Another Egyptian Enterprise 64

[rubber_jonnie]

When I got my Enterprise, it recommend replacing both tr2 & 3, so amongst other things I've done that as well.

Yep I'm planning to do that as well. As for T2 it's on it's 4th TR2...

And as per normal I have no BC212's...

[stephen_usher]

Seeing as TR2 is merely generating an AC resonance to cause an AC supply to the transformer and doesn’t carry the current used by the secondary 12V supply it’s likely that input ripple is hitting a resonance. This would boost the current going through the transistor.

The circuit coukd probably also do with a diode to redirect the back emf coming from the inductor when the field collapses.

[rubber_jonnie]

Seeing as TR2 is merely generating an AC resonance to cause an AC supply to the transformer and doesn’t carry the current used by the secondary 12V supply it’s likely that input ripple is hitting a resonance. This would boost the current going through the transistor.

The circuit coukd probably also do with a diode to redirect the back emf coming from the inductor when the field collapses.

Yes to the diode for sure!!!

But where would it go?

Can't recall, did the Speccy ahve a diode for back EMF protection?

[stephen_usher]

Seeing as TR2 is merely generating an AC resonance to cause an AC supply to the transformer and doesn’t carry the current used by the secondary 12V supply it’s likely that input ripple is hitting a resonance. This would boost the current going through the transistor.

The circuit coukd probably also do with a diode to redirect the back emf coming from the inductor when the field collapses.

Yes to the diode for sure!!!

But where would it go?

I'd have to look at a load of Big Clive videos to work that out. Basically, it's to stop backwards current trying to go through the transisotr, but around the coil area.

Can't recall, did the Speccy ahve a diode for back EMF protection?

No idea... probably not as that would have added costs. There are later, better, versions of the DC-DC circuit on the Issue 6 boards which can be retro-fitted which make it more reliable. I guess that may add diode protection.

[rubber_jonnie]

I'd have to look at a load of Big Clive videos to work that out. Basically, it's to stop backwards current trying to go through the transisotr, but around the coil area.

Yeah I've seen him mention it a lot in his vids.

No idea... probably not as that would have added costs. There are later, better, versions of the DC-DC circuit on the Issue 6 boards which can be retro-fitted which make it more reliable. I guess that may add diode protection.

There are power replacement boards so I've been told over on the Enterprise forum but I can't find anything about them yet.

[rubber_jonnie]

OK, so here's the plan for the power circuit.

I'm going to replace D4 and fit a bi-polar cap at C9 (As recommended on the Enterprise forum) and then re-solder the remaining components as the solder does look a bit dodgy.

So these:

L1
TR2
TR3
R13
R14
R15
R16

Will get de-soldered and re-soldered with fresh. I will also remove L1 and clean underneath it in case there are still silty remnants there.

I'll also replace TR3 just in case.

C10 has already been replaced and is correctly oriented.

Hopefully the parts will be here tomorrow though I can get the resistors re-soldered and L1 done before that.