[ARC5] AM modulating an "ARC-5" transmitter...

[Kenneth G. Gordon]

On 13 Oct 2015 at 10:33, Leslie Smith wrote:

>    Robert (and others)
>    I distrust this circuit.  I suspect the circuit was never built and
>    tested.

Which wouldn't be one bit unusual for that era. I found several modifications 
which were published in nationally known ham magazines of that general 
time period which were OBVIOUSLY never built since they could not have 
worked as published.

>   Does any-one here know of this circuit being used - either on a
>   test-bench or actually on-air?

"Not I", said the Little Red Hen. ;-)

>   The circuit lacks stability for the DC bias (operating) point.  If
>   used with a battery the operating point will drift as the battery
>   voltage drops.
>   The result?  Either a low degree of modulation or splatter.

OK. Possibly...
 
>   The claim that the circuit will operate with a crystal microphone
>   seems to be unfounded - only Q4 supplies voltage gain;  Q1 to 3 supply
>   only current gain.

Again, maybe so. I'm not nearly as familiar with SS circuits as I am with 
tubes.

>   The operating bias for the 1625 will be somewhere between -35 and -65
>   volts.  If this modulator is to work it will need to supply a peak to
>   peak signal AT LEAST 50% of that range. 

OK. Sounds good.

>    Without the benefit of "spice", - just looking at the circuit I would
>    say, "No way."   

Well, not with only 6 VDC on it, at least. That seems logical. But other issues 
may be at play here. Aren't these transistors actually acting as some sort of 
dynamic variable resistor?

>   That's my assessment  

Well taken. We need to run some tests.

As I said, I HAVE used the old tube-based "Simplest Modulator" and it works 
just fine.

As soon as I can, I'll bread-board one of these SS units up and see what 
happens in my DX-35, then will report here. I'll take "scope photos" too.

Ken W7EKB