[Coco] Coco Cassette interface help

[RETRO Innovations]

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