Repair one channel on Tektronix 535a oscilloscope

[jimthegreek]

Hello everyone,

I have a tektronix 535a oscilloscope that was given to me. I have full functionality on channel B including voltage and frequency measurements. I do not have anything working on Channel A. There isint even a trace. The vertical positioning and all the channel A switches are correctly positioned. I have the schematic for the scope.

Could it be the CA dual trace plug in unit not working for channel A only? I also have the schematic for the CA plug in unit too. Where do I begin to troubleshoot?

thanks,
Jim

[basinstreetdesign]

The fact that you have functionality on ch. B proves out the mainframe.  So that immediately localizes the problem to the ch A circuits of the plug-in.  With ch A switched on what do the beam lights (above the screen) show?  They will tell you which way the beam has been deflected to if not on screen, above, below, left, right.  If one of the up or down lights are lit then the ch A section of the plug-in has driven the beam that way with excess voltage.  Do the chop and alternate modes work on the CA?

Try going over all of the switches and pots internal to the plug-in with contact cleaner and see if that makes a difference.  Other things that may be wrong with the CA are:

bad tube - try swapping in corresponding tubes from the other channel to see if that fixes anything.  But always replace them back to their original sockets after each test!
bad capacitor - best to replace all electrolytics, anyway.  OOPS, just checked my unit and there ARE no electrolytics in it!

Tim

[Cyberdragon]

Try calibrating the vertical section, if someone screwdrivered it, it may have thrown the trace offscreen. Beyond that, check V3334, V3354, V3364, and V3374. Also check the associated resistors. Cleaning the switches might help (just make sure to leave no residue), but don't bother cleaning the module contacts, both channels share the same vertical output.


EDIT: Almost forgot the golden rule! Thou shalt check voltage!

[jimthegreek]

Thanks! I will check the chop and alternate modes. I noticed one of the resistors was broken on the switch for channel A vertical position( it has numerous resistors on it). Can that make a difference or would only that one position not work on the switch?

thanks,
Dimitri

[jimthegreek]

I will check the vertical section and check all corresponding components. Ill keep you posted.

thanks,
Dimitri

[Cyberdragon]

Vertical position switch? I know of the vertical position knob but not a switch. Unless you mean the volts switch (attenuator), in which case it would not affect the vertical position.

[jimthegreek]

I found one broken carbon resistor(333K) on the volts/div switch. I will replace it shortly and check all the others. Also, I found tubes for channel A at a very good price so I will order them. I will swap them out only one at a time to determine the problem tube. I checked the scope on chopped mode with no difference to seeing channel A work.  I also sprayed the switches with contact cleaner.

Any other suggestions until the tubes come in?

thanks,
Dimitri

[Cyberdragon]

Just remember if you replace tubes you'll have to recalibrate the unit. And did you check the trace indicator lights?

[jimthegreek]

I did check the indicator lights! They didnt make a difference. What do you mean that I have to recalibrate after changing tubes? How? Even if its one tube at a time?

thanks,
Dimitri

[basinstreetdesign]

What do you mean that I have to recalibrate after changing tubes? How? Even if its one tube at a time?

If after swapping tubes to get the ch A functional again, the new tube(s) will not match the old ones for calibration.  That means that after the repair, ch A should be re calibrated so you have some confidence in making measurements with it.  But while you're at it you might as well do the whole unit if you can.  For this you will definitely need the service manual for the plug-in.

Good luck and happy new year!

[Cyberdragon]

Odd, I would expect the trace indicators to light, maybe it's so far out it doesn't even detect it. I would recommend changing the tubes I mentioned first, as those are the most likely. If any replacement doesn't fix it, put the original back, just to make recalibration easier. You could always get the manual and the voltages to find the exact fault, since you're going to need it to recalibrate it anyway.

[jimthegreek]

The trace indicator arrows do light but I still was unable to get a trace for channel A with all the controls. I have the service manual with the schematic so thats a good reference. I understand now what you mean by recalibration after changing the tubes. The manual has that information in detail.

The main manual for the scope mentions lubricating the fan and shows a picture of the lubrication points. I took both sides and the back off but I can not find these points. Any idea where they are?

thanks,
Dimitri