ZX Spectrum 128k toastrack horizontal bars at power up

[opossum]

Hi all I have a problem with my ZX Spectrum toastrack...

Till some time ago everything was working fine...Now each time i power up the machine I get a thick white/black bands pattern with black frame (seldom with a few green dots) and the system is stuck. The strange thing is, if I reset the machine from the pattern screen the Spectrum boots normally (esxdos checks to white screen Spectrum 1982 etc..) and i can enter Divide/DivMMC menu through button. The machine often boots correctly also if I quickly switch off and switch on the Spectrum (I have a power switch)... Without Divide/DivMMC inserted the Spectrum quickly shows the pattern all the same and boots into menu.

I'm using an original European PSU, but i tried also With a 48k UK PSU and a compatible one...Same story.... I deeply cleaned the back connector with IPA, and i tried to power on without the keyboard attached...More of the same.

I have no problem in games once i launch them with Divide/DIVMMC (graphical glitches etc..) I was able to launch both B. Alford's "Tape Tests" utility and diagnostic rom through this method -> https://www.youtube.com/watch?v=wiPi8fVeYN8 ...The lower RAM tests result PASSED as all the other tests (I have a SLAM ULAPlus installed)...

What's wrong with my toastrack? I'm only getting suggestions, if a repairment is needed I'm going to pass everything to my modder/repoman.

Hope I was able to explain myself in a comprehensible way.
Thanks for attention.
A.

[Ast A. Moore]

The first thing I’d do is try it with the original ULA. If nothing changes, I’d check C27 and C26. And, of course, you should always check your voltages.

I’d also recommend upgrading to a modern regulated PSU. Look for one rated 2.5A or more.

[RMartins]

I would check the CPU Reset circuit.
There is a Capacitor connected to the reset pin, should be easy to spot, or follow the schematics (could be one of those pointed by [mention]Ast A. Moore[/mention].

I would also check the ULA, and the voltage levels (5V, -5V, 12V), because this could be related with the power not raising up to spec quickly enough, but once reached, it will work as expected. A quick power down/up, could provide this effect.

the 48K plus had a reset switch, but I'm not sure if the 128K has one.
If not, try an interface, that has a reset button (besides DivMMC), and check if a simple reset work ok, after power is stabiilized (was powered up for a few seconds already).

[opossum]

Thanks both for hints, Yes the toastrack has a *hidden* reset button and If i push it while on the screen pattern (machine stuck) the Spectrum boots normally...

[Ast A. Moore]

I would also check the ULA, and the voltage levels (5V, -5V, 12V)

The Toastrack doesn’t use the –5V, and the 12V rail is only used by the TEA2000 video encoder. All the logic ICs operate on the 5V rail.

[1024MAK]

Not sure. Horizontal bands are unusual.

As it works once you get it going, I would suggest that you try replacing capacitor C27. This is the capacitor that is part of the power on reset circuit.

Mark