Spectrum 128 Toastrack Mishap

[Arranyn]

Hi all,

New to the board but reaching out for help.

Recapped loads of Spectrums in the past and just got my first Toastrack. Recapped the whole board and all looked very neat and tidy - powered on to see a beautiful clear image and then it all of sudden just faded to black. A quick inspection of the board soon discovered that I'd popped C25 in the wrong way round. I could have kicked myself!

Popped the capacitor back in the right way round (after checking it was ok, of course) and now just have the black screen.

If I change from RF to SCART, I get the TV reporting a PAL 50hz signal and if I turn the brightness WAY up I can make out the beginnings of a screen, and pressing the up and down arrows gives me the keyboard click so it seems everything is working, just no screen.

I'm not familiar at all with the Toastrack so what might I have damaged/blown in the process of this tomfoolery?

Thanks for any help offered!

[acsi]

Hi

I have had similar behavior with the screen fading out to black with both a 48K and a MTX which use a LM1889 chip. I note though that the 128k uses a TEA2000 rather than the LM1889 but that would probably be my starting point.

[1024MAK]

The colour encoder chip is not involved with the RGB output.

Are the +5V and +12V supplies for the video section okay?

Mark

[Arranyn]

Morning all,

Suspecting the TEA2000 initially, I tested the VCC which was only receiving 1.5 volts.

Tracing the 12v line back from the TEA2000 I noted that R62 was only getting 1.5 volts, as was C131.

This led me back to TR4 which I knew was failure point on the 48k machines, and sure enough I'm getting a voltage drop of 0.6 on one leg, but not on the other, so it looks like TR4 is fried.

They're getting harder to come by now - does anyone know of a suitable modern replacement?

[1024MAK]

ZTX651 and ZTX653 are suitable replacements for the ZTX650 normally found in TR4 position and when I last checked earlier this year, were still in production.

TR4 normally only dies if there is a short circuit or other overload on one of the outputs of the DC/DC converter.

Mark

[Arranyn]

Thanks Mark,

I've ordered some spares from Retroleum and will report back!