Embarrassing newbie questions

[Dave]

Hi,

the serial numbers don't give a very good idea of what you have.

Photos are always good, but it appears that you have an MTX500 motherboard that has been upgraded with an extra 32k, this is very common. The RAM chips on the main board are probably OKI M3732H or L - 32k x 1 DRAMs. The extra card is an expansion memory board, usually only half the board is populated (with the same chips as the mainboard), giving another 32k to take the total up to 64K, making an MTX512.

Sometimes the memory board is just connected via a push-on edge connector, others have the memory board soldered to the main board - again, a photo would help.

From your description, it sounds like you have a Video RAM fault - it is more than likely that this will require changing one or more of the 8 x ITT 4116 RAMs below the modulator.
Video RAM failures are a very common problem, I won't make judgement on whether the machiine had the fault when it was sent to you
These pages may help . ..
http://www.primrosebank.net/computers/m ... _fault.htm
http://www.primrosebank.net/computers/m ... repair.htm


regards
Dave

[schombi]

Cheers Dave. I will take some pictures tonight or tomorrow.

[Bill B]

Is it possible that your lines of white blocks are simply due to a stuck key auto-repeating? I can't remember what character the function keys produce on the screen.

[JonB]

I've got a load of 4116 chips on their way to help fix my PET. there will be lots of leftovers and I intend to sell them off in lots for small money (to cover the cost). If you want a set of 8, let me know.

[Dave]

Thanks Jon,

but I have one or two already

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

regards
Dave

[JonB]

I wasn't offering them to you, Dave... you wily old fox....

And if I'd known you had some, I could have saved the shipping from the US, drat!

[Dave]

Hi Jon,

I bought mine from the US too - with a thought that as they seemed to have quite a high failure rate, other MTX'ers might be interested. I posted a couple of messages on Facebook and on here, but nobody seemed interested. It looks like I have enough spares to last at least another 30 years

If ever you need any DRAMs for the MTX , remember to check the page I referenced earlier, they may be suitable for your other retro computers

regards
Dave

[schombi]

Finally I was able to carry out some more troubleshooting. First of all I re-checked all connections and re-seated all socketed chips. I also tightened up the connection to the monitor output.

Looks like Bill B was right. There is something weird going on with the keyboard. With the keyboard removed, the MTX now boots up correctly. However, there´s still an annoying moving, horizontal bar on the screen. This is the same TV and PSU, that I use for my working MTX. Once the keyboard is re-connected, the MTX goes nuts, building up four lines of blocks. Some keys seem to respond, others doesn´t. I tried to catch this in a video, where I let it boot up and then I pressed a few keys:
http://www.youtube.com/watch?v=JGA4Y0kQ ... e=youtu.be (Note: the MTX in front of the telly is my working one, the bad unit is on the floor).

I wonder, is it "just" the keyboard or something more severe like a controller chip, how to fix it and how to get rid of the bar on the screen?

I also took a few pictures of this MTX512 with an old board and a high serial number.







Edit: Larger pictures can be found on: http://schombi.de/public.html

[Dave]

As they say, a picture really DOES paint a thousand words!

As the system starts up, you can see the white blocks filling the edit area on the screen, looking very much like a stuck key or other keyboard fault.

Have you looked closely at every key on the keyboard?
Do any appear to be stuck in the down position?
Try operating every key multiple times, this will help clean the contacts and the "feel" will help you determine if any are stuck.
Don't be too gentle when you "bang" the keys - you want to give them a good workout!

For the screen problem, try swapping the video daughter board from your good, or at least better, MTX and see whether the display improves.


regards
Dave

[1024MAK]

The function keys are the most likely suspects.
Next, the numeric keypad / curser keys.

On one of my MTX machines, I found one key that was held partly pressed down by the metal faceplate because the PCB was not correctly lined up.

On another of my machines, I found that a funtion key appeared to have an electrical short circuit between the two contacts. So I desoldered this key. Removed it and the tested it again. The short circuit had gone! So I refitted it. It has not caused any further problems. I have no idea what the cause of the short circuit was...

For the keyboard matrix, someone has already posted the details somewhere in the hardware section

Mark