Turning on STacy only 2 times out of 10 does it start.

[Roberto]

Hi guys, did you miss me? :-D
I think I sang victory too soon, because it seems I still have some small problems to solve.
After switching CPUs, my STacy boots up about 2 or 3 times out of 10 attempts. When it does boot it works fine, it loads the floppy, I can use games and utilities, I can even reset the system - it always restarts. The only problem is when I turn it off, in the sense that I don't know when it will want to start: in fact when it doesn't start it turns on and shows (like last time) a white screen (on a color monitor) or black with a monochrome monitor . I tried to use the diagnostic cartridge and it passes all tests, so the problem is just when you press the ON key immediately sending the CPU into HALT (0V oscilloscope signal, active low), while addresses and data are all high (exactly like the last time). I have two CPUs (I have a spare, just in case), and with both the situation is the same, so I think the CPU is good. Sometimes, with the diagnostic cartridge in, it won't even read it and act like it's not there, other times it will read it perfectly and run all the tests. Another thing: if I attach its original lcd display, it just doesn't start, not even the floppy starts reading (but perhaps this has nothing to do with the previous problem).

[exxos]

Reset circuit caps ?

[Roberto]

Reset circuit caps ?

When turning on any old electronic system, the capacitors play a fundamental role, true. Before the failure, however, it turned on regularly ... is it possible that I corrupted the operation of some capacitors when the CPU exploded?

[Steve]

Reset circuit caps ?

When turning on any old electronic system, the capacitors play a fundamental role, true. Before the failure, however, it turned on regularly ... is it possible that I corrupted the operation of some capacitors when the CPU exploded?

The whole damn thing needs re-capping tbh. 30 year+ old caps. They have a shelf life you know. But yes at *least* do the reset circuit caps...

[Steve]

Oh and here is a link to the mandatory fixes forum

[Roberto]

Hi guys, I waited for ALL the capacitors, both axial and vertical; they have finally arrived.
I then performed these steps, namely:

Changed all DC/DC board capacitors - Done.
Changed all capacitors on the motherboard - Done.

I used voltages equal to the originals (or higher) and of course the same capacity and respected the polarity.

Result: It always turns on and boots correctly without ever failing once.

However, I noticed that I no longer have the -12V and +5V (purple and green arrow), and I have a distorted sound, as if some voices work and others don't (they feel dull and distorted), between the other when you turn on you now hear a beep.

So, I decided to remove each capacitor one by one and test the STacy to figure out which capacitor is causing this.

In the end I brought the Stacy back to how it was before, with the same original capacitors, dragging me the sound problem (it still feels like in the first one even though the capacitors are the original ones, since before putting the new ones it felt good). And I still don't have the -12 and +5V at the marked points.

If anyone has an idea of what I could do because I'm out of ideas...
PS: I'm 100% sure that the job was done to perfection, i.e. respected capacity, polarity, perfect soldering (I've been soldering electronic boards for years).

ingrandimento.jpg

[exxos]

I cannot really find a decent diagram for it but

I would guess that the transistor which is on the left and half of the schematic is what is pulsing the L101 transformer. Which seems to be powered from the 5V rail. So that transistor could be bad.. What is driving it is bad.. Would need to see if the switching or not.. And check resistance on the coils on the transformer to see if they have voltage. As you have lost both negative rails I assume the transformer is either blown or the drive transistor has.

There is a 7912 which generates the -12 from the -20V supply. That could may well be short. But indeed any silicon or coil in that circuit could be bad.

Do you have the -20V voltage there ?

I also cannot see anything but -12.

Capture.PNG

EDIT:

Just re-read and I thought you said -5V... so indeed if you have not got 5 V then you're not going to get negative rails either.. But how can you have distorted sound if the 5V is not working ?! The system would not even attempt to start up without the 5V rail. s Your arrows point to -12V and 12V from what it looks like, not 5V ? so slightly confused and seemingly conflicting information..

Please just retest and write the voltages on your picture instead of arrows to it , it will be easier to understand..

[Roberto]

Just re-read and I thought you said -5V... so indeed if you have not got 5 V then you're not going to get negative rails either.. But how can you have distorted sound if the 5V is not working ?! The system would not even attempt to start up without the 5V rail. s Your arrows point to -12V and 12V from what it looks like, not 5V ? so slightly confused and seemingly conflicting information..

Please just retest and write the voltages on your picture instead of arrows to it , it will be easier to understand..

In that photo I have all the correct voltages except the pin that should give me -12V (purple arrow, here I have a -0.7V) and the green arrow pin (it gives me 0V). All the other pins give me the correct voltages, therefore, also the 3 pins above the red arrow have the canonical +5V and I have the -23V (under the purple arrow). This of course allows me to turn on the computer and use an external monitor but it doesn't allow me to use the STacy's lcd screen. Where did you get this DC/DC schematic??? I didn't have it!

[exxos]

In that photo I have all the correct voltages except the pin that should give me -12V (purple arrow, here I have a -0.7V) and the green arrow pin (it gives me 0V). All the other pins give me the correct voltages, therefore, also the 3 pins above the red arrow have the canonical +5V and I have the -23V (under the purple arrow). This of course allows me to turn on the computer and use an external monitor but it doesn't allow me to use the STacy's lcd screen. Where did you get this DC/DC schematic??? I didn't have it!

So you do not have 12V and -12V ?

-12V comes from -20V. So if you have -20V , then I assume the 7912 is faulty.

The diagrams are on this site https://docs.dev-docs.org/

[Roberto]

So you do not have 12V and -12V ?

-12V comes from -20V. So if you have -20V , then I assume the 7912 is faulty.

The diagrams are on this site https://docs.dev-docs.org/

I screwed up. I misread my notes. I have -12 volts and -23V and also +5V but I don't have +12V. What a mess I am! :mrgreen:

ps: even the translator gives me problems! he swapped the + for the - and vice versa! Now I corrected