MSXmakers support

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By RayneX

Resident (44)

Аватар пользователя RayneX

30-05-2022, 15:22

msxmakers wrote:

Try to identify the chips you've added and try to find a culprit, eg I would replace U17 and U18 with the other 2 74xx670's you have and power back on, and if the result is the same I'd cross them off the culprit list.
If with that you do not find a culprit, start backwards, insert the smoking test 3 chips and see adding the chips one by one, turn it on and locate if there is one from which you lose the signal.
I think that when I documented the steps I inserted one by one and it always turned on, although you have to keep one thing in mind. Should it react the same way with all ICs changed?
Whoever made that kit on ebay decided for himself which replacement IC to use, some are 74LS and others 74HC and theoretically they are incompatible. Some time ago I wrote to him and he said "I have sold several and nobody complains", but of course it does not mean that the steps will work the same.
So another solution is to go ahead and insert them all, because the steps may not work the same but the end result will.
I hope to be helpful.

Thank you. I've already swapped the 670's and that didn't do anything. I will try adding 1 by 1. I just didn't know if that would cause any issues with the circuit. And you are correct, there are a lot of LS chips in that kit. If all else fails in Smoke Test 4, I guess I'll order the HC chips for all. Thanks for the help.

By msxmakers

Champion (473)

Аватар пользователя msxmakers

30-05-2022, 22:47

RayneX wrote:

Thank you. I've already swapped the 670's and that didn't do anything. I will try adding 1 by 1. I just didn't know if that would cause any issues with the circuit. And you are correct, there are a lot of LS chips in that kit. If all else fails in Smoke Test 4, I guess I'll order the HC chips for all. Thanks for the help.

Hi Sir,
I mean you try as many replacement as you can to test every chip/block, not only 74LS670.
And as per what you mention... 74LS chips are not the problem, in fact if you replace all chips by 74LS series it should work.
74Fxx are also TTL so it doesn't matter to mix them.
If you mix 74LS with 74F and 74HCT or 74AHCT that's not a problem too.
the real problem is, buffer chips should be TTL (74LS or F) and they are incompatible with 74HC.
Even that, sometimes they work without problems, but it's hard to identify when you will have problems caused by this.

If you cannot try replacing more chips, try the following:
1) put all chips as smoking test 4 and probe it still works.
2) try adding another chip and power it again, if it works add more but if it doesnt just leave it out until later.
3) when you end testing the ones who work, add again the rest one by one.
4) if nothing seems to work try adding all the omega chips at once and power it on again.
let us know then the results.

so, you better order all chips as in the Kiselev's BOM or you can order all of it TTL (74LS).

By retrocanada76

Hero (538)

Аватар пользователя retrocanada76

30-05-2022, 22:53

I only use LS chips and it is working fine. As I told you I got blank screen on smoke test #4 and the reason was a bad z-80. I find it out by using a logic probe and found that at some point the z80 goes haywire. The first bytes of code it does well but then it jumps somewhere. Replacing the z80 and the omega booted fine.

By RayneX

Resident (44)

Аватар пользователя RayneX

30-05-2022, 23:04

msxmakers wrote:

Hi Sir,
I mean you try as many replacement as you can to test every chip/block, not only 74LS670.
And as per what you mention... 74LS chips are not the problem, in fact if you replace all chips by 74LS series it should work.
74Fxx are also TTL so it doesn't matter to mix them.
If you mix 74LS with 74F and 74HCT or 74AHCT that's not a problem too.
the real problem is, buffer chips should be TTL (74LS or F) and they are incompatible with 74HC.
Even that, sometimes they work without problems, but it's hard to identify when you will have problems caused by this.

If you cannot try replacing more chips, try the following:
1) put all chips as smoking test 4 and probe it still works.
2) try adding another chip and power it again, if it works add more but if it doesnt just leave it out until later.
3) when you end testing the ones who work, add again the rest one by one.
4) if nothing seems to work try adding all the omega chips at once and power it on again.
let us know then the results.

so, you better order all chips as in the Kiselev's BOM or you can order all of it TTL (74LS).

I did the 1-by-1 test and actually pulled out the chip that was causing the issue. The 74LS00 was causing the black screen. I pulled it out and I got the Memory Error screen. All other chips in Smoke Test 4 worked. So, I have ordered a new 7400 chip but the one from the BOM (well, a direct substitute as none was in stock on Digikey or Mouser). Lesson learned is it's better to just buy brand new when you can and if cheap enough. Honestly, Mouser/Digikey is quite cheap on these ICs. It's just really difficult to get ALL the BOM in one order, that's all. One step closer to getting an operational Omega. If I still have issues, I'll swap the CPU and other bigger ICs. All parts should be here Wednesday/Thursday. Can't wait.

By retrocanada76

Hero (538)

Аватар пользователя retrocanada76

31-05-2022, 06:10

RayneX if I get my 74LS00 out I get the no memory error as well because this chip is part of the memory circuitry. I am betting on the Z-80. Do you have a replacement ? I had the exact SAME black screen on smoke test 4. I replaced the Z80 and it worked. I got the same memory error screen. Looks like this Z80 has some issue talking to the SRAM but when running code straight from the Rom like the memory check diagnostic it is okay.

By retrocanada76

Hero (538)

Аватар пользователя retrocanada76

31-05-2022, 06:09

BTW if you have a minpro programmer you can test the 74LS00 but I am sure it is working otherwise you'd have gotten the no memory found error instead of a blank screen.

By RayneX

Resident (44)

Аватар пользователя RayneX

31-05-2022, 07:04

retrocanada76 wrote:

BTW if you have a minpro programmer you can test the 74LS00 but I am sure it is working otherwise you'd have gotten the no memory found error instead of a blank screen.

I tested all the ICs I could and they all Passed BUT the 7400, when auto detecting, came up with 4 different models. I'm not sure what that means beyond the logic gates indicating multiple models or defective gates somewhere.

By RayneX

Resident (44)

Аватар пользователя RayneX

31-05-2022, 07:07

retrocanada76 wrote:

RayneX if I get my 74LS00 out I get the no memory error as well because this chip is part of the memory circuitry. I am betting on the Z-80. Do you have a replacement ? I had the exact SAME black screen on smoke test 4. I replaced the Z80 and it worked. I got the same memory error screen. Looks like this Z80 has some issue talking to the SRAM but when running code straight from the Rom like the memory check diagnostic it is okay.

Replacement CPU is coming Wed/Thurs as part of the parts I ordered. I'll replace the 7400 first and see what happens. If C-BIOS doesn't load, then I'll swap the CPU and see if that fixes anything.

By GFX

Supporter (7)

Аватар пользователя GFX

31-05-2022, 16:39

Hi everybody thanks for your answers regarding the small noise on the image. I found a solution to remove it.
I did few turns (8) in a ferrite donut with the power supply wire and the noise is gone Smile

I have another question, On the Omega when I'm using SCC emulation with the Carnivore 2 or the SD Snatcher Sound Cartridges, the SCC doesn't work...only PSG is playing. It works nicely with original cartridge but I was not able to make it work when emulated. Any idea why ?

Thanks !

By retrocanada76

Hero (538)

Аватар пользователя retrocanada76

31-05-2022, 17:14

Does it need 12v ?

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