Pentagon 128k debugging

[Shockwav3]

Hi there,
I'm currently in the progress of building a Pentagon 128 (Revision 2011 produced by zorel). Good things first, it already boots, accepts keyboard input and even lists betadisk directories. Now comes the bad part, after a few seconds I get junk characters on the screen, but the machine remains operational for a few more seconds til it crashes (mostly after 30-45 seconds). Sometimes it reboots, but most of the time it just hangs requiring a hard reset. Any ideas what could be wrong?

Further info:
- IC type is all 74lsxxx except for dd02, dd11 & dd12 (used 74hct93 there). I used the translation table from here: http://sblive.narod.ru/ZX-Spectrum/P...ntagon128k.htm (except for dd100 which is wrong there and should be cd4069).
- RAM chips are 41256 and have decoupling capacitors
- random decoupling capacitors have been placed on the board
- resistor and capacitor values have been used according to pentagon.pdf from here http://zxspectrum.it.omegahg.com/doc/Pentagon.pdf

Here's also a picture:
http://img855.imageshack_.us/img855/...ntagon128k.jpg

So any ideas what to try next?

[Advertiser]