[Drake] Strange problem with T4XC

[pulsarxp at earthlink.net]

What about making sure the bias voltage is getting from the supply to the
final tubes?  Maybe the bias wire in the cable is not connected between the
AC-4 and the T4XC..
Lee
w0vt


> [Original Message]
> From: ed <ed.brooks at att.net>
> To: <DJED1 at aol.com>
> Cc: <drake at mailman.qth.net>
> Date: 10/12/2005 5:34:05 PM
> Subject: Re: [Drake] Strange problem with T4XC
>
> Are you operating the AC4 without the T4XC attached?  I would try that 
> (You'll have to jumper the switching pins, 1 and 2) and see what kind of 
> voltages you are getting in the AC4.  That high screen voltage could 
> explain the transmitter's runaway plate current.  First clue - two 
> transmitters doing the same thing on the same power supply.  Of course, 
> something in the AC4 could have damaged both transmitters. 
>
> The high voltage really isn't awfully significant,  but the screen 
> voltage is way out of line. 
>
> The high plate voltage is OK, considering your high line voltage.  Not 
> significantly over the standard of 650.  So I'd look for a problem in 
> the screen supply.  Not many components in there.   C4 may well be the 
> culprit.  C4a shorted could do this.  Unsolder it and put another 
> approximately 80 mfd (doesn't have to be really close) temporarily in 
> its place. 
>
> Quite possibly the screen supply went bad on you and has now damaged the 
> driver in the transmitter, though I think that tube would handle that 
> high plate voltage for a while.  But the next thing to do would be the 
> coupling capacitor from the driver to the finals, putting that low 
> voltage on the grids of the finals. 
>
> Anyway, I'd start with substituting a different cap for C4a.  If that 
> drops your LV back to 275 you may be OK.  But I'd do this with the AC4 
> not connected to anything but the AC line.
>
> A shorted coupling cap in the driver would be bad news for the finals, 
> but I doubt it would have done any damage to the screen supply.   I 
> would definitely, though, have it on my list of things to check!
>
> Ed
>
>
>
> DJED1 at aol.com wrote:
>
> >I was starting to operate this morning on SSB when I glanced over and
saw 
> >that the plate meter was almost pegged without any voice input.   I shut
it down, 
> >and investigated-   When I first turn the T4XC on, the plate current
will be 
> >at zero, then will go to 450 ma if I turn the switch to tune, even
though  the 
> >RF gain is all the way down, and it won't go back down if I turn the
mode 
> >switch back to SSB.   Even stranger, I substituted my other T4XC and it
did the 
> >same thing (I just checked out both last week, and they worked fine!).  
So I 
> >was sure I lost the bias or screen voltage and pulled the AC4.   The
bias 
> >voltage was OK, but both the HV and LV were very high.   Part of this
may be due to 
> >high line voltage (124V), but the HV was around 750 V, and the LV around
340 
> >V.   I tried feeding the AC4 with a variac to pull down the voltages,
but the 
> >strange behavior of the plate current persists.   Anyone have some ideas
as to 
> >what is wrong?   This one has me stumped.
> >Ed   WB2LHI
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