Subject: Re: [drakelist] T-4XB troubles
From: msmith{at}licor.com


msmith{at}licor.com made an utterence to the drakelist gang
----------------------------------------------------------------------




Hi there,

The 6EV7 is sort of a known trouble spot in the T-4XB when it comes to the
the relay turning on when you do not want it to.
But I think this may be something different since you are seeing the plate
currents that you state. I think you might have a problem with a filter
capacitor in the negative bias suply of your AC-4 (or AC-3). If that filter
opens up and is not doing a good job of filtering, the ripple that will be
on the bias supply can be rectified by D10 and then passed to the grid (pin
7) of the 6EV7 and turn on the relay. Also, lack of filtering will lower
this negative bias voltage and result in increased plate current in the
finals from lack of bias.

If the filter capacitor in the supply goes short you will lose bias and
have improper final amp current, but I do not think you will have the relay
turning on that way, unless there is something I am failing to remember,
which could easily be the case.

Measure the bias voltage under load (meaning from within the T-4XB) and see
if it is low. You ought to be able to set it all the way to -70V or so with
the pot on the AC-4. If you have an oscilloscope you could look at the
ripple voltage. It should be pretty low. If you see 10 volts of ripple,
replace the filter cap.

If you need a capacitor for that bias filter I can give you one to get it
running.

Let me know if I was right. If not, ... anyone else want to take a crack at
it?

Michael Smith
N0XY

owner-drakelist{at}www.zerobeat.net wrote on 11/30/2004 01:31:49 PM:

> Hey all, I am new to the group, and would like to start off by
> picking the collective brain :-)) I am having trouble with my T-4XB.
> I was working CW with it the other day when all of a sudden, it shot
> a fuse. I changed out the fuse and started it back up. The new fuse
> didn't blow, but when the set warms up, and the tubes start to
> conduct, the TX/RX relay kicks in and the plate meter starts to show
> a lot of plate current, around .25 to .3 on the meter. The
> transmitter won't tune, won't key, won't do anything except light
> up. I did a visual check underneath, and nothing is obviously
> burned. or out of place. I thought it may be the 6EV7 tube
> associated with the relay, but I have no way of testing the one in
> the set, or any of the others I got with it.  Anyway, I thought this
> may be a common problem associated with this particular set and
> decided to put the problem in front of the all knowing ones of the
> famous Drake list! Anybody got any ideas?
>
> 73's
>
> Greg Gifford / KG4MMY

----------------------------------------------------------------------
On Behalf of msmith{at}licor.com
Submissions:        drakelist{at}www.zerobeat.net
Unsubscribe:        majordomo{at}www.zerobeat.net - unsubscribe drakelist in body
Hopelessly Lost:    majordomo{at}www.zerobeat.net - help in body of message
Zerobeat Web Page:  http://www.zerobeat.net
Brought to you courtesy of TLCHost.net  http://www.tlchost.net/
----------------------------------------------------------------------

---