Tek 485 PS - Missing Comb Connector?

P

Pete Chrisbacher

Guest
Hello All -

I finally got myself a Tek 485 - complete with ticking PS. Just read the
back posts (thanks Jim, et al), but I have a problem...

When I took the metal cover off the switcher board, I noticed a spot for a
comb connecter in the bottom left corner, but no comb connector in it!
Without a manual (coming shortly) I don't know if there's supposed to be one
there or not (I'd think so, but better to not assume ;-)

Can anyone tell me whether there's supposed to be a comb connector installed
in that spot?

Thanks!

-Pete
 
Whelp - in a leap of faith I fashioned a makeshift comb connector out of
some bits of solid copper wire. Good news is the PS fired up and the scope
came to life. All low voltages are spot-on.

Unfortunately, I now also get an occasional loud "snap" sound (arcing) that
I'm afraid might likely be an internal CRT short. The first time it
happened I immediately cut power. But I couldn't find any obvious places
where it might have arced. I turned the power back on and once I got a
trace, it arced again after a couple of seconds. I disconnected the CRT
neck connector, then ran the scope for a couple minutes without any sign of
an arcing, which is one reason I'm thinking CRT short. :-( When it arcs
(with the CRT connected), there's a small flash on one side of the screen,
and the scope does what seems almost like a "reset," as if all the voltages
drop to zero for a split second, then come back up, though I haven't been
able to verify this b/c it hasn't arced since I connected up my AVM to the
50V TP. Weird. BTW, no obvious flashes from the HV area during arcing -
the sound seems like it's coming from the center of the scope, near the CRT.

Anyone seen anything like this before with this scope? Sound like a CRT
short? Are there causes of this other than internal CRT flaws? Excessive
HV? Any chance the CRT arcing (if that's what this is) oxidized the arcing
points which is preventing further arcing? The scope's been running for 20
minutes now without a snap, where when I first got it running it arced a
total of maybe 5 times - once every 20 seconds or so.

-Pete

"Pete Chrisbacher" <pxbacher@netaxs.com> wrote in message
news:bOXkb.6411$Vf7.1564@nwrdny02.gnilink.net...
Hello All -

I finally got myself a Tek 485 - complete with ticking PS. Just read the
back posts (thanks Jim, et al), but I have a problem...

When I took the metal cover off the switcher board, I noticed a spot for a
comb connecter in the bottom left corner, but no comb connector in it!
Without a manual (coming shortly) I don't know if there's supposed to be
one
there or not (I'd think so, but better to not assume ;-)

Can anyone tell me whether there's supposed to be a comb connector
installed
in that spot?

Thanks!

-Pete
Click to expand...
 
"Pete Chrisbacher" <pxbacher@netaxs.com> wrote in
news:Ev2lb.8222$Fc5.6287@nwrdny01.gnilink.net:

Whelp - in a leap of faith I fashioned a makeshift comb connector out
of some bits of solid copper wire. Good news is the PS fired up and
the scope came to life. All low voltages are spot-on.

Unfortunately, I now also get an occasional loud "snap" sound (arcing)
that I'm afraid might likely be an internal CRT short. The first time
it happened I immediately cut power. But I couldn't find any obvious
places where it might have arced. I turned the power back on and once
I got a trace, it arced again after a couple of seconds. I
disconnected the CRT neck connector, then ran the scope for a couple
minutes without any sign of an arcing, which is one reason I'm
thinking CRT short. :-( When it arcs (with the CRT connected),
there's a small flash on one side of the screen, and the scope does
what seems almost like a "reset," as if all the voltages drop to zero
for a split second, then come back up, though I haven't been able to
verify this b/c it hasn't arced since I connected up my AVM to the 50V
TP. Weird. BTW, no obvious flashes from the HV area during arcing -
the sound seems like it's coming from the center of the scope, near
the CRT.

Anyone seen anything like this before with this scope? Sound like a
CRT short? Are there causes of this other than internal CRT flaws?
Excessive HV? Any chance the CRT arcing (if that's what this is)
oxidized the arcing points which is preventing further arcing? The
scope's been running for 20 minutes now without a snap, where when I
first got it running it arced a total of maybe 5 times - once every 20
seconds or so.

-Pete
Click to expand...
There could be a problem at the CRT anode connector,perhaps the silicone
seal is breaking down.Or the CRT faceplate is building up charge,then
discharging to the chassis.Those 'snaps' can destroy the DC restorer
diodes,too,taking out the intensity and focus controls.

Or you could have a failing HV multiplier,or the negative cathode voltage
(-2960V,IIRC) has a HV cap thats breaking down,perhaps a pinhole in the
coating.This assy is buried in the middle rear of the scope.

The 485 PS is a series-resonant type of switching supply,and has many
voltage and current sense loops.(it's a real bitch to troubleshoot,too.)



--
Jim Yanik,NRA member
jyanik-at-kua.net
 
Thanks Jim -

This morning when I fired it up, it arced again (twice). Two additional
bits of information:

1. When it arcs, the 50V rail DOES drop momentarily to zero (fan also stops,
so I'm assuming all LVs drop to zero)
2. When I turned it off just now, it arced a second time about one second
AFTER I had powered it off using the front panel button.

Not sure where the above clues are pointing me... I'll have to think about
them in combination with Jim's list before I start rooting around in this
thing.

-Pete

"Jim Yanik" <jyanik@abuse.gov> wrote in message
news:Xns941B65C048036jyanikkuanet@204.117.192.21...
There could be a problem at the CRT anode connector,perhaps the silicone
seal is breaking down.Or the CRT faceplate is building up charge,then
discharging to the chassis.Those 'snaps' can destroy the DC restorer
diodes,too,taking out the intensity and focus controls.

Or you could have a failing HV multiplier,or the negative cathode voltage
(-2960V,IIRC) has a HV cap thats breaking down,perhaps a pinhole in the
coating.This assy is buried in the middle rear of the scope.

The 485 PS is a series-resonant type of switching supply,and has many
voltage and current sense loops.(it's a real bitch to troubleshoot,too.)



--
Jim Yanik,NRA member
jyanik-at-kua.net
Click to expand...
 
On Tue, 21 Oct 2003 04:21:56 GMT "Pete Chrisbacher"
<pxbacher@netaxs.com> wrote:

Whelp - in a leap of faith I fashioned a makeshift comb connector out of
some bits of solid copper wire. Good news is the PS fired up and the scope
came to life. All low voltages are spot-on.
Click to expand...
Deane Kidd would probably be a good source for a replacement comb
connector: dektyr@teleport.com

Unfortunately, I now also get an occasional loud "snap" sound (arcing) that
I'm afraid might likely be an internal CRT short. The first time it
happened I immediately cut power. But I couldn't find any obvious places
where it might have arced. I turned the power back on and once I got a
trace, it arced again after a couple of seconds. I disconnected the CRT
neck connector, then ran the scope for a couple minutes without any sign of
an arcing, which is one reason I'm thinking CRT short.
Click to expand...
Can you measure the capacitance of the CRT and substitute a HV
capacitor of approximately the right value? That might be a better
test than just leaving the HV lead open. Be sure to ground the low
side of the cap to the same ground area as the CRT uses.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
One note: I would think that if the arc were actually IN the CRT, you
might see it but it definitely wouldn't be very loud. Sound doesn't
travel in a vacuum so it would be by conduction.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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contact me, please use the Feedback Form at repairfaq.org. Thanks.
 
Good point, Sam. Yep, this is definitely a clear, loud "snap," which would
seem to indicate the arcing is in one of the places Jim Y suggested. The
other half of the HV circuitry is about as buried in this scope's innards as
it could possibly be, and to top it off, you have to take all the comb
connectors out to remove the boards, which means once I have it open, I
can't fire it up to see where the thing is arcing. I guess I could make a
bunch of jumpers, but I'm hoping when I finally get in there, there'll be
some burn marks pointing out the offending part. More likely when I get in
there it'll be clean as a whistle. Why couldn't I just have had a couple of
shorted tantalums to hunt for? :-(


"Sam Goldwasser" <sam@saul.cis.upenn.edu> wrote in message
news:6wu162hu1p.fsf@saul.cis.upenn.edu...
One note: I would think that if the arc were actually IN the CRT, you
might see it but it definitely wouldn't be very loud. Sound doesn't
travel in a vacuum so it would be by conduction.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work.
To
contact me, please use the Feedback Form at repairfaq.org. Thanks.
Click to expand...
 
Sam Goldwasser <sam@saul.cis.upenn.edu> wrote in
news:6wu162hu1p.fsf@saul.cis.upenn.edu:

One note: I would think that if the arc were actually IN the CRT, you
might see it but it definitely wouldn't be very loud. Sound doesn't
travel in a vacuum so it would be by conduction.

--- sam | Sci.Electronics.Repair FAQ Home Page:
http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info:
| http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer
work. To contact me, please use the Feedback Form at repairfaq.org.
Thanks.
Click to expand...
I suspect the snap is coming from the inner HV PCB assy,the one buried in
the middle of the scope(accessible by removing the rear panel).

--
Jim Yanik,NRA member
jyanik-at-kua.net
 
"Pete Chrisbacher" <pxbacher@netaxs.com> writes:

Good point, Sam. Yep, this is definitely a clear, loud "snap," which would
seem to indicate the arcing is in one of the places Jim Y suggested. The
other half of the HV circuitry is about as buried in this scope's innards as
it could possibly be, and to top it off, you have to take all the comb
connectors out to remove the boards, which means once I have it open, I
can't fire it up to see where the thing is arcing. I guess I could make a
bunch of jumpers, but I'm hoping when I finally get in there, there'll be
some burn marks pointing out the offending part. More likely when I get in
there it'll be clean as a whistle. Why couldn't I just have had a couple of
shorted tantalums to hunt for? :-(
Click to expand...
Yeah, I sure hope I never have a similar problem with my 485! Why
couldn't Tek design these things to be serviceable?!

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the Feedback Form at repairfaq.org. Thanks.
 
Good news: Only took 45 minutes or so to get to the HV board inside the
scope.
Bad news: No burns or marks of any kind.

I put the scope back together (loosely) using a few screws and the combs to
hold everything in place. Ran it for 20 minutes staring at it in the dark -
no arc. This is MY KIND OF INTERMITTENT! :-( Grrrrrr!


"Jim Yanik" <jyanik@abuse.gov> wrote in message
news:Xns941BF05E6F6B4jyanikkuanet@204.117.192.21...
Sam Goldwasser <sam@saul.cis.upenn.edu> wrote in
news:6wu162hu1p.fsf@saul.cis.upenn.edu:

One note: I would think that if the arc were actually IN the CRT, you
might see it but it definitely wouldn't be very loud. Sound doesn't
travel in a vacuum so it would be by conduction.

--- sam | Sci.Electronics.Repair FAQ Home Page:
http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info:
| http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer
work. To contact me, please use the Feedback Form at repairfaq.org.
Thanks.



I suspect the snap is coming from the inner HV PCB assy,the one buried in
the middle of the scope(accessible by removing the rear panel).

--
Jim Yanik,NRA member
jyanik-at-kua.net
Click to expand...
 
"Pete Chrisbacher" <pxbacher@netaxs.com> writes:

Good news: Only took 45 minutes or so to get to the HV board inside the
scope.
Bad news: No burns or marks of any kind.

I put the scope back together (loosely) using a few screws and the combs to
hold everything in place. Ran it for 20 minutes staring at it in the dark -
no arc. This is MY KIND OF INTERMITTENT! :-( Grrrrrr!
Click to expand...
Hopefully the bug (six legged variety) that caused all the trouble fell
out during the process. :)

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the Feedback Form at repairfaq.org. Thanks.
 
Sam Goldwasser <sam@saul.cis.upenn.edu> wrote in
news:6wznft5q97.fsf@saul.cis.upenn.edu:

"Pete Chrisbacher" <pxbacher@netaxs.com> writes:

Good point, Sam. Yep, this is definitely a clear, loud "snap," which
would seem to indicate the arcing is in one of the places Jim Y
suggested. The other half of the HV circuitry is about as buried in
this scope's innards as it could possibly be, and to top it off, you
have to take all the comb connectors out to remove the boards, which
means once I have it open, I can't fire it up to see where the thing
is arcing. I guess I could make a bunch of jumpers, but I'm hoping
when I finally get in there, there'll be some burn marks pointing out
the offending part. More likely when I get in there it'll be clean
as a whistle. Why couldn't I just have had a couple of shorted
tantalums to hunt for? :-(

Yeah, I sure hope I never have a similar problem with my 485! Why
couldn't Tek design these things to be serviceable?!
Click to expand...
You can't imagine the 'discussions'(arguments) I used to have with the
Service Support people about serviceability.

Then TEK did away with Service Support,piece by piece.

(I mourned the loss of 'Mod Summaries',their one great work.You folks saw
the edited version,Maintenance Notes if you had a subscription,back in the
'Great' days of Tektronix.)



--
Jim Yanik,NRA member
jyanik-at-kua.net
 
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