Sinclair ZX80 / ZX81 / Z88 Forums

It's difficult to see whether the signal is right, when you don't have a CRT, as your LCD won't display the loading signal.

mrtinb wrote: ↑Tue Aug 30, 2022 2:34 pm It's difficult to see whether the signal is right, when you don't have a CRT, as your LCD won't display the loading signal.

Yes, the LCD monitor will disable when receiving the flickering signal.
Could the difference between failure and succes be the shortage in voltage? Your Arduino generates signals 0,6V higher than my Maxduino.

Next step: I will somehow try to amplify the signal from 0,0-3,8V to 0,00-4,4V

jesperp wrote: ↑Tue Aug 30, 2022 2:48 pm Next step: I will somehow try to amplify the signal from 0,0-3,8V to 0,00-4,4V

It is worth a try.

Maybe you could save a small P file on an emulator. Try to load it without the RAMpack attached. This will rule out any problem with the RAMpack.

Just a simple:

Hi Mrtin

“ Maybe you could save a small P file on an emulator. Try to load it without the RAMpack attached. This will rule out any problem with the RAMpack.”

Just wrote a small program (2 lines) in EightyOne, and generated a p-file.
Disconnected the RAM pack, and tried loading the p-file via MaxDuino - no change.

I will make a little amplifier able to generate up to 5V rectangular signals, based on the 3,8V MaxDuino output.
Earlier i tried a small music amplifier, wich amplified the signal around zero up to 3,5V (7,0V peaktopeak). I dont think the Lambda can see signals below 0V.

The MaxDuino problem is that it is supplied via USB, so 5V is max output voltage and some of this voltage is used by the output circuit. Probably why i only have 3,8V output.

Your Arduino is (i think) supplied with 9 or 12V and therefore able to deliver a larger output.

No. It’s just connected to my Mac over USB.

jesperp wrote: ↑Tue Aug 30, 2022 2:48 pm

mrtinb wrote: ↑Tue Aug 30, 2022 2:34 pm It's difficult to see whether the signal is right, when you don't have a CRT, as your LCD won't display the loading signal.

Yes, the LCD monitor will disable when receiving the flickering signal.
Could the difference between failure and succes be the shortage in voltage? Your Arduino generates signals 0,6V higher than my Maxduino.

Next step: I will somehow try to amplify the signal from 0,0-3,8V to 0,00-4,4V

Hi again

Made a small OpAmp amplifier, raising the MaxDuino signal with a factor 1,2. (raising the signal level from approx 4,2V to 5,0V)
Pictures showing the amplifier, MaxDuino output signal before amplifier and signal after the amplifier.
All measurements with the output connected to the Lambda EAR input.

Unfortunately without any succesfull program loading into the Lambda. Tried with different p-files.
I am beginning to state that my Lambda is totally deaf (the EAR input is blown away)?

Whilst I cannot work out what you have done on your bread board, the pictured hand-drawn schematic has the input and negative feedback resistors reversed. Should be 10k input, 110k feedback, you'd be lucky to get unity gain with the schematic as shown. You are using a 9v battery and this type of circuit uses or should be using a virtual earth not shown in your schematic the voltage swing is only 4.5 volts + - two batteries in series would be more suitable or a stabilised 12v power supply. A full schematic would also be useful.

Also which op-amp are you using? As most only raise an audio signal to the line level required to drive a power amp not a computers tape input socket directly and seeing as the schematic shows the output connected to pin 2 used as the input then I suspect this is a low power pre-amp chip therefore unsuitable for this purpose.

If it's an LM386 ( the schematic suggests it''s not due to the biasing method used ) then the only reason your are seeing any rise in signal is because this particular chip is internally biased to X20 which is probably cancelling out the negative or unity bias of the incorrectly placed resistors but still on the low side to drive the ear input, the 386 can amplify up to X200 suitably when biased by fitting a 10uf capacitor between pins 1-8 rather than a resistor though a potentiometer may be needed on the input to control the input signal to be boosted.

I'm basing this only on the schematic as seen and the attached picture would be a good start, leaving out the the cap and resistor outlined in red.

Should any excess noise or "motor-boating" effect be heard then a 10uf cap between pin 7 and earth is in order.

Hi Moggy

Thx for replying.
I am using a LM358 OpAmp, and the breadboard circuit is not identical with the drawn schematic. I will post the build circuit schematic later.
I could try building a better amplifier, but when looking at the result (output) from the present amplifier - i gain the result i wanted, amplifing MaxDuino output by factor 1,2. Never the less, i realize i have no success loading into the Lambda. But will another (better) amplifier make the load successful? I doubt.
If you think a nicer (more square signal, like the amplifier input), will make any diffrence - i will give your amplifier circuit a try.

The LM358 op-amp whilst it amplifies voltage doesn't really have the "punch" to drive say a speaker or headphones and isn't suitable for audio use whereas the 386 is a genuine audio power amplifier that outputs the kind of signal that computers like the ZX81/Lambda expect to hear. The earphone output of a cassette recorder normally used with our computers, gives a signal strong enough to power a small speaker/headphones, the 386 does the same and is the chip usually found on commercially sold boards used for what you are trying to achieve.

The LM358 should be regarded as a pre-amplifier at best and again not really suitable for audio work.


Should you try the 386 on your breadboard and it works for you then an example of the commercial product can be found at this link which is just one I picked at random and an internet search will find many other similar items.
https://www.ebay.co.uk/itm/193581039197 ... %3A2047675

Maybe we should find out if the signal reaches the CPU.

How about 2 oscilloscope pins connected to D7 (tape in data) and /IORQ (I/O access).

Then we can see tape data when /IORQ is low. I don't know how many port reads/writes happens during tape read, but tape data read would be the most frequent.

Any suggestions elsewhere?