This past year, user mijk on grex.org introduced me to the
world of home-built z80-based computers. It's old news to
many, but it was new to me, and I found it fascinating and
exciting. Many of the modern arduino projects hold little
interest for me, and as a result I really haven't done much
with electronics, beyond a few small things here and there.

I stumbled on Grant Searle's "Simple z80"[1] and "CP/M"
designs online and got started. You can read about all that
by browsing through the "projects" folder in my grex gopher
hole if you wish, but I'll say here that I managed to build
the Simple z80 design successfully a few times. The only
issue that I have personally with the design is that it
doesn't have a clear path for storage. Of course, it is a
"simple" design; and to be fair, it does include expasion
in the form of simple I/O... but I really hoped to find a
way to add storage.

Being a beginning in the electronics hobby, I searched the
net, and managed to find a design for a simple cassette
interface[2]. After finally getting all the pieces for it,
and even building an external clock to drive the serial at
300 baud, I built the thing. Now, I'm having trouble
getting it to work properly.

This is where the hobby gets a little frustrating, but at
the same time, this is where I'm learning the most. When
things don't work "out of the box" I'm forced to go and
learn a bit more. The people who make these designs are
very friendly and helpful, but they're also very busy. So,
I've been attempting to troubleshoot on my own.

So far, with this cassette interface build, I've been able
to narrow the problem down to one side/circuit in the
design (the "audio in" side, where it takes audio and turns
it back into serial data,) and even to one component (the
100k resistor.) With an oscilloscope (DSO138, cheap&simple)
probe at a certain point, the circuit works, at least
partially. Apparently, scopes alter the circuit necessarily
every time, so this type of thing is not uncommon.

Sadly, I'm not exactly sure what to try next. I'm using
multiple resistors to get up to 100k: a 75k, a 22k, plus a
10k POT. The design works only when the probe is sitting at
the point right between the 75k and 22k resistors, no where
else. I've tried re-soldering that point. I've tried adjusting
the 10k pot- I can get a range  of ~95k-104k. 

Next, after a bit of reading online, I tried putting a small
capacitor from that same point between the 75k/22k resistors
to ground. I can get a little bit of functionality that way,
but couldn't get a resistor that make the circuit "work."
Plus, I really want to know *why* the behavior is occurring,
more than I want a working circuit.

It may be that I need to join a forum of some kind. The
issue then might be that folks may think I'm in way over
my head. And I probably am; but that's how I like to learn.

For now, I've got a few more parts on order (obviously I'm
out of the values of resistors I need to make this thing
in a simple way,) so I can rebuild the circuit on
breadboard again (the soldered one is hard to tweak...)
That way I can maybe try out some theories to get the thing
working. 

Once it works, I'll of course have a z80 computer that can
save/load basic files to/from tape. I have a few basic
programs written already to take advantage of that. I won't
have anything amazing or particularly useful, but I'm
enjoying the process to much that it doesn't matter to me
in the least. And when it's done, I'm going to find a PCB
version to move on to, hopefully with CP/M, so I can have
a litle more fun with the hobby, and hopefully a little
more learning.


[1] http://searle.hostei.com/grant/z80/SimpleZ80.html
[2] https://diyhacking.com/make-uart-cassette-tape-interface/