# Research an electric schematic of the mother board for msx vg-8020

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Ok Eugeny, i want measure with ohm multimeter hooked in j7 pin 1 and 4 for check shortcircuit,
but may be every integrated circuit than can't function: how can i understand what's I.C. than can't function or in shortcircuit ?

At this time we need to see _if there is_ a short circuit. Take your multimeter and measure how many ohms it shows in both directions between pint 1 and 4. The switch multimeter in another mode (it is usually indicated by diode sign on multimeter's switch) and measure in both directions again.

If a powerful +5V supply gets short circuited (close to 0 Ohms), then your output reading is ~0V, and the power supply is heating itself up.

Or in protective shutdown state. STK770 can provide maximum 2Amps. It should feature shortage autoprotect, thus I assume there either no contact in the system, or a short-circut.

However, if there is a short-circuit, pin 1 should have potential close to ground, and then all other voltages would read more or less properly (+12, -12). As they are wrong (7v and 17v) then there should be problem with ground, and not a short-circuit.

Let's wait for Fabio's findings.

step1-Rpin1-4=400 ohm
Rpin4-1=400 ohm

V1-4=2,61v
V4-1=3,49v

step2-Rpin2-4=10Mohm
Rpin4-2=13Mohm

V2-4=infinity ohm (open-circuit)
V4-2=infinity ohm (open-circuit)

step3-Rpin3-4=from 248 ohm to infinity ohm(capacitor-load-process)
Rpin4-3=from 248 ohm to infinity ohm(capacitor-load-process)

V3-4=infinity ohm(open-circuit)
V4-3=infinity ohm(open-circuit)

Would you tell me whitch is the I.C. with any problem ?

step1-Rpin1-4=400 ohm
Rpin4-1=400 ohm

That's not a shortage! My MSX has about 100 ohms (changing from 58 to 100 as capacitors charge).

How do you connect lab power source? It should be +5v, with "+" connected to pin 1 and "-" connected to pin 4. Please connect it, and take a picture of the connection.

Turn power supply on for several seconds, and touch chips (VDP, processor, other logic). Are there any which unusually hot?

In addition look into every connector (joystick, printer, both slots) to see that there're no shortages, stuck objects or bent pins.

BTW, what is a dark box in the corner of the board with UE3603 label on it?

Eugeny, i've toutch the CPU, the TMS9129, the EPROM of the Microsoft operative system, the S-3527 and other chips of the periferal, but they aren't hot.
The black box in the corner of the mother-board is the UHF modulator.
What is the method for search broken I.C. for you ?

What is the method for search broken I.C. for you ?
I do not see clear breakdown condition yet.
Can you please carefully remove pin 1 from the power supply connector, put your multimeter in current measurement mode (A±, 10A max), and measure current which flows between power supply's brown wire and pin 1 of the motherboard?

And also - can you inspect if this board had been already repaired - I see some splashes of the solder near the slot connector (from soldering side) and it seems for me that some pins of S3527 are shorted (particularly /CAPS and STB2)...

Eugeny, may be the splashed solder than make shotcircuit or problem at s-3527 or CAPS/LOCK.
Splashed solder are mine: i've maden them me, for solder a 22 PIN flat for connect RS-232 card,maden myself, to the slot but now it has been sconnected from the slot.

step 1-At the switch on power supply
I1-4 at begin=+1mA
I1-4 at final=0mA

step 2-At the switch off power supply
I1-4 at begin=-0,5mA
I1-4 at final=0mA

V1-4 at void=+1,03v

For me nothing I.C. is broken, may be the splashed solder would make any problem at s-3527:i want control that solder.
What do you think about that ?

Splashed solder are mine: i've maden them me, for solder a 22 PIN flat for connect RS-232 card,maden myself, to the slot but now it has been sconnected from the slot.
Was your VG8020 working before you made this modification? If yes: reverse (undo) it.

PS. I gather you made some hi-res pix, Fabio? Feel free to send those to my e-mail... (see contacts list, or Bitcycle.org homepage).

RetroTechie, my vg8020 working befor and after that modification, therefor don't dipends from
that for me (it's logic).
It wasn't working when i've riconnected the keyboard (i've rebuilded the keyboard in the printed-circuit) at the mother-board and i've switch on the power supply.

Do you think the problem is caused from the keyboard in printed-circuit ?

Not possible if the keyboard isn't connected, as I don't know any MSX that needs its keyboard to work (apart from not being able to type anything / press CAPS LOCK or CTRL-G, of course). If you removed something soldered to a slot-connector, it might be a good idea to connect the keyboard temporarily, just to re-check if machine responds to CAPS LOCK presses or gives out beeps when pressing CTRL-G. Btw: you can also check this by taking a loose wire (stripped ends) and hold it between correct points of the keyboard connector(s) - see schematic for keyboard matrix & -wiring.

Do you think the problem is caused from the keyboard in printed-circuit ?
Possible, but because of above reason only if you modified the mainboard somehow. Could you explain what you mean with "i've rebuilded the keyboard in the printed-circuit" Again, if you modified anything on the mainboard: please post pix (preferably hi-res, and/or some close-ups) of current situation.

Note that this would only be interesting to determine the point of a broken track / short circuit somewhere, as it relates to 1st priority: to have normal power supply voltages on the mainboard. For example a short circuit between a few keyboard X- or Y-signals shouldn't stop the MSX from showing to a BASIC message.

On the power supply issues, I suggest this: connect the original power supply, but nothing else, so that you have only: orignal power supply - connector - mainboard. Don't switch anything on! Now take multimeter, put it on lowest resistance range (not diode test!), and hold the test pins together to confirm that you have a <1 Ohm total resistance when short-circuited (and make mental note of the short-circuit value). Now for ground and each supply voltage: put 1 pin on a point in the power supply that should be connected to it, and other pin on a point on the motherboard that should be connected to it. For suitable test points: see schematic / follow circuit board tracks. If connections are as they should be, that should give you very close to the short-circuit value in all cases (voltmeter pin -> circuit board track -> connector -> circuit board track -> voltmeter pin = still <1 Ohm typically).

Some test points:

Ground - PS: middle one of a (3-pin) 7812 regulator. MB: modulator case, lower left pins of ROMs or 74xx logic IC's, pins 41+43 on cartridge slot.
+5V - PS: output pin of switching regulator, + side of electrolytic capacitor on that supply. MB: upper right pins of ROMs or 74xxx logic IC's, pins 45+47 on cartridge slot.
+12V - PS: right pin of 7812 regulator, cartridge slot pin 48.
-12V - MB: cartridge slot pin 50 (not important, probably not used in VG8020).

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