Sinclair ZX80 / ZX81 / Z88 Forums

1024MAK wrote: ↑Wed Nov 25, 2020 5:54 pm TR2 - the transistor used has to have a very quick switch-off time. Most general-purpose transistors take a small amount of time to turn off, especially if the base becomes saturated. If a BC548 is used the value of R6 may need to be adjusted. If the transistor used switches off too slowly, the Z80 clock input will stay low instead of being a ‘square’ wave.

Sorry, I should have asked if you have a suggestion for a better transistor?

Unfortunately I have not found a good replacement for ZTX313 (or ZTX312 or ZTX314) transistors where the replacement has a very fast turn off time. Previously the plastic TO92 version of the 2N2369 (often called PN2369) was the equivalent, but the 2N2369, 2N2369A and PN2369 are all discontinued

However, a BC548 (or equivalent) will work. But you may have to use a higher value resistor for R6. Especially if the transistor used has a high gain (hFE).

One problem with finding transistors that have turn off times that are fast, is that a lot of manufacturers don’t provide the turn off time in their data sheets. And suppliers don’t include this figure in their selection tables.

Mark

I was told to use MPS2369 as a replacement, and it works great in my ZX81.

mrtinb wrote: ↑Sun Nov 29, 2020 1:41 pm I was told to use MPS2369 as a replacement, and it works great in my ZX81.

Thanks I will check the footprint and get some in.

Yup the emitter and collector are the opposite way round and I think the part is obsolete too!

Yeah, unfortunately the MPS2369 from ON Semiconductor is also obsolete

Mark

Well, it's a mess but everything is connected!

Tidy up, then DRC!

Slowly fixing issues.

Next stage buying the bits!

Well, the bits have turned up and the PCBs have been made and are in transit.

In the meantime, I have made this webpage to help with making the boards:
https://fordp2002.github.io/

Well, we have PCBs and so far all the bits seem to fit the footprints.