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Tek 454 Ch1 Vertical Drift


terry.brown6@...
 

Stan (or others), you have helped me with this scope just recently. I
picked up an original users manual which is great! But, my scope has
just started giving me trouble with Ch1. When I turn the unit on, and
even after an ample warmup period, the Ch1 trace doesn't show up. I
locate it with Beamfinder but can't get it stabilized on the screen.
When I can get it to position, it continues to drift vertically, both
up and down, and will not completely stabilize (even after 30 minutes
or more). That is with leaving the Volts/Div control in the same
position. Any ideas?
Also, I discovered the Ch2 Cal control has absolutely no effect. I
can't find a detent position either. I thought it might be a loose
knob but that wasn't the problem. What do you think? Thanks, much.

Terry


Stan or Patricia Griffiths <w7ni@...>
 

terry.brown6@att.net wrote:

Stan (or others), you have helped me with this scope just recently. I
picked up an original users manual which is great! But, my scope has
just started giving me trouble with Ch1. When I turn the unit on, and
even after an ample warmup period, the Ch1 trace doesn't show up. I
locate it with Beamfinder but can't get it stabilized on the screen.
When I can get it to position, it continues to drift vertically, both
up and down, and will not completely stabilize (even after 30 minutes
or more). That is with leaving the Volts/Div control in the same
position. Any ideas?
Also, I discovered the Ch2 Cal control has absolutely no effect. I
can't find a detent position either. I thought it might be a loose
knob but that wasn't the problem. What do you think? Thanks, much.

Terry
Hi Terry,

Here is an educated guess. I suspect you have two problems. The drift is
probably due to a bad nuvistor in the Ch 1. You might just try reseating it
in the socket.

The lack of detent in the Ch 2 Variable Volts/Division control could be due
to a broken shaft coupling. I can't remember if there are shaft couplings
here or not. This may be a real tough one to repair because it will be
buried under the CRT shield. I might have a part for this if you can
determine what you need and can get to it. If this turns out to be one of
those white plastic pots with a crack in the part with the set screw in it,
one way to fix it is to back the set screw out and epoxy the plastic. You
have to let it sit for a couple of days to make sure the epoxy is real hard,
then retighten the set screw.

Stan
w7ni@easystreet.com


Terry Brown <terry.brown6@...>
 

Thanks, Stan. I do see the plastic part you described and, yes, it is
cracked. Would any of the "superglues" work in place of expoxy? Any fairly
easy way to get to it?
I'll try reseating the Nuvistor for the other problem. If it's a bad one,
any ideas where they are commercially available. Thanks, again.
Terry

Stan or Patricia Griffiths wrote:

terry.brown6@att.net wrote:

Stan (or others), you have helped me with this scope just recently. I
picked up an original users manual which is great! But, my scope has
just started giving me trouble with Ch1. When I turn the unit on, and
even after an ample warmup period, the Ch1 trace doesn't show up. I
locate it with Beamfinder but can't get it stabilized on the screen.
When I can get it to position, it continues to drift vertically, both
up and down, and will not completely stabilize (even after 30 minutes
or more). That is with leaving the Volts/Div control in the same
position. Any ideas?
Also, I discovered the Ch2 Cal control has absolutely no effect. I
can't find a detent position either. I thought it might be a loose
knob but that wasn't the problem. What do you think? Thanks, much.

Terry
Hi Terry,

Here is an educated guess. I suspect you have two problems. The drift is
probably due to a bad nuvistor in the Ch 1. You might just try reseating it
in the socket.

The lack of detent in the Ch 2 Variable Volts/Division control could be due
to a broken shaft coupling. I can't remember if there are shaft couplings
here or not. This may be a real tough one to repair because it will be
buried under the CRT shield. I might have a part for this if you can
determine what you need and can get to it. If this turns out to be one of
those white plastic pots with a crack in the part with the set screw in it,
one way to fix it is to back the set screw out and epoxy the plastic. You
have to let it sit for a couple of days to make sure the epoxy is real hard,
then retighten the set screw.

Stan
w7ni@easystreet.com


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Terry Brown <terry.brown6@...>
 

Stan, I removed and reseated all the Nuvistors I could find and also checked
transistors in the sockets to see if loose. That seems to have solved the
drifting issue. I'll let it cool down completely and retry later.
Are the 454's very sensitive to movement...i.e., do the
transistors/Nuvistors and other parts move around with just normal carrying and
storing of the scopes?
I also noticed that the leading edges of the calibrator square waves have a
slight upward/downward curve to them (even using a straight patch cord in place
of probes). Is that normal or does it indicate some need for recalibration?
Thank you, again, for all your help and sharing of knowledge.
Terry

Stan or Patricia Griffiths wrote:

terry.brown6@att.net wrote:

Stan (or others), you have helped me with this scope just recently. I
picked up an original users manual which is great! But, my scope has
just started giving me trouble with Ch1. When I turn the unit on, and
even after an ample warmup period, the Ch1 trace doesn't show up. I
locate it with Beamfinder but can't get it stabilized on the screen.
When I can get it to position, it continues to drift vertically, both
up and down, and will not completely stabilize (even after 30 minutes
or more). That is with leaving the Volts/Div control in the same
position. Any ideas?
Also, I discovered the Ch2 Cal control has absolutely no effect. I
can't find a detent position either. I thought it might be a loose
knob but that wasn't the problem. What do you think? Thanks, much.

Terry
Hi Terry,

Here is an educated guess. I suspect you have two problems. The drift is
probably due to a bad nuvistor in the Ch 1. You might just try reseating it
in the socket.

The lack of detent in the Ch 2 Variable Volts/Division control could be due
to a broken shaft coupling. I can't remember if there are shaft couplings
here or not. This may be a real tough one to repair because it will be
buried under the CRT shield. I might have a part for this if you can
determine what you need and can get to it. If this turns out to be one of
those white plastic pots with a crack in the part with the set screw in it,
one way to fix it is to back the set screw out and epoxy the plastic. You
have to let it sit for a couple of days to make sure the epoxy is real hard,
then retighten the set screw.

Stan
w7ni@easystreet.com


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Stan or Patricia Griffiths <w7ni@...>
 

Hi Terry,

I have not had any luck with any superglues sticking to that white plastic which I
think is really nylon. I have only had luck with epoxy and then I used a fair
amount of it and filled in the areas around the set screw part to give it a lot of
extra strength. To get to it, you can remove the CRT still inside the CRT
shield. There is a procedure for doing this in the manual. Don't forget to
disconnect the CRT neck pins before removing the CRT. It really only takes about
10 minutes and you have unsolder a few wires . . . just remember where they went.

To help make sure the epoxy will stick, I would clean the area to be glued with a
cotton swab and alcohol. To bring the area of the pot to be glued to the top
where you can reach it, you can actually mechanically rotate the pot by by using a
pair of needle nosed pliers or a soldering aid to push on the front part of it
where the set screw is. In the event this attempt to glue it does not work, I
would suggest that you turn the pot to the detent position and leave it there.
Then at least that channel will be calibrated . . . it just won't have a
functioning variable control.

I think you can buy nuvistors from Deane Kidd <dektyr@teleport.com> or
503-625-7363. Deane will need the Tek part number of the nuvistor you are looking
for.

Look luck on getting this fixed and let me know how it works out for you.

Stan
w7ni@easystreet.com

Terry Brown wrote:

Thanks, Stan. I do see the plastic part you described and, yes, it is
cracked. Would any of the "superglues" work in place of expoxy? Any fairly
easy way to get to it?
I'll try reseating the Nuvistor for the other problem. If it's a bad one,
any ideas where they are commercially available. Thanks, again.
Terry


Stan or Patricia Griffiths <w7ni@...>
 

Hi Terry,

Comments inserted below:

Terry Brown wrote:

Stan, I removed and reseated all the Nuvistors I could find and also checked
transistors in the sockets to see if loose. That seems to have solved the
drifting issue. I'll let it cool down completely and retry later.
That may be all that is required to fix the drift.

Are the 454's very sensitive to movement...i.e., do the
transistors/Nuvistors and other parts move around with just normal carrying and
storing of the scopes?
No, not normally. But you have to remember all 454's are now at least 30+ years old
and things like nuvistor sockets and transistor sockets can get a little oxidized
after that much time. Sometimes all that is required to fix an intermittant socket
is to remove and reinstall a transistor or nuvistor a couple of times.

I also noticed that the leading edges of the calibrator square waves have a
slight upward/downward curve to them (even using a straight patch cord in place
of probes). Is that normal or does it indicate some need for recalibration?
You may looking a the calibrator signal in the "AC" postion of the vertical input
coupling switches. (These are the horizontal lever switches near the input BNC
connectors.) Try the "DC" position and see if it doesn't look a little better.
The calibrator will look a little slanted in the AC position.

Glad to help, Terry. Let me know if I guessed this one right . . .

Stan
w7ni@easystreet.com

Thank you, again, for all your help and sharing of knowledge.
Terry