MSXmakers support | MSX Resource Center (Página 3/15)

Hero (551)

28-05-2022, 10:37

RayneX wrote:

I followed the schematic with a help of a friend. We're starting on Pin 6 of the VDP getting 16.23khz signal. I get the same signal on one side of R27. R27 shows a resistance of 1k, which is correct, but I get 0 frequency on the other side of the resistor. I'm not understanding how that's possible. I also checked C85 and also get 0 frequency on both sides of the capacitor. Capacitor is tested to be .1nF, so that's probably good. I don't have a sensitive enough Multimeters to get it down to pF. All continuity of the circuit are good as well. Not sure what we're missing here. Something is blocking the signal at that R27 resistor.

Hi again.
Did you installed Q2? Are you using a V9958? those two are incompatible.

Por RayneX

Resident (44)

28-05-2022, 14:31

msxmakers wrote:

RayneX wrote:

I followed the schematic with a help of a friend. We're starting on Pin 6 of the VDP getting 16.23khz signal. I get the same signal on one side of R27. R27 shows a resistance of 1k, which is correct, but I get 0 frequency on the other side of the resistor. I'm not understanding how that's possible. I also checked C85 and also get 0 frequency on both sides of the capacitor. Capacitor is tested to be .1nF, so that's probably good. I don't have a sensitive enough Multimeters to get it down to pF. All continuity of the circuit are good as well. Not sure what we're missing here. Something is blocking the signal at that R27 resistor.

Hi again.
Did you installed Q2? Are you using a V9958? those two are incompatible.

Q2 is not installed and I am using a V9958.

Por msxmakers

Hero (551)

29-05-2022, 11:37

Please, measure if in the first leg of R27 do you have about 2,5v altern current, if you have less than 1v your problem is caused before that (in your VDP perhaps).
Check also R28 is 1K.
Then, what if you just replace the R27 resistor?
you can de-solder those (R27 & R28) and replace they by new ones and better if you measure their resistance before to solder them.

it could be possible you have the problem in other place, I'm not sure how munch voltage should you have after R27 (I will try to measure mine when I got time).
please, follow all measurements in my blog to deduct if 82c55 is working properly or Slot systems won't do it.

Por retrocanada76

Hero (575)

29-05-2022, 19:57

I was getting a blank screen as well. (strangely it produced video showing the memory not found in the previous smoke test) and the result was a faulty Z-80. I got my Z-80 from an ebay seller (they sell all chips together) and this Z-80 seems to work in other projects but not omega. I don't know why maybe a bad timing or it is not a CMOS. Anyway I replaced to a different Z-80 (i pulled from an apple 2 CP/M card) and it booted nicely.

Por retrocanada76

Hero (575)

29-05-2022, 20:03

Did you get your chips from this place ? https://www.ebay.ca/itm/114105900693 I start to think their Z-80 are not suited for Omega...

Por RayneX

Resident (44)

29-05-2022, 21:39

msxmakers wrote:

Please, measure if in the first leg of R27 do you have about 2,5v altern current, if you have less than 1v your problem is caused before that (in your VDP perhaps).
Check also R28 is 1K.
Then, what if you just replace the R27 resistor?
you can de-solder those (R27 & R28) and replace they by new ones and better if you measure their resistance before to solder them.

it could be possible you have the problem in other place, I'm not sure how munch voltage should you have after R27 (I will try to measure mine when I got time).
please, follow all measurements in my blog to deduct if 82c55 is working properly or Slot systems won't do it.

Well, bad news. Something is dead and I'm not sure what. I tested R27 voltage and it was 3.85v on both sides, but also noticed that I lost "signal" to the monitor. I swapped the Resistor out for a new one; no change in voltage. Not sure when I lost the signal, but I did. I re-tested Smoke Test 2 and black screen with no signal. So I removed the ICs back to Smoke Test 1. Smoke Test 1 now fails. I have the Halt-Bios and I'm getting 5V with JP1 Disconnected and testing Pin 18 of the CPU. I should be getting 0v like before but not anymore. Does this symptom lead you to some conclusion on which IC went bad? (maybe it was bad all along causing all the issues?).

Por RayneX

Resident (44)

30-05-2022, 00:31

Now good news. I found the culprit. U26, SN74F245N was defective it seems. I swapped it and Smoke Test 1 passed again. Tried Smoke Test 2, I got a garbled screen finally. Added VRAM and I finally got the "Error: Memory Not Found" Error Screen.
Now I just tried Smoke Test 4 and that one now fails. Black Screen but signal detected. There was a lot of ICs added in the 4th test. Is there a way to break that down further so I can test where it stops showing a screen? I went back to Smoke Test 3 and it works, so that's great news.

Por RayneX

Resident (44)

30-05-2022, 03:35

retrocanada76 wrote:

Did you get your chips from this place ? https://www.ebay.ca/itm/114105900693 I start to think their Z-80 are not suited for Omega...

Sorry, I didn't even see this reply. Yes, that's the same seller I bought my IC set from. The CPU actually matches the BOM exactly, so unless it's just a faulty CPU in general, I'd like to think it's fine. But I guess it couldn't hurt to buy a new one from Mouser as well just to eliminate possibilities if nothing else could be the cause.

Por retrocanada76

Hero (575)

30-05-2022, 10:32

Here the tool you needed (but you need a MX29F040): https://www.youtube.com/watch?v=99BE0DRaV44

Por msxmakers

Hero (551)

30-05-2022, 15:15

RayneX wrote:

Now good news. I found the culprit. U26, SN74F245N was defective it seems. I swapped it and Smoke Test 1 passed again. Tried Smoke Test 2, I got a garbled screen finally. Added VRAM and I finally got the "Error: Memory Not Found" Error Screen.
Now I just tried Smoke Test 4 and that one now fails. Black Screen but signal detected. There was a lot of ICs added in the 4th test. Is there a way to break that down further so I can test where it stops showing a screen? I went back to Smoke Test 3 and it works, so that's great news.

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.