[GreenKeys] Fwd: UPDATE- shockingly basic question - Teletype Model 33 ASR

[hwhall at compuserve.com]

Assuming that the loop is grounded at some point, why not look at the keying voltages vs ground?

WayneWB4OGM

-----Original Message-----
From: Jim Cooper <jim.w2jc at gmail.com>
To: Nick England <navy.radio at gmail.com>; Andy KN4UCL <kn4ucl at gmail.com>; Greenkeys <greenkeys at mailman.qth.net>
Sent: Thu, Feb 11, 2021 4:09 pm
Subject: Re: [GreenKeys] UPDATE- shockingly basic question - Teletype Model 33 ASR

On 11 Feb 2021 at 12:29, Nick England wrote:

> *Across* the resistor???? Do you
> mean scope probe to one side and
> ground clip to the other? 
> 
> Careful! If part of the loop is
> grounded, and you hook up the scope
> ground to the wrong spot, you can
> short out the power supply. Sparks
> sparking! Fuses fusing! Transistors
> blowing quickly to protect the fuse! 

Well, yes ...  but never a good idea to 
stick an instrument that's not 'floating' 
into any circuit. 

> I am a M33 ignoramous, but isn´t
> there a selector magnet driver?
> That´s where Russ was talking about
> ripple, not a signal loop. 

Well, the complaint is bad characters 
being printed ...  scoping across the 
selector magnet should give the same 
waveform as scoping across the current 
limit resistor, except you might also see 
spikes from the inductance ...  and wouldn't 
your caveat about ground hold for clipping 
on the SM as well ? 

> I could be reading this all wrong.
> Nick

or maybe I am ...  

> 
> On Thu, Feb 11, 2021 at 11:55 AM Jim Cooper <jim.w2jc at gmail.com>
> wrote:
> 
> > On 11 Feb 2021 at 11:20, Andy KN4UCL wrote:
> >
> > > *Well that makes sense. Looking at
> > > my ring binder (Tech bulletin 310B
> > > Vol2) I have a power supply circuit
> > > diagram, but I am not sure where I
> > > would physically plug in my highly
> > > underused budget Hantek scope...*
> >
> > If your scope can handle the voltage,
> > one simple place to put the scope is
> > across the current limiting resistor
> > in the loop (usually about a 2k resistor).
> >
> > You will then see the square pattern of
> > the character code.
> >
> > w2jc
> >
> >
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> -- 
> Nick England K4NYW
> www.navy-radio.com
> 


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