Re: Tek 464 CRT tube presenting "double-peaking" - Is rejuvenation recommended

Fabio Trevisan
 

Hi guys,
Thank you all David, Ed, Leon, Brad and Richard for your comments (not in
any particular order :-) )

Indeed I had visited most of the sites and links you indicated, and have
been researching the subject since I first stumbled in the term
"double-peaking" and realized it was very descriptive of my own 464's CRT
behavior.

I got convinced by Ed's (Breya) arguments to use an external rejuvenation
device (instead of trying to do it "in-circuit" of the scope).

Since I don't have a BK470 and didn't find any (or similar) being sold in
Brazil, I decided to copy the BK470 circuit...Not everything but only the
Clean+Balance and Rejuvenate function (the only difference between the 2
functions is the current limiting resistors... (5k for Clean and 2.5k for
Rejuvenate)

Later I will post the simplified circuit and properly document the process,
before and after, with photos (and possibly a video).

After having studied the BK470 circuit, the only thing that's not specified
in its service manual, is the amount of boost that it applies to the the
heater voltage.
The boost is implemented in the form of a "boost tap" in the transformer's
primary winding. When either function "Clean+Balance" or "Rejuvenate" is
engaged, it feeds the transformer primary through this tap, instead of the
normal tap, thus increasing the voltages in all the secondaries of the
device by "x" %.

Unfortunately, there's no indication in the B+K 470 S.M. of how much is
that increase when the "boosted" functions are engaged.
From all I read on the net on this subject, it can be as little as 15% to
as much as 50%... (41% boost already doubles the power) but there's no
common ground here to make any assumption.

So I have 2 questions:

1. Do any of you know beforehand how much this "boost" is (for the BK470)?
or...
2. If Ed or any of you still have a BK 470, if you can be so kind of
measuring the heater voltage in the following 2 conditions (DON'T CONNECT A
CRT, as I`m not sticking to any CRT particular voltage and, as set below,
it won't matter):

2.1. Set function selector to "Set Heater", set Heater Range selector to
"4-7", set heater adjust to full CW - measure (with external AC voltmeter)
the 1st heater voltage output at CRT socket.

2.2. Set function selector to "Clean+Balance", leave everything else the
same - measure (with external AC voltmeter) the 2nd heater voltage output
at CRT socket (don't push the button while in this function as this action
will disconnect the voltage to the heater).

From these 2 values I will know how much of boost is "hard-wired" in the
transformer and can build my circuit to do exactly that.

Before you mention, I know that I don't really need to do exactly like BK
did, but doing so will add value to this experiment as it will make it
consistent to Ed's previous experiment in where he used specifically this
model, only that now on a different CRT.

Besides... Since there's no right or wrong in this subject, it's better to
stick to boost values that were built into products like the BK470, with
some reputation to be effective at doing its thing.

Rgrds,

Fabio

2016-10-18 2:48 GMT-02:00 David @DWH [TekScopes] <
TekScopes@...>:



On Mon, 17 Oct 2016 19:45:51 -0400, you wrote:

Hello--

I dimly recall reading that in a television CRT, electron emission took
place from
a small spot on the cathode's surface. As the tube aged, the cathode
spot would decrease
its emission, thereby dimming the picture.
That is my understanding for CRTs as well. The Tektronix Circuit
Concepts book on CRTs has some details.

A short-term cure consisted of installing a "booster"-- it's a stepup
transformer spliced into a short wiring harness that plugged into the
existing CRT
socket and and the CRT's base. The booster would increase the CRT's
heater voltage
by a few percent and thus increase the cathode's temperature and emission
by
enlarging the cathode spot's area..
They also made these for general purpose tubes. I had one installed
in my cabinet sized combination AM/FM receiver and phonograph when I
was little.

The new emission "spot" now consisted of an emitting ring around the
original
depleted cathode spot. This larger emitting area affected beam sharpness
and reduced picture
quality.

...

Brad AA1IP
I wonder though how double peaking occurs.

There are a fair number of web pages discussing CRT rejuvenation to be
found now. This one shows the general idea for the schematic:

<http://www.thevalvepage.com/teletech/crt_rej/crt_rej.htm>

And this one discusses various cases and mentions that while a
rejuvenated CRT may only have 100s to low 1000s of hours of operation
before failing again, that is a lot of time for home use:

<http://www.ke5fx.com/crt.html>


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