Damaged HP 9810A

[douze-bis]

Hello,
I would like to fix an HP 9810A that apparently got damaged during shipment from USA to Europe.
There is a problem with the display.

- Instead of having the three display lines show the x, y and z registers respectively, all three show the same pattern, which seems to be the merging of x, y and z.

- In addition, on the three display lines, only the 8 left hand side digits seem alive, the 7 right hand side digits stay black.

The calculator logic however seems fine, and the printer works well. It is able to print results consistent with what I key in.

It appears the calculator was not packaged safely enough and got shocked during shipment. The bezel got broken in several places, and the metal plate at the bottom of the calculator is slightly bent.

I did a quick visual inspection of the boards inside the calculator, they seem to be in good shape though.

Does this ring a bell ?
Any help greatly appreciated!

Frederic

[Tony Duell]

The HP9810 CPU has a 16 bit I/O register, when driving the display the bits are used as follows :

DO(0)-DO(3) : Select one of 15 digits in a particular line (binary coded)
DO(4) : Enable X register display (active low)
DO(5) : Enable Y register display (active low)
DO(6) : Enable Z register display (active low)
DO(7), SO(0)-SO(3),CO(0)-CO(2) : segment drives (7 segments + decimal point)

It is very likely the problem is on the display PCB itself. I had thought that damaged connections to it could cause this problem, but since TTL inputs float high, this would
not explain why all register displays were enabled together, a bad connection to DO(4) would prevent X from being displayed, I think.

Anyway, with a 'scope or logic probe, I'd start to make sure all of the DO(n) lines are toggling (changing state) on the display PCB edge connector. Then check round the 7404 inverter, U15, on the display PCB. This IC both handles the register enable lines and the high bit of the digit selection.

You can download my schematic for the HP9810 from the Australian museum site, the display PCB starts on page 60 of the .pdf

[douze-bis]

Thanks a lot Tony !
I was secretely hoping you would be around
Indeed, it looks like the high bit digit selection and x/y/z enablers are stuck together.

---
I could not find the 7404 inverter on the display PCB. Is it the one identified on the attached picture ? It has the marking 1820-0174 on the picture.

I took the machine apart, including the motherboard. I removed the dust. All boards look clean.

---
I'm really out of luck... My old scope died on me yesterday...
Without a scope, I still tried a few experiments with a little piece of paper : "cutting" some electrical connections at the edge of the display PCB.

Here are my findings:
a- DO(3) disconnected : all three lines stay black
b- DO(4), DO(5) or DO(6) disconnected : 3 identical lines, the 7 right hand side digits stay black
c- DO(7) disconnected : 3 identical lines, all dots showing, the 7 right hand side digits stay black

Does this confirm 7404 as the likely suspect ?

---
I did not try to force the DO(x) to 0V or to 5V. I was afraid I might damage some components.
Would it actually be safe to do such forcing ?
Would it likely tell me anything ? (like if I force DO(4) to 5V, maybe the x register line will stay black ?

---
If 7404 eventually proves dead, do you know whether and where I can supply such components nowadays ?

---
OK. I need a logic probe if I am to do any further testing. I have found item 281303705304 on TAS : "PC USB 20MHz 2CH Oscilloscope 16Channels Logic Analyzer Data logger 3in1 MDSO-LA".
Do you think this would be useful ?

---
Well that's a lot of questions.
Again, thanks a lot for your help,

Frederic

[Paul Berger (Canada)]

Acoording to the parts crossref at http://www.sphere.bc.ca/download/hp_xref-free.pdf 1820-0174 is a 7404. They should be pretty easy to find but if you really can't find one I could send you a couple.

[douze-bis]

Thank you Paul.
Thanks for your offer, but you're right, they seem pretty easy to supply actually.
Mine are already on their way

[Tony Duell]

(04-27-2014 01:56 PM)douze-bis Wrote:  Thanks a lot Tony !
I was secretely hoping you would be around

FWIW, I don't mind e-mail (or PMs here) from people repairing old HP desktops :-)

(04-27-2014 01:56 PM)douze-bis Wrote:  Indeed, it looks like the high bit digit selection and x/y/z enablers are stuck together.

---
I could not find the 7404 inverter on the display PCB. Is it the one identified on the attached picture ? It has the marking 1820-0174 on the picture.

Yes I am pretty sure that's the IC. You have the old display board (with separate display devices for each digit, the later one has 3 display modules for each row, those modules are like the ones used in 'Classic' handhelds, but with the pins formed differently), but the circuitry is much the same. If you trace the tracks from the IC pins they seem to go to the right places. And 3 of the gates are even labelled 'X', 'Y', 'Z' in the PCB etch :-). Those are the register display enables.

(04-27-2014 01:56 PM)douze-bis Wrote:  I took the machine apart, including the motherboard. I removed the dust. All boards look clean.

---
I'm really out of luck... My old scope died on me yesterday...
Without a scope, I still tried a few experiments with a little piece of paper : "cutting" some electrical connections at the edge of the display PCB.

Another repair project for you, then :-)

(04-27-2014 01:56 PM)douze-bis Wrote:  ---
If 7404 eventually proves dead, do you know whether and where I can supply such components nowadays ?

I can't believe they are hard to get. In London (England), I would try Cricklewood Electronics first. I think a 74LS04 or even a 74HCT04 would work, but try to get the original if you can.

(04-27-2014 01:56 PM)douze-bis Wrote:  ---
OK. I need a logic probe if I am to do any further testing. I have found item 281303705304 on TAS : "PC USB 20MHz 2CH Oscilloscope 16Channels Logic Analyzer Data logger 3in1 MDSO-LA".
Do you think this would be useful ?

It's probably a lot more than you need, but it will work. 16 channels of logic analyser is a bit on the small side, though.

I use a LogicDart most of the time, that has 3 channels. But I also have larger benchtop analysers with up to 40 channels. You need that if you need to look at the address and data lines in an HP9800 at the same time :-).

Even a simple logic probe -- a thing with LEDs that show 'high', 'low' or 'pulsing' is useful for the display testing. You are really just looking to see if signals are changing at all..

-tony

[douze-bis]

The postman brought a couple of Motorola SN74LS04N ICs this morning
Desoldering the old one... resoldering a new one ... and then a moment of pure bliss
The old boy's talking to me : three nice display lines !

Thanks a lot Paul and Tony for your help ... and supernatural intuition.

[douze-bis]

Now I guess the next steps are:

1- The magnetic card drive
The drive looks a bit dirty, and the "capstan wheels" have disintegrated of old age (see the attached picture).
Any advice how to deal with it ?

2- The fan
The fan seems to be tired and requires "help" on each power up or it won't move. Then it is noisy.
Any idea where to get a spare, or how to fix it ?

3- The bezel
The bezel is broken in several places.
Any advice on the best way to glue it back together ?

Cheers

[Paul Berger (Canada)]

For the fan, looking at Tony's schematic it looks like this is and AC fan, and this kind of behaviour was common, on an old system I used to service almost every time you turned one off you ended up replacing fans because they would not start again.

The best solution would be to find a suitable replacement fan, but on some older equipment that is not always possible.

The starting part can probably be fixed, a lot of these old fans used a sintered bronze sleeve bearing and what happens is the bearing goes dry plus there is a build up of debris on the surface of the bearing that impairs movement. The fix involves taking apart the fan. On one side of the fan there will be a cap over the end of the shaft under this cap you will usually find there is a clip on the end of the shaft holding it in, remove this clip and you should be able to pull off the rotor from the other side, there will also likely be assorted plastic spacers on the shaft as well so be careful when pulling it out. once you have it out clean off the shaft and also clean the black debris from the bearing. Oil the bearing with a light machine oil, a sintered bronze bearing is designed to absorb some oil as well, reassemble and test. This should fix the starting problem but may not help with the noise.

For the case the first thing I would try would likely be Methyl Ethel Ketone (MEK) often called plastic weld, it is a solvent glue so be careful about getting it on the surface as it will mar the finish.

[Katie Wasserman]

(05-02-2014 11:17 AM)douze-bis Wrote:  1- The magnetic card drive
The drive looks a bit dirty, and the "capstan wheels" have disintegrated of old age (see the attached picture).
Any advice how to deal with it ?

Here's what I did. This repair has worked flawlessly for many years and I virtually never get a bad card read.

[douze-bis]

(05-02-2014 03:42 PM)Katie Wasserman Wrote:  Here's what I did. This repair has worked flawlessly for many years and I virtually never get a bad card read.

Thanks Katie,
I will try your method.
One question : did you consider using a cut of tygon tubing instead of O-rings ?
It would give a "square" section instead of a "round" section, and might be closer to the original wheel ?
Maybe it would be more difficult to fit onto the wheels ?

Frederic

[Katie Wasserman]

(05-03-2014 12:02 AM)douze-bis Wrote:  One question : did you consider using a cut of tygon tubing instead of O-rings ?
It would give a "square" section instead of a "round" section, and might be closer to the original wheel ?
Maybe it would be more difficult to fit onto the wheels ?

I use silicon rubber tubing for the 65/67/97/41 card readers iinstead of the common o-ring replacement and find it works great. If you use it for the 9810 reader making sure it stays in place without gluing it down might be an issue.

[Tony Duell]

(05-02-2014 11:17 AM)douze-bis Wrote:  Now I guess the next steps are:

1- The magnetic card drive
The drive looks a bit dirty, and the "capstan wheels" have disintegrated of old age (see the attached picture).
Any advice how to deal with it ?

I see that Kattie has already replied.

My method is a lot more work, but I think it should be more reliable. I dismantle the reader, and get the old hub off the spindle. I then turn a new hub from brass (Outside diameter 7/8") with a couple of grooves for 1" diameter O-rings. I fix this hub on with a grub screw.

There are some photos in my flickr account (tony_duell). The album is called 'HP9820 Card Reader', but it's the same unit.

(05-02-2014 11:17 AM)douze-bis Wrote:  2- The fan
The fan seems to be tired and requires "help" on each power up or it won't move. Then it is noisy.
Any idea where to get a spare, or how to fix it ?

This is a 120V AC fan, run off the primary winding of the mains transformer. They are
still available, but you might not need to replace it. All my HP9800 machines still have the original fans.

Remove the rear panel from the machine. To do this, first take out the PSU boards, then unscrew the 2 edge connectors that are fixed to the case (these carry the AC outputs from the transformer). Loosen the nut each side near the top, you need an 11/32" spanner for this which is not easy to find (in the UK). Then take out the 3 screws on the bottom and pull the panel off, feeding the mains switch cable through the chassis.

On the face of the fan are 4 Phillips screws which hold the fan housing toghether. Take these out and lift off the top complete with the motor and blades. The motor is fixed with 2 screws under the label, feel for these with a screwdriver and take them out. Then you can see a circlip on the end of the spindle. Take this off, and it all comes apart. Clean the parts and put some light oil (I use clock oil) on the spindle and bearings.

Again there is a photo in my flickr account, in the 'HP9820' album.

(05-02-2014 11:17 AM)douze-bis Wrote:  3- The bezel
The bezel is broken in several places.
Any advice on the best way to glue it back together ?

These bezels get very brittle with age, I am afraid.

To mend them you need a solvent that will disolve the plastic. I use dichloromethane (methylene chloride) which is sold in the UK as 'plastic weld'. What you do is put the bits together and run a brush dipped in the solvent along the cracks. This will soften the plastic and the parts will weld together.

Now get a piece of cotton fabric and cut a bit to fit over the back of the damaged area. Put it on and 'paint' it with the solvent. When the plastic has softened, force the fabric into the plastic. This will strengthen the repair. The bezel will still be fragile of course.

The screws that hold the keyboard to the bezel were locked with 'threadlock' adhesive. Unless you plan on shipping your machine around a lot, I feel it's best to get rid of this to reduce the torque in fitting and removing the screws. I cleaned mine up with a suitable tap and die (4-40 UNC I think). They now go in very easily.