Tektronix Scope Repair Help

Try the astig control. And there's a Tek scopes Yahoo group that's full of gurus.

Cheers

Phil Hobbs

Worked on a few of these. As I recall, replace a group of electrolytics...was a common problem. As others suggested, also check the Yahoo group.

Brilliant, thanks Phil & Rick

What's your definition of "spot-on"? Some of the tek scopes have a spot-size way bigger than you used to find on the old stuff. There's typically a high voltage bleeder resistor that's often made up of a bunch of resistors in series. One or more may be open. Be careful. Without a bleeder, the charge can hang on for a LONG time.

Mike is correct here. If you don't have the service manual, you can get it here.

Open it up and check the five 1 megohm resistors in the focus divider circuit. You may find one or more have drifted high. These would be R-878 through R-884 and check the focus control. Hopefully you have measured all the low voltage supplies and found them good.

These scopes are nice lightweight 60 MHz bandwidth units.

hth tm

Anti-Spam wrote in news: snipped-for-privacy@4ax.com:

easy one,I think. there's six 1MegR 1/2w carbon film resistors in the focus string,that change value and screw up your focus.they are located around the focus pot on the main board,you'll have to remove the power supply cage to access them. R878,879,880,881,882,884.

TEK had a mod that chnges the carbon film resistors to carbon composition,they hold up better.

don't forget the astigmatism adjustment.that goes hand-in-hand with focus.

If that doesn't fix it,your CRT may be getting old,as the tube ages,the cathode emits from a larger area and thus can't focus as well. you may see "double-peaking" as you increase intensity,where the brightness increases as you turn up the intensity control,but then DIPS as you keep going,then brightens again. it's another sign of a dying CRT.

Any hints for a model 475 that no longer has beam current? It packed up just before an Xmas lecture and has been sat in a corner ever since (not worth paying to mend it). Waiting for a Roundtuit to come along.

Regards, Martin Brown

Thanks Mike, Jim & TM, I will open it up at the weekend.

For the last 30 years, always had Textronix Scopes (always second-hand) they are works of art inside. But never had to repair one before now.

Mark in Spain

How do you know that it "no longer has beam current?"

It packed up

The trace vanished spontaneously on a previously reliable scope. It was working at the time and hasn't shown a glimmer since.

I presume some part of the HT side is no longer at voltage. I don't like HT stuff much. It bites.

Regards, Martin Brown

Open it up and check the power supply test points on the interface board (the main PC board on the bottom). There is a 1.5 amp fuse on the rear left that should have unregulated 15 volts and should be at about 21 volts. This powers the HV supply.

A good copy of the service manual is here:

I would suspect one of the electrolytic caps has opened up. A loss of any supply can cause a loss of the beam. If you do find a bad cap, be careful removing it. There is track under the can and it pulls off very easily.

hth

tm

So, you know that the "trace vanished". Jumping to conclusions tends to limit the advice one gets.

yep, could be, but could also be a blanking problem...or a low voltage power supply, or any number of other things unrelated to HT.

Or trace deflected off screen due to an amplifier problem.

Thank you very much for this info. Armed with a decent service manual I might well get around to opening it up and taking a look over Xmas.

Thanks again. It was a nice scope in its day - though physically large.

Regards, Martin Brown

Fair point. Guilty as charged.

Could be and I accept that I can't prove it isn't no beam current, but none of the things that usually at least causes the screen to glow a bit worked in this instance so I do suspect it is no beam at all rather than beam not reaching the screen. Armed with a service manual I will take it apart. I had been quietly ignoring it for a while now.

Regards, Martin Brown

Anti-Spam wrote in news: snipped-for-privacy@4ax.com:

Fair warning; TDS model TEK scopes do NOT have component-level schematics available,they were intended for board-exchange repair only.,and for "obsolete" products,that service is no longer offered.

That is not true. You can download a PDF file called 'TDS520B component service manual' from Tektronix. The diagrams in this PDF cover most of the TDS500, 600 and 700 series. The PCB layout varies between the various models.

There's a huge difference between the TDS500 and 600/700 series. That schematic is useless for them.