~Tektronix~ oscilloscope repair 2246 ModA (but others may help too)

On Mon, 19 Nov 2012 08:20:04 -0800 (PST) snipped-for-privacy@gmail.com wrote in Message id: :

it back on, channel 1 is deflected off screen, below it.

this ghost" for a while now.

Probably caps, but I've never worked on one. A place where you'll likely get help:

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 aga> On Mon, 19 Nov 2012 08:20:04 -0800 (PST) snipped-for-privacy@gmail.com wrote

it back on, channel 1 is deflected off screen, below it.

this ghost" for a while now.

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.

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.

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

"chase this ghost" for a while now.

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 ?

it back on, channel 1 is deflected off screen, below it.

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).

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

"chase this ghost" for a while now.

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.

does the trace come on screen and closer to center or does it remain off screen regardless ?

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.