Troubleshooting MVS 2F video problems

channelmaniac

Mr Neo Fix-it
15 Year Member
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OK...

This post is from the perspective of repairing a MVS 2F board - the newer version of the 2 slot - but will work for most other MVS boards.

The output of the video subsection will go through the NEO-G0 chip then on to the Palette RAM (8k x 8bit SRAM) and the 74LS273 latches. From there it goes to a resistor ladder per color with a couple of the lines being buffered/amplified (higher current) by a 74LS05 IC.

When troubleshooting MVS color issues take a moment to divide the problem into sections.

1. Palette, or Color RAM error
2. No Sync (either horizontal or vertical)
3. Missing or incorrect colors

Part 1: Palette, or Color RAM error

If there are Palette RAM problems this can be caused by a problem in any of the ICs connected to that data bus: NEO-G0 chip, Palette SRAM, or the 74LS273 latches. The easiest thing to do is to power OFF the board and check the data lines on the SRAM ICs for shorts to +5v or ground connections. If there are none then replace the SRAM first.

If there is a short, remove the 74LS273 ICs first as they will be the most likely culprit. Check the data lines again. If the short is gone, replace the latches. If not, move on to the next chips, the SRAM. Repeat tests and replace the NEO-G0 last.

Part 2: No Sync (either Horizontal or Vertical)

The sync line goes from the NEO-I0 chip through a 100 ohm resistor to the JAMMA connector. It's also tied high to +5v through a 470 ohm resistor. Check both resistors. Replace if bad. If there is still no sync, replace the NEO-I0 chip.

Part 3: Missing or incorrect colors

Missing colors can be caused by any of the parts in the output section. Unless there's an esoteric problem with the Color RAM the digital part can be mostly taken out of the equation.

The output is made up of the 74LS273 latches, a series of resistors forming a ladder network, and 74LS05 Hex Buffer/Inverter IC.

With the power OFF, check the inputs and outputs of the 74LS05 IC for shorts to either +5v or ground connections. Replace the IC if any shorts are found. If there are shorts but the 74LS05 IC isn't it then replace the 74LS273 ICs. Next check the resistors for any that are burned.

If the RGB outputs are shorted the most likely culprits will the the 220 ohm and 150 ohm resistors as these are the lower value resistors. (The lower the value the higher the current that will flow through during a short.) check these resistors with an ohmmeter and replace any that are not reading correctly.

The next step is to check the outputs of the 74LS273 ICs with a logic probe. On the datasheet there are D and Q pins listed. The D is the input, the Q is the output. Check for signals on both sides. Replace the ICs that have inputs but no outputs between their D and Q pins.

Hook everything back up after replacing the parts and give it a test!

I had a board in the shop with missing red colors. There were no other symptoms on this board such as lines through images, etc... just screwy looking colors.

I replaced the NEO-G0 IC before stepping back and taking a better look at the circuit. After analyzing the circuit I replaced a shorted 74LS05 IC and two bad resistors to fix the board.

The 150 ohm and 220 ohm resistors were bad and the 74LS05 gate driving the 150 ohm resistor was shorted to ground.

I hope this helps you keep your Neo boards running!

RJ
 
Last edited:

channelmaniac

Mr Neo Fix-it
15 Year Member
Joined
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MKL,

Yes, but it depends on the version of the Neo board you are fixing. Not all revisions use the 74LS86 ICs. I said it was written from the aspect of fixing a newer version 2F board. ;)

RJ
 

MKL

Basara's Blade Keeper
20 Year Member
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The point is that the sync is outputted from the LSPC chip no matter what board we're dealing with while your post implies that it starts at the NEO-I0. In the link I posted I mention that on a MV1FZ the sync is on pin 150 of the LSPC2 which also holds for the 2-slot (it goes from there to pin 22 of the NEO-I0). So if the NEO-I0 goes bad (or at least the part of it that deals with the sync) the obvious solution is not to replace this custom chip but to bypass it and use a LS32 instead which is readily available and easy to work with.
 

channelmaniac

Mr Neo Fix-it
15 Year Member
Joined
Nov 29, 2005
Posts
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Re-read the post. I traced it back to the NEO-I0. I did not say it "originated" there. I only tracked it back that far for this thread.

Yes there may be other ways to fix a board. I for one prefer to use original parts and not "hack" in some kind of repair unless absolutely necessary.
 

MKL

Basara's Blade Keeper
20 Year Member
Joined
Mar 8, 2002
Posts
3,686
channelmaniac said:
Re-read the post. I traced it back to the NEO-I0. I did not say it "originated" there. I only tracked it back that far for this thread.

OK... so how about changing the thread title to *partially* troubleshooting...?
Of course I will delete my posts since they contain info that falls outside the scope of the thread.

channelmaniac said:
Yes there may be other ways to fix a board. I for one prefer to use original parts and not "hack" in some kind of repair unless absolutely necessary.

Ahh, so this thread is about what *you* prefer and is not meant to help *other people* fix their faulty hardware, right? For a moment I thought you were suggesting they desolder a 64-pin SMD custom chip (after buying a SMD rework station?), track down another 2-slot from which they desolder the same 64-pin SMD custom chip (what they don't grow on trees??) and solder it on their faulty board in order to fix it. But now I know you're not suggesting something that retard I'm relieved.
 
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