ZX81 composite mod PCB build transistor question

[Chrille]

Hi,

I'm assembling one these composite mod PCB's for my ZX81:
https://github.com/redhawk668/ZX81-Composite

The BOM says that you should use two BC337 transistors. Does it matter if I use BC337-16, BC337-25 or BC337-40?

According to the data sheet the BC337-40 has a higher gain than the other variants. Does this mean that video signal get amplified more? Does it affect the video output quality? Which one is the most suitable to use?

[1024MAK]

That's a copy of an existing circuit design (although the PCB layout may be original) from JoulesperCoulomb.

His PDF is here.
Ignore the security warning, the file is safe.

Note however that R3 and R4 are connected differently compared to the original circuit design, which I presume is a mistake.

These transistors are only amplifying current not voltage. And even then, not by much. Hence the gain is not critical. In fact, many small signal NPN transistors will work. Any of those you listed will be fine.

With that circuit, the most important thing is to get a genuine NE555 (or better yet, a 7555) chip. It's been found that cheap parts (that may be fake or out of specification) from the Far East don't always work in this circuit. For all the details, read through this topic...

Mark

[Chrille]

Thanks!

I built the circuit with a BC337-40 and it works.