Topics

9825A Troubleshooting #VintageHPComputers #HP9825


Rhys Lowry
 

Hi all, I'm a newish vintage electronics repair enthusiast having recapped and done basic troubleshooting and the such in the past. No expert, although I aspire to be. After picking up this HP 9825A last week from a local recycler who thankfully didn't throw it out, I gave it a nice cleaning before powering up (it was full of broken glass..) Upon powering up, the fan spun and the speaker gave out a single tone that was on as long as the calculator (computer?) was on.

I decided to check the voltages coming from the power supply board and it was partially off where the +12V rail was 12.43V, which I figured might have made the logic upset so I twisted one of the potentiometers on the power board while it was off. Upon turning it back on, one of the power diodes (possibly one of the voltage regulators) gave some magic smoke (I didn't see exactly where the smoke came from).

I'm planning on getting some replacement parts from Mouser and other sources but my question to you all is what should/shouldn't be replaced on this to give a better shot at this thing working? Any idea on what the original problem might have been?

Any help is appreciated.

Thanks! Rhys Lowry, student @ UMO studying EE


 

On Mar 1, 2021, at 6:53 PM, Rhys Lowry <rhys.lowry@maine.edu> wrote:

Hi all, I'm a newish vintage electronics repair enthusiast having recapped and done basic troubleshooting and the such in the past. No expert, although I aspire to be. After picking up this HP 9825A last week from a local recycler who thankfully didn't throw it out, I gave it a nice cleaning before powering up (it was full of broken glass..) Upon powering up, the fan spun and the speaker gave out a single tone that was on as long as the calculator (computer?) was on.

I decided to check the voltages coming from the power supply board and it was partially off where the +12V rail was 12.43V, which I figured might have made the logic upset so I twisted one of the potentiometers on the power board while it was off. Upon turning it back on, one of the power diodes (possibly one of the voltage regulators) gave some magic smoke (I didn't see exactly where the smoke came from).
12.43 V was within the spec and probably didn't need adjustment immediately. Did you look at the schematic before you turned the potentiometer? Doing so without understanding what you were adjusting or being able to measure was not a good idea. What were the voltages of the other supply rails?

I hope that you did not do this while it was connected to the logic boards, as the version of the power supply you have does not have the crowbar circuit modification that would have protected the rest of the unit from a short circuit in one of the power supply's voltage regulating pass transistors.


I'm planning on getting some replacement parts from Mouser and other sources but my question to you all is what should/shouldn't be replaced on this to give a better shot at this thing working? Any idea on what the original problem might have been?
Have you identified the failed component(s)?

As to the original problem, when you cleaned the unit out, are you sure you reconnected all of the cables correctly?


Rhys Lowry
 

I was measuring the 12V rail when it happened and was confused as to why it wasn’t changing the voltage when I turned the pot. As for dead components, the 2N4398 is shorted and it looks like some solder mask was blown off the back. The 2N3055 still tests fine with a 1.2V drop across it. As the now dead diode connected to pins 1 and two of the card, I searched downstream for any damage. I’m sorry to say I did leave it in while I was testing. Lesson learned. I did not see any obvious damage to the main boards (I didn’t have the front section attached). Following both the rails from the blown mask pin and the diode rail was inconclusive. There may be damage I can not see but I can not test it until I get the power supply board up and working again.

The pins I am referencing are the first, second and fourth from the bottom of the picture of the card connector. Blown mask can be seen in the other.


 

If you haven't already, download a copy of the service manual: http://www.hpmuseum.net/document.php?hwfile=1808
Also two sets of schematics are available, one are the hand drawn ones by Tony Duell: http://www.hpmuseum.net/document.php?hwfile=1809
The second are scans of the HP schematics, starting with: http://www.hpmuseum.net/document.php?hwfile=4141
These are easiest to find by searching on www.hpmuseum.net for "9825_schematic".
Then if you can match up the components you are describing with their component names on the schematic, we'll have some common information to help you move forward.

On Mar 2, 2021, at 11:23 AM, Rhys Lowry <rhys.lowry@...> wrote:

I was measuring the 12V rail when it happened and was confused as to why it wasn’t changing the voltage when I turned the pot. As for dead components, the 2N4398 is shorted and it looks like some solder mask was blown off the back. The 2N3055 still tests fine with a 1.2V drop across it. As the now dead diode connected to pins 1 and two of the card, I searched downstream for any damage. I’m sorry to say I did leave it in while I was testing. Lesson learned. I did not see any obvious damage to the main boards (I didn’t have the front section attached). Following both the rails from the blown mask pin and the diode rail was inconclusive. There may be damage I can not see but I can not test it until I get the power supply board up and working again.

The pins I am referencing are the first, second and fourth from the bottom of the picture of the card connector. Blown mask can be seen in the other.
<34E66E18-3BD8-465A-A3C8-F7514CA9CBAD.jpeg><21CA3946-1DB7-4022-96EA-22AB7D52EE09.jpeg>


 

By head I say it's the AC return line to the transformer.
So it seems your very lucky.

-Rik


Paul Berger
 


On 2021-03-02 3:13 p.m., Rik Bos wrote:
By head I say it's the AC return line to the transformer.
So it seems your very lucky.

-Rik

_._,_._,_

The trace that burned some of the solder mask off is the leg of the transformer feeding the +20 unregulated.  I am surprised it did not blow the fuse before doing that damage, maybe there is a nail in the fuse-holder.  I have had two power supplies fail one in a 9825 and one in a 9835  and neither one suffered that kind of damage. 

Paul.


Paul Berger
 


On 2021-03-02 2:23 p.m., Rhys Lowry wrote:
I was measuring the 12V rail when it happened and was confused as to why it wasn’t changing the voltage when I turned the pot. As for dead components, the 2N4398 is shorted and it looks like some solder mask was blown off the back. The 2N3055 still tests fine with a 1.2V drop across it. As the now dead diode connected to pins 1 and two of the card, I searched downstream for any damage. I’m sorry to say I did leave it in while I was testing. Lesson learned. I did not see any obvious damage to the main boards (I didn’t have the front section attached). Following both the rails from the blown mask pin and the diode rail was inconclusive. There may be damage I can not see but I can not test it until I get the power supply board up and working again.

The pins I am referencing are the first, second and fourth from the bottom of the picture of the card connector. Blown mask can be seen in the other.
_._,_._,_

The 2N4398 is the pass transistor for the +5V supply if it failed short (mine did too) it will put +20V in places it really should not be.  I am afraid the damaged trace would suggest the fuse did not blow quickly and damage could be extensive.  When repairing your power supply I would recommend disconnecting the power connectors from the circuit boards until you are confident all the voltages are correct.

You may want to make sure the fuse in the power supply is the correct rating, I had the +5V regulator fail on a  9825 much the same as yours did and did not have any damage to the circuit traces on the board.

When the 5V regulator failed on my power supply it took out CR5 as well that is the big stud diode 1N6096  I have spares of 2N4398 and 1N6096 left over from repairing by power supply if you need them.

Lastly I have most parts for a 9825 if you find you need some, the only thing I don't have is a good print head for the strip printer, mine got damaged when the +5V regulator failed and tehre was a big black splotch on the paper under the print head.

Paul.



Rhys Lowry
 

I just ordered new filter caps as well as a new 2N4398. I'll let you know if that fixes things or if I need any more help. I'll make sure to test it safely this time.

Rik, is your crowbar circuit available online in a Gerber file or some CAD file? I'd definitely like to implement that, although I see in the other thread that you haven't put together a reference build as of a month or so ago. Any updates on that?

Thanks for all of the help.

Rhys


 

Rhys,

 

I uploaded a slightly optimized gerber, you’re free to use it non-commercial 😉

 

-Rik

 

Van: VintHPcom@groups.io <VintHPcom@groups.io> Namens Rhys Lowry
Verzonden: donderdag 4 maart 2021 19:15
Aan: VintHPcom@groups.io
Onderwerp: Re: [VintHPcom] 9825A Troubleshooting #HP9825 #VintageHPComputers

 

I just ordered new filter caps as well as a new 2N4398. I'll let you know if that fixes things or if I need any more help. I'll make sure to test it safely this time.

Rik, is your crowbar circuit available online in a Gerber file or some CAD file? I'd definitely like to implement that, although I see in the other thread that you haven't put together a reference build as of a month or so ago. Any updates on that?

Thanks for all of the help.

Rhys


Rhys Lowry
 

Thanks Rik,

This is my first time ordering a PCB from a file (pretty non-commercial if I'd say so). I'm having some trouble finding the size of the board/number of layers etc from the file. Any help would be appreciated.
The design looks great, I'll get that ordered once I know more details on dimensions.

New filter caps and NPN coming in today, I'll see if that can remedy whatever issue was happening. Will update.

Rhys