[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: Saturday, November 19, 2011 11:19 PM
> To: amradio at mailman.qth.net
> Subject: Re: [AMRadio] DX-100B, output falls off gradually
> 
> At 10:24 PM 11/19/2011, Gary Schafer wrote:
> 
>  >>>>>
> >Also check to see if you have the mode switch in AM or CW.
> >Only in the AM position does the plate switch ground/open the cathode
> >circuits. In the CW position the cathodes are connected directly to the
> CW
> >key jack which has shorting contacts when the key is unplugged.
> >
> >So if you have the mode switch in CW and the key unplugged, then the
> >cathodes of the VFO and buffer are always grounded.
> <<<<<
> 
> Right, I saw that in the schematic.  Pin 1 of the accessory socket is
> always grounded regardless of the position of the Phone/CW switch.
> 
> Rick WA1RKT

Hi again Rick,

Have you tried plugging in a key into the key jack and placing the rig in
the CW position? 
If you are able to key the rig as expected with the key while the high
voltage is on then that eliminates 6AU6 VFO and 12BY7 driver problems (could
be cathode to filament short in one of those).
In the CW mode the plate switch and rear panel jack are not supposed to be
connected.

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.
Bad plate switch half staying stuck in closed position.
Mis-wired mode switch.

Also if it was used with an SB10 side band adaptor they might have wired a
switch goofy somehow.

You mentioned a "spotting" switch. The DX100B never had a spotting switch
like the Apache had. If someone added one then that might be stuck closed as
it would have to ground those cathodes of the VFO and buffer. It would
create the symptom that you have.

The power supply is not the problem but what you are seeing is the
oscillator/buffer running all the time and when the high voltage is turned
off the delay in output reduction is due to the bleed down time of the high
voltage.
If the oscillator were getting turned off, as it should, the bleed down time
on the power supply would be of no consequence. 

When drive is removed by opening the key circuit output will immediately
cease. The clamp tube pulls the 6146 screen voltage down and cuts off the
final to protect it if there is still high voltage present (as there always
is in CW mode).

73
Gary  K4FMX