Another MTX Fault - Jon Riglar's MTX

About original Memotech hardware.
jonriglar
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Re: New member saying hello

Post by jonriglar » 14 Aug 2015 23:40

No that's an interesting question!
I bought 4116 chips - which is what was/is fitted as VRAM on my board (x8).
Is there a 'aha' moment coming? (Something fundamental I've got wrong!?)

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Dave
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Re: New member saying hello

Post by Dave » 14 Aug 2015 23:48

Ooops - the "aha moment" is all mine :-)

I have 4116 and 4164 for sale, I confused which ones you had bought.

The 4116 are good for the VRAM
The 4164 are what's needed for the main RAM

regards
Dave

p.s., if you wanted to save us both the rip off ebay fees, and you pay the Paypal fees, I could do sets of 8 for £8.20

Martin A
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Re: Another MTX Fault - Jon Riglar's MTX

Post by Martin A » 15 Aug 2015 10:32

The black screen and tone are the normal power up status of the video and sound chips.

So the only thing that's certain is that the system didn't managed to reach the part of the rom that sets up the blue screen and turn off the sound.

I've had this on both my MTX's at different times over the years, one of them just needed the Z80 replacing for a 2 min fix.

The other one took months to fix http://primrosebank.net/computers/mtx/r ... air_ma.htm

So if you've got one spare, replacing the CPU for a known good on is worth trying, the MTX service manual recommends replacing all the socketed chips for good ones. Though these days' that's not so easy as spare VDP's and system roms are none too common

The CTC isn't actually needed to start the system so if you don't have a spare it can just be pulled temporarily. An MTX with no CTC can be programmed too, of course anything interrupt driven won't work, IE the clock, break key and likely other things....

jonriglar
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Re: New member saying hello

Post by jonriglar » 15 Aug 2015 16:35

Hi Dave
I'll take you up on the memory offer; my email is jonriglar@outlook.com if you want to send me a Paypal invoice direct?
Thanks
Jon
Dave wrote:Ooops - the "aha moment" is all mine :-)

I have 4116 and 4164 for sale, I confused which ones you had bought.

The 4116 are good for the VRAM
The 4164 are what's needed for the main RAM

regards
Dave

p.s., if you wanted to save us both the rip off ebay fees, and you pay the Paypal fees, I could do sets of 8 for £8.20

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

Re: Another MTX Fault - Jon Riglar's MTX

Post by 1024MAK » 15 Aug 2015 17:43

Now that the confusion over names, people, places, secret services and DRAM chips is sorted.... :lol: :lol: :lol:

How many ROM chips does your system have / or to put it another way, which issue PCB do you have? And does it have any "yellow" wiring on the underside?

Only, Andy Key can supply a very basic test "ROM" that can show if the CPU is running. It needs no RAM to run, so as long as any failed RAM is not shorting out the data bus, it will not care about the RAM.

Also, did you already say that you have checked the three regulated DC supplies? The video DRAM needs +12V, +5V and -5V. If the -5V is missing the DRAM chips used for the video DRAM may die. If the +12V is missing they will not work at all and the video board will have no power.

What test gear do you have?

Oh and don't give up. Although some chips are hard to get hold of, the lack of custom chips in the design means it's reparable :mrgreen:

Mark

jonriglar
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Re: Another MTX Fault - Jon Riglar's MTX

Post by jonriglar » 15 Aug 2015 22:03

Hi Mark
The board appears to have 3 ROMs in sockets. It has 4000-06 printed topside.
There are no link wires soldered on the underside.
I've checked the main voltages and they are present. I have a multimeter, soldering kit, even one of those usb powered small digital storage scopes, but that only shows frequencies up to 1MHz.
Regards
Jon

jonriglar
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Joined: 14 Aug 2015 08:46

Re: Another MTX Fault - Jon Riglar's MTX

Post by jonriglar » 29 Aug 2015 17:44

An update on my attempts to get the MTX back to life;

New Z80A CPU fitted
New Z80A CTC fitted
New TMS9929A fitted
New main memory fitted
New video memory fitted (which still runs hot - how hot is too hot?!?)

No output on screen (from either RF or video out), continuous tone.

Re-checked voltages, they seem all ok.

Beginning to suspect the video daughter board(?); may have to send it to some kind forum member to swap out/test in their machine??

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Dave
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Re: Another MTX Fault - Jon Riglar's MTX

Post by Dave » 29 Aug 2015 18:27

I think that it's highly unlikely that it is a video daughter board problem if you are not even getting sound, but, you can eliminate it by taking mono composite video straight off the VDP by removing the video board and connecting to the plug pins as mentioned on this page

http://www.primrosebank.net/computers/m ... _video.htm

Regards
Dave

Martin A
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Re: Another MTX Fault - Jon Riglar's MTX

Post by Martin A » 30 Aug 2015 15:11

Dave wrote:I think that it's highly unlikely that it is a video daughter board problem if you are not even getting sound,
I agree with Dave, it's VERY unlikely that it's the video daughterboard.

The MTX start-up sequence is basically
Check/setup the ram
Run any auto-boot rom
Setup the VDP
Turn off the sound chip.
Setup the CTC
Start Basic

So if it were just a faulty daughterboard, you'd have a black screen but no sound.

Black screen and tone is the trickiest fault to pin down, as all you really know from the symptoms is that the CPU hasn't managed to run the boot sequence far enough to reach step 3.

As Dave said earlier it's usually an indication of a memory related issue, however that covers quite a lot of the board.

You could try the scope on the data pin pins and chip selects for the ram, I took this shot scoping pin 2 of the ram chip at C5. It's not that fast a signal that a 1mhz scope shouldn't display it. After all what it shows is irrelevant you just want to see a wiggle to confirm it's not stuck high or low.
Ads00001.jpg
Scoping D0
Ads00001.jpg (29.39 KiB) Viewed 2224 times

jonriglar
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Re: Another MTX Fault - Jon Riglar's MTX

Post by jonriglar » 01 Sep 2015 10:08

Thanks Dave, thanks Martin,
I'll get busy with the 'scope.
Regards
Jon

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