Hi Tom and all
The clue may be that one of the 3-500s has gone west with an internal short.
I have had quite a few go over the years.
3-500Zs are prone to blowing up with an internal short. The brand or the
supplier doesn't seem to make a lot of difference. The age of the tubes
does. One of the reasons that they go is that flakes fall off the plate and
cathodes and short to the grids. It's just a natural occurrence as the tubes
age. If you are aware of it... no problem as you should put a 1 amp quick
blow fuse in series with each cathode so you don't blow the power supply.
Pull the 3-500s carefully without knocking them around. If you put an
ohmmeter from grid to plate or grid to cathode you will often see a "short"
A normal 3-500Z will show almost infinity ohms.
How do you get rid of it? two methods work. The first is a very sharp rap or
knock which will dislodge the flake and it will fall to the bottom of the
glass. If that doesn't work you can hook up 3 or 4 or more KV from cathode
to grid or from plate to grid and ionize the damn flaky thing!!! It may
sound drastic but in most cases the cure actually works and you can get
hundreds more hours out of 3-500s. I have often wondered how many perfectly
good 3-500Zs have been tossed and could have been restored. I bought a pair
a few years ago at a junk sale (flea market for you non-ZLs) They were in
original boxes and had a label "condition not known".
I ran the ohmmeter over both and one showed a short. The other seemed fine.
I bought them very cheaply indeed and a sharp rap on the side of the glass
with a rubber hammer dislodged the short and both worked rather well for a
long time in my amp.
Sound a bit drastic? You have nothing to lose and everything to gain by
trying it.
73, Lee ZL2AL
----- Original Message -----
From: "Georgens, Tom" <tom.georgens@engenio.com>
To: <amps@contesting.com>
Sent: Wednesday, June 01, 2005 11:39 AM
Subject: [Amps] TL-922 question.
> Hello -
>
> I am looking for some help on a TL922 problem. At my contest station in
Barbados I had a TL922 fail in February. Looking at it, I saw that the
doorknob blocking cap had split and was burned inside. I also noticed that
the HV fuse resistors I had installed had been blown. I reasoned that the
cap shorted, the output RFC took the HV to ground and the fuse resistors
blew protecting the power supply and plate choke.
>
> Only this past weekend have I been back to work on it. I took down a
spare doorknob cap and output RFC, if the prior one had been damaged. I had
looked at the plate choke before I left and it showed no sign of damage.
>
> The first issue was that the TL922 has soldered over all of the mechanical
connections and I did not have a sufficient heat source to take them apart.
Nonetheless I did find a place to mount the new cap. The output RFC still
looked good.
>
> When I turned it on, the amp made power but the output was only about
500W. I looked in the cage and both filaments were definitely on. I power
it all down to take a look inside to see if some connection is missing. The
connections looked fine but I could feel that one tube was still scorching
hot and the other was merely warm. It seemed that only one tube was making
power. It is possible some tube failure was the precipitating event and not
the blocking cap so I replaced the tube with a spare I tested about a year
ago. The result was the same. Once again it seemed like only one tube was
active. I was out of time here and I should have swapped the tubes but I
did not.
>
> What I did do is bring back the original tube to test at home. Assuming
that it works, I am open to ideas on what may be wrong. I am assuming that
the plate voltage is connected since my experience with RF drive and no HV
is high grid current. This was not the case. All readings were normal, for
one tube. About 100 ma Grid and 450 mA plate. One possibility is that RF
is not getting to one of the tubes. As it turns out, the rf is coupled
through a cap to the first tube (the one that is working) and then another
cap couples to the other tube (the one that is not). It could be a simple
as the cap failing. I will bring a replacement next time but I want to be
prepared in case it is not that easy.
>
> My question is whether there could be another reason for the symptoms I am
seeing. My only other theory is the biasing circuit could be damaged and
the tubes are indeed both on but not at the right bias point and they are
not sharing the load equally. I did not measure the zener voltage but the
part was not obviously damaged.
>
> Thanks in advance for any suggestions.
>
> Fortunately I had three other amps that work fine
>
> Tom W2SC 8P7A
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>
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