Tektronix 485 scope

[mick]

Hi, have a Tektronix 485 scope.   Have a issue where it does not seem to sync with any signal.  Can any one tell me where I can start looking on the scope to address.   I am thinking either the horizontal amp or trigger circuits.

thx

[sluckey]

Is this the old '70s vintage 485 or is it the later TAS-485? Are you an experienced scope user? Can you get it to sync on the calibrator output?

If you can get a horizontal trace and vary the speed with the timebase switch, the horizontal circuits are probably good. Look at the trigger circuit.

[mick]

Steve it is a older 70's scope.   I am not a experienced scope user.  I can not get it to sync on the calibrator output.  Varying the time/dev switch will not bring it in.  This scope was in storage for about 10 years.   I don't think that  helped much. 

Mick

[sluckey]

Tek 485 manual available here...
http://bama.edebris.com/manuals/tek/485/


Changing the time/div switch does not affect scope sync. Set the scope as follows...

1. Release the CAL FREQ button so the CAL output will be 1kHz and 5volts amplitude.

2. Connect the CAL output to the CH 1 vertical input using a BNC cable or 1X probe.

==============================================================
Controls for steps 3-6 are located below the display.

3. Select CH 1 VERT MODE pushbutton. Select 1MΩ.

4. Set CH 1 VOLTS/DIV to 1. Be sure the small variable knob is fully CW and detented. The 1X LED should be lit. Just turn the knob to put "1" adjacent to the 1X LED. (Since the cal signal is 5 volts, you should have about 5 centimeters vertical deflection.

5. Use the POSITION knob (just above CH 1 input jack) to center the display vertically.

6. Set INT TRIG pushbuttons for CH 1.

===============================================================
Controls for these steps are located to the right of the display.

7. Set TIME/DIV switch to .5ms and Select HORIZ DISPLAY pushbutton A.

===============================================================
These controls are for A TRIGGERING (lower right)

8. Set SWEEP MODE to NORM TRIG. Set COUPLING to DC. Set SOURCE to INT.

9. Set SLOPE switch (tiny knob) to +.

10. Adjust LEVEL knob for stable display. The green TRIG'D light should be on.

11. Set the horizontal POSITION so the displayed trace starts at the 1st horizontal graticule.


That should get a stable display that is 5 divisions tall. Each half wave of the squarewave signal should take up 1 horizontal division.

[mick]

Steve,

I tried that.   All I have are 10x probes.   I compensated by turning downt the volts/div to get vertical deflection.   It wont sync up to the 1K calibrate signal. 

[sluckey]

Are you saying you have no horizontal deflection, just a vertical line? (This means the horizontal osc/amp is not working)

Set SWEEP MODE to AUTO and SOURCE to LINE and LEVEL to 0. Rotate the TIME/DIV switch several times and push all the HORIZ DISPLAY buttons several times. Note that the TIME/DIV switch is actuall two switches and a variable pot as well. Make sure the two switches are locked together and turn as one. Put the HORIZ DISPLAY switch back to A. Make sure the A TRIGGER HOLDOFF knob is in the NORM position. Do you get horizontal deflection?

===============================================
Or, do you have horizontal deflection but the signal is not stable? (This is a trigger problem.)

Operate all the A TRIGGERING switches and pushbuttons several times. The method I gave you earlier would result in a blank display until the TRIG'D light comes on. When you vary the A TRIGGERING LEVEL, does the TRIG'D light ever come on? Try setting SWEEP MODE to AUTO TRIG. Try setting SOURCE to LINE. Also try setting SOURCE to EXT and connect the CAL jack to the EXT TRIG jack. Don't worry about the display. Just watch the TRIG'D light. Does it ever come on?

This is the pic I'm looking at...
http://home.comcast.net/~seluckey/tek485.jpg

[tubesornothing]

Mick, posting a pic of what you see and the scope setting will help a ton.

[mick]

ok,  posting some pics - a good idea...

Files are bigger than 128K so here is the url...

http://www.frontiernet.net/~stutler/ocilloscope1.jpg
http://www.frontiernet.net/~stutler/ocilloscope2.jpg

The second pic,  I can make out a signal ( I hope)..

Thx guys..

Mick

[sluckey]

Let's eliminate the probe. Just use a piece of 22AWG wire. Strip 1/4" insulation from each end. Stick one end in the CAL jack (freq set to 1KHz) and the other in the CH 1 input. Or stick a paper clip in each of those jacks and connect with a gator clip lead.

[mick]

Ok, so had time to do re do the steps with 22 awg wire.   Here is the pic url:

http://www.frontiernet.net/~stutler/485signal1K.jpg


I tried asjusting the slope.   It never seems to synch on the signal.

Thx