~Tektronix~ oscilloscope repair 2246 ModA (but others may he

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Hello everyone,

I have a Tek 2246 ModA which has a slight but annoying problem:

If I leave the scope off for some time, more than a couple hours, when I turn it back on, channel 1 is deflected off screen, below it.
All other channels are there and appear fine, but not channel 1.
Then after a couple minutes it comes back and everything works fine then.

I know this is minor but is kind of annoying, so I have been trying to "chase this ghost" for a while now.

Does anyone here know what it can be? Maybe had the same problem before?

Thanks a lot everyone for even reading this far.

Cheers,
Marcelo.
 
On Mon, 19 Nov 2012 08:20:04 -0800 (PST) marcelo.f.dantas@gmail.com wrote
in Message id: <774ced29-e3cc-4c8d-af11-b8e04f923a82@googlegroups.com>:

Hello everyone,

I have a Tek 2246 ModA which has a slight but annoying problem:

If I leave the scope off for some time, more than a couple hours, when I turn it back on, channel 1 is deflected off screen, below it.
All other channels are there and appear fine, but not channel 1.
Then after a couple minutes it comes back and everything works fine then.

I know this is minor but is kind of annoying, so I have been trying to "chase this ghost" for a while now.

Does anyone here know what it can be? Maybe had the same problem before?

Thanks a lot everyone for even reading this far.
Click to expand...
Probably caps, but I've never worked on one.
A place where you'll likely get help:
http://tech.groups.yahoo.com/group/TekScopes/
 
If I leave the scope off for some time, more than a couple hours,
when I turn it back on, channel 1 is deflected off screen, below
it. All other channels are there and appear fine, but not channel
1. Then after a couple minutes it comes back and everything works
fine then.
Click to expand...
A similar problem I had with an HP scope deflected the beam to the
right. It turned out to be an open collector resistor. Since yours
appears to be a repeatable problem, it's likely to be a semiconductor
and not a dry joint (not with a TEK scope, anyway) or PS problem since
the other channel works. Deflection plates are fed with complementary
balanced DC coupled stages, so an intermittently open transistor
somewhere in the vertical deflection circuit is my best guess. I know
I'm stating the obvious but you really do need a schematic to tackle
that one. Failing that, you might try shocking each transistor with
cold spray after the thing warms up to isolate the bad component.
of course that assumes you can get to the area in question. I've
heard there are extender boards to allow access. Note also that some
parts are very carefully matched pairs and many are proprietary and
hard to get. Good luck!

Ray

PS No flames please. I know I'm a novice at scope repair.
 
Thanks for the reply ... yes it may be cap related ... I will check this other group, or maybe someone here that could maybe have the "rosetta stone" for this.

Thanks again,
Marcelo.

On Monday, November 19, 2012 11:28:16 AM UTC-5, JW wrote:
On Mon, 19 Nov 2012 08:20:04 -0800 (PST) marcelo.f.dantas@gmail.com wrote

in Message id: <774ced29-e3cc-4c8d-af11-b8e04f923a82@googlegroups.com>:



Hello everyone,



I have a Tek 2246 ModA which has a slight but annoying problem:



If I leave the scope off for some time, more than a couple hours, when I turn it back on, channel 1 is deflected off screen, below it.

All other channels are there and appear fine, but not channel 1.

Then after a couple minutes it comes back and everything works fine then.



I know this is minor but is kind of annoying, so I have been trying to "chase this ghost" for a while now.



Does anyone here know what it can be? Maybe had the same problem before?



Thanks a lot everyone for even reading this far.



Probably caps, but I've never worked on one.

A place where you'll likely get help:

http://tech.groups.yahoo.com/group/TekScopes/
Click to expand...
 
Hi Ray,

Yeah ... novice here too :)

I do have the schematics and I have been looking for the obvious issues first. I am getting to the point though where I will have to start looking ar less obvious stuff.
Freezing some transistors seems like a good idea though. It has to be temperature related, so maybe I can make the problem reoccur under my control.

Thanks for the post.

Cheers,
Marcelo.


On Monday, November 19, 2012 12:13:14 PM UTC-5, Ray Carlsen wrote:
If I leave the scope off for some time, more than a couple hours,

when I turn it back on, channel 1 is deflected off screen, below

it. All other channels are there and appear fine, but not channel

1. Then after a couple minutes it comes back and everything works

fine then.



A similar problem I had with an HP scope deflected the beam to the

right. It turned out to be an open collector resistor. Since yours

appears to be a repeatable problem, it's likely to be a semiconductor

and not a dry joint (not with a TEK scope, anyway) or PS problem since

the other channel works. Deflection plates are fed with complementary

balanced DC coupled stages, so an intermittently open transistor

somewhere in the vertical deflection circuit is my best guess. I know

I'm stating the obvious but you really do need a schematic to tackle

that one. Failing that, you might try shocking each transistor with

cold spray after the thing warms up to isolate the bad component.

of course that assumes you can get to the area in question. I've

heard there are extender boards to allow access. Note also that some

parts are very carefully matched pairs and many are proprietary and

hard to get. Good luck!



Ray



PS No flames please. I know I'm a novice at scope repair.
Click to expand...
 
<marcelo.f.dantas@gmail.com> wrote in message
news:774ced29-e3cc-4c8d-af11-b8e04f923a82@googlegroups.com...
Hello everyone,

I have a Tek 2246 ModA which has a slight but annoying problem:

If I leave the scope off for some time, more than a couple hours, when I
turn it back on, channel 1 is deflected off screen, below it.
All other channels are there and appear fine, but not channel 1.
Then after a couple minutes it comes back and everything works fine then.

I know this is minor but is kind of annoying, so I have been trying to
"chase this ghost" for a while now.

Does anyone here know what it can be? Maybe had the same problem before?

Thanks a lot everyone for even reading this far.

Cheers,
Marcelo.
Click to expand...


Have you seen it in the process of drifting? smooth or jerky?
I would suspect any switch contacts like in the attenuator
Put a DC monitor line in somewhere, while still using it (comparing with the
same point in ch2) , plus trying
freezer spray/ low setting hot air gun
 
First question :

When you switch the deflection facor, say all the way down to 5 volt/div., does the trace come on screen and closer to center or does it remain off screen regardless ?

J
 
On Monday, November 19, 2012 8:20:05 AM UTC-8, marcelo....@gmail.com wrote:

I have a Tek 2246 ModA which has a slight but annoying problem:

If I leave the scope off for some time, more than a couple hours, when I turn it back on, channel 1 is deflected off screen, below it.

All other channels are there and appear fine, but not channel 1.

Then after a couple minutes it comes back and everything works fine then.
Click to expand...
The most likely reason for this, is a power supply sequencing problem.
If it's only channel 1, there's possibly an RC filter in a channel one
supply that has either a faulty resistor (high value) or faulty capacitor
(leakage current too high). It could even be a shorted capacitor
that heats up and behaves better (but that isn't as good as a working capacitor).

It's also possible that offset adjustments of all your channels run off a
highly regulated reference supply, and that secondary supply is
slow coming up (but other channels have a smaller adjustment, so show
less effect).
 
<marcelo.f.dantas@gmail.com> wrote in message
news:774ced29-e3cc-4c8d-af11-b8e04f923a82@googlegroups.com...
Hello everyone,

I have a Tek 2246 ModA which has a slight but annoying problem:

If I leave the scope off for some time, more than a couple hours, when I
turn it back on, channel 1 is deflected off screen, below it.
All other channels are there and appear fine, but not channel 1.
Then after a couple minutes it comes back and everything works fine then.

I know this is minor but is kind of annoying, so I have been trying to
"chase this ghost" for a while now.

Does anyone here know what it can be? Maybe had the same problem before?

Thanks a lot everyone for even reading this far.

Cheers,
Marcelo.
Click to expand...

I'm assuming as Tek that it has 2 separate CRT beams . Is it possible to
swap ch1 and ch2 somewhere mid-process to localise to early or late circuit
stages? ie
problem stays on the same beam or swaps over
 
Assuming this is a single-beam CRT, the problem can't be with the CRT.

I'd look for an offset somewhere in the gain stages.
 
Hi J,

Is remains offscreen regardless.
I have tried messing around with the knobs and switches initially thinking it could be some bad contact.

Thanks,
Marcelo.


On Tuesday, November 20, 2012 12:28:22 PM UTC-5, jurb...@gmail.com wrote:
First question :



When you switch the deflection facor, say all the way down to 5 volt/div., does the trace come on screen and closer to center or does it remain off screen regardless ?



J
Click to expand...
 
On Wednesday, November 21, 2012 7:42:55 AM UTC-5, William Sommerwerck wrote:
Assuming this is a single-beam CRT, the problem can't be with the CRT.



I'd look for an offset somewhere in the gain stages.
Click to expand...
Hi William and N_Cook,

Yes I will have to start looking further on the circuit now.
The big problem is that I have only this 1 minute window to troublehsoot. Because when it comes back it works perfectly from then on. So I would have to leave it off for hous to see the problem again.

Thanks all for the answers.
And happy thanksgiving.

Cheers,
Marcelo.
 
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