One of them has a trimpot already installed (original HP ?) and
someone had played with it. This unknown guy has forced me to
think out the above procedure.

Comments / success stories invited !

Regards,
Bernhard


#14

Wonderful!
But, What is the value of the speed-resistor and the trimpot? (more or less)

#15

I seem to remember that the HP97 service manual (on the MoHPC CD-ROMs) contains a procedure for setting the motor speed. You record a card, then coat it with 'magnasee' (a coloidal suspension of iron oxide, used to show the magnetic pattern). Measure the total length of the recorded bit. If it's too long (the lengths are given in the manual), you slow the motor down, if it's too short, you speed it up.

The manual also gives the possible resistor values to use.

Since the 67 uses essentially the same card reader circuit, I think the procedure would work for that machine too.


#16

The 97 service manual has a simple procedure for adjusting the eccentric cam -- the little white screw-like piece that determines the pressure of the card against the roller. It tells you to turn this cam to get a specific current draw (180ma +/- 20ma) from the motor. It seems to me that this adjustment has a big impact on motor speed, perhaps it's the primary determinant.

I've repaired many card readers and find that the current draw is an excellent way to get the cam pressure correct and a good test to see if the card reader will work before you've got it fully reassembled. But I've also found that the reader can work well even if the current draw is off by a larger amount (in the range of 130ma - 220ma).

I've never had to resort to looking at bit patterns to get the motor speed right -- the cam adjustment has always worked. I'm I just lucky? I don't think so, rather there must be a huge tolerance for bit pattern length built into the firmware because my luck isn't that good :)

-Katie


#17

Basically, the card reader will work just fine at just about any reasonable speed while reading. The same is true for writing as long as the card is not moving so fast that you run out of card before you get to the end of the bits.

The silicone rubber fuel line seems to be soft enough that setting the cam tension has very little effect on the card reader. I do go through the motions of trying to set the cam about the middle of its pressure range. I seldom bother measuring motor current any more.

#18

Torque of a small DC motor is roughly proportional to motor current.
Speed of a small DC motor is roughly proportional to motor voltage
minus motor current * armature resistance.

Bit density is influenced by many factors (including clock speed),
so my desire was not to resort to second order effects but to
measure bit density as directly and precisely as reasonably possible.

It's also a matter of philosophy - if you invest some of your valuable lifetime into restoring the best calculators ever built by mankind, why settle for less than the best possible precision.

Same thing if you choose a mechanic for restoring the engine of your vintage italian sportscar. The parts do cost the same. The labor is about the same. But hell, you better want to have the work done by a true master who is proud to deliver an absolute masterpiece, an engine that is better than the original from the factory.

This way of thinking is the only one (for me). All those who want to settle for less do just waste their money, ressources and lifetime.

Regards,
Bernhard

#19

I've repaired many, but I don't recall a white screw-like bit of plastic. Could you be more precise, or provide a photo? Geez, this might be a good time to order the museum CD, if they contain the service manual.

Andreas


#20

The MoHPC DVD or CD set does include the 97 service manual. Unfortunately, no service manuals have turned up for the 67, the other Topcats, nor any other handhelds prior to the 41C.

#21

The eccentric cam is simply the shaft that the pressure roller rotates on. If you look at it closely you'll see that it's not symmetric it's "eccentric". The slot in the end of this shaft allows you to use a screwdriver to rotate it and by doing so you'll move the pressure roller very slightly closer to or further from the R/W head.


#22

KAtie,
I did not realize it is slightly excentric. It IS! I never paid attention to this detail. But I suppose the power rates indicated in the service manual are for the original gummy wheel, and do not necessarily apply to our O-rings or fuel tube repairs. Or do they?

Andreas


#23

Torque of a small DC motor is roughly proportional to motor current (see message #6). So if you adjust current, then you adjust torque,
not bit density, but you are sure the motor and drive gears are not
overloaded.

Using different materials for the friction roller may influence the torque needed to pull a card through. IMHO, the following procedure
I use seems to make sense:

1. Adjust friction roller pressure such that you can gently push a card in and pull it out again, such that you can feel the friction roller just grips the card (calculator power off).

2. Then increase pressure by 1/8 turn. Now it should be impossible
to gently push a card in while power off, but with power on the
card should run through smoothly.

3. Measure motor current to make sure it does not exceed the spec given in the manual, just to be on the safe side.

4. Apply the bit density adjustment procedure (message #1). If adjustment is needed, use a resistor-trimpot series connection that allows adjustment to a resistance +/- 20% of the measured original value. This range should be sufficient in most cases. When the required value is found, replace by a series or parallel connection of temperature stable metal film resistors rated 250 mW and of course measure bit density again just to be sure.

5. Test the card reader with reading / copying a few cards from the standard pac.

I think that besides thoroughly cleaning the whole card reader and carefully greasing the gears (the teeth only, very very sparingly with a fine brush under a magnifying lens !!!) with silicone based grease this is all you could do to restore the card reader to the best possible condition (mineral oil based grease may destroy plastics).

One final note to those who think that the card reader always works even without proper adjustment I'd recommend you first read the papers on the card reader design from the HP Journal available at this Museum. They also give some numbers on the required precision.
These are quite demanding, and with my procedure you can get the bit density just right near these requirements, but not much better.

It all boils down to bit error rate. A card reader out of adjustment may appear to work, but with all the various factors thrown in, such as magnetic flux shift (over time), head alignment, head wearout, card flutter, drive train jitter, speed deviations, battery voltage fluctuations, etc., it is unlikely it will deliver reliable service especially if cards are to be interchanged between different machines. So it is advisable to have the best possible adjustment.

Regards,
Bernhard

#24

Oh yes, Magnasee magic potion. Makes a mess all around.
Contaminates all the workbench and all calcs that come
even near the contaminated spots.

And then the imprecision of measuring length with a
ruler.

I tried all that and I wanted a clean procedure using
just common clean electronics lab equipment.

Bernhard


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