General Stuff(tm)

[rubber_jonnie]

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.

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!

[JezC]

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!

Ouch!

[stephen_usher]

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.

[rubber_jonnie]

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.

Scorchio...

[stephen_usher]

Right, the ZTX651 transistors have arrived from Retroleum, so I can see if the 12V and -5V rails spring to life.

There are definitely no shorts there.

[stephen_usher]

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.

[rubber_jonnie]

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.

UGH, dead ULAs are a pain in the backside...

Sounds like maybe bad RAM under all the other bad stuff. Those 4116's are bad juju...

[stephen_usher]

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]

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.

Upper or lower RAM?

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

[stephen_usher]

Upper or lower RAM?

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

Lower RAM. I'll see what the upper RAM is like once I have the CPU running.