Re: ZX Spectrum +2 booting problem
Posted: Wed Nov 07, 2018 9:22 am
I just read this again and makes sense, but I had high voltage drop also without using the resistor...This method gets increasingly impractical as the current increases in a design. As more power is lost in the resistor, and you also have the problem of the voltage ‘lost’ across the resistor means that the circuit that is being supplied no longer sees the correct supply voltage.
In the meantime I found that the Composite Sync is using 1.2v and that is just the current value on TEA2000's pin11. I guess it could be a sign that TEA2000 may be functional? If is true, is curious that it receives 1.2v without any loss , compared to the ram chips. The only chip that was getting warm was the CPU.
Ok maybe the plan with using an external source was too ambitious with my current tech and experience...
What if I'll do like this:
1. connect the Spectrum as usual from a 48k power supply - will be safe to leave TR9 out just for testing 5v functionality ?
2. improvise a wiring from the RGB port to some SCART jack (don't have a cable for this) - do you think bare wires will work for this or I have to put some resistors on R,G,B?
3. observe if something is going on screen
I'm still a bit surprised that I had +12v dead at some point but still had some fuzzy image with analog output (when voltage regulator gone berserk). I remember that I still had something like 1. something volts on TEA2000 so in theory analog video should be dead?
LATER EDIT:
I just read on some comments on other sources that the Serial Transceiver Chip (I think is about that MC1488P ?) is common to kill the +12v line. It is safe to remove this chip (or just some pins) just to test if is indeed the short's cause ?
EVEN LATER EDIT:
I got my hands on some TIP31C (to replace ZTX653 / TR9), do you think this could handle better the short and survive the tests?