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HP25C repair, help needed
01-15-2019, 09:24 PM
Post: #1
HP25C repair, help needed
Dear All,
while waiting for some spare parts for our third HP-65, we have began the repair project for an HP25C.
This is a old purchase made on TAS, mostly for the collateral that came with the unit, pouch, manuals and battery charger.
The unit was not working, and we would like to give it a try.
It had some light corrosion that we cleaned with lemon juice and rinsed with alcool.
We know that the keyboard has corrosion under the keys but we will deal with that later,
we can see it in transparency, underneath and we will probably dismantle the keyboard
once we will have figured out how to rivet it back together.
We charged a spare battery pack, but the unit looks like dead.
Can anyone one give us in indication of where can we measure voltages and signals to check
what's wrong ? (ROM, RAM, ACT or PSU) ?
Does this board have any test pins ?
Thanks for help !!!!


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Edoardo & Alberto
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01-15-2019, 09:53 PM
Post: #2
RE: HP25C repair, help needed
One last thing,
we have left the unit with the battery connected and turned on,
after a while we have discovered the the two transistors in the PSU and the two capacitors
where very hot.
Pictured on the right is a also an HP25 board from which I have already scraped some spare parts.
Question is : how confidently can we transplant the two transistors from the HP25 to the HP25C ?
Why is on the HP25 present only a large capacitor (gray) and on the HP25 there are two ?
Can we sort of replace them ?
Anyone knows values for the PSU components specifically for the HP25C ?
Thanks a lot !!

   

Edoardo & Alberto
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01-16-2019, 11:37 AM
Post: #3
RE: HP25C repair, help needed
(01-15-2019 09:24 PM)albertofenini Wrote:  Dear All,
while waiting for some spare parts for our third HP-65, we have began the repair project for an HP25C.
Can anyone one give us in indication of where can we measure voltages and signals to check
Thanks for help !!!!

Try this...

http://home.citycable.ch/pierrefleur/Jac...W_main.htm

cheers

Tony
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01-16-2019, 11:39 AM
Post: #4
RE: HP25C repair, help needed
Link did not appear to transfer properly

This is all one link

http://home.citycable.ch/pierrefleur/JacquesLaporte/

Woodstock/ws_HW_main.htm

cheers

Tony
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01-16-2019, 11:46 AM (This post was last modified: 01-16-2019 11:46 AM by Didier Lachieze.)
Post: #5
RE: HP25C repair, help needed
The link above doesn't work for me. What works is the archived version of Jacques Laporte site on hpcalc.org available here: https://archived.hpcalc.org/laporte/Wood...W_main.htm
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01-16-2019, 11:49 AM
Post: #6
RE: HP25C repair, help needed
(01-15-2019 09:53 PM)albertofenini Wrote:  One last thing,
we have left the unit with the battery connected and turned on,
after a while we have discovered the the two transistors in the PSU and the two capacitors
where very hot.
Pictured on the right is a also an HP25 board from which I have already scraped some spare parts.
Question is : how confidently can we transplant the two transistors from the HP25 to the HP25C ?
Why is on the HP25 present only a large capacitor (gray) and on the HP25 there are two ?
Can we sort of replace them ?
Anyone knows values for the PSU components specifically for the HP25C ?
Thanks a lot !!

Not sure about transistors, if they have the same markings, maybe they are the same.

The link I presented earlier has transistor equivalents hand drawn in on the circuit.

Capacitor changes probably due to circuit modifications between early and later models.

If the transistors are hot, then the power supply may not be oscillating properly and the transistors are probably passing high current through the toroid coil.

If the capacitors are hot, they are probably short circuit, so a good idea to replace them before too much more use. Check any diodes as well. The big danger is that the malfunctioning power supply might be delivering too high a voltage to the rest of the circuit.

cheers

Tony
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01-16-2019, 11:54 AM
Post: #7
RE: HP25C repair, help needed
Thank you very much !!
I also had visited this link but I can't find a specific reference to the HP25C.
It looks, to me, that the PSU and some of the circuitry is slightly different form the HP25.
Also, if you look at the picture we posted of our HP25C and compare it to this :
[Image: 25cinter.jpg]
it looks like our is missing an IC, just above the ACT chip.
Is that possible ?
Our board doesn't look tampered, removing a chip would have left some traces,
nevertheless it's different from the picture from the Forum, could it be a different HP25C ?
Thanks for help !!

Edoardo & Alberto
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01-16-2019, 12:21 PM
Post: #8
RE: HP25C repair, help needed
(01-16-2019 11:54 AM)albertofenini Wrote:  it looks like our is missing an IC, just above the ACT chip.
Is that possible ?

According to the very useful HP Calculator Chips table, there are two types of RAM chips for the 25C:
  • single density (1820-1630 or 1820-1843 - 8*56).
  • double density (1820-1886 or 5061-0469 - 16*56)
The board above has two single density 1820-1630 RAM chips. So if you have a double density RAM chip on your board it's normal to have only one RAM chip.
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01-16-2019, 01:55 PM
Post: #9
RE: HP25C repair, help needed
Very good point, thank you !
Tonight we'll check it and we will post the result,
we also will post Voltages and will see
take care Alberto

Edoardo & Alberto
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01-16-2019, 09:55 PM
Post: #10
RE: HP25C repair, help needed
Dear All,
we worked on the unit, but we are sorry to say we made little to no progress ...
I can confirm our unit has a double density RAM 1820-1886, which means no chips are missing.

First we measured the Vss, Vgg and Vdisp values, with the following results :

Vss (should be 6V) was 2.33V
Vgg (should be -12V) was -0,37 V
Vdisp (should be 4V) was 2.32

It must be said that differently from Jaques Laporte documentation being this a HP25C unit
the Anode Driver is not a 1818-0153 but is a 1818-0186, so for Vdisp we have assumed the
same pinout for both Anode Drivers and we have measured Vdisp at PIN 5 of our Anode Driver.

Given that the values were totally wrong we worked on the PSU.
So far we have substituted : both the transistors, most of the diodes and the little
transformer with components from an HP25 donor machine.

After that, we checked, and the values for Vss, Vgg and Vdisp were exactly the same as before.

Then we took apart the keyboard and we cleaned all the oxidation.
We did this because we suspected it was somehow impacting in the process.

Nothing changed.

Then we removed the ACT, we wanted to see if without the ACT the PSU was generating
the correct values for Vss, Vgg and Vdisp.

Which it wasn't because without the ACT the three values dropped close to 0 V (all of the three).

Then we re soldered the ACT and the values for Vss, Vgg and Vdisp remained close to 0.

What can we check ?
What can we do to isolate PSU and try make it working ?
Can we use components from a Classic Calculator PSU ?
Also, does anyone has more detailed information specifically about the HP25C ?

As always, thank you very much for you precious support !!!
Take care !

Edoardo & Alberto
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01-16-2019, 10:35 PM
Post: #11
RE: HP25C repair, help needed
Check out: http://www.panamatik.de/html/produkte.html
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01-17-2019, 12:08 AM
Post: #12
RE: HP25C repair, help needed
Thanks Hewlpac
we know that solution,
however PSU must be working, and also anode and cathode driver must be working,
so we need help in making PSU generating good Vss, Vgg and Vdisp, than we could think
of installing this.
Thanks again Alberto

Edoardo & Alberto
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01-17-2019, 01:04 AM
Post: #13
RE: HP25C repair, help needed
(01-16-2019 09:55 PM)albertofenini Wrote:  What can we check ?
What can we do to isolate PSU and try make it working ?
Can we use components from a Classic Calculator PSU ?
Also, does anyone has more detailed information specifically about the HP25C ?

As always, thank you very much for you precious support !!!
Take care !

You mentioned the capacitors were getting hot. I would replace those with new ones as they are probably unusable now. If they are faulty, then replacing other parts may not show any change in symptoms and could even damage them.

There are only a handful of passive components in the power supply plus the transistors. Checking each of them individually would certainly help.

cheers

Tony
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01-17-2019, 02:59 PM
Post: #14
RE: HP25C repair, help needed
Tony,
do you have any specs or schematics specifically for the HP-25C PSU ?
Thanks in advance Alberto

Edoardo & Alberto
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01-17-2019, 05:39 PM
Post: #15
RE: HP25C repair, help needed
(01-17-2019 02:59 PM)albertofenini Wrote:  Tony,
do you have any specs or schematics specifically for the HP-25C PSU ?
Thanks in advance Alberto

Hi Alberto,

Sorry, I don't, although I would imagine the component values are much the same as in the HP25 PSU. The only difference that I know of between the 2 models is the constant memory feature which has nothing to do with the actual power supply circuit.

cheers

Tony
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01-17-2019, 08:29 PM
Post: #16
RE: HP25C repair, help needed
(01-17-2019 12:08 AM)albertofenini Wrote:  Thanks Hewlpac
we know that solution,
however PSU must be working, and also anode and cathode driver must be working,
so we need help in making PSU generating good Vss, Vgg and Vdisp, than we could think
of installing this.
Thanks again Alberto

These C machines are easily damaged by overvoltage coming from the charger. A bad contact between the batery pack and thr pcb and the ram chips can be destroyed, even with the machine in powered off state.
Not to mention the other chips when in power on state.

So the first thing to do is to not use the charger at this stage. And do not use the vattery pack at all for now.

If you do not have a lab power supply, use a battery holder with 2 x 1.5V AA battery cells. This is a cheap and safe way to test and troubleshoot. Use two wires and solder them to the machine battery terminals.

Next, you need a current meter inserted in series with one of the battery holder leads and the machine , so now you can measure the current consumption.

As the power supply voltages are low, and assuming that the 3volt AA batteries voltage is above 2.5 volts under load, a fault exists either in the power supply or at least one chip is shorted, or both conditions can be true.

You need a oscilloscope for proper troubleshooting as well.

Several things to do besides what other fellow members said above:
- Measure the resistance between each output power supply and ground. A lowish value means a short in that line.
- cut the copper traces for the 3 power supply lines and connect one 1kohm resistor between each output and ground to load a litle the powet supply., and check the voltages.

- If you do not want to cannibalize the pcb as described above, and you are skilled using a desoldering iron, then remove all the chips at once. Take carevto not overheat the chips, meaning that you need a powerful iron able to maintain high temperature to ket you remove the solder in the shortest amount of time.
This is an opportunity to do a proper full cleaning using IPA. I had similar symptoms in several machines whete the battery electrolytes were creating shorts everywhere.

Jose Mesquita
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01-18-2019, 01:48 PM
Post: #17
RE: HP25C repair, help needed
Dear Jbem,

that's exactly what we have done so far.

When we try to troubleshoot a machine we always attach a pack of rechargeable batteries that we charge externally, so no original battery pack nor charger here either.

We have both a multimeter and an oscilloscope but differently from the HP-65s we have recently repaired here we are missing a correct scheme so that we don't know where we can intercept the Vss, Vgg and Vdisp in the nearby of the PSU.

We know where we can get them at the pin out, but we suffer from the lackness of a PSU scheme with correspondence with physical components. The HP25 PSU is different, has more components and are connected differently.

PCB is clean, it has been checked but it has no broken traces nor oxidation whatsoever.

We will try over the weekend to test the PSU components, diodes and transistor, and we can definitely de solder the ACT and the RAM in order to see if Voltages and Current consumption are stable.

We will be more than happy to hear from anyone that may have additional suggestion, on how to check the components

Thanks again for the precious tips !

Edoardo & Alberto
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01-18-2019, 05:51 PM
Post: #18
RE: HP25C repair, help needed
(01-16-2019 11:39 AM)teenix Wrote:  Link did not appear to transfer properly

This is all one link

http://home.citycable.ch/pierrefleur/JacquesLaporte/

Woodstock/ws_HW_main.htm

cheers

Tony

Tony was nearly right, but misses a '-'.

http://home.citycable.ch/pierrefleur/Jac...W_main.htm
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01-19-2019, 03:39 AM
Post: #19
RE: HP25C repair, help needed
(01-18-2019 01:48 PM)albertofenini Wrote:  We will be more than happy to hear from anyone that may have additional suggestion, on how to check the components

Thanks again for the precious tips !

Resistors are easy, get a color code chart from web and check against the component. Read with an ohmmeter. Remove components from the circuit first or you will get a false reading, unless you know what to expect.

Capacitors are best tested with a capacitance meter, or you can still use an ohmmeter. Some have capacitor checkers built in.
They should read a brief short circuit followed by open circuit as they charge up. Reverse the leads and the same should happen. You may not notice the short, depending on how big the cap is, the important thing is being open circuit.

You mentioned, the capacitors were hot, so they are probably destroyed. These will only get hot because of higher than normal current flow through them. They should only pass current with an AC voltage unless they are shorted. They can also be destroyed, quite literally, if they are of a polarised type and the applied voltage is reversed, such as the tantallum types in the power supply. I would replace these regardless.

Google "check transistor with analog multimeter" for checking transistors unless you have a proper checker. You mentioned yours were hot, so replacement might be a good idea, considering their age. Jacques Laporte mentioned for the HP25, 2N3704 and 2N3904, both of which seem available from Mouser.

Diodes should have a low ohms reading in one direction only. The cathode is marked with a line at one end of the component case, the other is the anode. An ohmmeter black lead connected to cathode, red to anode should pass current. The other way around should be open circuit. Some ohmmeters have a diode setting, or use around the 2K range. In the HP25 circuit, it looks like 2 different types, but IN4002 types might do.

One of the diodes in the HP25 circuit, connected to the base of a transistor and the 6V output, is a zener diode. These can still be tested as per a normal diode, but they are used to provide voltage regulation. You can test it by placing a 1K2 resistor in series and apply a rising voltage. Test with the +ve supply via a series resistor to the Cathode, and the anode connected to ground. Measure the voltage across the zener and when it reaches its breakdown voltage, it should stop rising, probably somewhere around 3-5V. I'm not sure what this voltage will be, so don't apply too high a voltage, say < 7. This diode should conduct almost no current until the breakdown voltage is reached, then any extra voltage will be dropped across the resistor.

The toroid coil has 3 coils wound on it, all with low resistance, so hard to read with an ohmmeter, but probably won't be shorted. Make sure no coils are open circuit.

If you remove the chips, de-solder the power supply pins last. If the chips have inbuilt protection from static, the power supply circuit will help absorb nasties. Solder these pins first when fitting back to the board. Best not to touch the pins with your fingers either unless you are protected against static build up. Store them wrapped in foil.

Your Pico Scope should help identify if the power supply is working. If it is similar to HP25, and if you have it isolated from the rest of the circuit, then you should read around 4V, 6V and -12V with respect to ground.

You could load up each supply output with say a 470ohm resistor for 4V, 680ohm resistor for 6V and 1200ohm (1K2) for 12V, all connected to ground. These will all draw about 10mA of current.

Be careful pulling out components when de-soldering them. Some of the vias in old double sided PCBs that connect one side of the board to the other can pull out with the component lead. These can be repaired with wire links, but messy.

cheers

Tony
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01-19-2019, 10:04 AM
Post: #20
RE: HP25C repair, help needed
Thanks Tony
Your tips were very helpful !!

I studied electronic at high school some thirty years ago so I recall some of them.

We have put together a bunch of useful documents about hp25 especially from Jaques Laporte website

Still the Hp25c is different in the PSU scheme

There are few less diodes and just to name a difference between the two models, below the display in the Hp25c there is no capacitor nor diode differently from the hp25 where part of the psu is located there.

Also, the big capacitor close to the transistors in the hp 25, in the Hp25c has evolved in two capacitors connected, so this is also a difference

there is an additional transistor close to the ACT which is not present in the Hp25

This makes more difficult to recover the testing points for the three voltages

We have a working hp25 and a Working hp25c so that we may open up those and try to locate those points

We will try to strip all the components following your advices and check them out

Still, if someone from the forum can tell us where are the testing points on an HP25C board it will really help us

If we will discover them will publish it here

In reality we now where to find this points once they arrive at the IC PINS as this is well documented but we would prefer to check them closer to the PSU components

Thanks again everyone!!!

Edoardo & Alberto
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