Tek 465B overheats?

J

Jay Levitt

Guest
I just bought a Tektronix 465B on eBay. It's my first scope, and it
seems to be in pretty good shape - good physical condition, DM44 option,
timebase slightly off but I'm sure that can be easily corrected by
following the service-manual procedures.

However, after it's been on for about an hour, the display disappears,
replaced by a smooth, diffuse glow. (No beacons, sorry.) One time
there was a fairly bright, diffuse spot on the right-center edge of the
screen; another just the overall glow. I have to turn it off and leave
it off a while before it will work again.

Presumably something's heating up; can anyone give me any pointers to
common failures, or should I just start randomly spraying Freon till I
see what was broke? I've never actually been able to catch the moment
of failure, but I can sit down for a boring evening of scope-watching if
that'll help track this down. This is my first scope, I'm really more
of a software guy, and I'm not sure where to start; there's not much
troubleshooting data in the service manual. I can probably borrow a
working scope to troubleshoot it if necessary.

In other news, can anyone tell me how the GND REF button on the probe is
used? Is it just an alternate way to charge the input caps, like
setting the AC-GND-DC switch to GND? I don't have the probe manual.

--
Jay Levitt |
Wellesley, MA | Hi!
Faster: jay at jay dot eff-em | Where are we going?
http://www.jay.fm | Why am I in this handbasket?
 
"Jay Levitt" <jay+news@jay.fm> wrote in message
news:MPG.1ab09ba16ed2c04698970f@news-central.giganews.com...
In other news, can anyone tell me how the GND REF button on the probe is
used?
Click to expand...
I find it useful when I've forgotten where the trace is centered and I'm
interested in DC level. All it does is short the signal to ground.
(Actually, I don't know whether it shorts the probe tip or the output, which
would be quite different in the case of a x10 probe. Suppose I should find
out before I inadvertently ground something that shouldn't be grounded.)
 
Jay Levitt <jay+news@jay.fm> wrote in news:MPG.1ab09ba16ed2c04698970f@news-
central.giganews.com:

I just bought a Tektronix 465B on eBay. It's my first scope, and it
seems to be in pretty good shape - good physical condition, DM44 option,
timebase slightly off but I'm sure that can be easily corrected by
following the service-manual procedures.
Click to expand...
Yes,if you have an accurate (digital)signal generator or time-mark
generator to calibrate it.(IOW,some known -standard- to compare to)
However, after it's been on for about an hour, the display disappears,
replaced by a smooth, diffuse glow. (No beacons, sorry.) One time
there was a fairly bright, diffuse spot on the right-center edge of the
screen; another just the overall glow. I have to turn it off and leave
it off a while before it will work again.

Presumably something's heating up; can anyone give me any pointers to
common failures, or should I just start randomly spraying Freon till I
see what was broke? I've never actually been able to catch the moment
of failure, but I can sit down for a boring evening of scope-watching if
that'll help track this down. This is my first scope, I'm really more
of a software guy, and I'm not sure where to start; there's not much
troubleshooting data in the service manual. I can probably borrow a
working scope to troubleshoot it if necessary.
Click to expand...
First thing is to check the power supplies for the right voltages and
ripple(need a scope for this).The manual has the specs.Then,to duplicate
the failure,take it out of the cabinet,wrap it in plastic sheeting,let it
warm up and fail.Then check the CRT pins to see what direction the trace
has gone off-screen,vertically or horizontally,or if the sweep ramp is
missing and it's just in holdoff or 'ready to sweep' mode.You may need
another scope to troubleshoot this one.
Read the circuit description in the manual,its a great help in
understanding how the scope works,especially the trigger and sweep circuits
where your problem may be.

In other news, can anyone tell me how the GND REF button on the probe is
used? Is it just an alternate way to charge the input caps, like
setting the AC-GND-DC switch to GND? I don't have the probe manual.
Click to expand...
The GND REF button is just to let you see where you set the trace before
applying a signal,without lifting the probe tip off the DUT or using the
scope's ground switch. A convenience.
--
Jim Yanik
jyanik-at-kua.net
 
In article <Xns94A274D9A358Ejyanikkuanet@204.117.192.21>,
jyanik@abuse.gov says...
I just bought a Tektronix 465B on eBay. It's my first scope, and it
seems to be in pretty good shape - good physical condition, DM44 option,
timebase slightly off but I'm sure that can be easily corrected by
following the service-manual procedures.

Yes,if you have an accurate (digital)signal generator or time-mark
generator to calibrate it.(IOW,some known -standard- to compare to)
Click to expand...
I figure good-quality audio D/A converters should be enough for hobbyist
use, at least up to 22kHz. I'm not knowledgeable enough to build an
oscillator myself, and I presume that any used calibrators I might find
on eBay are themselves equally suspect.

First thing is to check the power supplies for the right voltages and
ripple(need a scope for this).The manual has the specs.Then,to duplicate
the failure,take it out of the cabinet,wrap it in plastic sheeting,let it
warm up and fail.Then check the CRT pins to see what direction the trace
has gone off-screen,vertically or horizontally,or if the sweep ramp is
missing and it's just in holdoff or 'ready to sweep' mode.You may need
another scope to troubleshoot this one.
Read the circuit description in the manual,its a great help in
understanding how the scope works,especially the trigger and sweep circuits
where your problem may be.
Click to expand...
Sure, if I understood electronics :) Guess there's no time to learn
like the present, and I did just order the Don Cannon book. Right now,
a schematic is as useful to me as assembly source code is to my mom...
I'll check out the PS and the CRT pins and see what I find.

--
Jay Levitt |
Wellesley, MA | Hi!
Faster: jay at jay dot eff-em | Where are we going?
http://www.jay.fm | Why am I in this handbasket?
 
On Thu, 4 Mar 2004 01:49:54 -0500 Jay Levitt <jay+news@jay.fm> wrote:

However, after it's been on for about an hour, the display disappears,
replaced by a smooth, diffuse glow.
Click to expand...
Make sure that the fan is running. These actually have a small fan
pulling air in the back end.

In other news, can anyone tell me how the GND REF button on the probe is
used? Is it just an alternate way to charge the input caps, like
setting the AC-GND-DC switch to GND? I don't have the probe manual.
Click to expand...
It grounds the input to the scope. It does NOT short the probe tip to
ground. (The probe tip is 9 MOhms away from the point that is
grounded.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
Jay Levitt <jay+news@jay.fm> wrote in
news:MPG.1ab15cb9272f9299989710@news-central.giganews.com:

In article <Xns94A274D9A358Ejyanikkuanet@204.117.192.21>,
jyanik@abuse.gov says...
I just bought a Tektronix 465B on eBay. It's my first scope, and
it seems to be in pretty good shape - good physical condition, DM44
option, timebase slightly off but I'm sure that can be easily
corrected by following the service-manual procedures.

Yes,if you have an accurate (digital)signal generator or time-mark
generator to calibrate it.(IOW,some known -standard- to compare to)

I figure good-quality audio D/A converters should be enough for
hobbyist use, at least up to 22kHz. I'm not knowledgeable enough to
build an oscillator myself, and I presume that any used calibrators I
might find on eBay are themselves equally suspect.
Click to expand...
Yeah,but 22 Khz is not going to cover much of a scope timebase range.
First thing is to check the power supplies for the right voltages and
ripple(need a scope for this).The manual has the specs.Then,to
duplicate the failure,take it out of the cabinet,wrap it in plastic
sheeting,let it warm up and fail.Then check the CRT pins to see what
direction the trace has gone off-screen,vertically or horizontally,or
if the sweep ramp is missing and it's just in holdoff or 'ready to
sweep' mode.You may need another scope to troubleshoot this one.
Read the circuit description in the manual,its a great help in
understanding how the scope works,especially the trigger and sweep
circuits where your problem may be.

Sure, if I understood electronics :) Guess there's no time to learn
like the present, and I did just order the Don Cannon book. Right
now, a schematic is as useful to me as assembly source code is to my
mom... I'll check out the PS and the CRT pins and see what I find.
Click to expand...
Actually,for a scope timebase calibrator,one of those small clock
oscillators and a few 74C90 divider ICs,and you've got a great TB
calibrator;Electronic Goldmine used to sell a kit for making one,don't know
why they stopped.You can get a nice 1,2,5 divider ratio from the 7490s,and
use a 1 or 10 Mhz clock oscillator module. 100 PPM is more than enough
accuracy for a 3% scope timebase.

--
Jim Yanik
jyanik-at-kua.net
 
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