[Drake] Strange problem with T4XC

[ed]

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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