Zx81plus38 rev1.10 video issues

[ghhann]

I tried the single cable and it made no difference

I think I might have enough parts to make a tape arduino unit and I can try the loading process with it

All my best and thank you for the suggestion

[netuffgeb]

Thanks for information.
I tried the whole save/load with the ZX-81and digital recorder and an amplifier

AC_AMP_V200USB.pdf

(63.63 KiB) Downloaded 183 times

in between which was working, but not with the Speccy with Sponzy. Guess the amplitude is too low. An other forum say the ZX-81 uses the whole 5V whereas the spectrum does only use lower than 4V.

[ghhann]

So I finally assembled a maxduino and put a .p file it.

I tried the load process and it didn’t work

Did anyone put tougher these boards and have load and save
Functions working ?

[lardbloke]

Hi there,

I am not an electronics wiz by any stretch of the imagination, hut just wanted to bring the ZX81into a modern age. So I have also built the ZX81+38. I followed the instructions to the letter from the GIT Hub files. Below is some the text from another thread I have posted in. It is probably no help, but one.musy start from first principles.

'Yes, the loading definitely works as it should. I built a Maxduino nano. Upon loading, I enter the usual Load "" command on screen and press enter The screen will go black and look like it has failed, but it hasn't. You can press play on your tape device and it will still load. This can be tested by not using the Load command and just pressing play on your tape device. Then you will see the usual loading lines, but it will not load the program.
I have not saved anything in the true sense. I have played 1 bit music out externally using the external port to a portable speaker, so in theory that does work.'

So for me, the loading works as it should. I know there are issues with the wrong transistors and this is mentioned in the build instructions. It could.be anything with the amount of IC's on board.

Good luck,

Jim

[1024MAK]

Getting the “correct” volume (voltage level of the signal) level is critical.

With original Sinclair ZX80, ZX81 and ZX Spectrum computers, they were designed for use with “shoe box” mono cassette tape recorders. A lot of these can produce a rather strong output signal on their Ear jack socket outputs.

With the ZX81+38 design, the tape input stage includes a single transistor amplifier to make the input more sensitive. But you still need to get the volume level in a “sweet” spot. If the signal is too low, the digital chip that follows the transistor amplifier stage will not always toggle between zeros and ones. Similarly if the signal is too high, again it will always toggle between zeros and ones.

So it’s a case of saving a short program “on tape” (or equivalent) and then playing it back multiple times, each time trying a different volume level.

Mark

[netuffgeb]

Basically the destination is to have LineIn/Out level. For this, it is necessary to have a pre-amplifier like this:

AC_AMP_V200USB_wiring.pdf

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I use a Microchip Audio Development Board as a recorder and set it to line in. Then two mono cables with 3.5mm jacks to the amplifier, see attachment, then a Y-cable to the ZX-81 and it work perfect with the Zx81plus38 (ZX-81_V202 by me) without any trouble after adjusting the levels. To do so, read what lardbloke has written, I can confirm.

[ghhann]

If I read the schematic right ncas_in goes to the cd74hc541e on pin 4. If I put an oscilloscope on that pin I should see a typical 5v to 0v squared movement coming from either the maxduino or a properly volume controlled analog tape output in order to get a reasonable chance of a program to be loaded.

The problem could be a bad 541e chip… keep you posted

[1024MAK]

Well, kind of. Because the preceding circuitry is analogue, the waveform may be far from a nice square wave. And for the 74HC541 to change state, you only need to go just beyond its switching thresholds.

These may vary but a logic high is at least 3.15V to 5V, but may be as low as 2.4V.

A logic low should be no higher than 1.35V but may be as high as 2.1V.

Mark

[ghhann]

Thank you

Further observations

When using the maxduino and without type load “”” I do see the loading lines in the background and they look as expected. When the file is finished loading the screen clears up without issues

When using the maxduino with typing load “” the screen goes blank then when the maxduino finishes the file the screen never returns to normal. Sometimes break will work and sometimes not…

The Maxduino was tested with a spectrum program and loaded a program fine into my harlequin spectrum

Since CAS-IN appears to be working fine, I’m looking at nCAS after the split that creates CAS-IN. So nCAS-IN next step is CD74HC541E. Looking to see if I have another one to replace before breaking out the oscilloscope.


After the 541e (U22) I would need to know how the load command works and which chips it selects on the data lines

[1024MAK]

After the 74HC541, that's the data bus of the Z80A. Data is only valid when the ROM (or other) code executes a IN instruction to read the keyboard and tape input. If the keyboard works okay, all the control logic for the 74HC541 must be okay. The data line itself must be okay, otherwise the computer would not work. So that narrows it down to two things:

• The 74HC541 chip or the socket, solder joints or PCB track,
• Not all input pulses are causing the voltage to pass the thresholds for the input on the 74HC541.

Mark