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

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By sp4

Master (214)

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26-04-2010, 22:36

I've a last one thing to ask: how can i delete one of my post ?

By sp4

Master (214)

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27-04-2010, 21:52

Eugeny i wish to repair my vg8020.
I' ve followed this step:

1-I've sconnected all the priferal connector in the mother-board, i've keep connected only j7 for connect the power supply and the connector near the VDP tms9129, for connect the little card over it.

2- When i swiched on the the power supply, +vcc 5v get down at 0v, +12v get down at +6,80v,
-12v get down -17,16v.

3-VDP pin 37 and Z80 ck-pin 6 steady 0Hz.

4-If i connect the television-monitor with the UHF aerial cable at the UHF output in the
mother-board, i read no input signal in it.

5-If i repeat step 2 with external laboratory power supply the voltage is as follow: +5v get
down between +2,5v and +3,6v, +12v get down at about +11,6 and -12v was sconnected.
The second ,the third and the fourth steps result unvaried.

Why are there this problems ?
What's succed ?

By Eugeny_Brychkov

Paragon (1184)

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27-04-2010, 23:10

Very strange, but at least something to start with.

Step 1:

Take original power supply, disconnect it from motherboard and measure its output:
+5v between pin 1 and pin 4
+12v between pin 2 and pin 4
-12v between pin 3 and pin 4

Be cautious as this type power supply does not have automatic protection and shortcircuit may damage its components (STK770, 7812 and 7912).

Step 2:

disconnect EVERYTHING except power supply (disconnect even that little card) and try again measuring voltage with only power supply connected and powered on.

Step 3:

with everything disconnected (even power supply) take multimeter and measure voltrop on motherboard's power connector (J7): between pins 1 and 4, between pins 2 and 4, between pins 3 and 4 and report your findings.

By RetroTechie

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28-04-2010, 14:42

5-If i repeat step 2 with external laboratory power supply the voltage is as follow: +5v get
down between +2,5v and +3,6v, +12v get down at about +11,6 and -12v was sconnected.
The second ,the third and the fourth steps result unvaried.

If you connect an external lab power supply and its +5V gets pulled down to +2.5~3.6V, then it would be delivering a lot of current (a typical lab supply can pull many Amps). Read: it would be pumping a lot of power into the circuit, and that power (=heat) would have to go somewhere. With those readings, I'd expect to see something glowing hot and/or smoking. Crazy

Which makes me suspect your readings somewhat... so I have to ask: how have you hooked up that lab supply (to which points)? What else is connected (preferably nothing)? Between which points are you measuring the various supply voltages? Does the lab supply give nice +5, +12 / -12V outputs when not connected (read: is your voltmeter working) ?

3-VDP pin 37 and Z80 ck-pin 6 steady 0Hz.
And how are you measuring that clock frequency (that is not a simple voltage reading, you know) ?

Schematic for VG8020/00 shows external oscillator (14 MHz), not using VDP clock... if you have 14 MHz. xtal on that small add-on board, you probably need it connected. TMS9129 has CPUCLK on pin 38, I believe (not 37)? And most systems with S3527 use the S3527 for clock buffering (pin 30 = clock in, pin 29 = clock out, a couple of corner pins).

For video output, I suggest you use the composite video (DIN) output at the back. That takes UHF modulator out of the picture, so you can skip any channel tuning on TV. Easier methods to see if it's alive: keep [CAPS LOCK] pressed - on a working machine, the CAPS LOCK led should flicker while you do that. Or press CTRL + G, each time you do that, should give a 'beep' sound (audio output on the DIN connector).

By sp4

Master (214)

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28-04-2010, 18:10

RetroTechie, i've hooked lab power supply in only +5v +12v pin and i've keep sconnected
-12v pin.
Lab power supply has been hooked at j7 connector, i've measured the two voltages with voltmeter hooked again in j7 connector: j7 +5v +12v pin was get down when lab power supply has been connected at j7.

By sp4

Master (214)

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28-04-2010, 18:57

Eugeny, i've maden the follow measuring:

step 1- Measuring with j7 at void (connector j7 sconnected from mother-board).
+5,04v
+12,02v
-12,09v

step 2-Measuring at load (connector j7 connected at mother-board) and little card ck-generator
connected (at j10).
0v
+6,7v
-17,06v

step 3-Measuring at load(j7 connected at mother-board) and little card ck-generator sconnected.
0v
+6,78v
about -17,07v

By RetroTechie

Paragon (1563)

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28-04-2010, 19:29

17,06 = ~5 + 12
6,7 = ~ 12 - 5

Are you sure you haven't mixed up your power supply connections? Shocked! Let's hope you haven't fried this 8020 already...
One hint: modulator case (the 'tin can') is usually ground, and an easy reference point to put your voltmeter's black pin on Cool

By Eugeny_Brychkov

Paragon (1184)

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28-04-2010, 20:12

Eugeny, i've maden the follow measuring:

step 1- Measuring with j7 at void (connector j7 sconnected from mother-board).
+5,04v
+12,02v
-12,09v

I think this confirms that Fabio knows what pin corresponds to which voltage level.

Fabio, now take Ohm-meter (or any multimeter putting it in resistance measurement or voltrop measurement mode) and measure value between corresponding contacts on the motherboard's power connector with power supply disconnected.

Are you sure you haven't mixed up your power supply connections? Let's hope you haven't fried this 8020 already...
Alwin, no, I have pictured of the board, it does not look like fried Smile And thank you for noticing pattern! Well done!

Fabio, I think you will find that there's something wrong with ground wire in the connector. If ground is not connected, then of course measuring between 1 and 4 gives 0 (as both have 5v level), 2 and 4 gives 7v, 3 and 4 gives 17v.
I do not see anything wrong on the pictures of the board, and I leave it to you to inspect connectors and use multimeter to identify where's connection problem or short-circuit.

By sp4

Master (214)

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28-04-2010, 20:37

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 ?
Tell me how please ?

By RetroTechie

Paragon (1563)

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28-04-2010, 21:24

I think this confirms that Fabio knows what pin corresponds to which voltage level.
Yes, but I think Fabio is missing the point:

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.
If there's no short circuit, power supply can relax & circuit is okay (enough) to do further tests.
But in between (say, 2~4V) means the circuit IS heating up somewhere significantly - you can't pull down a power supply's 5V without drawing some Amps at the same time. Amps x Volts = Watts...

So if these readings are correct, then Fabio should be able to see something happening, even without voltmeter. I mean: put 5 Watts into a ROM chip, and it will burn... Crazy

@sp4: Try making measurements with just the original power supply, or just lab supply's +5V connected - have a look at +12V when +5V is okay & stable.

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