Busted OScope, I think the transformer is fried!

[smeezekitty]

Its a tektronics, it used to work.
Heres the story:
One day, i plugged it in and turned it on and noticed the screen wouldnt light.
So i checked the fuse, it was open.
I replaced the fuse and turned it on and still nothing.
Checked the fuse again and it was open again so i replaced the fuse again and got out the ohm meter.
The AC line resistance is 0.1 ohms LOL.
The transformer is green and has a ton of wires commin out.
No pix avil right now.

[Steve Anderson]

The AC line resistance is 0.1 ohms LOL. The transformer is green and has a ton of wires commin out.

Using a multi-meter the DC resistance of the primary of a transformer will read quite low. But 0.1 Ohms does sound a bit too low. Disconnect the primary of the transformer and try again. It should then read a virtual open circuit. Model No. would be useful.

Transformers are usually one of the most reliable components. If they do fail it's often due to something else failing causing them to be overloaded and then overheat.

Tektronix stuff is good quality and made to last so I would be surprised if the tranny has failed.

Steve A.

[smeezekitty]

Model No. would be useful.

Model 561
edit:it looks kind of like http://www.chiark.greenend.org.uk/scope ... 4frmed.jpg
-edit
How would you disconnect a transformer?

[Steve Anderson]

I've had a look at the Tek 561a manual and from the transformer primary 117V AC is fed to both plug-in units. Remove both plug-ins and check again. If the short goes away one of the plug-ins is faulty. Replace one at a time to identify which one.

I would expect the AC line resistance to be somewhere around 10 Oms normally when read on a multimeter.

Steve A.

If you want a copy of the manual I'll send it on, I won't post it here...it's 40MB! And that doesn't include the plug-ins.

It should look like the one below...your plug-ins might be different though..

[smeezekitty]

Whats a plugin?

[Steve Anderson]

Whats a plugin?

The two removable units at the bottom as shown in the photo above. Unscrew the knurled knob at the bottom of each unit....and pull hard! They were a very 'captive' arrangement.

[smeezekitty]

Removed both modules and now its measuring in at 1.8 ohms.

[Steve Anderson]

That sounds about right, I forgot that you're on 110/120V in the US so the resistance will be less than when set for the rest of the world. With both plug-ins removed briefly power it up (10 seconds, before the tube warms up) and see if the fuse blows again.

I suggest you stock up on 2A slow-blow fuses!

Steve A.

[smeezekitty]

That sounds about right, I forgot that you're on 110/120V in the US so the resistance will be less than when set for the rest of the world. With both plug-ins removed briefly power it up (10 seconds, before the tube warms up) and see if the fuse blows again.

I suggest you stock up on 2A slow-blow fuses!

Steve A.

I really dont know about that because a 1500 watt heter i 10 ohms.

[Steve Anderson]

I really dont know about that because a 1500 watt heter i 10 ohms.

Yes, correct. But a heater is resistive, here you're dealing with an inductance. An inductance has a very low resistance when measured with a multimeter, i.e. DC. At 60Hz the transformers' reactance will be much higher.

Steve A.

[smeezekitty]

I turned it on for 7 seconds and the CRT backlight lit and some of the tubes started to glow.
(No modules inserted).

[Steve Anderson]

I turned it on for 7 seconds and the CRT backlight lit and some of the tubes started to glow. (No modules inserted).

There you go. So now we know it's one of the plug-ins that is faulty. At the top of each plug-in is printed its Model No. Let me know which ones you have and I'll see if I have the data for them.

Steve A.

[smeezekitty]

Its working! one of the wires on the modules was broken and shorting to ground.
Although all i get is a dot on the screen.

[Steve Anderson]

Although all i get is a dot on the screen.

Have you replaced the plug-ins?

Steve A.

[smeezekitty]

No, i resoldered the wires.