*** M E M O R U M ***

So, at this point, the only thing that is not working is the VGA output?

Yes just the VGA

If so, can you check the short VGA adapter cable that the internal CFX-II uses

Belled out to end of VGA lead from Propellor pins OK (resister value show as they should)

It’s probably worth double checking that the white plug is inserted properly too.

White plug? VGA lead is soldered to the board on mine. Just want to make sure I'm not missing something here?

I do have a logic analyser of sorts. My knowledge on such things is minimal though but I get this from pins 5 and 6



It seems like it reads the rom then both stay low.

Obviously all 3 of those would happen if the propeller was plugged in backwards, and #3 if the eeprom was backwards. Leaving the obvious aside:

Checked. Never under estimate the obvious especially with me.

Out co curiosity I hooked up pins 1,2 and 3 of my VGA lead and get this... does it look like a normal output? I used the ground from the CFXII not from the pin BTW

Re the white plug . . . .

I did the internal board in two ways, one with a plug and socket arrangement and one with the cable soldered directly on the board. I couldn’t remember which yours was, but if you have continuity, then that’s fine.

Another “clutching at straws” idea . . . . .

But have you tried another monitor? (I know that the monitor works with another video source, but do you have another?)

I think I've found the problem. Pin 1 of connector 6 is GND and should bell through to pins 6, 7, 8 and 10 of the VGA plug which it does not.

Those pins are however connected to the shield ground. I've been tested to the main GND until now Nope scrub that my mistake


I cannot replicate the VGA signals for some reason?

But have you tried another monitor? (I know that the monitor works with another video source, but do you have another?)

Yep 2 monitors and both tested with a known source.

I also just tried desoldering the lead from the CFXII and sacrificed another lead to solder that in as a test. Still nothing.

Now I'm wondering if I've programmed the serial eprom correctly? I load the MTX_Monitor_11.eeprom file as a binary and have programmed that.
Is that the correct file?
And is Binary correct?

OK so I'm getting the hang of this logic analyser thing I think.

I have hooked up and test the Propeller shows activity on A0-A7 and DS0-DS7) however as per the below Pin 30 (Channel 0) is showing a constant Low, I believe this is a CLK so shouldn't I see the frequency pulses or is that just on an oscilloscope?

Channel 1 is Pin33 (IORQ)
Channel 2 is Pin34 (RD)
Channel 3 is Pin35 (WR)



To test I hooked up either side of the 2 XTAL's on the MTX board. Channel 0 and 1 is XTAL1 (4 MHz) and Channel 2 and 3 is XTAL2 (10.6875)
CFX was unplugged. The 10.6875 looks interesting?



Sorry if I'm on a wild tangent here but PHI also gets fed to the PSG (Audio Hi I believe) via a 74LS04. Given the sound issues started at the same time I now wonder about the 74LS04 chip? Seems this chip splits the XTAL1 signal to feed the CPU PHI, CTC PHI and PHI. Next step for later will be to see what's coming out of pins 8, 10 and 12.
Or I'm just on a wild tangent.

Hmm that chips been changed before by the looks of it. Currently its a 74HCU048I.

Removed the old chip and put in a socket. Found a PC74HCT04P I already had. Took a chance its specs are close enough



And now to switch on the power and... We have VGA output



Sound is also now working correctly

I am now a really happy chap

Great stuff - congratulations!

Yes, that chip had been changed. Memotech used 74S04, a TTL chip with low gate delays (3ns). I am not familiar with the replaced chip that you had in there, but it seems to be CMOS, probably with longer gate delays. However, it had been working, so must have been close to compatible, but it sounds like it failed (again). Fingers crossed that your replacement keeps working

Your new chip is a 74HCT, broadly similar to 74LS with similar gate delays. You might find it gives problems, Memotech obviously had a reason for using the 74S04, rather than LS. If it were me, I would source an S04, now that you have the socket, it’s an easy replacement

Regards
Dave

I would source an S04, now that you have the socket, it’s an easy replacement

Thanks I will certainly try and find one. And thanks to you and Martin for all your help, it's been really appreciated and without it I probably would have given up ages ago.

Congratulations. another one fixed !

There's an official timing mod change (pg 47 of the repair manual) that replaces the S04 with an HC04 - note no "U".

Your board looks like it's had that mod, big clue is the 2 missing resitors at R35/R36

This is the standard set up: And the official mod: Resistor chnages are in the yellow boxes.

HCT instead of HC should be fine. The major difference between the 2 types is in the input charactaristics and not the output.