Saturday, November 18, 2017

Repair Log: Tektronix SC-502 Oscilloscope

[Note:  My "Repair Log" blog posts contain my notes on equipment I've recently repaired.  Posted here in case someone else might find them useful.]

A couple of days ago I was thinking it would be nice to add a small oscilloscope to my FPGA SDR station for monitoring my transmit RF, and I remembered that, several years ago, I had picked up a broken Tek SC502 oscilloscope module at the De Anza swap-meet.


It had been languishing in my "projects for the future" pile (a very large pile!), and I thought that it, coupled with a TM503 chassis, could be a great addition to the station.

So I retrieved the SC502 from the pile, plugged it into the TM503, turned it on...and started to hear periodic snapping.

Oh oh -- something is arcing!

Also, although I could see a glow on the CRT (which was a sign that the CRT was probably good), there was no trace.

Well, I knew it was a project when I bought it.  Time to dig deeper...

After removing it from the TM503 chassis, I noticed that some of the pots were missing their wipers while blowing out the accumulated dust with a can of compressed air.

How could I tell?  Here is a picture of a "good" potentiometer (left) with its wiper "top" and a "bad" potentiometer (right), missing its top.


Below are the four bad boys (with one unattached wiper top) that I replaced, using various trimmer pots from my junk box (none of these replacement pots were of the original (and apparently flakey) style used by Tek in this module):


[After I had reassembled the SC502 I discovered that the wiper-top of R473 was turning freely -- in other words, it was also broken, but I had not noticed this earlier because the top had not fallen off of the pot.  I thought about replacing this one, too, but removing the CRT assembly to get at the PCB pads is a bit of a pain, and I decided that, for my application (monitoring my modulated waveform), R473 isn't that critical (it is used to set the frequency response of the cascode stage driving the vertical deflection plates).]

With the four potentiometers replaced and the SC502 reassemblied, I plugged this module back into the TM503 chassis and powered it up.

No snapping!  A good sign!

And now I could see a trace -- but it was very bright -- much too bright even with the INTENSITY knob turned to minimum intensity.

And another problem -- although there was a trace, I could not get it positioned above the lowest graticule marker on the CRT.  In other words, I could not move the trace up to the vertical center of the screen.

Attacking the latter problem first -- there was clearly a problem with the vertical deflection.

With no input signal, it seemed to me that the voltage on the CRT's vertical plates should be equal if the trace were to be centered vertically on the CRT screen, otherwise the electron beam would be deflected vertically in one direction or the other.

But measuring the collectors of the cascode plate drivers (Q470 and Q475), it was clear no amount of position-pot rotating would make these voltages equal.

So, with the SC502's MODE switch set to CH 1, I first adjusted the CH1 POSITION potentiometer so that the voltages at the collectors of Q320 and Q325 would be equal (in my case, roughly -2.3 volts) -- my thought being that, if these two voltages were equal here (assuming equal collector loads), this equality-of-voltages should apply to the differential signals along the entire vertical amplification chain.  And problems would be revealed if the two complementary voltages were ever unequal.

I've annotated Tek's schematic with my voltage measurements, below.  You can see that the complementary-voltages become unequal in the second schematic, at the bases of the Q460/Q465 pair (in fact, Q460 isn't even forward biased!).

(Click on image to enlarge)

(Click on image to enlarge)

With power off I made a quick in-circuit resistance measurement of R447 and R454 -- being in circuit, each should measure no more than their 698 ohm values (and possible much less, depending upon what is in parallel with them), but their values should be equal.

Instead, I discovered that both values measure significantly greater than 698 ohms (by at least 1K ohms),  and that the two measured values differed significantly, too.

So I removed R447 and R454 from the circuit and remeasured their values.  R447 was 1.78K, and R454 was open!

Per the manual, these are both 1/4 watt resistors, and, seeing how they and the board below them had darkened from heat, it seemed pretty clear that heat from power dissipation had probably affected their values.

Interestingly, using the voltage measurements above, the heat dissipation across each resistor should have been about 0.3 watts with the trace centered vertically on the CRT screen -- in other words, they were operating above their power-rating specification!

This too-high power dissipation raises the possibility -- perhaps the -5 volts I measured at the emitters is out-of-spec, and that this voltage should actually be lower -- if no base current were being drawn by Q450/Q455, then the voltage their bases would be about -8 volts (i.e. R449/R450 voltage divider), putting the emitters at about -7.3 volts.  In this ideal case, though, R447/R450 power dissipation would be about 0.23 watts -- too close to the 0.25 watt resistor specification for my tastes.  And with actual base current (rather than an ideal base current of 0 mA), the base voltage of Q450/Q455 will only become more positive, thus raising the emitter voltages and the power dissipated by the two resistors.

Concerned about their heat dissipation, I replaced R447 and R454 each with a series-connection of three 232 ohm, 1/8 watt resistors (to improve the overall power rating), and mounted them a bit away from the PCB so that they would get some air circulation around them and not discolor the board further.  (Note, these mods could be made without disassembling the SC502).

With this mod, the power rating of R447 and R454 becomes 0.375 watts.  But if more power-dissipation margin is desired, perhaps a better choice would be to replace R447 and R454 each with a series-connection of four 174 ohm, 1/8 watt resistors.

(Click on image to enlarge)

With the SC502 plugged back into the TM503 and powered up, success!  The trace(s) could now be centered vertically on the CRT!

The blindingly bright intensity was fixed by adjust the beam-current potentiometer (R873).  Although the manual says the current should be adjusted so that the test-point measures 0.4 volts, I found 0.2 volts to be a bit better, in my opinion).

Here's the TM503 and SC502 undergoing some bench checkout, just after I finished adjusting the beam current...


One final note -- the SC502 is a 15 MHz scope, and I had planned to used it for monitoring my transmit RF at frequencies up to 30 MHz.  Because I am more interested in seeing how the waveform looks rather than making accurate voltage measurements, it does not bother me if the response rolls off above 15 MHz, but I was curious how much this roll off would be.

Using my HP 3335 signal generator set to +10 dBm and feeding the scope directly via a length of coax (terminated in 50 ohms at the scope input), I measured the following frequency response.  Note, the amplitude measurement is in vertical division of the scope's CRT.


So, at 30 MHz the frequency response is only down about 3 dB from the response at 1 MHz.  Not too bad, and certainly acceptable for my application!

That's it for this post.


Resources:

Instruction Manual PDFs (which include schematics) can be downloaded here:

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/SC502%20.pdf

http://bama.edebris.com/manuals/tek/sc502/



Standard Caveat:

I might have made a mistake in my designs, equations, schematics, models, etc.  If anything looks confusing or wrong to you, please feel free to comment below or send me an email.

Also, I will note:

This information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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