Date   

Re: Low-Bandwidth CRT - why?

Stan & Patricia Griffiths <w7ni@...>
 

You are right that the 564B CRT has a bandwidth much greater than the 10 MHz
it is specified at. And yes, it might be possible to develop plugins with a
lot more than 10 MHz bandwidth. There was a scope (Type 506) and a couple
of plugins for it (9A1 and 9A2) that was almost exactly like the 561A with
more bandwidth (23 MHz) and used normal 561A timebases.

One of the problems with building a wideband amplifier for the 564B (or 561B
or any of that series) is that you will want faster sweep speeds to observe
those faster signals so now you have to build a faster timebase, too. Next,
you will actually want to SEE those waveforms and you will find there is too
little energy in the beam with only 3.3 KV of accelerating potential to see
the trace at sweepspeeds that are compatible with viewing 50 MHz signals.
After you have redesigned the HV for, say, 10 KV, you will discover that the
sensitivity of the CRT, both horizontally and vertically is now much less
than it was and now you have to redesign the vertical amplifier and timebase
AGAIN to be compatible with the new sensitivity.

The 564B was designed as a fairly low frequency SYSTEM and all parts of the
system interact to make it a decent scope. A major change in one part
implies nesessary changes in the other parts to make it operate well as a
system. For 50 MHz, Tek had the 547 or 647 and if you wanted 80 MHz, there
was the 585 and 581.

Stan
w7ni@easystreet.com

----- Original Message -----
From: "adenton" <adenton@bear.com>
To: <TekScopes@yahoogroups.com>
Sent: Thursday, December 04, 2003 5:26 AM
Subject: [TekScopes] Low-Bandwidth CRT - why?


I have a 564B which has a stated mainframe bandwidth of only 10MHz --
a remarkably low figure IMO. Can anyone here tell me what aspects of
physics can limit the bandwidth of the CRT? In this unit the
vertical and horiz defl plates are driven *directly* from the plug-
ins, so I (being naive?) would guess that, if I could produce a plug-
in that could deliver the required 160Vp-p at, say, 400MHz, into the
capacitance of the vertical plates, that I should expect to be able
to view 400MHz on that CRT (assuming I drove the horizontal
accordingly, which BTW is also driven directly from the [right side]
plugin). The distance from plug-in to CRT is about 6 inches, fed
thru 2 pairs of thin wires each about an inch apart, so I would not
expect any transmission-line effects (well maybe at 400MHz but, say,
50MHz should be fine).

It's an electron beam, so if the plates have potential, it's gotta
deflect, yes?

(of course it's rather difficult to deliver 160Vp-p at 400MHz but
that's another story....)

Or is it just Tek marketing... :-/

-Adam





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Re: Vitamin Q capacitors (paper or plastic)

Miroslav Pokorni
 

Hello Mark,

Awhile back, on this forum, someone told me that throughout life of Sprague
Electric, there has always been a 'Vitamin Q' capacitor. It looks like,
Sprague's marketing department ran out of imagination after they came up
with that name and then reused it many times, so 'Vitamin Q' does not even
designate type of cap, it appears to be the most used type of cap for a
given time period, be it paper or plastic film.

The Vitamin Q that I know of, and bought it six or seven years ago, were
plastic film with orange conformal coating. I do not remember, neither I can
find it now, what type of plastic was that.

Paper itself has an unfortunate property that every practical paper
production process makes the foil with pinholes. As paper dielectric
constant is higher than that of air, electric field is higher in those
pinholes and at right voltage there would be a breakdown in air, leading to
an avalanche that destroys the cap. Because of unavoidable pinholes,
dielectric for paper caps is made from several layers of paper, usually
three and up. The caps can be screened for this type of failure mode by
applying those 150% rated voltage, and I believe that is done by better
manufacturers.

Another failure mode, which is not exclusive province of paper, is breakdown
at resonance. All rolled caps have quite low resonant frequency because of
significant inductance, so resonant frequency is one to several megaherz. If
signal applied is around that frequency range, for that matter it can be
subharmonic or harmonic proper, current gets amplified by Q factor of
circuit and can cause damage. The cap's Q factor is 30 or so and thick foil
used for rolled cap does not spoil parasitic inductor's Q factor, so circuit
is quite resonant.

However, those two failure modes are apparent immediately, not a subject to
long term deterioration. The first one is most likely screened for by
(reputable) manufacturer and the second one is left to user to apply cap
wisely. If a cap is applied around resonant frequency, there would be a high
rate of failures quite quickly and problem becomes quite obvious.

My guess on the nasty failure mechanism of paper caps is water, rather,
water vapor penetration. In most cases, 'dry paper caps' are made from oil
impregnated paper, with electrodes either vapor deposited or rolled foil. At
one time, there were true dry paper caps, but they seem to have fallen out
of favor long time ago. In modern days, from 50s on, dry craft paper was
used for oil caps only and oil treated paper for all other. An oil cap was
rolled up with (very) dry paper and then oil impregnated in a vacuum
process. Whoever did not try to keep water vapors out of a containment
vessel might think that the job is trivial. As a gentleman pointed out to
me, water molecule is damn small and penetrates through most plastics that
are used for sealing packages, epoxy included. Plastics do just fine for six
months or so and then God help. It seems that only packaging materials that
would keep water out for good are glass and ceramic, but those are expensive
and fragile propositions.

At any rate, water that enters capacitor works on both, paper and oil. The
oil gets contaminated and becomes slightly conductive, while (undesired)
chemical residues in paper make conductive mess of their own. Remember, pure
paper is pure cellulose and water does not break it down, it takes other
chemicals to make a mess. So, under voltage, especially higher than around
15 V, those processes take on electrochemical effects and degradation speeds
up, leading to either exhausting available chemicals and ending with a leaky
cap or if there are enough chemicals to feed the process, cap will
completely deteriorate, i.e. would break down.

The fact that there are some old paper caps that still operate normally,
shows that paper does not degrade by itself, it needs some help. Paper used
for caps is probably the highest grade of paper manufactured, rinsed and
neutralized for umpteen times, but there must be some production runs, as
well as manufacturers, when not everything is done right. I suspect, those
production runs might produce caps where paper breaks by itself, but still,
that is not fault of paper, it is poor processing that gets to the cap.

Metalized paper caps are also known as 'self healing caps'. The metal foil
(deposited on paper by evaporation) is very thin and when cap breaks down
piece of foil around breakdown point evaporates and cap heals. Those caps
would have a reduced value as result of repeated failures. Self healing caps
are widely used in power factor correction and to start electric motors.
None of those applications would suffer too much if 50% or so of capacitance
is lost along the way.

Regards

Miroslav Pokorni

----- Original Message -----
From: "Mark Anton" <heightstv@hotmail.com>
To: <TekScopes@yahoogroups.com>
Sent: Monday, December 15, 2003 12:41 AM
Subject: [TekScopes] Vitamin Q capacitors (paper or plastic)


I am aware that paper and the black beauties were leaky and caused alot of
problems...and I have just replaced all of them in old radios i work on
too.
I have learned lesson hard way sometimes. In old radios and amplifiers
is
easy to have a cap short and upset the bias on output tubes and shorten
life
or destroy it.

Anyways in some equipment (including my hickok clone 310A) , there are
alot
of vitamin Q capacitors. These are in a silver metal can with a white
epoxy?....glass? seal on each end.
Are these paper inside too. I would suspect that moisture wouldnt get
inside....but wondering if part of problem is that paper breaks down over
time anyways....even if no moisture.
Kind of hoping these are plastic caps inside.....looks like date code on
whats in scope is 1962

Mark

_________________________________________________________________
Get holiday tips for festive fun.
http://special.msn.com/network/happyholidays.armx


Wavetek 188 Off Topic

regman10
 

I have a Wavetek 188 function generator and the -15V and -5V are non
existent in the power supply. Does anyone have a schematic?


R&S question answered

Craig Sawyers <c.sawyers@...>
 

Hi all

Thanks for the steer from a couple of listers, I have worked out what this
odd "Neumannzelle" is in my old sweeper. It is basically a cathode bias
battery. As such, it effectively supplies a bias voltage, but also has an
effectively extremely high "capacitance" - IOW an ac current hardly changes
the voltage at all.

This circuit needs to work down to 0.2Hz, so a grid bias arrangement would
need an extremely large bypass capacitor across the cathode resistor. Using
a cathode bias battery gets neatly around the problem. The alternative
approach of using either a zener or other diode (like an LED) doesn't work
here because of the very low cathode current of ~200uA - the effective ac
forward resistance of a diode at these low currents means that the
"capacitance" is too low.

I almost hate to mention this given the audio thread - but the tube audio
fraternity have gone across to this approach. Usually they use Ni metal
hydride or NiCd, chosen so that the nominal anode current is about 0.1 to
0.2 of the AH rating. In power amps this means a chunky battery. I guess
for low currents (like my application) Lithium manganese coin cells would be
a good choice, although the voltage is a touch high at 3V. And yes - there
are supposedly audible differences between different battery technologies.
What *has* been demonstrated is that there is a different distortion
signature (measured!) between battery cathode bias and bypassed resistor -
but only at high order harmonics.

I have also found that Neumann were the first company (in 1948) to make a
sealed NiCd battery. I have no idea if these little tubular beasts are
ancient NiCd's or not - they only hold a very small voltage (about 0.2V)
when the unit is switched off. But when operating, they both lock at 1.5V
and stay there. The currents flowing (deduced from the specified [and
measured] circuit volatges) are about 0.25mA - which is absolutely correct
for an EF86 with 180V on the anode and 60V on the screen grid.

Interestingly, the un-bypassed screen grid bias chain also runs into the
battery - supplying 1.5mA charging current, so maybe the 0.25mA anode
current isn't enough to get the cell charged quickly. And the cell will
discharge quite rapidly through resistor chains and bypass electrolytic
leakage. Hence I guess 0.2V measured after 24 hours of off time.

I still have my problem - the vertical gain is correct at switch on, but
after ten minutes or so it slowly drops to a few percent. But the rest of
the circuit is more or less conventional, so I should be able to nail the
problem now I understand what a Neumannzelle actually is!

Once I have fixed this beast I'll post some pics on the tekscopes pictures
section - it is at, or beyond, Tek construction standards. Even uses
ceramic strips in some sections.

Cheers

Craig


Re: tek 468 no trace

 

In message <bqt2if+phiu@eGroups.com>, sreaves22655 <sreaves@visuallink.com> writes
Hello All,


Anyway I hope that you have a manual for this scope for without it it can be a bear to fix.
But fixed it is a really nice and simple beast. I love mine.

Also good luck. Although you said that it had woken up after an overnight rest, I would check it through thoroughly. One thing that does occur to me is that you may have a loose connection somewhere.

Robin

Good luck,

Sam

W3OHM


--- In TekScopes@yahoogroups.com, "Gary Allsebrook" <regman10@c...> wrote:
ALL troubleshooting has to start with verifying the power supply voltages -
nothing will work right unless they are all present. It is suspicious that
both channels are affected. After power supply verifications, I would start
with circuitry common to both channels and work from there. It sounds like
a horizontal sweep problem.
-----Original Message-----
From: bjapundza [mailto:bjapundza@y...]
Sent: Thursday, December 04, 2003 11:22 AM
To: TekScopes@yahoogroups.com
Subject: [TekScopes] Re: tek 468 no trace


OK, I'll look at the input preamp board as a place to start... I
have my fingers crossed, as both channels aren't working...hopefully
my ignorance didn't completely toast my scope. The manual says to
start at the power supply when troubleshooting nonstorage problems,
I've been thinking that the preamp would be a better place to start
given what I did.

Regards, Bob

--- In TekScopes@yahoogroups.com, "Robert Morein" <morepub@c...>
wrote:
> There are.
> However, I fixed a 434 that had a blown input, a cracked resistor,
and a blown vertical amp -- and the clipping diodes were OK !
>
> The diodes are mounted in a small metal can that looks like a
three or four legged transistor.
> ----- Original Message -----
> From: bjapundza
> To: TekScopes@yahoogroups.com
> Sent: Thursday, December 04, 2003 1:19 PM
> Subject: [TekScopes] Re: tek 468 no trace
>
>
> Yes, I am wondering if there are any diodes in the input stage
that
> would have caught the spikes...hopefully that would be simple to
fix.
>
> Regards, Bob
>
> --- In TekScopes@yahoogroups.com, "Robert Morein" <morepub@c...>
> wrote:
> > What a tragic story!
> > I would never use a scope like that, simply because I have no
> confidence in the limits of inductive kickback. I keep a 310A
for
> that purpose.
> >
> > A 100X HV probe, like a 6139, might provide acceptable
protection.
> >
> > A while back, I asked the question as to how far back in the
> circuitry a surge on the input could cause damage. One person
stated
> the belief that it would be limited to the input stage, but I'm
not
> sure I believe that.
> >
> > In this case, it appears that the surge has caused extensive
> damage.
> >
> > ----- Original Message -----
> > From: bjapundza
> > To: TekScopes@yahoogroups.com
> > Sent: Thursday, December 04, 2003 11:40 AM
> > Subject: [TekScopes] tek 468 no trace
> >
> >
> > Hello All,
> >
> > I recently purchased a beautiful used Tek 468 DSO scope and
I'm
> > afraid I broke it.
> >
> > Last evening I was trying to capture the inductive kickback
on
> the
> > primary side of an ignition coil, somewhere between 400-
500v. I
> was
> > using a 10x probe. With my 475 scope I could see the
waveform
> and
> > never had a problem. When I did the same with my 468, about
10
> > seconds fiddling to trigger the trace disappeared. I shut
the
> scope
> > off for a few minutes, disconnected it from the target, and
> tried to
> > use it again. All I get is a rather large dot (about the
size
> of
> > one div) on the scope which I can move around. Won't
trigger or
> do
> > anything. When I turn on the storage mode, I can see the
> cursors
> > but the dashed lines are much thicker.
> >
> > The digital portion appears to pass the self test.
> >
> > It seems the problem is in the analog portion of the scope.
> >
> > I do have vol.1 of the service manual, but I don't have
vol.2.
> I
> > have opened the scope up, and when I have some time I am
going
> to
> > probe the power supplies.
> >
> > My questions are: what is the typical failure mode for
these
> scopes
> > when an overvoltage condition occurs at the probe? What
should
> I be
> > looking for? I am not well versed in component
troubleshooting,
> is
> > there anyone I can send the scope to for repairs that would
be
> > reasonable for a hobbiest, if I can't figure out the problem
> myself?
> >
> > Thanks in advance,
> > Bob
> >
> >
> > Yahoo! Groups Sponsor
> >
> >
> >
> > Your use of Yahoo! Groups is subject to the Yahoo! Terms of
> Service.
> >
> >
> >
>
>
> Yahoo! Groups Sponsor
> ADVERTISEMENT
>
>
>
>
> Your use of Yahoo! Groups is subject to the Yahoo! Terms of
Service.
>
>
>


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--
Robin Birch


Re: tek 468 no trace

Sam Reaves
 

Hello All,

One thing to keep in mind on the 468. All of the triggering goes thru some digital circuitry. There is a cable that connects from the
sweep circuitry and connects to the processor boards in the rear of the instrument. I can't find my manuals at the moment but while in
the process of repairing mine some year back I mistakenly left this cable disconnected. While troubleshooting a no trigger problem I
assumed that if I ran the scope in its analog only mode that all of the digital circuitry is excluded - wrong-. Without this cable
connected the scope won't trigger in any mode. My point here is if somehow something in the digital processing was damaged by the
zap the scope may not trigger. Just my two cents here.

Anyway I hope that you have a manual for this scope for without it it can be a bear to fix.

Good luck,

Sam

W3OHM

--- In TekScopes@yahoogroups.com, "Gary Allsebrook" <regman10@c...> wrote:
ALL troubleshooting has to start with verifying the power supply voltages -
nothing will work right unless they are all present. It is suspicious that
both channels are affected. After power supply verifications, I would start
with circuitry common to both channels and work from there. It sounds like
a horizontal sweep problem.
-----Original Message-----
From: bjapundza [mailto:bjapundza@y...]
Sent: Thursday, December 04, 2003 11:22 AM
To: TekScopes@yahoogroups.com
Subject: [TekScopes] Re: tek 468 no trace


OK, I'll look at the input preamp board as a place to start... I
have my fingers crossed, as both channels aren't working...hopefully
my ignorance didn't completely toast my scope. The manual says to
start at the power supply when troubleshooting nonstorage problems,
I've been thinking that the preamp would be a better place to start
given what I did.

Regards, Bob

--- In TekScopes@yahoogroups.com, "Robert Morein" <morepub@c...>
wrote:
> There are.
> However, I fixed a 434 that had a blown input, a cracked resistor,
and a blown vertical amp -- and the clipping diodes were OK !
>
> The diodes are mounted in a small metal can that looks like a
three or four legged transistor.
> ----- Original Message -----
> From: bjapundza
> To: TekScopes@yahoogroups.com
> Sent: Thursday, December 04, 2003 1:19 PM
> Subject: [TekScopes] Re: tek 468 no trace
>
>
> Yes, I am wondering if there are any diodes in the input stage
that
> would have caught the spikes...hopefully that would be simple to
fix.
>
> Regards, Bob
>
> --- In TekScopes@yahoogroups.com, "Robert Morein" <morepub@c...>
> wrote:
> > What a tragic story!
> > I would never use a scope like that, simply because I have no
> confidence in the limits of inductive kickback. I keep a 310A
for
> that purpose.
> >
> > A 100X HV probe, like a 6139, might provide acceptable
protection.
> >
> > A while back, I asked the question as to how far back in the
> circuitry a surge on the input could cause damage. One person
stated
> the belief that it would be limited to the input stage, but I'm
not
> sure I believe that.
> >
> > In this case, it appears that the surge has caused extensive
> damage.
> >
> > ----- Original Message -----
> > From: bjapundza
> > To: TekScopes@yahoogroups.com
> > Sent: Thursday, December 04, 2003 11:40 AM
> > Subject: [TekScopes] tek 468 no trace
> >
> >
> > Hello All,
> >
> > I recently purchased a beautiful used Tek 468 DSO scope and
I'm
> > afraid I broke it.
> >
> > Last evening I was trying to capture the inductive kickback
on
> the
> > primary side of an ignition coil, somewhere between 400-
500v. I
> was
> > using a 10x probe. With my 475 scope I could see the
waveform
> and
> > never had a problem. When I did the same with my 468, about
10
> > seconds fiddling to trigger the trace disappeared. I shut
the
> scope
> > off for a few minutes, disconnected it from the target, and
> tried to
> > use it again. All I get is a rather large dot (about the
size
> of
> > one div) on the scope which I can move around. Won't
trigger or
> do
> > anything. When I turn on the storage mode, I can see the
> cursors
> > but the dashed lines are much thicker.
> >
> > The digital portion appears to pass the self test.
> >
> > It seems the problem is in the analog portion of the scope.
> >
> > I do have vol.1 of the service manual, but I don't have
vol.2.
> I
> > have opened the scope up, and when I have some time I am
going
> to
> > probe the power supplies.
> >
> > My questions are: what is the typical failure mode for
these
> scopes
> > when an overvoltage condition occurs at the probe? What
should
> I be
> > looking for? I am not well versed in component
troubleshooting,
> is
> > there anyone I can send the scope to for repairs that would
be
> > reasonable for a hobbiest, if I can't figure out the problem
> myself?
> >
> > Thanks in advance,
> > Bob
> >
> >
> > Yahoo! Groups Sponsor
> >
> >
> >
> > Your use of Yahoo! Groups is subject to the Yahoo! Terms of
> Service.
> >
> >
> >
>
>
> Yahoo! Groups Sponsor
> ADVERTISEMENT
>
>
>
>
> Your use of Yahoo! Groups is subject to the Yahoo! Terms of
Service.
>
>
>


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ADVERTISEMENT




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Re: Falling brightness, and still slogging through the Tek 310

stan mcintosh <mcintosh@...>
 

The 310 HV transformer is not potted, but I am getting the same problem. I looked up the referenced message (5293?) on falling brightness. Any chance this is merely one or both of the rectifiers gone gassy? How best to tell the difference? The transformer isn't potted, but I'd rather not try rewinding.

On the 310, I had some minor troubles getting through the vertical amplifier chain. The 6CL6's weren't amplifying. After some checking, the 5.6k bias dropping resistor had drifted slightly out of spec to 8.3k (grin). Vertical amps are now FB. My HV section is probably next on the agenda. There does not seem to be a loss in oscillator voltage to correspond with a loss in brightness.

Also, what about power supply capacitor 'upgrades?' I mean in terms of disc cap bypassing the electrolytics. Sure, I know that you want to stay close to the original value for filter caps, but designs through the '70's often contained no disc bypass caps parallel to electrolytics. We now know that electrolytics are not as good as discs for bypassing high frequency noise. Should I go on and disc-bypass electrolytics the way I normally do? Well, I have to admit that I already did in the power supply.

When the brightness is right, the trace is beautiful. Square waves have perfectly flat tops, and the trace is sharpened-pencil thin.

Thanks.

stan


Falling brightness

elie_shvarzman
 

Dear Mr. Aberto
to know if its H.V transformer problem note to the size of the display and the
brightness, if as long as the display loosing intensity the display size
increase (both vertical and horizontal) and during time the intensity keep
reducing and the size incresing its 99% a H.V transformer problem, but if the
size remain the same even from the start its looking like a CRT bias or
unblanking circuit problem.
Elie


Another 485 Question

w1ksz <w1ksz@...>
 

I too like the 485, and it makes a nice addition to the lineup I
have (7104, 7704A, 7603, 475A, 485).
But the 485 has a small problem. When I turn it on cold the trace
drifts slowly to the right (about 1 cm) and stabilizes there. I can
move it to the left with the position knob, but that needs to be
fixed.
Any thoughts on where to start ? I checked the Power Supply voltages
and they do not drift. Maybe one of the Horizontal Xstrs changes ?
Short of replacing them one by one, any thoughts ?

Thanks, Dick, w1KSZ


Re: Help needed for Tektronix 2246, and with the Blackouts (OT)

ghpicard <ghpicard@...>
 

What I think may be the reason is:
There are two basic types of Electrical Power Plants, seen from a
demand point of view (and from 10.000 feet altitude):

1 - Base Power Plants, that are designed to provide power in a
continuous way, all day long during all year. This is the kind of
power plant that have a long power-on delay. Examples are some
hydroelectrical plants, nuclear plants (putting one of this online
takes months), and some thermal ones.

2 - Peak Power Plants, specifically designed to come online to cover
peak power demands, like the early night, when all domestic lighting
comes in, office lights are still on, plus heating or air conditioning
is fired up at home, etc. This ones come online quite quickly, and
many times use internal combustion engines to drive the generators. Or
used to be so... now a lot of this ones use mixed cycle engines that
use natural gas as fuel...

Most probably, during the oil crisis, the Base Power Plants were kept
on as they weren't powered by oil derivatives (some of them are, but
also accept a mixture of powdered charcoal and air).
What left the Peak Power Plants offline because of an economic
decision. So the base centrals had to cope with the augmented load of
the peak hours, and then the network reacted as a nice overloaded
power supply... The line voltage dropped down and when the current was
too much for the interruptors, cables, and all installations being
able to handle it safely... hehe... blackouts...

Yes... I know it is OT, but at least I snipped out the unnecesary
parts, and I'm not beating on the audiophiles... :-)

Regards
Gaston

--- In TekScopes@yahoogroups.com, "Craig Sawyers" <c.sawyers@t...> wrote:
Sorry to say this, but almost all electric equipment (even
lightbulbs) works at the same level of *power* comsumption, regardless
of voltage level. At least, if we talk of +/- 20% variations.
Darn. I've been trying to think of some major exceptions to your
statement - and mechanisms I can think of are second order effects, or
consume relatively little power. Perhaps the biggest exception is
kettles -
the elements do not run hot enough to give the same effect as a
light bulb.
But then you have to leave them switched on longer to boil the water
- so
the same *energy* is delivered - and your argument still holds up.

So what *was* the reason behind browning out the UK on a frequent basis
during the oil crisis? Or maybe it was a second order effect as a
result of
mothballing power generation to save oil and coal - so when demand
peaked,
the pruned-down generation system couldn't cope.


Re: 485 Volts/Div switch maintenance

John Miles <jmiles@...>
 

The 485 is my all-time favorite Tek scope for nostalgic reasons; I
started my career using one in 1981. It was the pride and joy of the
owner of the small engineering company I worked for. He paid like 9
grand for it, and entrusting me to use it was an significant event.
(I down to owning only 2 of them at the moment)
It's probably my all-time favorite, too. The 485 has the preternatural
triggering and control precision of the 453/454 and a stable,
well-engineered front end like the later scopes, yet it lacks the
microcontrolled bells and whistles that get between the scope and the
operator. The difference between using the 485 and the 2465/7 is the
difference between driving a Porsche and a Cadillac with a broken leaf
spring.

Unfortunately, the vertical attenuators on the 485 make it just too annoying
to live with, in my opinion. (This is actually the first time I've ever
heard of someone fixing them successfully. I could never even figure out
how to take mine apart with any confidence.)

-- john KE5FX


Re: Falling brightness

benj3867
 

Hi,

When I switch on my 475 and display a waveform, I adjust the
brightness for a normal viewing level. If I leave the room and return
after 15 minutes, the trace is barely visible. I can touch the knob
and restore it to a viewable level, but I am wondering if this is just
a sign of bad things to come in a near future.
Or is it normal for a 475 to lose some brightness during initial warm
up ?
This sounds like the famous high voltage transformer potting problem
but you will have to do some measurements to be sure. You should read
message 5293 on the forum which describes a problem a friend of mine
had with his 545B (the 547 has the same HV Transformer). Follow the
answers to his message (messages 5294, 5297, 5303 and 5328) or just do
a search for "545B looses picture after 1/2 hour" (yes, with the
spelling mistake).

Good Luck,
Benjamin


Re: 485 Volts/Div switch maintenance

Jeff W <vwthingy@...>
 

Royce wrote:
Are any of these areas supposed to be
lubed?
No, I don't believe they should be lubed. In fact, there is quite a
debate about using DeOxit or other residual type contact cleaners on
these gold plated leaf-type switch contacts in the attenuator
sections. Tek designed many of these gold leaf contacts with very
little wiping action for longer life. This makes them prone to noisy
action in the presence of dirt & dust. Cleaners like DeOxitleave a
residual film which attracts dirt and dust, which then makes the
contact intermittent again in short order, because there is little
wiping action. (Believe me, I am a big fan of DeOxit and use it all
the time on pots and switches, but won't use it on these contacts)

I also have a question about the trace I'm seeing when checking
probe compensation.....snip ... At first I thought Tek didn't show
the
vertical lines in the handbook illustration in order to clarify it,
but I'm not sure. If I turn up the trace intensity, what would be
the right vertical edge of a square wave does show up, but not the
left edge. Is this a problem?
No, it is not a problem, it is a feature! (I'm now in Marketing!)
Jokes aside, this is typical of a high bandwidth scope (ie, it is a
good thing, not a problem). Low repetition rate square waves with
fast rise and fall times will appear as sets of dashes, due to the
fast writing speed of the scope. For probe compensation it obviously
doesn't matter anyway; you are evening out the tops of the "dashes".
Simply increase the sweep speed if you really want to see that left
edge! Or if you want more fun, get a tunnel diode pulser and really
put your scope thru the paces!

The 485 is my all-time favorite Tek scope for nostalgic reasons; I
started my career using one in 1981. It was the pride and joy of the
owner of the small engineering company I worked for. He paid like 9
grand for it, and entrusting me to use it was an significant event.
(I down to owning only 2 of them at the moment)

Jeff


Falling brightness

Alberto I2PHD
 

When I switch on my 475 and display a waveform, I adjust the
brightness for a normal viewing level. If I leave the room and return
after 15 minutes, the trace is barely visible. I can touch the knob
and restore it to a viewable level, but I am wondering if this is just
a sign of bad things to come in a near future.
Or is it normal for a 475 to lose some brightness during initial warm
up ?

TIA
Aberto


Tektronix Plug Ins

Richard Oglebay <richardoglebay@...>
 

Hello: To anyone out there, I am still looking for 1l5 plug in for the 500 series Tektronix scopes. this is a spectrum analyzer plug in and low freq. type , if you have one forsale please contact me, Thank You , richardoglebay@yahoo.com





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Rohde & Schwarz question

Craig Sawyers <c.sawyers@...>
 

Hi all

This is almost off-Tek topic ('cept that Tek own R&S now) - but I'm hoping
that someone will be able to answer a question. I'm fixing up an R&S
Polyskop SWOB II - this is basically a sweeper plus TV display. In spite of
being tubed (34 of them) it does 500kHz to 1.2GHz in 6 bands - which is
pretty darned good.

Problem is in the vertical amp, which has very low sensitivity. I haven't
done too much quantitative diagnostics yet, but there is a component that is
baffling me. In the cathode circuit of the input tube there is what the
manual describes as a "Neumannzelle", and the parts list as a
"Stabilisationzelle". The manual adds "A voltage stabiliser cell
("Neumannzelle") is connected as a cathode resistor to provide the bias
voltage for the first stage. This cell has an equivalent capacitance of
40,000uF."

It is a wire-ended small metal tubular component about the size of a 2W
carbon resistor, with a + on one end.

Anyone got any ideas?

Craig


485 Volts/Div switch maintenance

rollsroyce1_92394
 

Hi. I just bought a Tek 485 scope on eBay which is in remarkable
shape (very clean, front cover , owner's handbook, and accessory
pouch included, etc.). It starts up fine and produces a good bright
trace. The volts/div switches on both channels were quite noisy so I
removed the switch assemblies. I then removed the upper and lower
covers and the smaller circuit board from each switch. I cleaned all
the contacts on both boards with isopropyl alcohol (recommended by
the 485 maintenance manual) and a small camel-hair brush. I noticed
that the entire length of the camshaft-for lack of a better word--on
both switches seemed to be dry and not lubricated. This included the
detent teeth and plungers. Are any of these areas supposed to be
lubed? I started searching the manual but so far cannot find any
reference to this. I'm purchasing some DeOxit and intend to go back
in there when it arrives to treat the contacts and that would be an
ideal time to also do any required lubrication.

I also have a question about the trace I'm seeing when checking
probe compensation. Most how-to's on probe compensation describe a
complete square wave seen on the scope during the procedure. The
owner's handbook for the 485 shows only two vertically separated and
horizontally offset lines of dashes, like a square wave with the
vertical portions of the signal not drawn. This is exactly what I
see for both channels. At first I thought Tek didn't show the
vertical lines in the handbook illustration in order to clarify it,
but I'm not sure. If I turn up the trace intensity, what would be
the right vertical edge of a square wave does show up, but not the
left edge. Is this a problem?

Thanks for all your help.

Royce


Re: tek 468 no trace

Miroslav Pokorni
 

No, Bob, you must have meant P6009. Not only that P6139 is X10, but it has
that tiny hibrid that would go up in flames. Two probes of old generation
that might do are P6007 and P6009, first one good to 25 MHz and the second
up to 120 MHz, both rated to 1.5 kV. Until recently, few years ago,
Tektronix also offered P5100, which was good to 2.5 kV up to 250 MHz and
last price that I saw in Tektronix catalog was $200. The load is a bit low
on that probe, only 10 MOhm.

Regards

Miroslav Pokorni

----- Original Message -----
From: "Robert Morein" <morepub@comcast.net>
To: "TekScopes" <TekScopes@yahoogroups.com>
Sent: Thursday, December 04, 2003 10:06 AM
Subject: Re: [TekScopes] tek 468 no trace


What a tragic story!
I would never use a scope like that, simply because I have no confidence
in the limits of inductive kickback. I keep a 310A for that purpose.

A 100X HV probe, like a 6139, might provide acceptable protection.

A while back, I asked the question as to how far back in the circuitry a
surge on the input could cause damage. One person stated the belief that it
would be limited to the input stage, but I'm not sure I believe that.

In this case, it appears that the surge has caused extensive damage.

----- Original Message -----
From: bjapundza
To: TekScopes@yahoogroups.com
Sent: Thursday, December 04, 2003 11:40 AM
Subject: [TekScopes] tek 468 no trace


Hello All,

I recently purchased a beautiful used Tek 468 DSO scope and I'm
afraid I broke it.

Last evening I was trying to capture the inductive kickback on the
primary side of an ignition coil, somewhere between 400-500v. I was
using a 10x probe. With my 475 scope I could see the waveform and
never had a problem. When I did the same with my 468, about 10
seconds fiddling to trigger the trace disappeared. I shut the scope
off for a few minutes, disconnected it from the target, and tried to
use it again. All I get is a rather large dot (about the size of
one div) on the scope which I can move around. Won't trigger or do
anything. When I turn on the storage mode, I can see the cursors
but the dashed lines are much thicker.

The digital portion appears to pass the self test.

It seems the problem is in the analog portion of the scope.

I do have vol.1 of the service manual, but I don't have vol.2. I
have opened the scope up, and when I have some time I am going to
probe the power supplies.

My questions are: what is the typical failure mode for these scopes
when an overvoltage condition occurs at the probe? What should I be
looking for? I am not well versed in component troubleshooting, is
there anyone I can send the scope to for repairs that would be
reasonable for a hobbiest, if I can't figure out the problem myself?

Thanks in advance,
Bob


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Re: tek 468 no trace

robin.birch@...
 

Do you get a vertical signal if you put some input in. If you do then it
is probably horizontal swseep and trigger, else the input amps are sick as
well.

Robin




"bjapundza"
<bjapundza@yahoo To: TekScopes@yahoogroups.com
.com> cc:
Subject: [TekScopes] Re: tek 468 no trace
04/12/2003 21:12






Gary,

Good point. I am able to control the position of the spot
vertically on the crt. And horizontally for the matter too. It
won't trigger though.

Regards, Bob

--- In TekScopes@yahoogroups.com, "Gary Allsebrook" <regman10@c...>
wrote:
ALL troubleshooting has to start with verifying the power supply
voltages -
nothing will work right unless they are all present. It is
suspicious that
both channels are affected. After power supply verifications, I
would start
with circuitry common to both channels and work from there. It
sounds like
a horizontal sweep problem.
-----Original Message-----
From: bjapundza [mailto:bjapundza@y...]
Sent: Thursday, December 04, 2003 11:22 AM
To: TekScopes@yahoogroups.com
Subject: [TekScopes] Re: tek 468 no trace


OK, I'll look at the input preamp board as a place to start... I
have my fingers crossed, as both channels aren't
working...hopefully
my ignorance didn't completely toast my scope. The manual says
to
start at the power supply when troubleshooting nonstorage
problems,
I've been thinking that the preamp would be a better place to
start
given what I did.

Regards, Bob

--- In TekScopes@yahoogroups.com, "Robert Morein" <morepub@c...>
wrote:
> There are.
> However, I fixed a 434 that had a blown input, a cracked
resistor,
and a blown vertical amp -- and the clipping diodes were OK !
>
> The diodes are mounted in a small metal can that looks like a
three or four legged transistor.
> ----- Original Message -----
> From: bjapundza
> To: TekScopes@yahoogroups.com
> Sent: Thursday, December 04, 2003 1:19 PM
> Subject: [TekScopes] Re: tek 468 no trace
>
>
> Yes, I am wondering if there are any diodes in the input
stage
that
> would have caught the spikes...hopefully that would be
simple to
fix.
>
> Regards, Bob
>
> --- In TekScopes@yahoogroups.com, "Robert Morein"
<morepub@c...>
> wrote:
> > What a tragic story!
> > I would never use a scope like that, simply because I have
no
> confidence in the limits of inductive kickback. I keep a 310A
for
> that purpose.
> >
> > A 100X HV probe, like a 6139, might provide acceptable
protection.
> >
> > A while back, I asked the question as to how far back in
the
> circuitry a surge on the input could cause damage. One person
stated
> the belief that it would be limited to the input stage, but
I'm
not
> sure I believe that.
> >
> > In this case, it appears that the surge has caused
extensive
> damage.
> >
> > ----- Original Message -----
> > From: bjapundza
> > To: TekScopes@yahoogroups.com
> > Sent: Thursday, December 04, 2003 11:40 AM
> > Subject: [TekScopes] tek 468 no trace
> >
> >
> > Hello All,
> >
> > I recently purchased a beautiful used Tek 468 DSO scope
and
I'm
> > afraid I broke it.
> >
> > Last evening I was trying to capture the inductive
kickback
on
> the
> > primary side of an ignition coil, somewhere between 400-
500v. I
> was
> > using a 10x probe. With my 475 scope I could see the
waveform
> and
> > never had a problem. When I did the same with my 468,
about
10
> > seconds fiddling to trigger the trace disappeared. I
shut
the
> scope
> > off for a few minutes, disconnected it from the target,
and
> tried to
> > use it again. All I get is a rather large dot (about the
size
> of
> > one div) on the scope which I can move around. Won't
trigger or
> do
> > anything. When I turn on the storage mode, I can see the
> cursors
> > but the dashed lines are much thicker.
> >
> > The digital portion appears to pass the self test.
> >
> > It seems the problem is in the analog portion of the
scope.
> >
> > I do have vol.1 of the service manual, but I don't have
vol.2.
> I
> > have opened the scope up, and when I have some time I am
going
> to
> > probe the power supplies.
> >
> > My questions are: what is the typical failure mode for
these
> scopes
> > when an overvoltage condition occurs at the probe? What
should
> I be
> > looking for? I am not well versed in component
troubleshooting,
> is
> > there anyone I can send the scope to for repairs that
would
be
> > reasonable for a hobbiest, if I can't figure out the
problem
> myself?
> >
> > Thanks in advance,
> > Bob
> >
> >
> > Yahoo! Groups Sponsor
> >
> >
> >
> > Your use of Yahoo! Groups is subject to the Yahoo! Terms
of
> Service.
> >
> >
> >
>
>
> Yahoo! Groups Sponsor
> ADVERTISEMENT
>
>
>
>
> Your use of Yahoo! Groups is subject to the Yahoo! Terms of
Service.
>
>
>


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Re: tek 468 no trace

robin.birch@...
 

Hi,
I don't know the failure modes but somewhere I have got a copy of Vol 2.
If you get stuck I can probably dig it out over the weekend.

Robin

**********************************************************************
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If you have received this in error, please contact the sender and then
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