738 ULA's

By LS120

Expert (95)

LS120's picture

29-03-2017, 09:20

Has anyone ever tried to backward engineer the 2 ULA's ?? as they seem to cause problems..
I was thinking a CPLD should be ok for one of them.... what do you think??
but i have no idea how to reverse to know what is in the ULA's..
or..
I think 1G0003 can be replaced with a 8255A and several logic ic,
and 1G0002 can be replaced with a CPLD chip.

any help...

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By RetroTechie

Paragon (1563)

RetroTechie's picture

09-04-2017, 20:27

Reverse engineering is mostly a matter of re-constructing the function of a component - and then replace that function with other component(s). Sometimes requiring very high-tech, expensive test equipment. Or not (like in this case). Depending on what's being reverse engineered, not necessarily difficult. But often very, very time-consuming. Sad

I suspect that given the number of SVI-738 machines & users, it's just not worth the effort. Less so as number of machines & users declines. And thus nobody bothered so far.

LS120 wrote:

I think 1G0003 can be replaced with a 8255A and several logic ic,

If you mean IC52 "ULA9RA041" in schematic: agreed, looks like it.

Quote:

and 1G0002 can be replaced with a CPLD chip.

If you mean IC35 "ULA5RA087" in schematic: yes, probably.

Quote:

any help...

...will be hard to find. Either you'd have to figure this out yourself, or find some other way to fix the problem.

Btw "as they seem to cause problems..": have you investigated this closer / excluded other causes? Electrolytic capacitors that have gone bad? Tested with an external power supply? Bad contacts in IC sockets? Cracks in circuit board traces? Swapped IC's with another SVI-738? Tested IC's in other computer with same IC inside? Asking because this kind of IC is generally quite reliable.

By Eugeny_Brychkov

Paragon (1097)

Eugeny_Brychkov's picture

09-04-2017, 21:00

RetroTechie wrote:

have you investigated this closer / excluded other causes? Electrolytic capacitors that have gone bad? Tested with an external power supply? Bad contacts in IC sockets? Cracks in circuit board traces? Swapped IC's with another SVI-738? Tested IC's in other computer with same IC inside? Asking because this kind of IC is generally quite reliable.

I watch posts regarding this issue, seems some components were replaced, but troubleshooting was not performed. I asked for logic probe, with no follow up.

By LS120

Expert (95)

LS120's picture

13-04-2017, 12:09

yes sorry have a skope and probe comming to check..

By LS120

Expert (95)

LS120's picture

09-06-2017, 11:13

hey RetroTechie, i have recapped the board and psu, all voltages are ok.i have also replaced the GPU,video ram, roms and CPU with a working unit. i have 3 doing the same thing.. the other thing i have to try is replacing the main ram.

i get the power on led, caps lock led on all the time and cassette relay clicks on power on. have pluged in a sunrise ide card and still nothing.. am waiting for a cart to test them with..

also just got my new 8 in 1 scope,tracer so will try going through the manual checks in the tech section. but i need the test cart to do it proper.

By Alexey

Guardian (2407)

Alexey's picture

09-06-2017, 11:24

I noticed that in my Yamaha the relay clicks in case of some bad event, for example if some data/address lines are shorted to the ground for whatever reason. Check the cartridge slots and the entire board for possible shorts. And yes, RAM could be the main suspect. I would recommend to use a RAM expansion cartridge first and some simple game cartridge. If the machine boots with them, then the problem is surely in main RAM. However please check if some data or address lines are shorted to the ground with a multimeter.

By LS120

Expert (95)

LS120's picture

09-06-2017, 12:14

will try.. i'm going to go through the svi diag manual it has what to look for with a scope so will see
oO

By LS120

Expert (95)

LS120's picture

16-06-2017, 09:23

ok some more info.. i tried a cart i found and ti's still no go i even replaced the 64kb ram.. i did notice that when i had the cart in and powered on the caps led went out but still nothing..

will get chance this week to try the scope.
Question

By l_oliveira

Hero (529)

l_oliveira's picture

25-06-2017, 16:50

I suggest you try to test the VDP chips on a working MSX computer. I had some computers which had bad TMS99xx/91xx IC which would give blank screen image but would make the Z80 crash during boot. Replacing the VDP cured the issue.

The VDP is easier to souce than those dedicated ULA ICs.