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Got a new scope... am too stupid to use it.

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AndrewM

Mon Jun 28 2010, 06:30AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Today I went to test a new widget for LonelyBot and dragged out my 10-year-old starterscope, a Velleman HPS40 - dead. So I pulled out my 8 year old BitScope - dead.

So I said screw it, time to get a real scope for once, and picked up a Techtronix 468.

Problem is... I think it might be broken! I've never used a real scope before, I figured it couldn't be all that difficult, but I'm having trouble. I've got everything set as it should be per the manual for a "normal sweep display"... but all I'm getting is a dot in the center of the screen. If I feed ti a square wave, I get a dot in the center plus another dot above it, properly located per the voltage scale. And if I sweep the horizontal position knob back and forth, I can make out my signal. Whatever makes the horizontal bit work is... well... not working.

Physically the horizontal offset functions work, as I can position my dots left and right at will. I suppose this is a trigger problem, then?

I'm so confused, I don't even know where to start.

This fellow has helpfully posted a picture of the scope:

Can someone explain what good "starter" settings would be so I can see something and tinker from there?

Corey

Mon Jun 28 2010, 06:58AM

Registered Member #1902 Joined: Fri Jan 02 2009, 07:59PM
Location: Lancaster, NY
Posts: 75

First off make sure that your not in X-Y mode which will prevent sweeping. Also I would check to make sure that your triggering on the right channel, and auto triggering with the trigger level set, otherwise I would say that there is something very wrong with your timebase/trigger logic. If you are interested I do have a very nice Tek 2335 Scope or 2225 scopes for sale and I will price to sell. Feel free to PM me if your interested I really need to get rid of them! Good luck!

-Corey

AndrewM

Mon Jun 28 2010, 07:19AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Corey wrote ...

First off make sure that your not in X-Y mode which will prevent sweeping. Also I would check to make sure that your triggering on the right channel, and auto triggering with the trigger level set, otherwise I would say that there is something very wrong with your timebase/trigger logic. If you are interested I do have a very nice Tek 2335 Scope or 2225 scopes for sale and I will price to sell. Feel free to PM me if your interested I really need to get rid of them! Good luck!

-Corey

Def not in X-Y mode.

Here is what I'm doing, or think I am:

300mV square wave into CH1
.2 V/DIV setting
CH1 vert mode
A trigger Auto, DC coupling, CH1 source, rising edge slope, min sensitivity

I get that all set up and then slooowly turn up the trigger sensitivity and BOOM, at a certain point the dots shoot off the right side of the screen and disappear. The triggering light then sticks on regardless of the sensitivity setting, and I have to turn the scope off and on again to refind the dots.

Corey

Mon Jun 28 2010, 07:28AM

Registered Member #1902 Joined: Fri Jan 02 2009, 07:59PM
Location: Lancaster, NY
Posts: 75

Hmmm not sure man, sounds to me it triggers once and for some reason won't after the first time. Regardless your either going to have to find a repair manual for your scope and find a working scope and find out whats wrong internally or simply buy a new scope. As for me I am out of ideas.

-Corey

AndrewM

Mon Jun 28 2010, 09:21AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Ok, I did some reading. Turns out the problem isn't that I'm stupid, the scope really does have problems.

I found a service manual and it told me that the digital section of the scope should go through a self check procedure when it starts up... which mine doesn't do.

I found a procedure in the manual for disabling the digital section of the scope, and did so... and voila, signs of life! Sorta. CH2 and the A trigger work fine. CH1 displays nothing, however, though the trigger light does appear to be functioning with CH1 input.

So I guess thats it. If anyone happens to be a scope repair expert chime in, otherwise I'm gonna hafta find a tektronix forum.

Corey

Mon Jun 28 2010, 10:27AM

Registered Member #1902 Joined: Fri Jan 02 2009, 07:59PM
Location: Lancaster, NY
Posts: 75

Yahoo groups has a very good forum for old tek scopes, they were very helpful when I fixed mine. (works great now)

-Corey

AndrewM

Tue Jun 29 2010, 05:16AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Tinker tinker tinker... got it!

+5, +/-12V supplies to the digital storage unit tested bad. All are supplied via the 30V tap on the transformer, and therefore affected by the completely demolished filter cap and blown 5A fuses on the same line.

:)

Corey

Wed Jun 30 2010, 08:39PM

Registered Member #1902 Joined: Fri Jan 02 2009, 07:59PM
Location: Lancaster, NY
Posts: 75

Nice to hear you've fixed it! One of my old scopes did also have bad filtering caps on the logic psu. The electrolytics sure do take a beating over the years, mine is from 1983! Replaced them all and bam perfect waveforms.

-Corey

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