My second MTX

[acsi]

Well I definitely over paid for this but not to worry it's just got added challenge.

Wasn't exactly packed very well (two piece's of paper in a box with the MTX were the PSU, Tape deck and a box of tapes) but I guess it was only packed to go across the country (UK) not also from there to New Zealand (That's the price for being so far away unfortunately).

That being said an added bonus I didn't expect was the expansion port cover

Video output isn't great (picture shows it at its best) and garbled screen so I'm guessing 4116 video chips?

Just awaiting my shipment of caps from Element14 before delving inside the machine. OK I had to open it up at least.

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[Dave]

Hi,

The photos are a little small, but yes, looks like VRAM

From my web page http://www.primrosebank.net/computers/ ... repair.htm

The Service Manual also describes how to disable each of the VRAMs in turn by earthing Pin 14 (via a 10-15 ohm resistor!) and checking whether the screen corruption reduces, the chip with the least screen corruption when it is being earthed is likely to be faulty and should be replaced.

Regards
Dave

[Martin A]

You've got enough of a display there to suggest the CPU and main ram are OK, as an MTX won't get to setting up the text mode if either are dead. If the cursor is flashing, even if corrupt, then the CTC is working too.

With a decent resolution picure it's sometimes possible to work out what ram chip to suspect.

[acsi]

With a decent resolution picure it's sometimes possible to work out what ram chip to suspect.

Sorry force of habit making the pictures small. I have replaced them with larger ones.

Just need to find some 4116 chips now.

[acsi]

So it seems I just can't leave it alone until the parts arrive.

I thought I'd just do a quick check on the 4116 chips to see if any got really hot (Never found this to work ever for me).

Low and behold Murphys Law is working overtime and as I had already ordered some 4116's from a local supplier the Garbage has gone.

I even managed to load from Tape as a test.

Space bar doesn't work well. Can get a space after about 20-30 bashes of it.

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[Martin A]

Swap the space key with the line feed key if it works, that one's almost redundant!

[acsi]

Swap the space key with the line feed key if it works, that one's almost redundant!

Thanks I wouldn't have actually considered that.

I did make some progress on the keyboard today.

I desoldered the Space Bar mech and pulled it apart as per Daves instructions and reassembled it as everything seemed fine inside. According to the meter it is now closing with every push but still to be tested.

I also cleaned up under the key cause and used some EML200F as mentioned in another thread by Stephen to lubricate the keys. The 3 key definitely does not seem to stick down any more at least.

Caps arrived today so that'll hopefully be tomorrows job.

Also one of the case screws has rusted in and in my attempts to remove it I've managed to snap off the head of the screw. Anyone have any ideas on a good way to remove it short of trying to drill it out?

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[acsi]

Something arrived on the doorstep today

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I had to do a quick test to satisfy my curiosity of course...

The board is requiring one more cap replaced where I got the wrong value.

Keyboard quick test shows the space bar works better but not 100%. And I still get some multiple results from a few keys but it's much better.

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[gunrock]

I've seen some dirty MTX keyboards in my time, but bloody hell! That is pure filth!

Well done on the repairs.

[stephen_usher]

As I've mentioned elsewhere, the best way of getting the key switches working properly non-invasively is to use Electrolube EML.

Take the keycap off, hold the stem down and squirt some EML down the hole. Operate the switch 20-30 times and it should be as good as new. If after a couple of days it starts to act up you can repeat the treatment. I had to do this on a couple of keys but never had any problems after that.

Far less dangerous than trying to desolder etc.