Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input frequency
increases. At 200MHz, the vertical amplitude is about half of what it
should be. CH2 is working fine. The fault seems to exist no matter what
the vertical sensitivity is so I don't think that it's the extenuator, and
the 100 and 20MHz bandwidth limiter also appear to be working. Does this
fault suggest anything in particular to the experts out there?

Thanks!

On Wed, 20 Feb 2008 05:46:44 -0500 JW wrote in Message id:
:

extenuator

Ahem.
Attenuator

Spell checkers...

JW wrote in
:

Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input frequency
increases. At 200MHz, the vertical amplitude is about half of what it
should be. CH2 is working fine. The fault seems to exist no matter
what the vertical sensitivity is so I don't think that it's the
extenuator, and the 100 and 20MHz bandwidth limiter also appear to be
working. Does this fault suggest anything in particular to the experts
out there?

Thanks!

If the problem is just on Ch1,then you have a problem in Channel 1.
Ch 1 and 2 don't combine until the delay line driver/channel switch,IIRC.

the tiny HF trimmer caps in the 400 series had a habit of developing a
black oxide underneath them that effectively insulates the rotor from the
solder pin,making the cap -open-.(and no effect when adjusted)
They also would freeze so they could not be turned without breaking the
screwhead loose from the rotor disc .

essentially,you have to replace the bad trimcaps.
Then recal per the cal procedure.

--
Jim Yanik
jyanik
at
kua.net

On Feb 20, 10:01*am, Jim Yanik wrote:
JW wrote :

Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input frequency
increases. At 200MHz, the vertical amplitude is about half of what it
should be. CH2 is working fine. The fault seems to exist no matter
what the vertical sensitivity is so I don't think that it's the
extenuator, and the 100 and 20MHz bandwidth limiter also appear to be
working. Does this fault suggest anything in particular to the experts
out there?

Thanks!

If the problem is just on Ch1,then you have a problem in Channel 1.
Ch 1 and 2 don't combine until the delay line driver/channel switch,IIRC.

the tiny HF trimmer caps in the 400 series had a habit of developing a
black oxide underneath them that effectively insulates the rotor from the
solder pin,making the cap -open-.(and no effect when adjusted)
They also would freeze so they could not be turned without breaking the
screwhead loose from the rotor disc .

essentially,you have to replace the bad trimcaps.
Then recal per the cal procedure.

--
Jim Yanik
jyanik
at
kua.net

Have you swapped probes??

"hr(bob) " wrote in
:

On Feb 20, 10:01*am, Jim Yanik wrote:
JW wrote

:

Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input
frequency increases. At 200MHz, the vertical amplitude is about
half of what it should be. CH2 is working fine. The fault seems to
exist no matter what the vertical sensitivity is so I don't think
that it's the extenuator, and the 100 and 20MHz bandwidth limiter
also appear to be working. Does this fault suggest anything in
particular to the experts out there?

Thanks!

If the problem is just on Ch1,then you have a problem in Channel 1.
Ch 1 and 2 don't combine until the delay line driver/channel
switch,IIRC.

the tiny HF trimmer caps in the 400 series had a habit of developing
a black oxide underneath them that effectively insulates the rotor
from the solder pin,making the cap -open-.(and no effect when
adjusted) They also would freeze so they could not be turned without
breaking the screwhead loose from the rotor disc .

essentially,you have to replace the bad trimcaps.
Then recal per the cal procedure.

--
Jim Yanik
jyanik
at
kua.net

Have you swapped probes??


you shouldn't be testing/calibrating BW using probes.
you use a levelled signal generator,50ohm coax,a 50 Ohm feedthru
termination,and appropriate 50ohm attenuators.

--
Jim Yanik
jyanik
at
kua.net

On 20 Feb 2008 16:01:59 GMT Jim Yanik wrote in Message
id: :

JW wrote in
:

Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input frequency
increases. At 200MHz, the vertical amplitude is about half of what it
should be. CH2 is working fine. The fault seems to exist no matter
what the vertical sensitivity is so I don't think that it's the
extenuator, and the 100 and 20MHz bandwidth limiter also appear to be
working. Does this fault suggest anything in particular to the experts
out there?

Thanks!

If the problem is just on Ch1,then you have a problem in Channel 1.
Ch 1 and 2 don't combine until the delay line driver/channel switch,IIRC.

the tiny HF trimmer caps in the 400 series had a habit of developing a
black oxide underneath them that effectively insulates the rotor from the
solder pin,making the cap -open-.(and no effect when adjusted)
They also would freeze so they could not be turned without breaking the
screwhead loose from the rotor disc .

essentially,you have to replace the bad trimcaps.
Then recal per the cal procedure.

Thanks, Jim. Given the age of this scope and lack of a schematic/service
manual, I think I'll just scrap it.

On 21 Feb 2008 02:47:02 GMT Jim Yanik wrote in Message
id: :

"hr(bob) " wrote in
:

[...]


Have you swapped probes??


you shouldn't be testing/calibrating BW using probes.
you use a levelled signal generator,50ohm coax,a 50 Ohm feedthru
termination,and appropriate 50ohm attenuators.

Yup.

JW wrote in
:

On 20 Feb 2008 16:01:59 GMT Jim Yanik wrote in
Message id: :

JW wrote in
m:

Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input
frequency increases. At 200MHz, the vertical amplitude is about half
of what it should be. CH2 is working fine. The fault seems to exist
no matter what the vertical sensitivity is so I don't think that
it's the extenuator, and the 100 and 20MHz bandwidth limiter also
appear to be working. Does this fault suggest anything in particular
to the experts out there?

Thanks!

If the problem is just on Ch1,then you have a problem in Channel 1.
Ch 1 and 2 don't combine until the delay line driver/channel
switch,IIRC.

the tiny HF trimmer caps in the 400 series had a habit of developing a
black oxide underneath them that effectively insulates the rotor from
the solder pin,making the cap -open-.(and no effect when adjusted)
They also would freeze so they could not be turned without breaking
the screwhead loose from the rotor disc .

essentially,you have to replace the bad trimcaps.
Then recal per the cal procedure.

Thanks, Jim. Given the age of this scope and lack of a
schematic/service manual, I think I'll just scrap it.


doesn't the BAMA site have any 475 manuals?
475's were good scopes;I doubt you'll find any new scopes as good.

--
Jim Yanik
jyanik
at
kua.net

JW wrote:

snip

Thanks, Jim. Given the age of this scope and lack of a schematic/service
manual, I think I'll just scrap it.

Where are you located? I'd love to take a shot at it as my only modern
portable as an ailing B&K 20 MHz scope.

Regards,

Michael

On 21 Feb 2008 14:11:35 GMT Jim Yanik wrote in Message
id: :

JW wrote in
:

On 20 Feb 2008 16:01:59 GMT Jim Yanik wrote in
Message id: :

JW wrote in
:

Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input
frequency increases. At 200MHz, the vertical amplitude is about half
of what it should be. CH2 is working fine. The fault seems to exist
no matter what the vertical sensitivity is so I don't think that
it's the extenuator, and the 100 and 20MHz bandwidth limiter also
appear to be working. Does this fault suggest anything in particular
to the experts out there?

Thanks!

If the problem is just on Ch1,then you have a problem in Channel 1.
Ch 1 and 2 don't combine until the delay line driver/channel
switch,IIRC.

the tiny HF trimmer caps in the 400 series had a habit of developing a
black oxide underneath them that effectively insulates the rotor from
the solder pin,making the cap -open-.(and no effect when adjusted)
They also would freeze so they could not be turned without breaking
the screwhead loose from the rotor disc .

essentially,you have to replace the bad trimcaps.
Then recal per the cal procedure.

Thanks, Jim. Given the age of this scope and lack of a
schematic/service manual, I think I'll just scrap it.


doesn't the BAMA site have any 475 manuals?

Yeah, but only operators...

475's were good scopes;I doubt you'll find any new scopes as good.

I'm sure that I can use parts from this one to fix the next one to come
across my bench. I work as a tech for a test equipment salvage company,
and I'd have to be able to justify my time and cost of calibration for
this to be worthwhile. Not having the service manual (or having to pay for
one) on a scope of this vintage just isn't worth it.

On Thu, 21 Feb 2008 11:08:00 -0600 msg wrote in
Message id: :

JW wrote:

snip

Thanks, Jim. Given the age of this scope and lack of a schematic/service
manual, I think I'll just scrap it.

Where are you located? I'd love to take a shot at it as my only modern
portable as an ailing B&K 20 MHz scope.

Well, as I said in my other post, this will be used as a parts unit. (It's
not my scope.)