Tektronix OS-245/P/U - Military spec 7603 (sortof) - suggestions

[JosephG]

Hello everyone,

So I'm starting to put together a small bench again after some years and am on a budget. This old boat anchor happened to fall in to my lap and who could resist a $20 price tag? Anyway, most of the scope seems to work. One of the verticle amplifiers had some minor solder joint issues on the volts/div-variable pot and some dirty sockets and the time base plugin had a triggering / holdoff issue due to dirty clocked latch IC sockets. I replaced one tantalum that was on it's way to a short, though others are difficult for me to test since I don't really have any other equipment yet other than a couple of inexpencive DMMs. I've grabbed all of the service manuals I could find, as well as the Tektronix oscilloscope troubleshooting manual but I'm sortof wondering about one problem that doesn't appear to be covered in the manuals. The time base appears fine until I exceed 2uS on the dial, where the trace begins to look like it's "doubling back" on itself, for lack of better words, onto itself from left to right about a quarter of the way over the CRT. The higher in frequency I go the farther it extends to the right. It's also accompanied by little notches at the far end of the overlapping section of the trace, almost like what you would see with extreme linearity problems with a television.  I'm planning on replacing the rest of the tantalums and probably should be asking this question if the problem still persists but I'm just curious if anyone else has ever seen anything like this, and has any ideas what it might be?

Also, anyone that might be familiar with these machines that has any other suggestions or information that may help me along the way are appreciated also.

Thanks in advance,

Joseph.

[med6753]

I also have a Tek OS-245(P)/U scope that was recently purchased and it works fine. So I could potentially provide some test points on a proper functioning time base.

Would it be possible to post pix of what you are seeing?

[JosephG]

Hey, thanks for the reply. I've attached a couple of pictures showing the trace at lower frequencies operating normally and at higher frequencies when it begins to become problematic. Sorry about the image quality, the only camera I have that I can use right now is an old Sony Mavica. The kind with a built in floppy drive! While poking around I also discovered something interesting that may be from factory, or more likely from the last time the machine was calibrated. In the sweep length area of the board the pot appears to be topped out with no travel left. I think I'm going to have to invest in something to test these capacitors, at the very least. I'm hoping _another_ scope isn't going to be necessary to troubleshoot this but I'm not holding my breath...

[capt bullshot]

Do you have (own or have access to) a function generator?
What does the trace look like if you apply a sine wave to one of the vertical inputs, with the frequency set to about one wave period per division?


The reason I'm asking: I've got a 7623A with a real strange behaviour:
http://wunderkis.de/tek-7/

Within his article, I wrote the problem goes away by adjusting the horizontal position to it's extremes. Yes, it did at the time I wrote that, but now it persists for some 10 minutes after turn on, then goes away. Can't find anything within the amplifiers, so my guess is there's a failure within the CRT, maybe a broken spot welding. Of course, this isn't repairable.

Edit:
Up to some extent, it is normal that you have to re-adjust the horizontal position for faster timebase settings, especially with the fastest timebase setting.

[JosephG]

Sadly, I don't have a function generator. Using an audio tone generator on the computer and the calibrator on the scope however everything appears perfectly normal and linear but I'm not able to generate frequencies high enough to test the trace enomoly. I'm going to see if I have enough parts laying around to throw something faster together, I'll post results as soon as I can. 

[David Hess]

What timebase plug-in are you using?  Standard for this model was a military version of the 7B53AN (TD-1085/U ??).

Assuming you have the standard dual timebase, does the delayed timebase function work?  When you pull the timebase control out to use the delayed timebase, does it have detents as expected?

It might be that the delayed timebase cam switch has broken lose from the control and is stuck or turning freely which can cause some odd behavior when using the main timebase.

Also, try swapping the timebase with one of the vertical amplifiers to generate a vertical sweep and see if it displays the same behavior.

[JosephG]

I'm using the TD-1085/U timebase. I tried putting it in a verticle slot but I couldnt get a trace no matter what I did. Even pressing the beam finder. The delayed time base appears to be working, though I have no experience using it so I'm unsure. I checked the internal switches, there is a leaf switch behind the knob as well as a little switch on the board operated by the cam when switched from delayed / normal sweep. They appear to be in the correct positions / operating fine (I also beeped them out to be sure).

Now here is where things get kind of questionable and I am really unsure. On my timebase both sweep modes are "tied" together, that is both selectors are activated and both detents can be felt. When I pull the knob out, the sweep has a brightened portion and when turned clockwise only one set of detents are felt and the bright trace shrinks while the "normal" trace is still visible behind it. if I rotate it counter clockwise there is a single detent felt until I reach the \_/ shaped indicator position where the knob was when I pulled it out. Now it begins to decrease the normal timebase down as well as the delayed sweep. I can stop and turn clockwise again and the normal time base remains and the bright trace begins to "shrink" again. Also, when the knob is pulled out and I rotate the delay time multi I can "move" the bright (delayed?) trace left to right as I think one would expect. Is this all normal behavior?

I hope I explained this well enough.

Thanks for your time.

[JosephG]

I just realized that the knob indicates "pull for intens" and "push for dly'd" however delayed mode seems to only work when pulled out and the trace appears normal when pushed in. I'm going to look at the manual, which I should have done already, and see. Either I'm not using it properly or something is not quite right here.

[David Hess]

I'm using the TD-1085/U timebase. I tried putting it in a verticle slot but I couldnt get a trace no matter what I did.  Even pressing the beam finder.

The beam only unblanks if an amplifier or timebase is installed in the horizontal slot so to use the timebase in a vertical slot, install one of the vertical amplifiers into the horizontal slot.

You should still do this test.  Based on what you describe below, the actual problem may be with the horizontal amplifier.

The delayed time base appears to be working, though I have no experience using it so I'm unsure. I checked the internal switches, there is a leaf switch behind the knob as well as a little switch on the board operated by the cam when switched from delayed / normal sweep. They appear to be in the correct positions / operating fine (I also beeped them out to be sure).

Usually the problem I see is that the delayed timebase cam switch breaks lose from the time/div control so when in delayed mode, the time/div control turns freely.  I have a couple of 7B53As which failed this way.  I repaired a 7B92A which had this problem and it generated a reverse sweep.

Now here is where things get kind of questionable and I am really unsure. On my timebase both sweep modes are "tied" together, that is both selectors are activated and both detents can be felt. When I pull the knob out, the sweep has a brightened portion and when turned clockwise only one set of detents are felt and the bright trace shrinks while the "normal" trace is still visible behind it. if I rotate it counter clockwise there is a single detent felt until I reach the \_/ shaped indicator position where the knob was when I pulled it out. Now it begins to decrease the normal timebase down as well as the delayed sweep. I can stop and turn clockwise again and the normal time base remains and the bright trace begins to "shrink" again. Also, when the knob is pulled out and I rotate the delay time multi I can "move" the bright (delayed?) trace left to right as I think one would expect. Is this all normal behavior?

That all sounds normal to me.

In order to see just the intensified zone, push in the time/div control knob.  On the front of the control is says, "PULL FOR INTENS / PUSH FOR DELY'D".  Now the time/div control sets the magnification and the delay time multiplier sets the position.

There is a tab on the clear plastic time/div ring which allows turning the main timebase counter clockwise while the time/div knob is pulled out.

I hope I explained this well enough.

You did fine.  Dual delayed timebases are kind of weird until you gain some experience using them.

I do not have a Tektronix OS-245/P/U or any of the military plug-ins for it but I have the non-military non-ruggedized 7603 which it is based on as well as several other 7000 mainframes.  I can probably answer any questions you have about this series of oscilloscopes.

These oscilloscopes are pretty old so suffer from various age related maladies but when working, they are very capable.

[David Hess]

I just realized that the knob indicates "pull for intens" and "push for dly'd" however delayed mode seems to only work when pulled out and the trace appears normal when pushed in. I'm going to look at the manual, which I should have done already, and see. Either I'm not using it properly or something is not quite right here.

The 7B53AN (TD-1085/U) timebase only displays one sweep at a time.  When the time/div control is pushed in and the main and delayed timebases are locked together, it displays the main.  When the time/div control is pulled out and the main and delayed timebases are not locked together, it displays the main with an intensified zone showing the delayed.  When the time/div control is pushed in and the main and delayed timebases are not locked together, it displays only the intensified zone.

There is a special "mixed sweep" mode activated by pulling the time/div CAL control out but ignore that for now.

[JosephG]

Doh. That makes perfect sense. I'll try swapping them as soon as I get a chance.

Thanks again for taking the time.

[JosephG]

Swapping the plugins appears to give me the same results, just with a verticle line as apposed to horizontal. It may be a little bit less noticable, however, but it's hard to tell due to the fact that the trace stretches off the top and bottom of the screen. When I use the horizontal position to bring it down, I still see some artifacts. Which brings me to the next question, should the trace fit within the graticule like the image above, or should it span slightly off the left and right of the screen, as I've seen on some older Tek scopes?

[David Hess]

Swapping the plugins appears to give me the same results, just with a verticle line as apposed to horizontal.

That is exactly what should happen.  The oscilloscope will work as before but of course the horizontal bandwidth is only a couple MHz and internal triggering will not work.

Placing a vertical amplifier in the horizontal slot is one way to implement X-Y mode.  Placing a timebase in a vertical slot is a way to generate a raster display.  In this case though, doing both is for diagnostic purposes.

It may be a little bit less noticable, however, but it's hard to tell due to the fact that the trace stretches off the top and bottom of the screen. When I use the horizontal position to bring it down, I still see some artifacts. Which brings me to the next question, should the trace fit within the graticule like the image above, or should it span slightly off the left and right of the screen, as I've seen on some older Tek scopes?

The timebase plug-in actually generates a trace slightly longer than 10 divisions and for the 7B53A, it is adjusted to 10.4 divisions during calibration but I do not think this is critical.

However the vertical and horizontal deflection should be identical with the same plug-in so the 7B53A should produce the same horizontal and vertical deflection which is not going to be apparent without a time-mark generator of some sort.  This might be easier to check using a signal generator and vertical plug-in.

I think you are going to need like a 100 to 500 kHz waveform generator to really see what is going on.  A triangle wave would be ideal but a sine wave will work.

[JosephG]

I've got some salvaged components together and I'm going to create a triangle wave oscillator with a dual op-amp. Do you think this will be sufficient? I know it's not ideal but it's the best I can do.

Also, I'd like to point out some new findings that may or may not offer some clues. As I mentioned above, the sweep length adjust pot is maxed out. Also, I've noticed at all frequencies using the 1khz calibrator that there is some linearity issues across the trace. It seems compressed 3/4 of the way to the right of the display then goes back to normal for the remaining divisions. I've also discovered that while panning the horizontal position to the left, the display is normal however at the very edge of the right hand side while panning to the right, the entire trace gets compressed instead of rolling off the display as it should.

Again, thanks for your time. This is the most complex analog device I've ever worked on and it's a little intimidating. :-)

[edited / additional info]

I just started reading Tektronix's "Troubleshooting Your Oscilloscope" and it does have an area covering the sweep compression when poisitioning to the left or right. It recommends checking for leaky diodes in the network at the bases of the amplifiers. What's not clear to me though is if this is in the time-base or in the verticle section of the mainframe itself. Perhaps I should try swapping the plugins again and see if the problem follows along to the verticle. The only problem with that is that there is no triggering but I should still be able to see if there is compression...

[David Hess]

I've got some salvaged components together and I'm going to create a triangle wave oscillator with a dual op-amp. Do you think this will be sufficient? I know it's not ideal but it's the best I can do.

Make a 500kHz sine wave oscillator instead; it will be almost as good.  Salvaged operational amplifiers will not typically be useful for 500kHz triangle wave operation.

Or there are all kinds of fast ramp generators that you can make with a comparator or a couple of logic gates.  A two transistor astable multivibrator would also work.

As I mentioned above, the sweep length adjust pot is maxed out. Also, I've noticed at all frequencies using the 1khz calibrator that there is some linearity issues across the trace. It seems compressed 3/4 of the way to the right of the display then goes back to normal for the remaining divisions.

This is why I mentioned swapping the timebase and vertical amplifier.  Operating the sweep vertically will reveal if the non-linearity is in the timebase or the horizontal amplifier.

I've also discovered that while panning the horizontal position to the left, the display is normal however at the very edge of the right hand side while panning to the right, the entire trace gets compressed instead of rolling off the display as it should.

One of the first things which should be checked is the low voltage power supply outputs.  Your multimeter set to DC can measure the levels and when set to AC, will measure the ripple which should be effectively zero.

Perhaps I should try swapping the plugins again and see if the problem follows along to the verticle. The only problem with that is that there is no triggering but I should still be able to see if there is compression...

To get the triggering to work, use two probes and connect one to the external trigger input on the timebase and set the timebase to external triggering.

[JosephG]

All of the low voltages look clean, though I still want to replace some of these capacitors anyway when I get a chance.

I swapped the plugins again and the problem does not seem to be within the timebase as there is no squishing and the trace is perfectly linear to the eye on the verticle so I'm going to focus on the horizontal amplifier and see what I can find.

In the mean-time if the funny trace aberration persists, i'll work on getting that oscillator working.

 

[David Hess]

Without knowing more, the first thing I would check are the base voltages of the horizontal output transistors which should be constant and about -9.4 volts for the bases of Q560 and Q578 and +113 volts for Q558 and Q578 and the 4 emitter resistances which are 2.49k and 100 ohms respectively as shown on schematic 5.  My hypothesis is that the constant current driving the output stage is low making it slow and these tests are easy enough to do with just a multimeter as the horizontal CRT amplifier board is exposed above the CRT.

[JosephG]

I think you are looking at a different board revision or perhaps the service manual for the actual 7603? Most of it is the same but the horizontal amplifier is quite different in the OS-245.

Here is what my board looks like:

[capt bullshot]

Appears to be just other component numbers: Q555 / Q580 (+112V) and Q559 / Q584 (-9.2V) are the output transistors. Emitter resistors are the same values as David mentioned. You may also want to check the voltage at the emitters, as noted in the schematic. Too low a current (to stay on David's hypothesis) in the output transistors could also result from resistors drifted to higher value, so check the emitter resistors (R555, R559, R580, R584) for their value.

There are many voltages noted in the hor. amplifier schematic, it's a good start to check them all.

[JosephG]

Thanks for the reply capt.

I'm going to go through the circuit as soon as I get done work. Hopefully I'll find something.

[David Hess]

I gave those transistor base voltages and the emitter resistors to check because that is easy enough to do with just a multimeter and they are a common failure point.  I figured the OS-245 documentation is different but the circuit should be close or identical.

Ideally a second oscilloscope would be used to check the horizontal signal between the input and output of the horizontal CRT amplifier.