HP25 repair - need advice (TESTING POINTS PICTURED)

[albertofenini]

Dear All,
we started today the repair of an HP25 SN/ 1511S46480
The unit was already repaired by me years ago, or at least I tried to repair it at the time.
It was obviuos that all of the ACT, ROM and RAM had been de-soldered and re-soldered.
It is equipped with an ACT 1820-1523, a ROM 1818-0154 and a RAM 1818-1564 of which we don't know if (some or all of them) are working or damaged.
The unit is not turning on, we use an external battery pack with two rechargeable 1.2 V batteries.
By not turning on we mean the display looks like dead.
Following the schematics we have identified some testing points, we share this picture as it may
be useful to someone.
We tried the following :
1. we measured Vss, Vgg and Vdisp with the ACT, ROM and RAM soldered and the values were Vgg: 1,45 V, Vdisp: 2,54, Vss: 2,54
2. we replaced twice the couple of transistors that form the PSU with transistors from other HP25 boards, and the Voltages didn't change
3. We "add" the diode highlighted in the picture by the yellow arrow, since it was present in a working HP25 and was "missing" in this unit, but the voltages didn't change
4. we de-soldered the ACT, ROM and RAM chips and the Voltages from PSU remained the same.

We plans to leave the ACT, ROM and RAM de-soldered and try to fix the PSU.
Question : should a good PSU provides correct Voltages with no ACT, ROM and RAM on board ?"

What can we do to identify what's wrong in the PSU ?
Thanks in advance for help !!

       

[teenix]

(02-23-2019 10:44 PM)albertofenini Wrote:  Dear All,
we started today the repair of an HP25 SN/ 1511S46480
The unit was already repaired by me years ago, or at least I tried to repair it at the time.
It was obviuos that all of the ACT, ROM and RAM had been de-soldered and re-soldered.
It is equipped with an ACT 1820-1523, a ROM 1818-0154 and a RAM 1818-1564 of which we don't know if (some or all of them) are working or damaged.
The unit is not turning on, we use an external battery pack with two rechargeable 1.2 V batteries.
By not turning on we mean the display looks like dead.
Following the schematics we have identified some testing points, we share this picture as it may
be useful to someone.
We tried the following :
1. we measured Vss, Vgg and Vdisp with the ACT, ROM and RAM soldered and the values were Vgg: 1,45 V, Vdisp: 2,54, Vss: 2,54
2. we replaced twice the couple of transistors that form the PSU with transistors from other HP25 boards, and the Voltages didn't change
3. We "add" the diode highlighted in the picture by the yellow arrow, since it was present in a working HP25 and was "missing" in this unit, but the voltages didn't change
4. we de-soldered the ACT, ROM and RAM chips and the Voltages from PSU remained the same.

We plans to leave the ACT, ROM and RAM de-soldered and try to fix the PSU.
Question : should a good PSU provides correct Voltages with no ACT, ROM and RAM on board ?"

What can we do to identify what's wrong in the PSU ?
Thanks in advance for help !!

Looks like a little camp fire site :-)

Try changing the caps, check resistors, open circuit coils, circuit traces etc. Not too much to check there.

cheers

Tony

[albertofenini]

ciao Tony, thanks for your reply
But in theory, if all the components should be ok, the PSU should work also without ACT, ROM and RAM, right ?
Thanks !!

[teenix]

(02-24-2019 09:01 AM)albertofenini Wrote:  ciao Tony, thanks for your reply
But in theory, if all the components should be ok, the PSU should work also without ACT, ROM and RAM, right ?
Thanks !!

In theory, yes, as long as the circuit board is ok. The voltages may be a bit higher with no loads.

You should be able to check the PSU activity with he scope.

cheers

Tony

[Dreato]

I've found bad tanatalum caps on these, and other HP boards. Try replacing those.

If all else fails. This could be a candidate for Panamatik's Woodstock LP board.

Regards,

Dave

[albertofenini]

Hi everyone, given the bad weather we have started to work again on this project.
We attached the Vbatt to the board and we have noticed that on pin1 of the Toroid there was no activity at all.
On a working HP25 with Vbatt attached we have found a square wave which means the the oscillator works.
We suspected then of a transistor, the 0550 which, has we learned from our PSU prototype forms the basic oscillator.
(if interested in this you can find more about the PSU we built, in this post http://www.hpmuseum.org/forum/thread-106...#pid114649)
However, both the transistor : the 0550 and the 071 had a good HFE.
We striped then all of the PSU components and tested them and all of them are good.
Diodes, capacitors, resistance, all of them, even the Toroid, that we tested with our PSU prototype !
Nevertheless all of Vdisp, Vgg and Vss have values equal to Vbatt ....
What could we possibly check ?
Remember the ACT, Rom and Ram have been removed, and only the cathode and anode/rom drivers are present on the PCB
Thank you very much !!!

[albertofenini]

Update : success, but ...
We checked all the traces and discovered that the Ground terminal from the battery had a bad
connection on the upper layer of the main PCB.
We fixed it by soldering a thin wire and rechecked all the traces.
At this point we mounted the Toroid, one transistor (0550), the 2,2nF capacitor and the resistor and
we checked that the basic oscillator was working, and it was !
We mounted all the components for the PSU and we checked the Vdisp, Vss and Vgg where good.
The PSU was working.
We installed then the ACT and the Rom and the HP25 finally turned on !
However, when we added the RAM, we weren't able to use memory registers nor programming
All the programming steps look like in the picture.
This make us think that the RAM chip 1820-1564 is defective.
Any way we can check this ?
We browsed this link http://www.brouhaha.com/~eric/hpcalc/chips/ but it looks like the only possible donors could be an HP22 or an HP91
Does anyone has found a different compatible chip ?
Thanks for all the support !