Sorry, I should have asked if you have a suggestion for a better transistor?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.
ZX21 Project ZX81 Replacement PCB
Re: ZX21 Project ZX81 Replacement PCB
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
- 1024MAK
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- Location: Looking forward to summer in Somerset, UK...
Re: ZX21 Project ZX81 Replacement PCB
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
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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: ZX21 Project ZX81 Replacement PCB
I was told to use MPS2369 as a replacement, and it works great in my ZX81.
Re: ZX21 Project ZX81 Replacement PCB
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.
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
- 1024MAK
- Posts: 5118
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
Re: ZX21 Project ZX81 Replacement PCB
Yeah, unfortunately the MPS2369 from ON Semiconductor is also obsolete
Mark
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: ZX21 Project ZX81 Replacement PCB
Tidy up, then DRC!
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
Slowly fixing issues.
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
Next stage buying the bits!
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
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/
In the meantime, I have made this webpage to help with making the boards:
https://fordp2002.github.io/
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!
Re: ZX21 Project ZX81 Replacement PCB
Well, we have PCBs and so far all the bits seem to fit the footprints.
FordP (Simon Ellwood)
Time is an illusion. Lunchtime, doubly so!
Time is an illusion. Lunchtime, doubly so!