Rebuilding an HP65 logic board from scratch - any advice welcome

[albertofenini]

Hi everyone,
we have been away from the forum for a while, sorry for that,
mostly it was because of Spring approaching, but also because our multi meter went broken and although we had a backup that was way too basic to be used.
So we purchased this model : https://www.amazon.it/dp/B00EYYJRC0?ref_...1_54845001
Even if we are in no way affiliated with the seller we wanted to share the good purchase, just in case someone else is in the need of a new tool
However, if this is inappropriate feel free to remove the link

This said, since this new tool has new features like a capacimeter we went back to an old
project we had left behind.
Last year we had stripped all the components from a non working HP65 logic board.
We now checked all the passive components like resistors, capacitors, diodes and found all good but
two diodes which we have replaced with some identical spares prior to check that the spares were identical and good.
The transistors had the following HFEs :
the F4 094 427 had hfe=13
the M4 550 had hfe = 40
We had two couples of these types of transistors and also the other couple had the same hfe values
Don't know if it is good or bad, but to us it meant that these were working.

However, once soldered back all of the components of the PSU (we left behind the ROMs and the CPU) the PSU was not working.
With this I mean that all of the Voltages Vgg, Vcc, Vss were all equal and a bit lower than Vbsw.
Given that all of the components had been tested and proven good we replaced the transistors
Based on some old thread, we replaced the following :
the M4 550 with a BC182
the F4 094 427 with a 2N2369
these have a different pinout, basically Collector and Emitter are inverted, but since we
suspected about the transistors from the beginning we had used some pins and this has made
easy to replace and change position without molesting too much the board.

Nevertheless the result is the same, Vcc, Vgg and Vss all are equal more or less to Vbsw.
We are now thinking to remove part of the PSU and leave the core of the PSU
then checking with the scope the waveform at the Base of transistor 2N3904 which should be the M4 550

Anyone has a better idea about what could be wrong ?
Keep in mind that all of the capacitors, diodes and resistors are good and of the right value.
Thanks in advance !!

   
   

P.S. we will reply soon to the other open threads about HP45, HP25 and HP65 back label ...