Getting an ACI (or cassette recorder) to work

I suggest you use a modern playback device on a prepared audio file that has been successfully used by others; you need to bisect your problem to narrow down the object of troubleshooting.

There is no significant difference between 0.1µF and 0.11µF, but the type of capacitor can matter a great deal. Some types have high enough ESR to change filter characteristics, and some change significantly with bias voltage. It would have been better to A/B test the setup before changing its circuit.

In post #1, 'njissen' wrote:

" Followed Uncle Bernie's instructions, one thing I did different:

- at the component side I put a 0.01 uF ceramic disc to make it look authentic

- at the solder side I put a 0.1 uF in parallel with this ceramic disc (reliability improvement)"

Uncle Bernie comments:

Ouch. The trick used in the Gen2 ACI (which fixes the fatal flaw of the original comparator circuit) is impedance balancing on the comparator inputs.

The 100nF capacitors in the kit were checked for sufficient matching (in the order of 2..3% or better).

If you parallel a 10nF capacitor to a 100nF capacitor you already have 10% mismatch if the capacitors have zero tolerances. The tolerance of the capacitors may add or subtract, so you could (randomly) get more or less mismatch.

The next important thing is that with the Gen2 improved ACI you should have a 300k resistor for the hysteresis and not the original 47k. This makes the circuit more sensitive and allows for a lower volume setting of the signal source. Which also helps with playback reliability (less distortions than a fully cranked up amplifier). (Don't try to reduce the hysteresis on a Gen1 or regular ACI - it won't work. You need the impedance balanced Gen2 circuit to be able to reduce the hysteresis).

As 'robespierre' has pointed out in post #2, try a known good audio source first, such as media player. Do not use MP3 files or any other compressed audio format with the ACI. It only works with WAF or AIFF files. You can find an AIFF file in my 'Codebreaker' thread.

The proper timing of the playback operation is also important. Set up the command line but don't hit 'return' yet. All addresses must be typed with 4 hex digits, including leading zeros. Then start the recorder and wait for the header tone (LED should light up) and then immedialtely press 'return'.

Properly built and operated, the Gen2 improved ACI works very reliably, there even is a 'scientific' investigation with bit error rate measurements on github, which was done by another Apple-1 enthusiast, and no, I did not bribe him to produce better results than real (I'm not the pharmaceutical industry claiming 'safe and effective' for useless but deadly products).

Hope you also did check the bias voltage generated by the LM4040 to be 2.5V

Be patient and follow the proper procedure and it will work .., but with old cassette recorders you need one which is in good shape. Believe me, it's a very tedious and expensive process to refurbish a 30-50 year old cassette recorder. All the rubber parts typically are hardened to become brittle, or turned into sticky goo, and all drive belts will need replacement. Oh, and if you think a recently produced cassette recorder 'Made in China' is any better, think again. They are complete trash new out of the box. Horrible wow and flutter and very lousy audio fidelity. Don't buy any of them. And don't ask me how I know they are so bad ;-)

- Uncle Bernie