Resurrecting an MTX500.

[stephen_usher]

I think my next step this weekend, pending the arrival of replacement LS273 chips is to swap 3A and 5A and put the run with the PAL and see if that fixes the ROM select line issue. I don't need the keyboard at the moment so it's no a major problem if it's not working.

As for the PAL board, if needs be I could always just borrow the one from my working MTX512 which is in the loft and protect the solder side with electrical tape so as to prevent accidental damage.

Unfortunately I'm running out of time for this project as next weekend I have to pack up everything in my hobby room, put it all in the loft and convert it back to a guest bedroom. After the Bank Holiday weekend I can convert it back again but I'll then be preparing machines to exhibit at a game convention (Lavecon) and then it's holiday season. So if I can't get anywhere by the end of the week it's probably on hold until September.

[1024MAK]

You can also feed the Y output from J12 pin 2 to a composite monitor or SCART TV. The signal needs to go through a resistor to form a voltage divider as the 9929 outputs a nominal 3v peak voltage, and composite 1v (on SCART at least)

470R is the resistor value I found "on the net". That to me seems a bit on the high side. If the load for the monitor or TV is 150R that reduces a 3v signal to 0.73 peak to peak. Which is fine, but SCART is set up for 75R. IF that's the case then I would be thinking to use 220R instead of 470R.

Mark, this is more your field than mine, any comments ?

Well, really a transistor is needed to provide a low impedance output at 13.3mA peak. But for short term test purposes a resistor is fine. For a TV or monitor with a 75 ohm input impedance (all domestic products), use a 150 ohm or a 180 ohm resistor, a 220 ohm should also work. Any decent TV or monitor should cope with a moderate over voltage.

Mark

[stephen_usher]

Getting somewhere at last!!!!

Part of the PAL board problem was due to the wire on the back of the video BNC connector coming off and I didn't notice. Doh! So now I have at least some video output.

Secondly, after swapping the LS273s over, with the PAL and diag ROM installed I get...

IMG_1453.jpg (1.45 MiB) Viewed 8725 times

So, it looks like I have video RAM issues, but I have something.

P.S. It works with the GAL and passes all main memory tests and the sound test!

P.P.S. That diag ROM is an excellent tool! Thank-you!

P.P.P.S. After replacing the CD4016s on the PAL board that's now working too. (New video RAM chips ordered from Retroleum and another 5m or "Soldermop" braid ordered from Farnell as I'd used up the last 5m desoldering those chips and I'll need it for the video RAMs.)

[stephen_usher]

With the VRAM fault it almost looks like it's in Italian.

IMG_1454.jpg (1.27 MiB) Viewed 8716 times

[1024MAK]

http://www.primrosebank.net/computers/m ... _fault.htm

Mark

[Martin A]

YAY Progress, I'd be looking at the 6116 providing bit 0 of the video memory 6G maybe ?

[stephen_usher]

As the system gets warm some of the memory bits come back to life. Using a freeze spray on 4G returns it to the worse state, which suggests that it's this chip which is knackered.

[Dave]

Re. martin’s Note. I think that the MSB/LSB are reversed from what you might expect. I think LSB is 4G

That seems to agree with your freeze spray results

[Martin A]

That's the reason for the "?" in that post

TI's crazy naming convention

[stephen_usher]

Hmmm... I've just seen a neat hack to use 4164 chips in place of 4116 chips...

http://atariage.com/forums/topic/257923 ... liability/

Basically cut off pin 1, bend pin 8 over the top of the chip and solder it to pin 9.