Electronics > Repair

What more i can do?

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Harry_22:

--- Quote from: 222Lab_Test222 on November 20, 2024, 07:21:56 am ---Here

--- End quote ---
Could you reduce time/div to see the pulses

222Lab_Test222:
One thing I do not understand, Please Clarify this..

Regarding the RTC ICs,
Here Red color are Inputs and we done lots of check on it, Its all good, same in working and non working board.

Yellow color are Outputs, This should be Ok as All inputs are ok right?
Then why Outputs are not going as exception,
-The Data Out should be constant High voltage BUT it Keeps Fluctuating.
-The TP should be 64Hz But it doesnt give frequency sometimes and sometimes it give 72Hz.
-The XTAL inverter seems good.

Why is this happening? Is RTC bad?
Or I miss something on the input of this RTC?

Image is below:

Harry_22:
Follow the logic, it is the most important thing here:
First we found where the Bios itself is located. It turned out that it is in EPROM together with MS-DOS system.
We checked whether it loads - it turned out that it doesn't. It doesn't even try.

It became clear that the malfunction occurs at the very start. In modern machines, when the machine is turned on, a special Power On Sequence testing procedure occurs and it is quite complex (see the picture). We also have some events from this procedure.

On the other hand, we approached it from the other side, not head-on. We tried to understand which elements can cause such a multiple malfunction.
These are capacitors. CMOS with power supply that could run down and "spoil the data". Such cases have repeatedly occurred when the computer refused to start with a black screen due to a dead battery.

We checked the RTC that is the only one powered here. We found that the working one board has no pulses at the output, but the faulty one does.
Here. The faulty one starts from zero. The C0, C1, C2 pins have a low level, the output frequency is 64 Hz according to the datasheet (Correction: C2 should be high!)
RTC on the working one board does not start from the zero. Now it stores the C'(0...3) settings recorded during the previous work cycle. Therefore TP (Timing Pulse) outputs are different.
But when you unsoldered the TP pin from the line, we were convinced that the TP signal does not affect the start.

Now about the important thing. I missed your message that "DATA OUT pin (Pin 10) is unstable in non working board", most likely this is interference on the bus in the Z state (high impedance). But the absence of CLK on pin 9 needs to be studied in more detail. Where does it come from?

PS
I remind everyone that we can't work faster because of the big time difference. It's like driving a Mars rover. From 3 to 20 minutes, radio waves fly in only one direction.

Harry_22:
Here you can see Beep behavior on working board as well as EPROM Output Enable.

Harry_22:

--- Quote from: fzabkar on November 19, 2024, 10:26:16 pm ---
--- Quote from: Harry_22 on November 19, 2024, 10:07:15 pm ---@222Lab_Test222, could you take out both EPROMS from working board and track the beep signal as usual.
--- End quote ---

I wonder if it might be an idea to scope the OE and CE pins of the EPROMs? The OP has reported that there is activity on all the address lines other than the high order A16 and A17. I'm wondering whether the pre-BIOS beeping is non-fatal and still allows the BIOS to POST, at least until it encounters a problem?

This is how I imagine the two EPROM layouts (the high order address space is occupied by MS-DOS):


--- Code: ---256K .--------. A17  .--------. 0x3FFFF
     |        |      |        |
     |        |      |        |
     |        |      |        |
     | MS-DOS |      | MS-DOS |
     |        |      |        |
     | v3.21  |      | v3.21  |
     |        |      |        |
     |        | A16  |        | 0x10000
 64K |--------|      |--------|
     |        | A15  |        | 0xFFFF
     |  BIOS  |      |  BIOS  |
     |        |      |        |
     |KDX 1.34|      |KDX 1.34|
  0K '--------' A0   '--------' 0x0000

      High byte       Low byte

      D15 - 8         D7 - 0
--- End code ---

--- End quote ---

Please check OE, CE, Address and Data busses on the non-working board with the same 500us/div. Attach here only meaningful ones.
All the lines are paralleled except Data bus.

Recheck CLK signal at Pin9 IC23 RTC.

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