[Plus / 4B] Power Issue (fried TR4)

[happycactus]

Yes, the note was about National Semi, I haven't found the reason. And yes, They are from Harris, and again yes it was bought by TI.

[happycactus]

Ok, I DID IT!

Changed the 74HCT00 (it's needed for address decoding), desoldered TR6 and TR3 and resoldered them (BC547 is not ok for them).
I noticed a change in the video pattern. Now there are a lot of black vertical stripes (like 1 every 8 pixels, sounds familiar?), or sometimes big black bands like when I mistakenly connected the 28AT256 EEPROM as it would be a normal EPROM, effectively disabling it.

So I decided to check the signals on the ROM, and found what? on pin20 (\CE) the low level signal was above 2V; the two diodes I added to drive it were switching correctly. That meant the pull down resistor was far too high to be driven at the correct level. Changed with a 4k7 made the board boot perfectly.

No faults in the lower RAM (we knew it), and none also in the HIGH RAM too!

Next is to change the keyboard membrane (I ordered it) and restore the original ROM. Last a full diagnostic and closing it

Thank you for all the support, especially to Mark for his precious suggestions!

[1024MAK]

Yes, TR3 and TR6 need to be transistors that have fast turn off times. Most transistors are not suitable in these positions, as they are far too slow. So where possible leave the original transistors (normally ZTX313 in U.K. made ZX Spectrum boards) alone. The failure rate of transistors in these positions is very low anyway.

Good news that you’re got your Speccy sorted . Well done

Mark