Trying to fix Mikro's wrecked TT board

[dml]

There's definitely something with the reset circuit, I sometimes see brief flashes and the monitor tries to sync but other times nothing.

I remember Mikro mentioning something about this too.

The reset circuit is a 556 timer and C108 seems to be the timing/delay cap... poking at that now.

Mmmm... C108 is 10uF/35v and reading about 2 ohms on the board. It's not dead - but not great either. Not getting a capacitance reading from it in-circuit.

...will tack another cap underneath and see if anything happens.

[dml]

So this is new ;)

Screenshot 2025-04-26 at 13.03.04.png

Blasted the reset switch with contact cleaner and added a cap under the reset timer. It's still slightly glitchy but now it's mostly coming up with a white screen \o/

[stephen_usher]

If you have ROMs in the machine check the basics... +5V, clock signal and reset, though I know that you're looking at the latter. If the CPU is coming out of reset is it trying to run code (check address bus for activity)?

[dml]

If you have ROMs in the machine check the basics... +5V, clock signal and reset, though I know that you're looking at the latter. If the CPU is coming out of reset is it trying to run code (check address bus for activity)?

Yep I'm moving onto that next - will see if there is any activity from the CPU/ROMs etc. I do have a diagnostic cart also but I'll have a poke around first.

[stephen_usher]

:dualthumbup: :crossed1:

[dml]

Trying to cook right now while wrangling a cat away from the postman - but I saw briefly signs of life from the address bus, before my old Tek analog scope died.

It did put on an interesting show of bendy, spiky squarewaves before the trigger stopped working - so there's that.

It was ancient anyway and novelty value for future repairs. I'll get the other, hopefully working one, post cooking.

[dml]

Current drama is that the trigger position encoder on my Fluke scope is not working now :-( and it's stuck in exactly the worst position to see anything but garbage.

So I'm down two scopes and looking for a third. TT situation might have to be a cliffhanger until I sort out a scope that actually works properly.

[dml]

Ignoring the scope situation for now, I found a couple of new things to look at.

There is still something funny going on with the reset circuit because it always gives a black screen when powered up - but I get a white screen after a warm reset.

The schematic shows there are two reset circuits using the same 556 timer, each with a separate startup-delay timing capacitor. I 'fixed' the one associated with the reset button. I think the other one also has a failing capacitor - that probably explains the black screen on startup.

But separately, the reset line is not moving to a high state properly. I read about 2.8v from the outputs of U102 (buffer/inverter) from the reset timer - I'm sure that should be closer to 4v or higher.

If that's the case, it seems possible some chips are coming out of reset while others are still stuck in reset.

This also makes me worry a bit that one of the larger ICs may be damaged and interfering with the reset line. :| or may be more board/trace damage somewhere.


Anyway I think I have a way forward with this but I'll try to swap the encoder on my Fluke first.

[dml]

That's weird... all of the buffer outputs from that IC read around 2.8v after reset - but they are not all connected together in the same net so it can't be a single downstream IC causing this.

I think I'll replace the timing caps properly and check both ICs involved in both reset circuits before I do anything else with it.

.....ok and guess what U102 is right next to?

.....it's next to one of the big red circled danger zones in the original post pic :p a few mm above the mysteriously missing ACIA in the lower-left corner. hmmm!

[dml]

How is it even possible I can't find a 74LS05 in this pile? :roll:

I might have to raid something if this IC tests bad :)

Screenshot 2025-04-27 at 09.47.36.png