Tektronix 465B problem

[el Casa Bertrand]

i recently purchased a dead Tek 465b scope (no lights except for power and
uncal, not even scale illumination) and opened it up to find no voltages
near spec at the test points on the main board. resistance checks showed
the +55v test point near ground. disconnecting the connector to the board
next to the crt (670-6385-?) allowed all voltages to come up. reconnecting
this board and disconnecting the coiled timing delay line that comes in from
the vertical preamp board to this board also allows +55v to come up. I
thought the delay line might be shorted out, and reconnected it to this
board, and disconnected the other end (in the vertical preamp board). Same
problem with the +55v.

I put an ohmmeter across the terminals of the delay line at both ends, and
there is no evidence of a short. yet with the delay line plugged into the
670-6385 board the +55v is loaded down, and thus the entire power supply
output is drawn down.

I'm confused. it seems that this coiled timing delay line is causing
problems, but it shows no sign of shorting or damage. am i missing
something? (besides a good service manual and schematics, which i am trying
to obtain) what is the likelihood that this line has failed?

and what does this board do? it has inputs from the vertical preamp board,
also power, sep. enable, and beam find thru the power connector, and outputs
to the crt tube.

 

[Jim Yanik]

"el Casa Bertrand" <lindbert@nbnet.nb.ca> wrote in
news:hu96c.119202$IF6.3982951@ursa-nb00s0.nbnet.nb.ca:

i recently purchased a dead Tek 465b scope (no lights except for power
and uncal, not even scale illumination) and opened it up to find no
voltages near spec at the test points on the main board. resistance
checks showed the +55v test point near ground. disconnecting the
connector to the board next to the crt (670-6385-?) allowed all
voltages to come up. reconnecting this board and disconnecting the
coiled timing delay line that comes in from the vertical preamp board
to this board also allows +55v to come up. I thought the delay line
might be shorted out, and reconnected it to this board, and
disconnected the other end (in the vertical preamp board). Same
problem with the +55v.

I put an ohmmeter across the terminals of the delay line at both ends,
and there is no evidence of a short. yet with the delay line plugged
into the 670-6385 board the +55v is loaded down, and thus the entire
power supply output is drawn down.

I'm confused. it seems that this coiled timing delay line is causing
problems, but it shows no sign of shorting or damage. am i missing
something? (besides a good service manual and schematics, which i am
trying to obtain) what is the likelihood that this line has failed?

and what does this board do? it has inputs from the vertical preamp
board, also power, sep. enable, and beam find thru the power
connector, and outputs to the crt tube.

It's not the delay line.It's common practice to short the delay line to
test the vertical trace centering,it does no harm.
It may be the vertical output amp (CRT driver amp) that's bad,or the
circuits feeding the delay line,causing a large offset that loads the
+55V.That's the PCB you seem to be describing.

Also,the +55 probably has a foldback current limiter,that may shut down the
supply when the VOut assy draws too much current trying to drive the trace
off screen.Maybe a bad series pass xstr,a bad B-E drop.Check the manual
circuit description for how that foldback works.

--
Jim Yanik
jyanik-at-kua.net

 

[Jim Yanik]

"el Casa Bertrand" <lindbert@nbnet.nb.ca> wrote in
news:5ym6c.119499$IF6.4000988@ursa-nb00s0.nbnet.nb.ca:

thanks... i was beginning to wonder if i'd gone nuts, or was missing
something obvious.
of course, by this time i'd already stripped the tube and yoke to get
at the delay line..

now that i've stopped worrying about several feet of wire being bad or
weird, it seems my problem is actually in this 'vertical output
amplifier' board. i can strip out every IC and transistor, yet the
+55v terminal is still showing 1-2 ohms to ground. removing a little
glass component partly covered in blue eliminates this problem.
but i cannot identify this little glass component as the blue coating
is damaged (overheated?). it looks like an old germanium diode but
measures the same 1-2 ohms across.

That sounds like a decoupling cap,it could be shorted,that happened a
lot.Probably a 0.1 uf ceramic,glass-encapsulated cap.Those things are a
PITA.

guess i'll just have to break down and get a manual!

Best idea yet!



--
Jim Yanik
jyanik-at-kua.net

 

[el Casa Bertrand]

it is indeed a 0.1uf cap.
i found and obtained a copy of the manual, and opened my eyes in wonder (i
keep hearing Tek doesn't give out component level schematics, yet there they
are).
hopefully it is the only component on the board that went south; will know
as soon as i put everything back in place and do a check on the voltages.

now i need to find someone locally who can sell me a 0.1 uf 100v capacitor
that isn't almost the size of the board itself.


thanks again. folks!

danny
"Jim Yanik" <jyanik@abuse.gov> wrote in message
news:Xns94B0C96CCB9F0jyanikkuanet@204.117.192.21...

"el Casa Bertrand" <lindbert@nbnet.nb.ca> wrote in
news:5ym6c.119499$IF6.4000988@ursa-nb00s0.nbnet.nb.ca:

thanks... i was beginning to wonder if i'd gone nuts, or was missing
something obvious.
of course, by this time i'd already stripped the tube and yoke to get
at the delay line..

now that i've stopped worrying about several feet of wire being bad or
weird, it seems my problem is actually in this 'vertical output
amplifier' board. i can strip out every IC and transistor, yet the
+55v terminal is still showing 1-2 ohms to ground. removing a little
glass component partly covered in blue eliminates this problem.
but i cannot identify this little glass component as the blue coating
is damaged (overheated?). it looks like an old germanium diode but
measures the same 1-2 ohms across.


That sounds like a decoupling cap,it could be shorted,that happened a
lot.Probably a 0.1 uf ceramic,glass-encapsulated cap.Those things are a
PITA.


guess i'll just have to break down and get a manual!



Best idea yet!



--
Jim Yanik
jyanik-at-kua.net

 

[JW]

On Fri, 19 Mar 2004 03:43:51 GMT "el Casa Bertrand"
<lindbert@nbnet.nb.ca> wrote in Message id:
<X%t6c.119809$IF6.4016751@ursa-nb00s0.nbnet.nb.ca>:

[465B]

it is indeed a 0.1uf cap.
i found and obtained a copy of the manual

Where did you find one?

 

[Jim Yanik]

"el Casa Bertrand" <lindbert@nbnet.nb.ca> wrote in
news:X%t6c.119809$IF6.4016751@ursa-nb00s0.nbnet.nb.ca:

it is indeed a 0.1uf cap.
i found and obtained a copy of the manual, and opened my eyes in
wonder (i keep hearing Tek doesn't give out component level
schematics, yet there they are).

No,TEK -stopped- putting schematics at the beginnning of the TDS (TEK
Digital scopes)line of scopes,intending that those scopes be repaired by
module exchange,or at a TEK service center.

hopefully it is the only component on the board that went south; will
know as soon as i put everything back in place and do a check on the
voltages.

now i need to find someone locally who can sell me a 0.1 uf 100v
capacitor that isn't almost the size of the board itself.

--
Jim Yanik
jyanik-at-kua.net

 

[nick]

On Fri, 19 Mar 2004 09:58:36 -0500, JW wrote:

On Fri, 19 Mar 2004 03:43:51 GMT "el Casa Bertrand"
lindbert@nbnet.nb.ca> wrote in Message id:
X%t6c.119809$IF6.4016751@ursa-nb00s0.nbnet.nb.ca>:

[465B]

it is indeed a 0.1uf cap.
i found and obtained a copy of the manual

Where did you find one?

The 465B service manual is at http://bama.sbc.edu/
It's in DejaView format so you will have to get djvu at
http://www.djvuzone.org/
djvu is FREE and available for Linux or the other OS and is a nice
doc viewer - you can print, etc.

Nick

 

[JW]

On Sat, 20 Mar 2004 06:44:45 -0700 "nick" <nick@francolini.com> wrote in
Message id: <pan.2004.03.20.13.44.44.369974@francolini.com>:

The 465B service manual is at http://bama.sbc.edu/
It's in DejaView format so you will have to get djvu at
http://www.djvuzone.org/
djvu is FREE and available for Linux or the other OS and is a nice
doc viewer - you can print, etc.

Thanks!