Help with repair

[1024MAK]

Recommended test points on a MTX board:

There is a schematic diagram on Dave's web site. I'll let him provide the link

All these tests are to be carried out with the external PSU disconnected from the computer.

The regulator chips and the power transistor are under the large electrolytic capacitor. When referring to the legs, this is with respect to viewing from the front of the computer (with the text on the chips the correct way up!).

A good 0V point is either the modulator case, or the ground on the video output (if the lead is plugged in to the MTX video board!), so connect the negative black meter lead to 0V.

The following tests are done using the positive red meter lead / probe.

Test the left hand pin of the 7812 regulator with the red meter lead / probe. If the electrolytic smoothing capacitor is connected and at least partly working, the resistance should start low and go upwards in value. Resistance should be >500 ohms.

Test the right hand pin of the 7812 regulator. Resistance should be >300 ohms.

Test the left hand pin of the 7805 regulator with the red meter lead / probe. If the fuse (FS1, 3.15A) is intact and the electrolytic smoothing capacitor is connected and at least partly working, the resistance should start low and go upwards in value. Resistance should be >500 ohms.

Test the right hand pin of the 7805 regulator. Resistance should be >300 ohms.

The centre pin of both regulators should be less than 1 ohm as they are the 0V connections.

Now remove the black meter lead from the 0V point.

With the fuse removed, test between the two outer legs of the power transistor (one probe to each leg). The resistance reading should be 10 ohms.

Test between the middle leg and the right hand leg of the power transistor (one probe to each leg). The resistance reading should be high.

Note that you may get slightly different readings depending on your meter.

If you are happy with the results of these tests, replace the fuse. Connect it to a TV (use a BNC to phono adaptor, then use a phono to phono cable to connect up the video and "Hi-Fi" audio) composite input. Ensure the TV is switched to the composite input.

Now power the MTX up. Do you get a blue screen with "ready"? A black screen with a constant tone on the audio or nothing?

If no blue screen with "ready", don't leave the MTX powered up for long, as the 16k video RAM chips can be damaged if one of the supplies is missing.

Good luck

Mark

[Retrocosm]

This is fantastic Mark, thank you so much. I'll go through your list and see what results I get.

Thank you again for taking the time to type this all out.

Charlie

[Dave]

Charlie,

the schematic from the MTX manual can be found here, along with soome notes on how the power regulation works on the MTX

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

regards
Dave

[Retrocosm]

Cheers Dave, is that your site, it is an excellent resource!

Charlie

[Dave]

Yes - it's mine

thanks for the kind words!

regards
Dave

[1024MAK]

Cheers Dave, is that your site, it is an excellent resource!

Remember, any and all the mistakes are Dave's fault. No mistakes were made by the people he pinched / extracted / obtained the information from

He does do a lot of work putting information on his site though [don't want to praise him too much, don't want to risk any over sized heads ].

Mark

[Dave]

mmm, let me see, how do we get users BANNED from here ?

[Retrocosm]

Well I shall no doubt soon be pinching lots of info for my blog, all referenced of course

[Dave]

Hi Charlie,

no problem!

regards
Dave

I bet someone gets onto me that this should be in the "OFF TOPIC" forum now

[Martin A]

Looking a the black capacitor, I don't much like the look of the joint of the "top" leg.