exxos's Atari, Amiga & retro forum

stephen_usher wrote: 19 Aug 2024 20:45 Whilst moving things around in the loft to make way for some of the things recovered from my Dad's house I found an unexpected item in the bagging area... a ZX Spectrum (Issue 4A) motherboard.

Turns out that there's no 12V or -5V so it looks like TR4 is dead. (I desoldered TR5 and tested it and that looks fine). Just got to wait for the spares from Retroleum to arrive. Annoyingly I have a spare for TR5.

rubber_jonnie wrote: 19 Aug 2024 20:57 That's given me flashbacks to fixing my own spare Spectrum with tr4 issue and getting CBE mixed up on an alternative transistor and getting 70v where I should have had 12v!

stephen_usher wrote: 20 Aug 2024 10:22 Indeed.. Oops and ouch!

Seeing as I don't currently have a ZTX561 I tried a lower power equivalent... 0.6A rather than 2A... It got a bit hot very quickly and didn't really work. Not a surprise... So that burning GIF from The IT Crowd would be apt. :-)

stephen_usher wrote: 20 Aug 2024 18:57 Fitted the ZTX651 and the voltages are correct.

Activity on the data bus but no /ROMCS signal and lots of the outputs from the ULA are floating... So deadish ULA.

Put an "incompatible" older ULA in and although there's no proper RF output I can see a picture if I touch a composite cable to the input to the modulator. The usual black/white stripe with colour blobs of a dead Spectrum, so at least some life.

After dinner I'll do a proper composite mod so I can easily see what I'm doing.

stephen_usher wrote: 20 Aug 2024 22:57 After desoldering the RAM and testing it, one dead one.

However, the real blocker is TR3, which acts as an amplifier for the CPU clock. So, no CPU clock no worky.

rubber_jonnie wrote: 20 Aug 2024 23:06 Upper or lower RAM?

And yes, without that clock you're in a bit of trouble.