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tek 7xx4 series brightness dimming after power on


Alberto, IZ2EWV
 

Hello dear friends,
I own three beautiful oscilloscopes from the series 7000: a 7904A, a 7104 and - the most recent arrival - a 7854.
Some of them had common issues, after powering on (failure at the RC snubber and the AC line filter - which I replaced with new components and everything come back to normality.
I ask you about a behavior that is common to the 7904A and the 7854: at power on, the traces and the readouts are bright, but after some minutes they dim: to restore the brightness it’s a matter of little correction on the related controls.
By the way, I ask you if it is a normal situation (e.g. “normal” at the time that the equipment was younger, by memory of somebody of yours) or if it is a sign of aging of some component in the main power supply or in the HV power supply. In this latter option, I ask you advice to let me get closer to the problem (I have the service manuals).

Tnx es 73 de Alberto, IZ2EWV


Tom Bowers
 

Hello,

I have a 7904A and it works exactly as you describe. I consider that
normal, nothing to worry about. Just a minor annoyance.

Tom Bowers

On Mon, Jan 6, 2020 at 8:29 AM Alberto, IZ2EWV <iz2ewv@...> wrote:

Hello dear friends,
I own three beautiful oscilloscopes from the series 7000: a 7904A, a 7104
and - the most recent arrival - a 7854.
Some of them had common issues, after powering on (failure at the RC
snubber and the AC line filter - which I replaced with new components and
everything come back to normality.
I ask you about a behavior that is common to the 7904A and the 7854: at
power on, the traces and the readouts are bright, but after some minutes
they dim: to restore the brightness it’s a matter of little correction on
the related controls.
By the way, I ask you if it is a normal situation (e.g. “normal” at the
time that the equipment was younger, by memory of somebody of yours) or if
it is a sign of aging of some component in the main power supply or in the
HV power supply. In this latter option, I ask you advice to let me get
closer to the problem (I have the service manuals).

Tnx es 73 de Alberto, IZ2EWV




Eric
 

The 7904A has a screen protect - anti burn circuit. Mine functions exactly
the same as well. The service manual calls this out though I forget which
page it is on.

On Mon, Jan 6, 2020, 10:33 AM Tom Bowers <pvhengineering@...> wrote:

Hello,

I have a 7904A and it works exactly as you describe. I consider that
normal, nothing to worry about. Just a minor annoyance.

Tom Bowers


On Mon, Jan 6, 2020 at 8:29 AM Alberto, IZ2EWV <iz2ewv@...> wrote:

Hello dear friends,
I own three beautiful oscilloscopes from the series 7000: a 7904A, a 7104
and - the most recent arrival - a 7854.
Some of them had common issues, after powering on (failure at the RC
snubber and the AC line filter - which I replaced with new components and
everything come back to normality.
I ask you about a behavior that is common to the 7904A and the 7854: at
power on, the traces and the readouts are bright, but after some minutes
they dim: to restore the brightness it’s a matter of little correction on
the related controls.
By the way, I ask you if it is a normal situation (e.g. “normal” at the
time that the equipment was younger, by memory of somebody of yours) or
if
it is a sign of aging of some component in the main power supply or in
the
HV power supply. In this latter option, I ask you advice to let me get
closer to the problem (I have the service manuals).

Tnx es 73 de Alberto, IZ2EWV






Chuck Harris
 

There is a 5uf, wet tantalum capacitor, C171, in the z-axis output amplifier
that sometimes degrades to a condition between open and shorted. It has direct
control over the intensity.

Also everything down stream of that capacitor is the very high impedance DC
restorer circuitry. Any resistor 1M or higher is a reliability problem... as
is any capacitor that it services.

-Chuck Harris

Alberto, IZ2EWV wrote:

Hello dear friends,
I own three beautiful oscilloscopes from the series 7000: a 7904A, a 7104 and - the most recent arrival - a 7854.
Some of them had common issues, after powering on (failure at the RC snubber and the AC line filter - which I replaced with new components and everything come back to normality.
I ask you about a behavior that is common to the 7904A and the 7854: at power on, the traces and the readouts are bright, but after some minutes they dim: to restore the brightness it’s a matter of little correction on the related controls.
By the way, I ask you if it is a normal situation (e.g. “normal” at the time that the equipment was younger, by memory of somebody of yours) or if it is a sign of aging of some component in the main power supply or in the HV power supply. In this latter option, I ask you advice to let me get closer to the problem (I have the service manuals).

Tnx es 73 de Alberto, IZ2EWV




 

On Mon, Jan 6, 2020 at 04:40 PM, Eric wrote:


The 7904A has a screen protect - anti burn circuit. Mine functions exactly
the same as well. The service manual calls this out though I forget which
page it is on.
In my service manual (070-4593-00), that's on page 2-9, Ïntensity Controls", second paragraph.

Raymond


Alberto, IZ2EWV
 

Tnx all!
Your answers were very reassuring! I will take a deeper look at the service
manual.

73 de Alberto, IZ2EWV

Il giorno lunedì 6 gennaio 2020, Raymond Domp Frank <@Raymond> ha
scritto:

On Mon, Jan 6, 2020 at 04:40 PM, Eric wrote:


The 7904A has a screen protect - anti burn circuit. Mine functions
exactly
the same as well. The service manual calls this out though I forget which
page it is on.
In my service manual (070-4593-00), that's on page 2-9, Ïntensity
Controls", second paragraph.

Raymond




Jean-Paul
 

Hello Alberto: Wonderful to have these vintage scopes, in perfect operation and on the bench!

Just checked, my 7904 (NOT A) has no variation in intensity after turn on, and the 7104 microchannel intensifier has a ferocious beam intensity timeout/limiter, I believe the 7104 time out is inversely proportioned to beam current. The more modern 2467/B also have the timeout but in a more advanced implimentation.

Chuck and Dennis, any comments?

Jon


Alberto, IZ2EWV
 

Tnx Jean-Paul!

Your bench seems to be well equipped, too! I love these plug-in
oscilloscopes, which I dreamt to play with when I was attending
high-school! Having space enough, nowadays they are a must in our benches
and... they have nothing to compare with the present instruments (not to
take anything away from my R&S HMO 3034...).

I cannot tell you about brightness stability of my 7104, because I got it
recently (it has been laying abandoned for at least 10 years in the lab
room of my ham radio club, and finally I decided to make an offer and give
it one chance again!). It was tested there, but for short time and I'm
pretty sure (for the adage: good things come in threes) that when I will
keep it powered on for some minutes - respectfully for the microchannel CRT
- that the filtered AC socket or the RC snubber or something similar in the
switching PSU will give news of them!

73 de Alberto, IZ2EWV

Il giorno martedì 7 gennaio 2020, Jean-Paul <jonpaul@...> ha
scritto:

Hello Alberto: Wonderful to have these vintage scopes, in perfect
operation and on the bench!

Just checked, my 7904 (NOT A) has no variation in intensity after turn on,
and the 7104 microchannel intensifier has a ferocious beam intensity
timeout/limiter, I believe the 7104 time out is inversely proportioned to
beam current. The more modern 2467/B also have the timeout but in a more
advanced implimentation.

Chuck and Dennis, any comments?

Jon