HP25 repair - need advice (TESTING POINTS PICTURED)

[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