MSXmakers support

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Por msxmakers

Hero (551)

imagem de msxmakers

07-06-2022, 11:31

RayneX wrote:

Well, I'm about at my wits end here. Currently asking a friend about a scope but I don't know how to use it. I've swapped the last LS IC with an HCT and the same result on Smoke Test 4. Remove an IC and I get memory error. I've reflowed about half the board and isn't changing the results.
I've gotten a black screen with a signal, blue screen with nothing on it, and even a few Gray screens. The gray screen is new, but only got it twice and haven't again. Does any of this point to a potential IC issue? VDP? CXA? 8255 PPI? something?

I think that U45 is used in many parts of the circuit. Sometimes a chip seems to be the culprit just because it enables a part of the circuit and this is currently the case, because basically this is a simple nand gate and sometimes it is used just as a buffer.
It's on the RESET circuit, acts on the 82c55 chip select (ppi_cs) and its result ends up in the slot selection GAL.
I think the problem could be here.
We can lose our whole life replacing chips because they are not 74HCT or overheating the solders and the problem may be right under our noses, what we have not changed or tested yet.
As I have commented before, I have received many error reports due to bad sockets in the GAL or false contact too, so I advise you to take out that specific GAL and check it on a breadboard to test its operation a bit, to see if works as expected.
The 82c55 is also a critical chip, so checking at what state the reset or PPI_CS signal is with a logic probe can give a clue if the Omega is hanged in permanent RESET or PPI never selected.
You can repeat the HALT test but with all the smoke test 4 chips inserted, it should also work as expected, and maybe it will, I'm not sure, but if it's a RESET problem it won't.
About the U23 chip and about the LS/HC controversy:
I believe there are many reputable shops at the US, although you may not find them at JAMECO or Anchor due to the current shortage. I don't recommend you buy at ebay because everyone sells anything, but all my tests with UTsource were good, and it's the only vendor that can offer all the chips at once.

Using all LS chips should work, and even with all HC chips too (according to Sergey Kiselev's comments).
It does not mean that you will get the same results as me at each smoke test, in fact you may insert all the chips at once and (maybe) it will work without problems.
My suggestion is you try with as many chips as possible according to the original specifications and, in those that cannot be found, replacing them in this order:
74AHCT can be replaced by 74HCT.
74F can be replaced by 74LS.
74HCT can be replaced by 74LS.
Only if there is no other option would I replace a 74HCT with a 74HC, and to know that this works I should keep track of the entire circuit by checking how many CMOS inputs the TTL outputs go to.

If nothing this works for you, sharing a high resolution picture of your motherboard would help us to have new ideas.

Por RayneX

Resident (44)

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14-06-2022, 05:56

Update. My friend showed me how to use a scope and in most cases, I'm getting similar frequency results in the test. I have not populated all the chips yet. One weird thing right now is that instead of a blank screen, I'm getting a "memory not found" error in Smoke test 4 and 5 every time. So something happened or changed, I don't know.
I'll populate the entire board and see where that takes me and then redo the tests as I'm assuming the frequency tests were on a fully populated board.

Por gdx

Enlighted (6436)

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14-06-2022, 12:25

It's not a good idea to change the chip type without knowing the fan-INs/OUTs and a few other specifications.

Por RayneX

Resident (44)

imagem de RayneX

15-06-2022, 05:10

Here are my results for the relevant probe tests.
Clock U3 - 3.57Mhz
Clock U1 - 3.57Mhz
U1 Pin 30 - 275khz
U1 Pin 21 - from 420khz - 800khz
U1 Pin 22 - no reading. the frequency bar just moves up one square and doesn't get a reading no matter the time/div setting.
U26 - Pin 1 - from 420khz - 715khz
Flash Rom and SRAM both does the same thing as U1 Pin 22. No reading.

I verified continuity of U1 Pin 22 to the 82C55 IC and to SN74F541N. There are more it has continuity with, but at least I know it's not a bad solder joint.

This is as far as I got. The only IC I have not replaced on the board is 82C55, VRAM, and of course the VDP.
Everything else is pretty much new. I will re-verify the GAL chips again tomorrow. I verified before and re-wrote the JED files.

Por RayneX

Resident (44)

imagem de RayneX

16-06-2022, 20:39

Additional results of my testing. still getting "Error: Memory not found" from Smoking Test 3 through all Chips being populated.
With all chips populated, these are the scoped results
ADM U56 - Pin 15 - 5V. Pin 16 - 0V
U2 IC - Pin 6 (PPI_CS) - 4.6V (my friend said this should be 0V based on his understanding of the quick reading of the schematics)
All address lines on U12 (Pin 3 - 9) have continuity and frequency pulses to Z80 CPU
GAL chips re-programmed again and re-verified again. All valid. No change in error screen.

U2 IC - Pin 6 (PPI_CS) - No frequency reading on the scope.
U12 - Pin 14 (PPI_CS) - Same as U2 results. No frequency reading on scope

My friend is asking what actually activates the PPI_CS. Do you know?
Does this help you narrow down where the issue could be?
Does it seem to narrow it down that the 82c55 could be bad?

Just for reference, all of my ICs I've bought for replacement came from Mouser or Digikey. They are very reliable here. I just can't get the 82c55, VDP, and VRAM locally that I'm aware of.

Por steswart

Supporter (9)

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17-06-2022, 03:33

Yes, i found the issue. Like you said the bios is for NTSC and i am outputting PAL. :-) Problem solved.

Por RayneX

Resident (44)

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17-06-2022, 19:04

On revision 1.4 board, there are RAM jumpers. Is there a default jumper that needs to be set on them or leaving them of defaults to 512k RAM from the board?
Just trying to eliminate everything we possibly can.

Por RayneX

Resident (44)

imagem de RayneX

18-06-2022, 02:41

Omega MSX Build

Here are semi-high resolution shots of my build. (Ignore the masking tape on the GAL chips. I left it on by mistake on the Chip Select; it is programmed correctly for Chip Select). My friend who finally was able to start the Smoke tests also has the exact same error at Smoke Test 4.

Por RayneX

Resident (44)

imagem de RayneX

23-06-2022, 07:16

For the DC12v adapter, how do you wire the switch? Middle pin to middle via, but how do you differentiate the pin 1 to pin 3 to the DC board? Never wired a 3 terminal switch before.

Por ahmsx

Rookie (31)

imagem de ahmsx

23-06-2022, 08:36


I did the wiring in the following way.

I put the switch in the OFF position and using a multimeter checked which of the non-middle terminal legs of the switch had continuity with the middle terminal leg of the switch. That terminal leg is the OFF terminal leg, and the remaining non-middle terminal is the ON terminal leg.

Then I wired the switch middle terminal leg to the terminal block pin number 2 (the middle pin of the terminal block), the switch OFF terminal leg to the terminal block pin number 1 (the terminal pin in the terminal block which is closer to the discharging resistor R1) and finally the switch ON terminal leg to the terminal block pin number 3 (the remaining pin on the terminal block).

This is the board before the assembly and cabling.

And this is the board after (a quick and careless...) assembly and cabling.


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