ZX81 First line of display distorted

[1024MAK]

This is essentially a timing issue. You’ve got me a bit stumped, as the ULA is the master in terms of timing and clocks for the video output and controlling the Z80A.

Before I run off speculating, which composite modification has been carried out? Please provide details.
Has the video input to the modulator been disconnected?

The ULA uses a resonator for it’s master clock (which is also the pixel clock) and they can sometimes go a bit off frequency (well, more than normal).
It’s also possible that transistor TR1 or TR2 are not working properly. But these need to be high speed types, which are difficult to find. So before jumping in and changing them, it would be good to see what’s happening with the ‘scope first.

The other possibility is a dry solder joint that’s causing weird problems. Or maybe a fault with one of the resistors.

Mark

[daveyj67]

That's quite a long list of possible causes

So, the composite mod uses a pcb from youmakerobots per the photo. Here's a link to the instructions https://www.youmakerobots.com/index.php ... tachment=4

The modulator is still present but has been disconnected from 5v, central pin of RCA and video-in so is no longer functional.

Thanks for your reply.

Dave

[1024MAK]

That's quite a long list of possible causes

That’s because, having run out of sensible suggestions, you’ve reduced me to guessing

First off, I would suggest using a good light and a good magnifying glass, carefully inspect all the solder joints and have a good look at the PCB tracks. Why? Because (1) it’s free, and (2) two, what you have done is to eliminate the ULA, the Z80A, the ROM and the RAM.

Also, I’m a bit tired and have to start packing for tomorrow, as I’m going away. No time to think…!

Mark

[daveyj67]

Sounds like a job for the weekend. Thanks for taking the time to look at this for me Mark.

Dave

[daveyj67]

Hello again

I took a close look at the PCB as suggested, to the untrained eye there was nothing apparent except some poor soldering to a couple of the wires on the composite mod, so I've fixed those.

I've borrowed a vLA81 to try, just in case it made a difference. The picture was good with only a slight distortion at the top of the first line of the screen, and it didn't change after time for the computer to warm up. However, when I added my 16K RAM pack (which has been tested on another zeddy) the display was much worse (as pictured). Once I reset to 1K, the display was OK again.

So I'm wondering, might the RAM pack be drawing extra power or causing interference to the display in some way?

Dave

[mrtinb]

Do you use the original power supply with 9V or do you use your own at 5V?

[daveyj67]

I'm using an original PSU.

[mrtinb]

Ok. I was afraid it did get too little power, but the original power supply is ok.

[daveyj67]

It's got a Traco Power TSR-1-2450 instead of the 7805. Might that make a difference?

[1024MAK]

It's got a Traco Power TSR-1-2450 instead of the 7805. Might that make a difference?

Shouldn’t do.

However, it is worthwhile testing the voltages, both the input to the Traco Power TSR-1-2450 (which rather than being 9V is likely to be between 10V and 12V) and the +5V rail.

Do this without the 16K RAM pack fitted, then again with it plugged in.

One additional question, did you just fit the Traco Power TSR-1-2450, or did you also fit any additional capacitors?

Mark