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About original Memotech hardware.
Steven.G
Posts: 42
Joined: 18 Jul 2020 14:03

No picture.

Post by Steven.G »

Hi everyone and thanks for the acceptance to the forum.
I've been given a MTX 512 and was feeling chuffed however on connecting to the TV couldn't find the output using the ariel lead.
Tried a BNC to phono connector plugged into the video output but still no signal.
On advice I Removed the keyboard to check the connections and found a wire disconnected.

Also none of the photos of main boards on primrosebank show the black wire on the UHF modulator.

Thanks in advance,

Steve..
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Dave
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Re: No picture.

Post by Dave »

The black wire is pretty usual - it's tying the A/V grounds to the ground plane and is required to reduce noise.

The wire off seems to be the Audio ground, you should resolder it. With the Audio connected, if you hit <Control><G> after you power on or reset, you should hear a "ding" from the sound chip, even if the video is not working.

If you just hear a constant tone with a blank screen, then it's a more serious, but common, problem and will require a bit more diagnosis. (The most likely cause would be memory, but it could be a host of things)

regards
Dave
Steven.G
Posts: 42
Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

I've re soldered the wire back on and still nothing, I tried <Control><G> ,, nothing.
It occurred to me that even though the Power supply red light is on how do I test if any power is actually going to the board.

Steve..
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Dave
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Re: No picture.

Post by Dave »

You'll need a multimeter.

You definitely have the sound connected and the volume turned up and hear absolutely nothing on the audio output?

Unlikely, but possible, that the fuse has blown - it's glass, so you should be able to see whether it is intact or not - you'll probably have to remove the cover and pop it out to get a good look at it

Also, if you leave the machine on for a while, say 10 minutes, the video chip will get quite warm - this is normal, and will confirm that the board has at least got some (5v) power

regards
Dave
Steven.G
Posts: 42
Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

I have a multimeter.

Sound connected and the volume turned up, even tried headphones in the ear socket and still no audio output?

Checked fuse yesterday visually and removed and with multimeter, not blown.

Is the video chip the TMS9929ANL chip? not that it matters as all chips are cold.

Steve..
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Dave
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Re: No picture.

Post by Dave »

The Ear socket is only used for tape loading, so no help in diagnosing faults.

Easiest check is for 5VDC on any of the TTL chips.

The dimple in the chip denotes the top. You should measure 5v between the top pin at the right hand side (+) and the bottom pin at the left hand side (0v).

Try on the chip in the top left in this picture

http://www.primrosebank.net/computers/m ... 0_crop.jpg

In this view, the top left position is 5v and the bottom right 0v

If you have no +5v, and the fuse is OK, it could be a failed regulator, but to check the PSU, see this page for the pin out of the plug

http://www.primrosebank.net/computers/m ... mtxpsu.htm
(Voltages are AC)

Regards
Dave
Steven.G
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Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

No +5v reading.

And the words "voltages are AC" worry me!!

Steve..
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Dave
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Re: No picture.

Post by Dave »

If you look at my PSU page, you’ll see that they are very LOW AC voltages.

Nothing to be afraid of, but be careful when measuring the voltages, you obviously don’t want to short the pin to the shell when trying to measure the voltages, but you’re not going to hurt yourself.

If you don’t want to do that, the best bet is probably to take the board out of the case and measure the input voltages on the regulators. They are usually obstructed by the capacitor that sits above them, so you’ll likely need to take the board out.

Underneath the capacitors you’ll find this
http://www.primrosebank.net/computers/m ... emoved.jpg

Use the circuit diagram and the regulator pin outs so that you know what you should measure on each pin of the regulators and power transistor

You might need to remove the video board to get better access

Regards
Dave
Steven.G
Posts: 42
Joined: 18 Jul 2020 14:03

Re: No picture.

Post by Steven.G »

Thanks Dave you've been a great help.
I did try to get the board out but it refused to budge, I'll try the again tomorrow along with the other tests.

Steve..
Martin A
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Joined: 09 Nov 2013 21:03

Re: No picture.

Post by Martin A »

I can see from the photo that there'sa 32k ram board in there. To might find the main board easier to move if you take the extra ram out first.
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