[1000mp] RASCAL and 1000MP

Tue, 30 Apr 2002 04:23:55 -0000

I guess that, if it works for you, it must be OK. But it sounds, from your
description, that your optoisolator may have too low a CTR (current transfer
ratio)--essentially the current gain of the device--so the phototransistor
cannot sink enough current. The parallelled resistor serves to divert some
of the current. You have essentially created a teeter-totter, biasing to
just one side of the balance point, so that a small "weight"---the opto
current---swings the seesaw.

A more robust solution might be to increase the gain by connecting another
transistor with the phototransistor, as a Darlington pair ( emitter of the
phototransistor to the base of, say, a 2N3904, the 3904 emitter to ground,
and both collectors to the PTT line. Minimum output voltage would be about
0.6V, but you indicate that that will work.

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Gorsline
> Sent: Monday, April 29, 2002 19:03
> To: [email protected]
> Subject: Re: [1000mp] RASCAL and 1000MP
>
>
> I had a similar problem with a homebrew PSK interface using
> serial port PTT
> keying using one of the many circuits with an optisolator.  It
> would only pull
> the PTT line down from 13.2V to about 2.5V - not enough for PTT
> "closure."  So
> added a resistor across the PTT line to ground (~50ohms range -
> YMMV - started
> with higher values, tried successively lower values until found
> one that worked
> and didn't seem to bother normal VOX/QSK).  The resistor value
> still keeps the
> voltage above what the PTT pull-in needs ( ~0.6V)  and then when the opti
> switches, the PTT "closes" just fine.  I had to think long and hard about
> current draw on the rig, etc., until it occurred that the PTT is
> a short, so the
> current limiting has to already be in the rig.  You could solder
> it across the
> PTT jack if needed.
> Hardly an elegant solution, but it works and got me on PSK.
> 73,  George  VE3YV / K8HI
>
> PS - not being a electronics techie - old IT type - if anybody
> who is feels this
> is potentially harmful, please let me know!  With the current
> turnaround times
> at Yaesu repair for parts to my local dealer, this baby is being
> treated with
> respect!
>
> Bill Evans wrote:
>
> > Todd
> >
> > About a year ago I bought the Rascal and connected it to My 1000MP.  I
> > had problems keying the rig through  the PTT.  The problem is that some
> > 1000MP's have a 1K pull up resistor to 13-14V.  The standard Rascal can
> > not sink this much current.  Below is an exchange of e-mail with Buck,
> > K4abt, on the problem.  You did not say if the PTT was your problem, but
> > the measured data in my e-mail may help.   K4ABT  made the upgrade
> > quickly and I have enjoyed many contacts on PSK31 since.
> >
> > Bill
> >
> > ***********************************************************
> > My note to K4ABT:
> >
> > "I recently purchased the Rascal R 1000D to be used with my
> Yeasu 1000MP.
> >
> > The PTT does not work.  Here's the data.  The Rascal LED
> > lights when the transmit button is pushed on the DigiPan screen.
> > The Output impedance of the Rascal drops from "open circuit" to less
> > the one ohm with no connection to the transceiver.  With the
> > Rascal connected to the transceiver, the voltage on the output
> drops from
> > about 14V ( which is the normal open circuit voltage on the FT 1000MP
> > PTT lead) to about 10V.  It appears that the Rascal cannot handle
> > the current form the FT 1000MP. ( The short-circuit current from the PTT
> > is about 15ma.)  What do you think? Was the unit designed to
> > sink this current?  Have other users had this problem?"
> > ***********************************************************
> > Reply from K4ABT
> >
> > YES, about 3% of the MP users report a problem with the PTT... seems all
> > MP's are not created equal.
> >
> > If you will return it, I will build a FET keying circuit into
> it that will
> > sink it to .1 ohm, and handle the 1kmp.
> >
> > I talked to a tech at Yaesu Monday....
> >
> > It appears that all the late model 1Kmp are TTL at the PTT
> jack.  So, I must
> > use another design to handle the change.
> > ************************************************************
> >
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