HP-65 - 1605A02841 - Weird display behavior

[albertofenini]

Hi everyone
We are working on this unit but we need your help !
It powers up, but there is a segment always lit no matter what.
Also, the - segment is missing in all the digits so that a 8 is shown as a 0
We thought it was a bad cathode driver but funny thing is that we used a spare keyboard pcb which works fine and it behaves as before
This makes us think that is the logic board faulty
What could we check ?
Pictures can be found here : HP-65 - 1605A02841 weird display behavior
Thanks for help !!!!

[teenix]

(02-09-2020 09:04 PM)albertofenini Wrote:  Hi everyone
We are working on this unit but we need your help !
It powers up, but there is a segment always lit no matter what.
Also, the - segment is missing in all the digits so that a 8 is shown as a 0
We thought it was a bad cathode driver but funny thing is that we used a spare keyboard pcb which works fine and it behaves as before
This makes us think that is the logic board faulty
What could we check ?
Pictures can be found here : HP-65 - 1605A02841 weird display behavior
Thanks for help !!!!

Hi Alberto,

The g segment being off all the time might indicate...

a) an anode driver problem
b) an open circuit on the keyboard pcb - anode driver to LEDs
(anode driver pin #20 to LED module pins #1)
c) a faulty inductor for the g segment
d) a faulty ARC in the CPU hybrid module

The c segment being on all the time could be (a) or (d)
This problem indicates the "C" drive line (anode driver pin #1) is logic HI all the time.

Does the problem occur with a different CPU connected?

The only way to see the problem clearer is to look at the 5 partially decoded anode driver lines on an oscilloscope. Looking at the CPU board top, counting from right to left, they are pins 1, 2, 3, 5, 6.

If you are unfamiliar with how the display drivers work, check out my Classic display PDF.

http://www.teenix.org/ClassicDisplay.pdf

cheers

Tony

[albertofenini]

Hi Tony, thank you very much for the tips.

The problem does not occur with another CPU connected,
on the other side, the problem occurs in other HP65s when we connect this CPU.

I think we can almost certainly say that the problem is in this CPU logic board.

We would like to take the values that you suggest, just to know better what happens,
but it is not clear to us where we should connect the scope, did you mean the pins
Disp0, Disp1, Disp2, Disp3 and Disp4 from the schematics ?

It appears that this unit was heavily tampered, the CPU and some of the RAM have been
de-soldered and re-soldered, this can be seen from the close-up we have attached.
Also the card reader has a gummy wheel rebuilt although is dirty and with some cables
disconnected.

However, tonight we worked on the card reader but with no success.
We don't know if it was working or not before our intervention.
We took all the components off, removed the oxidation and rebuilt from scratch the board.
We cleaned the engine, rebuilt the clutch, and remounted everything.
We also checked the that switches were closing correctly.

BTW what should be the correct sequence for a prerecorded card, MTR first anf HDS after ?

Anyway, if we power the motor directly the card reader can pull a card nicely, which
we know is not the proper way but it tell us that the thickness of the gummy wheel is
correct and at least mechanically speaking everything is quite ok.

However, when the power comes from the unit the motor is going very slow ...
More or less like the HP67 we have described in another thread.

To speed the motor we replaced the fixed resistor on the card reader with trimmer however without much improvement.
If you look at the link with the pictures you will see that we have added a short clip where the behavior can be seen.
Consider that the clip was recorded with an open circuit rather than the trimmer or the resistor,
so we were expecting a card to go at maximum speed although without reading the contents.

At the first try the card went thru
At the second try it went thru but stopped a little earlier
At the third try it went thru but stopped even a little earlier than before
From then on, you could hear the switch closing, but the card was not pulled nor the motor started.
Disconnecting the battery or disconnecting the card reader, or leaving the unit off for a while allowed a new cicle of operations but with the same result.

Card Reader Behavior

What could it be that goes so wrong ?

Thanks for helping us out !!!
Take care !!

[teenix]

Hi Alberto,

>but it is not clear to us where we should connect the scope, did you mean the
>pins Disp0, Disp1, Disp2, Disp3 and Disp4 from the schematics ?

Yes, they are the 5 partially decoded outputs from the CPU. The waveforms from these pins are shown in my Classic display notes and in the HP-45 patent document.


>It appears that this unit was heavily tampered, the CPU and some of the RAM
>have been de-soldered and re-soldered

I would resolder all connections that look in poor condition. Looking at the pictures, there are a few that look suspect. Maybe some Rosin solder gel applied to the contact might help the solder wet properly.

Take care here, as excess heat can damage the components and they may have already been damaged by previous soldering. The soldering iron should not be on a terminal for more than a few seconds. Waiting a bit after soldering a few pins can help to cool the heat build up inside the chip. Cool airflow helps.


>BTW what should be the correct sequence for a prerecorded card, MTR first anf
>HDS after ?

Unprotected card
MTS goes LOW, WPS goes LOW then HDS goes LOW

Protected (clipped) card
MTS goes LOW, WPS stays HI when HDS initially goes LOW, shortly afterwards WPS will also go LOW


>At the third try it went thru but stopped even a little earlier than before
>From then on, you could hear the switch closing, but the card was not pulled
>nor the motor started. Disconnecting the battery or disconnecting the card
>reader, or leaving the unit off for a while allowed a new cicle of operations but
>with the same result.

This open circuit resistor operating method appears to be overheating the sense chip motor drive circuits and if so could possibly damage it. It looks like the chip is failing to work when it gets too hot and luckily it works again when cooled. I don't think it was designed to provide 100% power to the motor, instead it will drive the motor with pulses to control the speed, and the resistor helps to set the pulse duty cycle.

It may be possible that the capacitor on the motor terminals also helps with the speed control so if damaged it might need replacing. I think a 3.3uF/16V tantalum was the correct value. Watch the polarity :-)

cheers

Tony

[albertofenini]

Hi Tony, hi everyone
we made a little progress with the card reader.
We tested the value of the resistor and it was correct, it was 4.66 KOhm vs 4.64KOhm,
so we re-soldered the resistor, we also replaced the capacitor with a new 3.3 microF one.
However nothing didn't change.
Then we replaced the sense amplifier with a spare 1826-0158 and it started to work.
Is reading consistently although we haven't tried to record a card.

Then we measured with a scope Disp0, Disp1, Disp2, Disp3 and Disp4.
We don't know how these should look like, but Disp0 is different from all the others,
while the remaining four are all equal.

Please look at the pictures, what do you think Tony ?

HP65 - 1605A02841 Disp0 thru Disp4 waves

Thanks again for all the suggestion !!

[teenix]

(02-11-2020 10:04 PM)albertofenini Wrote:  Hi Tony, hi everyone
we made a little progress with the card reader.
We tested the value of the resistor and it was correct, it was 4.66 KOhm vs 4.64KOhm,
so we re-soldered the resistor, we also replaced the capacitor with a new 3.3 microF one.
However nothing didn't change.
Then we replaced the sense amplifier with a spare 1826-0158 and it started to work.
Is reading consistently although we haven't tried to record a card.

Then we measured with a scope Disp0, Disp1, Disp2, Disp3 and Disp4.
We don't know how these should look like, but Disp0 is different from all the others,
while the remaining four are all equal.

Please look at the pictures, what do you think Tony ?

HP65 - 1605A02841 Disp0 thru Disp4 waves

Thanks again for all the suggestion !!

Good to see you found the card reader problem. Making a replacement for the sense chip has been playing on my mind but I think it would be a challenging task although possible.

Here is an excerpt from my Classic display notes showing the 5 wave forms and the RCD pulse while 0.00 is on the display. These waveforms will repeat every 280uS with a flat signal in between because of the other blank digits.



cheers

Tony

[albertofenini]

Thanks Tony !
What do you think of our waves ?
Do they suggest you anything we may check on the cpu ?
We have only measured Disp0 to Disp4
We didn’t have measured Rcd, should we ?
Please keep in mind we only have a two channels scope
Let us know and thanks again !

[teenix]

(02-11-2020 11:38 PM)albertofenini Wrote:  Thanks Tony !
What do you think of our waves ?
Do they suggest you anything we may check on the cpu ?
We have only measured Disp0 to Disp4
We didn’t have measured Rcd, should we ?
Please keep in mind we only have a two channels scope
Let us know and thanks again !

From what I can see, the wave forms appear to be ok with the exception of the first picture. If the vertical scale is the same in all pictures, it looks to be a third of the voltage of the others.

cheers

Tony

[albertofenini]

The scale is the same for all of the pictures
The first one is Disp0, pin 1 of the logic board
Would this explain the segment always lit and the one always missing in this unit ?
How could we fix it ?
Thanks so much !
BTW This is this a good candidate for a replacement of your ?

[teenix]

Hi Alberto,

If you put three 8's on the display then press [ENTER], you should see your last display change to the following image.

The trace on the left is for 0.00 showing, the one on the right should be similar for 8.88 showing.

If there is no change then the G segment data is not being sent out from the ARC in the hybrid module.

cheers

Tony

[teenix]

(02-11-2020 11:51 PM)albertofenini Wrote:  The scale is the same for all of the pictures
The first one is Disp0, pin 1 of the logic board
Would this explain the segment always lit and the one always missing in this unit ?
How could we fix it ?
Thanks so much !
BTW This is this a good candidate for a replacement of your ?

Take the CPU board out and power it up with 3.75V. Measure that pin again. It now has no electrical load attached, so if it is the same then the ARC in the hybrid is faulty or there might be a problem with the PCB trace from the hybrid to the hole that the keyboard connector mates with.

If it now looks the same amplitude as the others, then there is a fault on the keyboard.

Just as an addendum, I think disp0 on the HP65 Tony Duell's circuit corresponds to Data C on the HP diagrams and disp2 corresponds to Data A.

cheers

Tony

[teenix]

(02-11-2020 12:45 AM)teenix Wrote:  I don't think it was designed to provide 100% power to the motor, instead it will drive the motor with pulses to control the speed, and the resistor helps to set the pulse duty cycle.

It looks like the motor is not driven with PWM but more likely a constant current source to keep the speed steady.

Below is an oscilloscope trace of the HP-65 reading a card, showing the motor voltage (2V/div) and the lower one the voltage across the 4K64 trim resistor (0.2V/div). The resistor obviously sets the motor current.

cheers

Tony

[albertofenini]

Sorry, last night we didn't have time to sit at the bench ...
We'll do some tests today and we will get back with the results ...
We are thinking of de-soldering the hybrid and put some SIL sockets so that we can swap it
with a couple of spares we have, what do you think ?
Take care !

[teenix]

(02-13-2020 12:58 PM)albertofenini Wrote:  Sorry, last night we didn't have time to sit at the bench ...
We'll do some tests today and we will get back with the results ...
We are thinking of de-soldering the hybrid and put some SIL sockets so that we can swap it
with a couple of spares we have, what do you think ?
Take care !

Sounds like a plan :-)

cheers

Tony

[albertofenini]

Hi Everyone

SIL sockets arrived, Hybrid de- soldered, cleaning of the underneath PCB which was clean anyway,
sockets soldered and Hybrid re-positioned, with the same results.

The two hybrids that we have, see pictures attached, are not in the best condition:

one is known to be working but several pins are detached and the golden traces are
not there, has anyone ever restored something like this ?

the other one we don't know if it's working since I had received may years ago,
and when put in place it only makes the display flicker ...

Tony I think is time that we order an HP65 and an HP67 board of yours ....

take care !!!

Updated album is here : HP-65 - 1605A02841 - Weird display behavior

[teenix]

(02-18-2020 09:40 PM)albertofenini Wrote:  Hi Everyone

one is known to be working but several pins are detached and the golden traces are not there, has anyone ever restored something like this ?

Yikes, looks like a real pickle.

It might be able to be done but would be fiddly. If solder doesn't take to the golden traces and or the heat damages them then I guess not. It looks like a few traces have all but disappeared back to the can edge.

You might be able to glue the contacts back on and run very fine wire from the gold trace to the contact. I don't know how the glue will react to heat.

I notice there is an extra finger on the MTRS WPS and HDS contacts for the CRC. I wonder what this was supposed to be for. The 65 even has a contact pad for it on the circuit board but goes nowhere.

I wonder if it would be feasible to add an extra Forum category for repair topics. It might be easier to look up items later on.

cheers

Tony


cheers

Tony