Testing the HP65 or HP67 card reader sense amplifier

[albertofenini]

Hi everyone,

Since sometimes the problem with bad card readers could be the amplifier,
we were wondering the following ...

Most of the scopes can generate a waveform,
is it safe to inject that waveform into the input channels of the 1826-0322 or the 1826-0158
and see if the amplifier output RA RB (WA WB also ?) are correct ?

If yes, what kind of waveform should we use, which values and into which pins ?

I know that those chips also provide other functions, but if the motor starts correctly
then checking if the amplifier does amplify should do most of the work, right ?

Also, some of the problems we have experienced where also due to the keyboard assembly,
in that case, other than the traces continuity what could be checked ?

thanks for help and take care !!

[teenix]

You need to isolate the WA WB and WE pins from the sense and CPU boards. This will also allow you to keep the 6 volts and GND connected to the sense chip with the calculator turned on.

The signal generator will need its ground connected to the calculator ground rail.

For write mode, set WE = 5V otherwise the sense head outputs will be disabled.

You can apply a 4-5V square wave of approximately 1KHz frequency on WA or WB and see what appears on the head coil outputs from the sense chip. (HA and HAC) and (HB and HBC) I don't think the motor needs to be running for this test.

cheers

Tony

[albertofenini]

(11-17-2021 01:02 PM)teenix Wrote:  You need to isolate the WA WB and WE pins from the sense and CPU boards. This will also allow you to keep the 6 volts and GND connected to the sense chip with the calculator turned on.

The signal generator will need its ground connected to the calculator ground rail.

For write mode, set WE = 5V otherwise the sense head outputs will be disabled.

You can apply a 4-5V square wave of approximately 1KHz frequency on WA or WB and see what appears on the head coil outputs from the sense chip. I don't think the motor needs to be running for this test.

cheers

Tony

Thanks Tony for your reply

How can I isolate those pins ? Simply covering those pins where the card reader PCB interconnects with the CPU board ?
Would it be better to take the chip out of the PCB ? (I have some amplifiers already desoldered from their PCB)
Which pins are the the head coil outputs between HAC, HA, CA (I guess that's channel A) and HBC, HB and CB (I guess that's channel B)

Can I use the same technique to test RA and RB ?

Thanks !!


P.S. I'd like to convert the old soldered pcb to the more convenient versions with those micro single hole sockets where the
head wires and the motor/keyboard assembly wires can be connected and disconnected without soldering, what's the name
of those micro parts ?

Thanks

[teenix]

You can't really isolate them without unsoldering them from the sense board. Definitely no good bending them or they will break.

The HP-65 schematic has pin details for the sense chip.

http://www.hpcc.org/cdroms/schematics5.0...c/hp65.pdf

The RA RB outputs require AC input signals in the millivolt range from the magnetic head. If you had a signal generator that gives AC sine wave output in that low range you could probably feed it into the (HA CA) or (HB CB) pins with the head disconnected. Or much easier, you could just activate the motor and feed a card though.

A GND signal on the MS pin will activate the motor. 5 volts will stop it. The MI pin will have to be at battery voltage also. The chip could be powered up from 6 volt, and separate 3.75 volt supplies with appropriate signals on all the input pins. Overall, it would be easier and safer to just unsolder the WA WB and WE pins.

I have not seen any solderless single wire to board sockets like that these days. They may be around but may have been ordered by HP at the time of manufacture.

40-50 years later, corrosion and oxidization will become a problem for some of these nowadays

cheers

Tony

[albertofenini]

I Tony,

I may as well unsolder those pins you are right

When testing write capability, where should I measure the output of the
signals that I have injected at WA and WB on the sense amplifier ?

Thanks and sorry to ask so many questions ...

(11-17-2021 01:54 PM)teenix Wrote:  You can't really isolate them without unsoldering them from the sense board. Definitely no good bending them or they will break.

The HP-65 schematic has pin details for the sense chip.

http://www.hpcc.org/cdroms/schematics5.0...c/hp65.pdf

The RA RB outputs require AC input signals in the millivolt range from the magnetic head. If you had a signal generator that gives AC sine wave output in that low range you could probably feed it into the (HA CA) or (HB CB) pins with the head disconnected. Or much easier, you could just activate the motor and feed a card though.

A GND signal on the MS pin will activate the motor. 5 volts will stop it. The MI pin will have to be at battery voltage also. The chip could be powered up from 6 volt, and separate 3.75 volt supplies with appropriate signals on all the input pins. Overall, it would be easier and safer to just unsolder the WA WB and WE pins.

I have not seen any solderless single wire to board sockets like that these days. They may be around but may have been ordered by HP at the time of manufacture.

40-50 years later, corrosion and oxidization will become a problem for some of these nowadays

cheers

Tony

[AndiGer]

(11-17-2021 01:23 PM)albertofenini Wrote:  P.S. I'd like to convert the old soldered pcb to the more convenient versions with those micro single hole sockets where the
head wires and the motor/keyboard assembly wires can be connected and disconnected without soldering, what's the name
of those micro parts ?

I read on this page https://www.hpmuseum.org/howtohp/howtohp.htm it's called augat socket. HTH.

[teenix]

(11-17-2021 02:03 PM)albertofenini Wrote:  I Tony,

I may as well unsolder those pins you are right

When testing write capability, where should I measure the output of the
signals that I have injected at WA and WB on the sense amplifier ?

Thanks and sorry to ask so many questions ...

Sorry, this might not have been clear on earlier post in the thread...

You can apply a 4-5V square wave of approximately 1KHz frequency on WA or WB and see what appears on the head coil outputs from the sense chip. (HA and HAC) and (HB and HBC)

No problems :-)

cheers

Tony

[albertofenini]

Tony
We have received today the test clips and this has been a big improvement
Now we can read signals without soldering wires

We’ll do more tests in the next days
Using a working HP67 will try to figure out the baseline for most of the signals

However, if we want to start from scratch and we disconnect the magnetic head from the card reader pcb
when a card is read where can we check if the head is working ?
What we should see and where ?
The black cable from the head is ground connected
Then you have colored ones
Should we put the scope probe between the black and each of the colored or between the colored taken two by two ? Red/yellow and orange/blu ?

Thank you very much !!!!

[teenix]

(11-17-2021 10:30 PM)albertofenini Wrote:  Tony
We have received today the test clips and this has been a big improvement
Now we can read signals without soldering wires

We’ll do more tests in the next days
Using a working HP67 will try to figure out the baseline for most of the signals

However, if we want to start from scratch and we disconnect the magnetic head from the card reader pcb
when a card is read where can we check if the head is working ?
What we should see and where ?
The black cable from the head is ground connected
Then you have colored ones
Should we put the scope probe between the black and each of the colored or between the colored taken two by two ? Red/yellow and orange/blu ?

Thank you very much !!!!

Yes, they are pretty handy for this type of thing.

The Classic Notes PDF has waveforms and connections shown. However the waveforms will be slightly different depending on what is on the card. (If reading cards is how you are testing)

I think the head coils are 47ohm 14mH each as per the wire colors you mention.

You can test each read signal by connecting the scope probe to the CA[6] or CB[10] pin on the sense chip.

The sense chip is a bipolar circuit but it would still be wise to treat it as though it can be destroyed by static electricity.

cheers

Tony

[Kees Bouw]

(11-17-2021 10:30 PM)albertofenini Wrote:  We have received today the test clips and this has been a big improvement

What clips do you mean exactly? Maybe attach a photo?

Thanks

[albertofenini]

Sorry guys … here are the pictures for the test clips
Test Clips
We’ll post the results of our tests soon
take care !!!