VDP output monochrome and strobing

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gunrock
Posts: 245
Joined: 28 Oct 2020 21:17

VDP output monochrome and strobing

Post by gunrock »

Hi all,

I have just received my CFX-II and after it sitting for two days awaiting installation I managed to have a crack at it last night.

Whilst the CFX appears to work as it should :D (thanks Dave, Martin and Bill), I thought I would switch the TV to the VDP output to see what was being output and compare with the VDP shadowing.

To my surprise, the VDP output was monochrome, with a slowly strobing (maybe the wrong term?) white band moving over the screen. It was working fine, so I figured I may have damaged something. First, I tried my other TV and where I had colour output there, witnessed here
20210209_211717.jpg
20210209_211717.jpg (66.84 KiB) Viewed 5484 times
I got a brief monochrome screen, with the same rolling artifacts, then synchronisation is lost and has never returned.

I've opened the machine up and check the grounding strap, continuity checks out. I have removed the CFX to simplify the troubleshooting and the issue remains. Back to the old TV, this is what it looks like:
2021-03-20-1924-VC1_1.gif
2021-03-20-1924-VC1_1.gif (759.75 KiB) Viewed 5484 times
Any ideas what I've done? :(
Steve G
Danish Memotech MTX 512, MFX and loving it
Martin A
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Joined: 09 Nov 2013 21:03

Re: VDP output monochrome and strobing

Post by Martin A »

It could be the TIP2955 power transistor. If the 10 ohm resistor at R62 is hot then that's an indication the transistor has failed. If the transistor has failed then all the 5V supply is pulled through the 7805 regulator, and can be more than it's rated for.

See Dave's page on this here:
http://primrosebank.net/computers/mtx/r ... _power.htm
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gunrock
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Re: VDP output monochrome and strobing

Post by gunrock »

Thanks for replying Martin. I'll check it out. When you say hot, you mean hot enough to (really) burn my fingers? i.e should I use a thermometer or can I touch it?
Steve G
Danish Memotech MTX 512, MFX and loving it
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Dave
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Re: VDP output monochrome and strobing

Post by Dave »

You can touch it, then . . . . . either :-

1. Rush to the sink and cool your throbbing finger under the cold tap, or

2. It’s slightly warm, so likely OK

Seriously, if it is u/s, it will probably be VERY hot, but be careful and a fleeting touch will tell you that it’s not well

Regards
Dave
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1024MAK
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Location: Looking forward to summer, in Somerset, UK

Re: VDP output monochrome and strobing

Post by 1024MAK »

Or you could just test the relevant supply voltages, like the +5V, +12V and the -5V.

If any are out of specification, then the power supply circuitry likely has a problem.

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer in Somerset later in the year :D

Not as many MTXs as Dave! :lol:
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1024MAK
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Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: VDP output monochrome and strobing

Post by 1024MAK »

You can test all three voltages on the 4116 video DRAM chips:

Image

A copy of the full data sheet is here

Pin 1 is VBB, which is -5V (-5.5V to -4.5V).
Pin 8 is VDD, which is +12V (+10.8V to +13.2V)
Pin 9 is VCC, which is +5V (+4.75V to +5.25V)
Pin 16 is VSS, which is 0V (ground/GND)

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer in Somerset later in the year :D

Not as many MTXs as Dave! :lol:
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gunrock
Posts: 245
Joined: 28 Oct 2020 21:17

Re: VDP output monochrome and strobing

Post by gunrock »

Well, I looked at that page on the R62 and R60 resistors. R62 seemed ok, the voltage measured at 0.67V.

R60 was 11.07V which suggests we have an issue!
Steve G
Danish Memotech MTX 512, MFX and loving it
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gunrock
Posts: 245
Joined: 28 Oct 2020 21:17

Re: VDP output monochrome and strobing

Post by gunrock »

Checking on the vram, all is good except the 12v, which reads around 8v. :?
Steve G
Danish Memotech MTX 512, MFX and loving it
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1024MAK
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Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: VDP output monochrome and strobing

Post by 1024MAK »

What do you measure across capacitor C51?

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer in Somerset later in the year :D

Not as many MTXs as Dave! :lol:
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