31 Jan
2006
31 Jan
'06
2:48 p.m.
Hi
No, even the tube may be revived some. You can
often run the heater at a higher voltage for some
time and check the emission periodically. Usually,
you can safely run the filament at 1.5 to 2 times
the rated voltage for 5 or 6 hours. It is usually
done with no cathode current while at the higher
voltage and then return to normal levels to check
for the current. It would help to have a tube
tester but I suspect that just measuring the voltage
drop across a resistor in the cathode circuit
someplace would tell if there was any improvement.
You graph the increase in current. At some time,
you don't get any improvement so you stop there.
No sense wasting the filament.
You could control the voltage with a variac
and a transformer.
Do make sure that the tube is the cause. Raised
voltage on the filament can shorten the life of
the filament and at times cause cathode to
filament shorts.
If the image is well focused and not over sized,
the main rail voltages are OK.
Dwight
From: "Richard" <legalize at
xmission.com>
In article <200601311927.LAA29559 at ca2h0430.amd.com>,
"Dwight Elvey" <dwight.elvey at amd.com> writes:
So... if its the tube, there's nothing to do but replace the tube?
Egads... I wonder how hard it would be to find a compatible
replacement.
--
"The Direct3D Graphics Pipeline"-- code samples, sample chapter, FAQ:
<http://www.xmission.com/~legalize/book/>
Pilgrimage: Utah's annual demoparty
<http://pilgrimage.scene.org>
[...] I
do have one monitor. I haven't tried
to power it up, but written on the case is the word "DIM", presumably
identifying a failure in the monitor somewhere. I've never repaired
monitors, so I'm not sure what a dim image is suppose to indicate.
Failing HV drive circuitry?
Hi
HV would cause blooming of the image. Dim image is weak cathode
in the tube or problems in the circuits controlling the cathode
to gate voltage levels.
Dwight 
31 Jan
31 Jan
3:06 p.m.
Hi
No, even the tube may be revived some. You can
often run the heater at a higher voltage for some
time and check the emission periodically. Usually,
you can safely run the filament at 1.5 to 2 times
the rated voltage for 5 or 6 hours. It is usually
The method generally used on TV tubes of the period (and I don't see why
monitor tubes would be any different) was something like :
1) Over-run the heater. Typically a 6.3V heater would be run at 8V or 10V
2) Apply a fairly high (200V-ish) +ve voltage the control grid (wrt the
cathode), all other electrodes floating (no EHT applied either).
This, combined with the overheated cathode, stripped the top layer of
emissive stuff off the cathode, hopefully exposing a fresh surface. It
worked will on some CRTs, on others, it totally removed the emissive
material and you ended up with a totally useless CRT. But you've nothing
to lose by trying it on an otherwise useless tube
There was also a simple way to do an emission test. Again the only
electrodes you used were the heater, cathode, and control grid. I think
you ran the heater at the normal voltage and used the cathode/grid as a
diode and measured the current flow with a fairly low (if any) +ve
voltage on the grid. The higher the current, the better the cathode was
emitting.
Many designs for such devices have been published in the magazines. The
lethal ones get the 300V or so by rectifying the mains (or in your case
using a voltage doubler on the mains). More friendly ones used an
isolating transformer or a oscillator/step up transformer from a battery.
-tony
3:33 p.m.
On 1/31/2006 at 11:06 PM ard at p850ug1.demon.co.uk wrote:
The method generally used on TV tubes of the period
(and I don't see why
monitor tubes would be any different) was something like :
1) Over-run the heater. Typically a 6.3V heater would be run at 8V or 10V
In the old monochrome days, servicemen used to install "brighteners" inline
with the CRT socket. Usually just an encapsuled transformer set to give a
selectable boost to the heater. Bottles were expensive back then--now we
just throw the set away, it seems.
There were also jigs for popping inter-electrode shorts (usually due to
some cathode material flaking off and falling on the electrode supports.
Basically, you located the shorted electrodes and applied a charged
electrolytic to them. Most of the time, it worked. I've heard of, but
never tried, doing the same thing to revive NiCd cells with internal
shorts.
Cheers,
Chuck
3:50 p.m.
On 1/31/2006 at 11:06 PM ard at p850ug1.demon.co.uk wrote:
>The method generally used on TV tubes of the period (and I don't see why
>monitor tubes would be any different) was something like :
>
>1) Over-run the heater. Typically a 6.3V heater would be run at 8V or 10V
I was talking about over-running the heater for a short period while you
boosted the CRT, and then going back to the normal voltage to actually
use it.
In the old monochrome days, servicemen used to install "brighteners" inline
with the CRT socket. Usually just an encapsuled transformer set to give a
Those were not common over here, possibly because 99.9% of valved
monochrome TVs had a series chain of heaters which included the CRT. That
was typically at the chassis end of the chain and this led to the
'butcher's method' of putting a high wattage resistor between the live
side of the mains after the on/off switch and the non-earthed CRT heater
connection, so as to give said heater a bit more current. As I said, a
butcehr's method.
selectable boost to the heater. Bottles were
expensive back then--now we
just throw the set away, it seems.
They still are expensive, it's just that TVs have become too cheap. When
I needed a new CRT for a somewhat rare mono terminal, the cheapest (and
easiest) way to get one was to buy a monochrome portable TV and raid the
CRT from it.
There were also jigs for popping inter-electrode shorts (usually due to
some cathode material flaking off and falling on the electrode supports.
Basically, you located the shorted electrodes and applied a charged
electrolytic to them. Most of the time, it worked. I've heard of, but
never tried, doing the same thing to revive NiCd cells with internal
shorts.
That works too (or if you're brave, a quick flick with the bench
supply!). My expeirence is that the cells with then charge, but won't
last that long (the separators are probably damaged, they tend to short
again). But it's worth knowing the trick becuase it'll get a dead NiCd
pack going long enough to test whatever device it comes from, and lets
you know if it's worth buying new cells.
-tony
4:56 p.m.
On 1/31/2006 at 11:50 PM ard at p850ug1.demon.co.uk wrote:
Those were not common over here, possibly because 99.9%
of valved
monochrome TVs had a series chain of heaters which included the CRT.
That setup didn't manifest itself here until the 1960's. All 1950's sets
had very large (and desirable for other projects) power transformers. I
imagine that today the glass audio folks would really love to get some of
those old hunks of iron. Secondary winding was typically something like
450-0-450v rms, with several separate heater windings; usually 5v for the
rectifier (usually a 5U4) and 6.3v for most of the other bits and maybe an
extra winding for the horizontal output heater. It wasn't that uncommon
to re-wind the secondary for higher voltage and current for transmitter
use. A transformer from an old RCA color set was the golden ring--an
absolutely huge thing.
I spent many happy hours scavenging old TV chassis for parts, even after
the owners had removed all of the glass from them (with a thought toward
using them later, no doubt). The old 21 MHz IF turret-type tuners were
wonderful for VHF work.
Cheers,
Chuck
1 Feb
1 Feb
4:36 p.m.
On 1/31/2006 at 11:50 PM ard at p850ug1.demon.co.uk wrote:
A few very early UK TVs had mains transformers, and even produced the EHT
(about 7kV) from that. That supply could source considerable current and
was lethal.
A few later TVs had mains tranformers, but were often live-chassis too.
The mains tranformer might be for the heaters only, or might be an
autotransformer, or something like that. Be careful if working on one of
these.
Most B&W TVs had series string heaters (the P-series 300mA valves). There
were oddities with some 100mA nand 200mA strings in them them, but you're
not likely to find one of those. The HT (B+) line came from half-wave
rectifying the mains, either using a valve (U291, PY32, PY82, etc), or a
contact-cooled selenium rectifier, or a silicon diode depending on the
age of the set. The EHT came from the flyback transformer, of course.
Most colour TVs had a mains transoformer, if only for the CRT heater. But
again you might find a series string of heaters (possibly fed from a tap
on the transformer), and the HT line would come directly from the mains.
Of course modern TVs have an SMPSU and an isolated chassis, if only to
simplify the circuitry for the SCART connector (this has composite video
I/O, RGB in, audio I/O on it, isolating all those signals would be a lot
more work than an isolated SMPSU).
Those were not common over here, possibly because
99.9% of valved
monochrome TVs had a series chain of heaters which included the CRT.
That setup didn't manifest itself here until the 1960's. All 1950's sets
had very large (and desirable for other projects) power transformers. I imagine that today the glass audio folks would really
love to get some of
those old hunks of iron. Secondary winding was typically something like
450-0-450v rms, with several separate heater windings; usually 5v for the
rectifier (usually a 5U4) and 6.3v for most of the other bits and maybe an
extra winding for the horizontal output heater. It wasn't that uncommon
In UK sets, the horizontal output valve (line output valve, something
like a 30P4, PL36, PL81, PL504 depending on the age of the set) and the
booster diode (damper to you, soemthing like a U191 or PY800) were in the
normal series string.
-tony
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