Tektronics 2246A Problem...

[Malcolm Thomson]

I was recently given a Tek 2246A scope with a "small" problem. I hate to throw it out as I'm sure it's probably fixable. The problem is that the screen is distorted in both x and y direction. I've attached a picture which shows the problem. Has anyone seen this before or any ideas where to start to fix the problem. I do have the service manual.

Thanks in advance.
View attachment 67200

 

[KeepItSimpleStupid]

yikes! The first thing I would look at is the power supply, particularly the HV supply. Servicing anything at all usually means check the power supplies first. I do believe that there may be leaky tantalum caps in this series, so inspect these.

For detailed help, join the TEKSCOPES groups of groups.yahoo.com. Check the archives there before posting, but you will have to join first.

 

[Malcolm Thomson]

Thanks for the prompt response. I'll check the TEKSCOPES at yahoo.

 

[ModemHead]

Yikes, indeed. That looks a lot like internal CRT damage.

 

[cowboybob]

Yikes, indeed. That looks a lot like internal CRT damage.

I'll say. Either the vertical plates have sifted, or perhaps the little magnets on the CRT neck (if there are any) have moved.

Dropped maybe?

 

[Mikebits]

And such a nice scope. I would try to get the manual, it is worth fixing.

 

[ronsimpson]

I think the CRT is OK.
The problem is both horizontal and vertical. What is common? Power supply. And there are many different supplies.
I would start with the supply that powers the deflection plates. Might be 100 to 200 volts.
I don't think it is the HV supply, but huge ripple in the HV supply will look just like this.

Why is the problem on the right side? Maybe the horizontal amplifier has a broken transistor so it pulls too much power on the right side, collapsing the supply.

 

[Malcolm Thomson]

I have checked the DC voltages and they are within the tolerance in the manual. I guess I need to figure out the higher voltage stuff.

 

[ronsimpson]

I have checked the DC voltages and they are within the tolerance in the manual. I guess I need to figure out the higher voltage stuff.

Measured with a meter? I am looking for ripple on the supplies that probably can't be seen with a meter. It looks like the deflection supply is 100V on the left side and 75 volts on the right side. (25 volts p-p or 10 volts ripple)

If the HV supply has ripple......the amount a electron beam bends is related to the amount of voltage.

 

[KeepItSimpleStupid]

That sort of ripple might be seen on the AC scale. Good point though.

 

[Malcolm Thomson]

Although my DC voltages appeared good, I looked at the ripple and not so good.View attachment 67209.

 

[ronsimpson]

I can't see well. Looks like 46khz noise from the switching power supply. (that is OK) I can not tell what the vertical scale is. The noise must be at the same rate the scope is drawing the text at. My guess is the ripple is 30% of the DC value to cause this problem. If that is the problem. Now that I think about it.....I usually try to make the text update rate match the switching power supply frequency so maybe 46khz is the ripple rate. Could be bad power supply Ele. Caps.

 

[vtech]

Electrolytics are the number one problem with older Tek scopes; use the chart in manual.

**broken link removed**

 

[ronsimpson]

Electrolytics are the number one problem with older Tek scopes; use the chart in manual

Your chart is a good find!
Check the 130 and 58 first. 100mV is not what I am looking for. I am looking for volts of ripple, or 10 volts of ripple.

 

[Malcolm Thomson]

Maybe I am on a wild goose chase but I decide to take a look at the power supply board as several of the comments indicate possible power supply issues. The board has a couple of areas that look burnt. Not sure if the components are bad but it is probably easiest for me (with my limited experience) to replace most of the caps and the components that have been cooked. I've attached a couple of pics.View attachment 67233View attachment 67234

 

[cowboybob]

Looks like fried (or certainly excessively hot) diodes (rectifiers) and some over heating damage to leads going to the heat-sinked transistor possibly driving the flyback transformer. Both would spell high voltage breakdown probs.

And as vtech says, cap replacement is the norm with older Textronix scopes (with MOST older electronics) but I might start with C2244 and C2245. They don't look bad but the circuit their in is very high voltage and that takes its toll.

 

[Dean Huster]

The burned section of the PCB does indicate a problem, but I'm not sure that it's the problem that's causing the original complaint. When I first saw the CRT display, I noticed that it was irregularly pincushioned. Nothing was symmetrical about it, horizontally or vertically. If I'd seen this thing in the Tek Service Center, the first thing I'd've looked for was physical drop damage to the case. A nasty drop can screw up the internal scan-expansion mesh, a really fine, delicate screen (so fine that it holds water) that sits in the CRT's electron beam path to increase deflection sensitivity and reshape geometry for a more-linear and accurate screen display. By the way, this is a scope and not a television -- there are never any external magnets on the neck of a Tektronix CRT. Once you have any internal PCB problems resolved, you'll likely find that the original problem is still there.

 

[cowboybob]

Dean,

I will bow to your expertise on this. I wondered in my first post about deflection plate damage (by being dropped). Wasn't too far off, perhaps.

Used, repaired and calibrated Textronix scopes (as well as others) in the NAV back in late '60s and early '70s but never got into the CRT itself.

And your right, the Textrinix scopes did not have magnets on the CRT neck, but some of the other, much older scopes of various brands (especially off the older diesel subs) most assuredly did.

 

[Malcolm Thomson]

Thanks Dean. I'll let you know when I get the PS board fixed up with some new components. Is there anything on the CRT that I can look at to see damage? The case looks fine and no visible signs of it being dropped.

 

[Mikebits]

Just a note, when I worked as a tech rep on Navy ships, 9 out of 10 times my service calls were to repair equipment that had failed, but the original failure was masked due to the ship board tech shotgunning the problem and just swapped and replaced components, all the while the excessive swapping of parts did more damage. I would have to fix what the techs did before going after the original fault.

Point: Take care when replacing parts.