Electronics > Repair

What more i can do?

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asis:
Hi,

@222Lab_Test222

Sorry.

I can't see the wires well on the Back PCB picture, so I thought that IC7 74ACT08 is somehow connected to the X1 crystal.
But now I realized that the schematic is as in the .pdf.
So look at pins 14 or 15 Xtal through a 1:10 divider and use a 2-Ch scope to observe DOUT / TP.
However, have you tried adjusting C10?
Have you tried manipulating the button on the PCB.
Is it Reset?

222Lab_Test222:

--- Quote from: asis on December 17, 2024, 01:49:54 am ---Hi,

@222Lab_Test222

Sorry.

I can't see the wires well on the Back PCB picture, so I thought that IC7 74ACT08 is somehow connected to the X1 crystal.
But now I realized that the schematic is as in the .pdf.
So look at pins 14 or 15 Xtal through a 1:10 divider and use a 2-Ch scope to observe DOUT / TP.
However, have you tried adjusting C10?
Have you tried manipulating the button on the PCB.
Is it Reset?

--- End quote ---
Yeah thats reset.

m k:
You've been told to be methodological, for now you're not.
Earlier data and address scopes was the only exact thing lately, use it.

If scoping swapped ROMs is too difficult you can get one of those cheap Sigrok compatible 8 bit logic analyzers.
Don't forget mini grabbers for pins if they are missing.

Using the analyzer you measure CS, OE and two of D7-5, no matter which ones, of both ROMs simultaneously, you can do this to all faulty boards.
When CS and OE goes down the data bus contains ROM data, your earlier measurement says that the data is not correct, so all other parts are irrelevant until you get that data right.

First you measure directly from ROM pins, correct data is so that one pair is up and up when other is down and down.
If that data is present you change the measurement so that you measure one nibble(4 bits) at the time.
Correct data for first nibble quartet is E 9 0 D, order can be different but content not.

Next you can obviously do many things, but be methodological.
The methodological thing is to move the next measurement to the other side of the socket contact.
If socket is a through hole the underside is an easy target, sort of.

Keep in mind that measuring from the underside of a socket doesn't mean that the chip is getting the signal.
Note also that measuring from the chip's leg and pressing the probe can lead to connection of a bad contact, this can be your earlier measurement discrepancies, but in opposite way, the measurement was faulty.
Pushing and pulling can also be pressing, direction of contact defines.

222Lab_Test222:
 Sorry guys,
I will be quite busy these weeks, i will be back next week.

asis:
Hi,

@222Lab_Test222


Could you please read the inscriptions (if any) on the uPD4990 on the working PCB(s) indicating the batch number and year of manufacture and compare them with the inscriptions on the IC23 chips on all the non-working PCBs.

I suspect that this is a "2000Year" problem.

If you are confident in yourself, you can move the chip from the working board to the non-working one (or vice versa).

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