exxos's Atari, Amiga & retro forum

I'd assume the transistor voltage rating is only like 30v ? If the voltage is going higher that could be damaging it. But of course if there is excessive current draw that would also damage the circuit though I doubt you would get high voltage and high current the same time.. As high current would be more the short-circuiting the voltage would be much lower than expected not higher..

exxos wrote: 21 Sep 2023 11:50 I'd assume the transistor voltage rating is only like 30v ? If the voltage is going higher that could be damaging it. But of course if there is excessive current draw that would also damage the circuit though I doubt you would get high voltage and high current the same time.. As high current would be more the short-circuiting the voltage would be much lower than expected not higher..

On the spares board, the low 12v rail is almost certainly the result of bad RAM.

Before I removed the 4116's on the spares board, and with a new transistor, I measured 7v and clearly had excessive current draw, which was what popped the transistor.

Now the RAM is gone, the 12v rail is perfect, so I just need to install sockets and test with working RAM. I can either use a full set of 4116s that I now have, or just use the bodged 4164s from the + board.

The original + board is zooming up to 29/30v so that clearly has another issue of some sort. It doesn't seem to kill the transistor, but obviously isn't workable as I need 12v. As long as I can make one board work, that's all I need, though getting both running would be nice.

For the DC-DC circuit to work both TR4, TR5, the coil and the capacitors need to be within spec as I believe that it's using resonance in a feedback circuit to create the on-off cycling of the power. TR4 going bad can kill TR5 of course.

stephen_usher wrote: 21 Sep 2023 12:18 For the DC-DC circuit to work both TR4, TR5, the coil and the capacitors need to be within spec as I believe that it's using resonance in a feedback circuit to create the on-off cycling of the power. TR4 going bad can kill TR5 of course.

If I can get the spares board to work, then that'll go in the + for the fix.

Then I'll look at the circuit on the + board for and see what I can do, I think you're right, the feedback circuit isn't working as advertised and it's running freely. I'll have a look at the caps and probably do a recap, see if that fixes the output.

So far, I've not had a bad TR5.

Aaaand.....

The spare board lives:


Which is a billy bonus.

Works with bodged 4164's and 4116's.

:cheer: :girldance: :cheer:

:bravo:

Yay!

Well, I thought that was too good to be true. It seems that there is another fault I need to track down.

It boots and stuff works, but I can't load anything, it's failing, so now wondering if there is an upper memory issue too...

Meh!

Also interesting, but neither the original + board nor the spares board work with the DivMMC cart, so the plot thickens, even though the original board will load from tape.

And yes, the edge connectors have been cleaned, but I need to have another look and see if I can clean some more.

My Spectrum 48k, the original subject of this thread works just fine.

Ok, so despite the 30V on the 12v line, the -5v line works as advertised...