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  #1  
Old 06-10-2017, 09:59 PM
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Tektronix T 912 Sillyscope

Tektronix T912 Sillyscope,
Plug it in,hit the power button,fan comes on,power indicator light flickers,no
other signs of life. Get a flash on the screen on power-off.
My "What could it be" question might seem vague,but I was just given
4 of these scopes and they all do the same thing.The fella that gave them to
me has 4 more that do the exact same thing.He bought them new in 79-80.
So,here we have 8 scopes with the same symptoms,that is what led me to think/hope there is a known failure issue with this model.
I downloaded the schematic,but have not had a chance to open one up yet.
I am hoping some "back in the day" tech might remember this model and recall
what the common failures were.Thanks for reading this........
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  #2  
Old 06-11-2017, 12:47 AM
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Hard to believe it's 8 units all failed exactly the same. More likely there's some operator error here, but hard to say what without more indication of how you are adjusting it (or not).

Some things it could be: sweep not being triggered; intensity turned down.

Set trigger source to line.
Set trigger mode to auto.
Turn trigger level up and down to see if a sweep (green horizontal line) appears. If not, try turning up Intensity.
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Old 06-11-2017, 12:50 AM
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Also, set seconds/division somewhere in the middle of its range.
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Old 06-11-2017, 12:53 AM
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Also, set vertical positioning controls to midrange.
Press beam finder button and see if a line or spot appears.
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Old 06-11-2017, 01:14 AM
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This same question was asked on ARF in 2012 and Alan Douglas responded: "Open it up, remove the plug-in circuit boards and clean the connector contacts between the boards."

The poster never replied.
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Old 06-11-2017, 10:32 AM
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Make sure you have the store button released (out) before troubleshooting.

So many times I've seen these opened up for troubleshooting, only to see the operator had it in storage mode. Knobology strikes again.... Check for basic functionality first, then test the storage functions - they are inter-dependent and only when the analog basic (10MHz) scope is working should the storage features be tested.

The Yahoo! TekScopes and TekScopes2 groups may be of help too.
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Old 06-11-2017, 03:13 PM
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If its like their waveform monitors used in TV broadcast with the switching power supply, I'd say you've got some bad caps on the secondary side of the switching supply transformer.
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Old 06-11-2017, 10:19 PM
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<<721 Porthole ????
 
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Thanks for the replies all,
The guy that gave them to me was a medical equipment repair tech years ago...
When stuff actually got repaired,remember those days?

I got rid of 98% of my stuff,just kept a Tek 504,and an Owon 2 channel LCD
unit.
I wanted to get a 2 channel CRT scope for a " Scope Clock " kit I built,it is not as impressive on the LCD screen :-(
Naturally I was hoping for a quick fix as I do not have to much patience for a value-less solid state unit...
I'm going to have to open one of these up regardless... The dirty circuit board pins seems like the best answer sofar.As these scopes have been sitting for a long time in a garage,and the flickering power light is erratic like something breaking down under a load.
If/when I open one up I will post if anyone is interested....I will post even if
no one is interested

Last edited by grimer; 06-11-2017 at 11:36 PM.
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Old 06-11-2017, 11:49 PM
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Quote:
Originally Posted by kf4rca View Post
If its like their waveform monitors used in TV broadcast with the switching power supply, I'd say you've got some bad caps on the secondary side of the switching supply transformer.
T912 has a pure linear power supply and is easy to service. 90% of the ones I've serviced in Cal Labs (the Navy used them in the mine countermeasures shop) had pure knobology issues. Nothing Electrically wrong, just knob position/rookie issues. "The XYZs of Scopes" is a good tutorial for analog scopes. Try to find a hardcopy (hint:non-PDF) from prior to 1991, when Tek changed it to include the DSO (digital) and Mixed signal scopes/microchannel architecture scopes. They shortened the text in the analog side, convinced that everyone would be using their DSOs only.

They are dual channel, so if your scope clock works in X-Y mode, you are okay. All of the scope clocks I've seen are X-Y types, like curve tracer kits and TV-on-a-scope kits use. The TV-on-a-scope kits also use the "Z" channel, available on all Tek analog scopes.

The T912 manual (available free on the web) has the full schematics, operation info, and troubleshooting matrices.

One question - has anyone messed with these before? A lot of these are now hangar-queens that others have cannibalized, but offer up for parts. I got one (ex-NASA) that had most of the transistors pulled from it. Good CRT and power supply, but I passed it on to a friend. He got it working by merely ordering and replacing the transistors. The Storage board was thought to be toast, but it turned out to be a flood gun power supply issue - the 7805 regulator couldn't handle the initial surge. One visit to the junkbox fixed that.

These are among the most repairable scopes out there with ZERO custom chips - all are the 156-prefixed ICs, commercially available. Knobs and pots are hard to find for some of the T-series, but with your horde of scopes, maybe you can make several good ones. Working ones rarely go for more than 60 bucks - it is a 10MHz scope, after all....
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Last edited by Findm-Keepm; 06-11-2017 at 11:54 PM. Reason: added clarification on DSOs/dual channel oops
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Old 06-13-2017, 04:02 AM
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Quote:
Originally Posted by kf4rca View Post
If its like their waveform monitors used in TV broadcast with the switching power supply, I'd say you've got some bad caps on the secondary side of the switching supply transformer.
With it being said that they sat for a long time in a garage and the initial symptom description; my money would go on this quote. I seriously doubt that some setting is causing all of them to do the same thing. Especially since they are a tool that is generally used by people with some sort of formal training in the field of electronics. Certainly, training or no; the best of us make unmentionable mistakes at times. I guess it is possible that perhaps a group of soldiers in Advanced Individual Training set them up in some strange way and the instructor couldn't figure out what they did, so they scrapped them and bought all new ones. LOL! Hey, this IS a government scenario, but again; it screams power supply.

If ya just desire to learn and/or the challenge, then make one or two good ones out of the lot and put the rest to the home of solid state fix me not. The dumpster....
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Old 06-13-2017, 12:54 PM
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Is there any "background" light covering the whole screen when the "store" is pushed in? Make sure the "integrate" button is not pushed in.

jr
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  #12  
Old 06-13-2017, 08:34 PM
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<<721 Porthole ????
 
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Opened up the first unit,not a "knobology" problem,its a "toastolgy" problem.
The P/S board has hole burned right thru.Did a temporary patch on the affected curcuit,powered it up,the same symptoms. I was hoping a fuse would
pop,thus making it easier to trace down a short.
I put this one aside for the moment,I will get to it,but I'm going to open another one in the next few days...

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Old 06-13-2017, 11:01 PM
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Somebody plugged the boards together while powered up or mis-indexed them somehow. I've see that on the T922 scopes. Eats a resistor, and melts a transistor, IIRC. Pull the board, and do a junction check on all transistors - diode function on your DMM should do the job. On the power supply board, lift the base of each transistor before testing, as the low impedances/resistances on the board will give false readings.

Or open up another and cannibalize to make one good scope...
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Old 06-14-2017, 11:56 AM
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Quote:
Originally Posted by grimer View Post
The P/S board has hole burned right thru.Did a temporary patch on the affected curcuit,powered it up,the same symptoms. I was hoping a fuse would
pop,thus making it easier to trace down a short.
I put this one aside for the moment,I will get to it,but I'm going to open another one in the next few days...
In a way a fuse did blow, the circuit trace. Will be interesting if all of the scopes have the same failure.
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Old 06-15-2017, 08:29 PM
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Scope # 2 is apart,and no visible signs of damage. After an hour of cleaning the board interconnect pins,and the switches and controls,there is life in this one.
The controls are still scratchy.Channel one will not pass a signal,and the trace shows "noise",even with no probe attached.
At least progress has been made......Maybe the 3rd will be the charmer......
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