[Coco] Coco Cassette interface help

Dave Philipsen dave at davebiz.com
Tue Feb 28 12:07:47 EST 2017

True, the cassette output is designed to be plugged into a MIC in jack 
on the tape recorder as I remember.  So the output of the cassette 
interface apparently does not generate a signal with enough amplitude to 
drive the input of the cassette interface.  Sounds like a job for a 
simple op amp circuit where you can experiment with the gain.  A good 
book for op amp circuits is Forrest Mims' /Engineer's Mini-Notebook/ 
here: Engineer's Mini-Notebook 

Look on page 16 for a simple single-supply amplifier to feed the 
comparator circuit.


On 2/28/2017 1:16 AM, RETRO Innovations wrote:
> On 2/27/2017 11:19 AM, Dave Philipsen wrote:
>> Well, if you want to use a tried & true circuit just do what the CoCo 
>> does for the incoming data from the cassette player.  The schematic 
>> is on page 68 and involves an op amp and a few other components. 
> Sadly, I constructed the exact circuit as defined on page 68, and it 
> does not work with a Coco 3 driving it.  I even used an actual LM339 
> from my Coco1, to no avail.  I doublechecked the circuit, and it is 
> correct:
> incoming signal to 220
> 220 to ground
> then 8K2 in line
> 1N4148 to gnd
> 6K8 inline to pin 10
> 56K from 5V to pin 10
> 56K from 5V to pin 11
> 15K from pin 11 to gnd
> 1M5 from 13 to 11
> 1K from 5V to 13
> 12 to GND
> 3 to VCC
> output to pin 13
> I am not sure the Coco3 can drive the circuit used in the Coco.  I 
> assume someon could test by hokking a coco up to a coco (in to out, 
> out to in), and saving a file while trying to load it on the other coco.
> My theory is that the Coco expects the AGC of the tape record to do 
> the dirty work and create a strong 2V ptp signal.  I measured the 
> signal from the Coco3.  .9V PtP, centered around .45V.  The 15K/56K 
> voltage divider on the LM339 sets a bias voltage of 1.05V, which means 
> all the signal is below the bias.
> Jim
> I have struck out.  3 tries, 3 failures.
> I am open to other ideas.
> JIm

More information about the Coco mailing list