*** M E M O R U M ***

You guys were right! It was F1 (along with "o"), that was stuck. After using some IPA and "hammering" each key, I managed to boot up the MTX and get all keys working. That´s good progress!

I did not dare swapping the video boards, but I removed the one from this unit, had a close look and put it back, making sure all connection were tight. The "hum bar" is still there, constantly moving from the bottom of the screen to the top. It also seems to add some flickering (MTX blue to magenta-ish) to the area where the bar is.

Is it normal that the TMS9929 gets relatively warm?

The service manual also mentions that some video signal problems could be caused by a faulty 7805/7812.

Replacements should be UA7805/UA7812. These are hard to get, so I wonder if the standard 7805 used in the Spectrum would also work? For the 7812, would ST L7812CV 12V/1.5A work?

The video board actually converts the +12V supply (from the 7812) to a +5V supply for its own use. So yes, a fault in the 7812 or its smoothing capacitors could produce screen distortion or interference.
Most modern 7805 regulators can replace the 7805 regulator on the MTX. There is nothing special about the existing regulator chip. The same goes for the 7812 regulator.

If you have access to an oscilloscope, this can quickly tell you if the regulator is producing noise on the supply lines (+5V and +12V). If you do not have access to a 'scope, but have a good multimeter, use the mV AC range(s) to check the +5V and +12V supplies. There should be less than 50mV AC. But unless it is a sensitive (expensive) model, it will only detect 50Hz to 200Hz (typical) ripple and noise. The better quality meters can work up to and into the audio hi-fi range.

Normally however, 78XX regulars (like the 7805 and 7822) work okay and noise problems have another cause.

Mark

schombi wrote:The service manual also mentions that some video signal problems could be caused by a faulty 7805/7812.

Replacements should be UA7805/UA7812. These are hard to get, so I wonder if the standard 7805 used in the Spectrum would also work? For the 7812, would ST L7812CV 12V/1.5A work?

I used a MC7805BTG and MC7812ABTG when attempting to get rid of that video distortion.

Since it seemed to only occur with the RS232 plugged in, I thought it might be load related, so I upped the spec to 2A

It didn't totally clear the problem. But it IS better, and doesn't occur all the time.

Thanks for the suggestions. Unfortunately I don´t have access to a scope and my HP DVM is way too old and slow.

I think I should try the regulator replacement; just about to order the parts. I´ll report back if there´s any improvement.

It´s getting embarrassing again. How the hell does the board get removed? I took off the keyboard, the video board and the screw on the back, but both the main board and the memory board do not move a mm! I also cannot remove the rear plastic bar (on the connectors). This one moves a bit to the left and to the right, but always gets stopped by the RF connector.

In theory, once you've removed the bolt that runs through the regulator. The circuit board(s) and the plastic at the back should all slide out together.

The circuit boards are in slots front and back, and the plastic part has moulded bumps that run in the same groove that the lower rear side panel's bolts use.

There are also black "rubber" like pads on either the bottom of the PCB and/or inside bottom of the case. These sometimes do not want to separate. Combined with the friction between the edge of the PCB and the case, this can make it difficult to slide the PCB along.

Mark

Mark is spot on!

Those rubber spacers/insulators can be a pain. I find that flexing the board very slightly upwards can allow the protrusions on the solder side of the board to clear the rubber pads that might have fallen off the PCB

Regads
Dave

I removed the memory board, but still did not manage to remove the main board.

The memory board hardly moved in its slots, but the main board simply does not move a nm! I double-checked the plastic backplate again and found that it cannot be pushed to either side, since the metal heat exchanger on the board stops it from sliding any further. I have no more ideas...