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 ...

[albertofenini]

UPDATE :
We have been able to get good Vcc and Vss, but not a good Vgg..
Investigating we have discovered that the anode of CR1 was not connected to the negative of C4 as it should be.
We have noted that this connection is done with a layer inside the PCB.
We also think that this PCB has more than one hidden layer as also the Ground signal goes thru a hidden layer.
We tried to short connected the anode of CR1 (with an thin wire) to the negative of C4 but
after that we lost all the Voltages to a mere 1,7V,
They didn't get back to good values even after removing the wire.
More in general we have discovered that this PCB (which is not very flat), has a bad ground trace.
Points that on the diagram should be connected to ground and then interconnected, like the negative of C4 and the positive of C5
or the Transitors's Emitters in reality are not, unless the PCB gets gently bended.
It looks like an internal middle layer trace is damaged or may be a cold joint is the problem,
even if we have checked all the thru-layers joint and they are good.
Basically, it has two problems : non continous ground trace, and missing/broken trace between
CR1 and C4, which is just about Vss
What can we possibly do to fix this ?
Thanks in advance !!

   
   
   
   

[teenix]

(03-30-2019 10:01 PM)albertofenini Wrote:  UPDATE :
We have been able to get good Vcc and Vss, but not a good Vgg..
Investigating we have discovered that the anode of CR1 was not connected to the negative of C4 as it should be.
We have noted that this connection is done with a layer inside the PCB.
We also think that this PCB has more than one hidden layer as also the Ground signal goes thru a hidden layer.
We tried to short connected the anode of CR1 (with an thin wire) to the negative of C4 but
after that we lost all the Voltages to a mere 1,7V,
They didn't get back to good values even after removing the wire.
More in general we have discovered that this PCB (which is not very flat), has a bad ground trace.
Points that on the diagram should be connected to ground and then interconnected, like the negative of C4 and the positive of C5
or the Transitors's Emitters in reality are not, unless the PCB gets gently bended.
It looks like an internal middle layer trace is damaged or may be a cold joint is the problem,
even if we have checked all the thru-layers joint and they are good.
Basically, it has two problems : non continous ground trace, and missing/broken trace between
CR1 and C4, which is just about Vss
What can we possibly do to fix this ?
Thanks in advance !!

If the via connections to the inner PCB layers have been broken, possibly by PCB age and/or removing some components, then it will be very difficult to repair the PCB. The only practical repair method I can think of would be external wire connections to restore the connection(s).

cheers

Tony

[albertofenini]

ciao Tony
thanks for the idea,
we did it for a single connection and it didn't worked out,we'll try again, surely for all
the GND connections and also for the connection generating Vgg, if that works (I mean PSU
is reliable) we can try to proceed in the project ...

[Raymond Del Tondo]

Would it be an option to use another known-working 65 PCB?

[albertofenini]

Hi Raymond,
Absolutely, but we don't have one ...

In reality we also don't know the working status of the 3 ROM (we have all of them however)
and the CPU has lost some pins. This is a old version, pins were mechanically crafted
over a gold thin strip which is coming from underneath the top metal cover.
Repair is possible, but hard
However, where would be the fun if it would be easy to do ?

[EugeneNine]

That looks like a pretty simple board, wouldn't take too long to redraw it in kicad and get new made through oshpark or such.

[albertofenini]

Well, this was what I had thought, but is a multi layer board and there are at least
two hidden layers (if not more), would they be able to make it anyway ?