[AMRadio] DX-100B, output falls off gradually

[Gary Schafer]

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Rick Poole
> Sent: Sunday, November 20, 2011 1:01 PM
> To: amradio at mailman.qth.net
> Subject: Re: [AMRadio] DX-100B, output falls off gradually
> 
> At 12:42 PM 11/20/2011, Gary Schafer wrote:
> 
>  >>>>>
> >The fact that you say pin 1 of the rear jack is at ground no matter
> where
> >the mode switch is would point to:
> >Someone placing an internal jumper across pin 1 to ground.
> <<<<<
> 
> It appears that is what happened... more specifically, someone
> apparently placed a jumper across the Plate switch so that the VFO
> and buffer cathodes are always grounded.
> 
> Turns out this is part of the rework of the DX-100B so that it will
> work with the SB-10, at least according to one of the illustrations
> in the SB-10 manual.  I haven't figured out why, yet.  But
> regardless, I'm not going to be using the SB-10 anyway, at least not
> any time soon, so I'm going to remove that jumper and maybe
> substitute an internal switch until I figure out why the jumper is
> there to begin with.
> 
> It's going to be a couple of days before I can wrestle that big block
> of iron over to the workbench... having an arthritis attack so it's
> all I can do just to walk... and anyway I just blew up one of the
> modulator tubes (it developed an internal short) so I need to wait
> for a replacement to show up at my door before I can take this little
> project any further.
> 
> Rick WA1RKT

Hi Rick,

I was just looking at a page of notes on the DX100 that Glen, K9STH had
posted on his web site. It shows converting it to grid block keying but the
keying is only done on the buffer stage and the VFO runs continuously. 
There is a small schematic that shows a jumper across the plate switch
contacts that were used to key the keying circuit. If that jumper is in
place the VFO will run continuously as they still ground the VFO cathode at
that switch while grid block keying the buffer. They probably left the
buffer cathode still connected to that line? This of course adds VFO
stability. 

I don't have the SB10 manual here so I am not sure what all is changed for
it but the notes show the buffer grid block keying line brought out to
another terminal on that 8 pin plug on the back for use with the SB10.

That jumper on the switch in the schematic may be a mistake in the drawing.
I would remove it even if grid block keying was added it should still work
properly in AM or CW without the jumper. Unless they do have the buffer
cathode directly grounded, then you may have a problem with the buffer
overheating with no drive.

Good hunting
Gary  K4FMX