Bring Zeddy back to life

[zx81user]

Hi everyone,

While cleaning out the garage, I found my Zeddy that had been in a box stowed away for about 25 years or so. I decided to bring it back to life in the past couple of days. So far it's going quite well. I could only hook it up to my Compro DVB-T300 TV card as that is the only TV tuner I got in the house that can still receive analog TV signals.... At first power up, it wouldn't do much but after cleaning the Z80 & ULA contacts I got my promising "K". I then plugged in my 16k memory pack and my promising "K" disappeared. Cleaning the finger contacts wouldn't help. After opening the pack, I had lost the +12V and -5V voltages for the DRAM chips, so I decided to change the ZTX750 transistor used in the switched mode circuit. The new transistor is an NTE159 and brought the memory pack back to life! The next step would be to load some program into the Zeddy, and that's where I am stuck now. I saved a file to an MP3 player, but the Zeddy won't load it. The audio signal from the MP3 player seems very nice so I am not too sure why this doesn't work. The voltage (see attached picture) is about 300mVpp. That is the loudest the MP3 player will go.

Does anyone know what the optimal audio input voltage should be?

Cheers,
Michel

[Rink]

Does the ZX detect anything at all? i.e. Does the picture on screen change once you start playing the MP3?

[zx81user]

No, the screen won't change at all. I mean, I see the usual un-synchronized lines after I type "load", but they don't change once the audio comes in. It seems like the ULA doesn't detect anything at it's input.

[bbock]

Did you plug the MP3 player cable into the "EAR" socket?
Did you use a MONO cable (not a stereo one)?
You could use an audio software like Audacity to amplify the signal.

Bernd

[Rink]

There's one thing I usually refuse to check, but is often the problem - verify that your EAR socket is physically working.

I've spent ages looking for more technical problems before discovering that the EAR and POWER sockets have worn out and aren't making good contacts.

To answer your original question - no idea about voltages but I've used an iPhone for loading tapes, for ages now. So an MP3 player should be roughly right - have you tried starting with no volume after pressing Enter on your LOAD command... and then notching up the volume to see when (if at any point) the ZX81 responds?

And also: think back 25 years - are you 100% it ever worked? I bought a ZX81 recently which the owner claimed used to work and later discovered it hadn't been built properly.

[Moggy]

300mv is not even professional line level (750mv-1v) which is still not enough to trigger the input.
You have to remember that the zeddy when attached to the EAR socket of a tape recorder is relying on CURRENT not voltage to drive the input , LINE out sockets are voltage drivers , EAR/SPK sockets are current/amp drivers.
A rough rule of thumb is that if Your device can power a full size pair of headphones adequately (not the puny earbud things that line level can just about manage) then it should power a zeddy.


Moggy.

[PokeMon]

300mV is definetely too low.
But there are more hints to follow.
Do not use a stereo cable, that didn't work at all with my ZX81.
Still use the original mono cable - OR - a stereo audio cable with mono adapters on both (!) ends.
As my tests you need a voltage of approx. 3.5 Volts pp.

See here for more details loading audio via PC and sound card to ZX81:
viewtopic.php?f=6&t=1064#p10987

And in the first posting of this thread you can download the latest version:
viewtopic.php?f=6&t=1064

Good luck.
I always load programs via PC, onboard sound card with the ZX81-IDE. Works great.
Even the Fastloader is very helpful, loading up to 25 times faster but need a little extra power (voltage).

[Andy Rea]

i'd just like to add... Don't use any EQ at all if your playing back from a digital player be it PC or MP3 player, adding EQ does some very odd things to the waveform, Good old fashioned analog EQ on the other hand is quite acceptable

Andy

[zx81user]

Thanks guys, that is a big BINGO! I attatched a picture for your pleasure You got to admit that the real thing is better than the emulator!

I did go through all the problems listed, like swapping EAR and MIC (........ obvious mistake .........), using mono vs stereo cable etc etc. But then, yesterday I was sure I got everything right, it still wouldn't load a file. Apparently, the 300mVpp that I had was way to low and it should be more like 3Vpp. The ZX81-IDE does a fantastic job! Apart from transferring files, it looks like a nice application for some assembly programming. The last time I wrote some assembly code for this baby (about 25 years ago) was writing the code down on sheets of paper, then converted it to machine code using the table listed at the end of the ZX81 user manual .

I used to have an 8 bit input / output card that was listed in the "20 Simple electronic projects for the ZX 81 and other computers". I still have a few copies I made from pages of that book, but the I/O interface is nowhere to find, I may have given that to someone long time ago. I might make a new one, just for nostalgic purposes so that I have my ZX81 setup the way it used to be just before I packed everything into a box!

How does that fast loader work b.t.w.? 25 times faster?? I had to uncheck it in the IDE otherwise the ZX81 wouldn't load the file properly. The normal frequency of the pulses is around 3kHz, so if you would speed that up 25 times, you arrive at 75kHz which I am sure your sound card doesn't support.

Thanks again, will post some project in the next couple of weeks or so.
Cheers,
Michel

[sirmorris]

Woohoo!

Another one back to life Congratulations!

If you find the schematic/layout for the IO PCB I might be tempted to make one or two..!

C