MSX 1 Video Problem

By e_lisaaa

Supporter (5)

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27-06-2021, 18:41

Hello!
Recently i bought an Gradiente Expert 1.1, a brazilian made MSX 1, the seller said it had a video problem, but it was cheap and i thought replacing the old capacitors would solve the problem (it still had the original parts from when it was made in 1987 and was never opened, the warranty seal was intact).
But the problem is still there, the video starts scolling fast and after some time it "stabilizes", i also noticed the VDP (TMS9128NL) is heating more than i think it should.
Is my VDP dead or could this be caused by another defectuous part?

Here is a video where i put some isopropyl alcohol on the chip to hav and idea of how fast it is heating, sorry for the video being tilted, my phone some times doesn't film the right way.
https://youtu.be/yhACAdQxvrc

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By PingPong

Prophet (3788)

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28-06-2021, 00:45

if the chip is hot this is a good reason to replace it.

By sdsnatcher73

Prophet (2311)

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28-06-2021, 08:32

I’m not convinced it’s the VDP that is faulty. One can definitely recognize the BASIC startup message and the function key descriptions, so it is still doing something. Can you post a photo of the complete mainboard? It seems to be an NTSC machine, are you sure the TV supports NTSC? Also the TMS9128 does not output composite signal directly, so there is a chip somewhere that takes the luminance+color variation signals from the VDP and combines them to create composite / RF output. That chip is more likely to be at fault here.

By Grauw

Ascended (10135)

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28-06-2021, 10:17

Seems to be a sync signal issue. I agree with sdsnatcher73. Afaik Texas Instruments VDPs get pretty hot (they often have a heatsink mounted) so that doesn’t necessarily point to a defective VDP.

By gdx

Enlighted (4801)

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28-06-2021, 10:38

sdsnatcher73 wrote:

Also the TMS9128 does not output composite signal directly, so there is a chip somewhere that takes the luminance+color variation signals from the VDP and combines them to create composite / RF output.

According the wiki, this is on a board apart.
https://www.msx.org/wiki/Gradiente_Expert_GPC-1
Check the connector beside to the VDP.

By e_lisaaa

Supporter (5)

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30-06-2021, 22:03

sdsnatcher73 wrote:

I’m not convinced it’s the VDP that is faulty. One can definitely recognize the BASIC startup message and the function key descriptions, so it is still doing something. Can you post a photo of the complete mainboard? It seems to be an NTSC machine, are you sure the TV supports NTSC? Also the TMS9128 does not output composite signal directly, so there is a chip somewhere that takes the luminance+color variation signals from the VDP and combines them to create composite / RF output. That chip is more likely to be at fault here.


Here is the mainboard, it isn't an NTSC machine, it uses PAL-M (a version of PAL only used in Brazil). The singals going out of the VDP go to another board, that outputs a monochromatic signal (i was using this output in the video, since it has a sharper image), this board also outputs RGB and the RGB signal goes to another board to be modulated and sent out as RF and PAL-M Composite, using any of the outputs produces the same result, the image keeps scrolling and after some time it stabilizes but it's still unreadable. The modulator board uses the clock output from the vdp to generate color, since it's the frequency of the PAL-M subcarrier (3.579545 MHz).


This is the board that produces RGB and monochromatic signal.


And this is the modulator board, the schematic shows an oscillator in the board, but in reality it uses the processor clock.

By e_lisaaa

Supporter (5)

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03-07-2021, 02:09

I decided to record another two videos showing the problem, using both composite and monochromatic outputs, on a completely "cold start" the video "stabilizes" after about 1 minute
https://www.youtube.com/watch?v=w0UTZYJUwjk
As you can see at the beggining of the video, this TV supports NTSC, PAL and PAL-M inputs
https://youtu.be/8N8QqX3puGM

By sdsnatcher73

Prophet (2311)

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03-07-2021, 20:20

Okay, you have the TV set on AUTO, correct? What is you set it to PAL-M. The theory is if there is a slight deviation the auto detection might fail but forcing PAL-M might improve the situation.

You mentioned you replaced the capacitors. Did you also change them on the 2 video daughterboard?

By Parn

Hero (659)

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03-07-2021, 20:38

This is not what typically happens when the TV misidentifies the color standard. Plus it wouldn't explain the unstable monochrome video. So I would investigate the highlighted part in the schematics that handles the Y signal. Just my 2 cents.

By e_lisaaa

Supporter (5)

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04-07-2021, 06:00

Parn wrote:

This is not what typically happens when the TV misidentifies the color standard. Plus it wouldn't explain the unstable monochrome video. So I would investigate the highlighted part in the schematics that handles the Y signal. Just my 2 cents.

Just tested the Y amplifier circuit, I disconnected the Y signal from the parts that extract the sync and green signals, just to be sure that only the amplifier would be connected to Y, and the problem persisted, i tried to connect the Y output from the VDP directly to the TV, as expected it produces a very weak image, but I could see enough to notice that the problem is still there, this ends my doubts that the problem is not in the VDP, I will buy a new one next week and see if that solves the problem, and I ill replace the crystal too, just to be sure.