ZX21 Project ZX81 Replacement PCB

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fordp2002
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

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?
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1024MAK
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Re: ZX21 Project ZX81 Replacement PCB

Post by 1024MAK »

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.

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Re: ZX21 Project ZX81 Replacement PCB

Post by mrtinb »

I was told to use MPS2369 as a replacement, and it works great in my ZX81.
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Re: ZX21 Project ZX81 Replacement PCB

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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Yup the emitter and collector are the opposite way round and I think the part is obsolete too!
Last edited by fordp2002 on Sun Nov 29, 2020 6:26 pm, edited 1 time in total.
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1024MAK
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Re: ZX21 Project ZX81 Replacement PCB

Post by 1024MAK »

Yeah, unfortunately the MPS2369 from ON Semiconductor is also obsolete :(

Mark
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

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

Tidy up, then DRC!
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Re: ZX21 Project ZX81 Replacement PCB

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zx21_3d.jpg
Slowly fixing issues.
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

Next stage buying the bits!
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unnamed.png
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Re: ZX21 Project ZX81 Replacement PCB

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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/
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Re: ZX21 Project ZX81 Replacement PCB

Post by fordp2002 »

ZX21_Real.jpg
Well, we have PCBs and so far all the bits seem to fit the footprints.
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