help needed with sv-738 fault

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By LS120

Expert (76)

LS120's picture

08-03-2017, 10:01

Hi is anyone able to help me with trying to repair a 738 that wont startup (black screen) when i power on,

i get a click from the relay and the power led is on.. i have re-caped the board and replaced the vdp & all 3 x roms with working ones from a working sv-738 i have (v1.5).

all other chips are soldered in so any help.. please...

PM and we can email info if needed as i would like to get the 2 non working ones going and into new homes..

thanks

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By LS120

Expert (76)

LS120's picture

12-03-2017, 00:52

anyone keen??? i was going to make a diag blog/pdf with what to do step by step.. but i need help in doing this.. Wink

By mohai

Hero (622)

mohai's picture

12-03-2017, 23:47

It sounds like a short circuit (maybe).
Does floppy start?
Does CAPS led works as it should do?
Check the power from the power supply and the +5 and +12 volts.
An external RAM can help too.

By LS120

Expert (76)

LS120's picture

13-03-2017, 10:01

nope all is standard, waiting for a couple caps to finish the psu.. but it was giving 12v & 5v & 12v-, will check the caps lock when done.. and let you know

By mohai

Hero (622)

mohai's picture

13-03-2017, 11:01

Just an idea: you can try to check Z80 and other chips with a cheap analyzer.
There are some some Saleae clones on sale.
Try to find out if ROMS and Z80 are working.
If CAPS is working, then Z80 and BASIC ROM are working.
If CAPS stays always on, then seems a shortcut somewhere, maybe a chip, a capacitor or a transistor.
Hope you already have the schematics.

By LS120

Expert (76)

LS120's picture

14-03-2017, 22:23

thanks for the ideas.. will try today..
Smile

By LS120

Expert (76)

LS120's picture

15-03-2017, 03:02

ok on the one i'm doing the caps led does not work at all but the power led comes on and i can hear the relay click

the other one has led caps on all the time and relay clicks on start and power led is on.

any idea??

Question

By mohai

Hero (622)

mohai's picture

16-03-2017, 23:43

If caps led is always on, then it looks like a shortcut.
You can do another test: set the bright on your monitor a little higher and check is some image is being shown. Sometimes collums and borders can be seen even when the image ia black. That means that VDP is working.
Then check crystal oscilators and clock signal comming feom VDP to z80. Check if Z80 is working or not...
Check if correct power is reaching all components...
Sometimes is a faulty solder. Check and re-solder any suspicious solder.
If there is some kind of upgrade, check connections and solder points.

By LS120

Expert (76)

LS120's picture

21-03-2017, 00:24

ok so look at board with glasses and could not find any faults but did notice that IC36 had oxidization on pin near tp-12 so cleaned up and re flowed it. should i replace the green polys on the board and all the de-couple caps (small disc's). the only transistors are for the cassette and in the rf unit.. i noticed ic36 is used in 1/6 units.. around the board.. any more help?? i'm waiting for the full tech book to arrive to see where to test for power etc..

By Alexey

Paragon (1042)

Alexey's picture

22-03-2017, 00:27

A relay click on power-on is a bad sign. My Yamaha YIS503 does that when there's a problem with inserted cartridges. Probably this has something to do with power shorted to the ground by accident. The short is not enough to cause major drama, but prevents the machine from starting. So please check if there is a continuity between power and ground, whether there are any bent and shorted pins in the cartridge slots, whether any mounted components have shorted pins, etc.

By LS120

Expert (76)

LS120's picture

22-03-2017, 07:26

it's funny as it was working fine and then one day just did not turn on.. so have replace psu with a good working one but still no go..
i have replaced the cpu with a socket to put a Z80b in as going to do a turbo if it runs again..

just waiting for some ram sockets so i can put in some 256 x 1 for ram upgrade as already has the 128mb video ram in it

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