Just got a Tektronix 2235 (Opt 1)

[belzrebuth]

I've been searching for a scope for the past months and after a bad experience with a Tek 475 (there's a thread about it)https://www.eevblog.com/forum/beginners/about-to-buy-tektronix-475-first-oscilloscope-so-need-price-advice!/msg1011159/#msg1011159 ,
I finally got myself a 2235 USM version.

It's in pretty decent condition but unfortunately the Volts/Div knob is missing 1/3 of its backplate.(see attachement)
If someone knows where I could find a replacement knob please give me a clue.
There's a service center for old Teks in my country but doesn't have one in stock.

The other issue I'm having is that when I turn the scope on the traces seem to have a bit of noise (if I change the timebase value I can see that they are displaying something like a saw wave).
Traces slowly straight up afterwards.
What could be the issue here?
Does the scope just need a little bit of time to warm up?

[belzrebuth]

Here are some pictures with various settings on the scope.
Hopefully if someone has the same or similar scope can confirm that mine is fine.
I'm more concerned on what I see on the first picture.
Obviously I'm a noob regarding oscilloscopes but I intend to study the basics and learn how to use one (just need to ensure my scope is fine and it's me who doesn't have a clue about its operation)

If someone needs more info I can take additional photos with other settings and post them here.

[MosherIV]

Hopefully if someone has the same or similar scope can confirm that mine is fine.
I'm more concerned on what I see on the first picture.

Hi, I do not have that particular scope but it looks fine to me.
The first picture is just showing noise, not surprising since you have selected Channel2 and Chan2 is on 50mV/Div or 5mV/Div and it is AC coupled. I would expect some random noise pick up at that low volts per division.

Try changing the timebase to see what the period is, try turning electrical stuff off around the room/house and see if the noise goes away - you will figure out what is radiating lots of noise this way.

[belzrebuth]

At first I wondered whether this is DC ripple (from where you may ask and I don't know) or just plain noise .

Changing between AC/DC coupling (while the scope is set like the first picture) the wave remains the same.

So it must be mains noise?
I have a lamp connected to the same power strip.
I'll try without the lamp.

The trace should remain flat when nothing connected in all timebases and volts divisions?
Right?!
Not necessarily a flat line of course but flat nonetheless?!

[JPortici]

if i'm reading the panel correctly it looks like a 22kHz ripple, could very well be a residue from the power supply
could also come from the lamp. those are nasty

[belzrebuth]

It's not the lamp:(

So it's probably from the power supply,maybe I can move the scope to another power outlet (other room) and check if it behaves the same.

Channel 1 is slightly more noisy for some reason.

From where would this ripple come from ?Bad filtering?
Should I recap the power supply,maybe?

[JPortici]

i certainly would

[Tom45]

The first channel with the noise has the input set to 2 and input coupling set to AC. What does it look like if the input coupling is set to GND?

Before tearing into it I would suggest leaving it on for a day or so. Perhaps that might more fully form the capacitors in its power supply and also drive out any moisture that has settled in from not being used.

[belzrebuth]

Here's both channels with their Volts/DIV turned fully clockwise.(2m or 20m(for x10probe)).
Timebase is 20?s per division.
GND.

I don't very much like what I see.
I'll turn the intensity almost to zero and leave the scope on for a day like suggested.
Maybe remove the cover so that it doesn't get too hot or it's ok?

[KerryW]

I have a 2236, which is the same but with a computer module (freq counter, voltmeter, etc).
There is a similar signal on my display at those settings, but only 1/5 of a division high.

Try some experiments.  Short out the input BNC.  Still there?  Invert channel 2 and put it in ADD mode.  Still there?

[belzrebuth]

Same settings with ADD (first picture)
and channel 2 inverted (second picture,it's not straight it's still noisy)
ALT mode with channel 2 inverted(third picture)
.


@Kerry
maybe later you could try to apply these settings in your 2236 and post the results?!

[belzrebuth]

Forgot the shorted BNC input
(same settings as the first picture but with the bnc of channel 1 input shorted)
When shorted the amplitude of the signal is reduced.

[NexusKoolaid]

It's in pretty decent condition but unfortunately the Volts/Div knob is missing 1/3 of its backplate.(see attachement)
If someone knows where I could find a replacement knob please give me a clue.
There's a service center for old Teks in my country but doesn't have one in stock.

If you have access to a 3D printer or know someone who does, fabricating a replacement could be unsightly yet serviceable solution while you search for an official part. 

[KerryW]

Mine has a few idiosyncrasies of its own.  Channel 2 only shows the noise if inverted, non-inverted is a flat clean trace.  The GND position doesn't work on channel 1.  The noise is at the limit of syncing, so most tests (like ADD) just result in a slightly fuzzy trace.

[tautech]

@ belzrebuth
Good source for Tek bits and bobs is:
http://www.sphere.bc.ca/

[belzrebuth]

Mine has a few idiosyncrasies of its own.  Channel 2 only shows the noise if inverted, non-inverted is a flat clean trace.  The GND position doesn't work on channel 1.  The noise is at the limit of syncing, so most tests (like ADD) just result in a slightly fuzzy trace.

So if your scope is set like mine in the photo (both volts/div fully clockwise and timebase at 20?s ,only ch1 active ;you're getting some kind of noise or just a slightly fuzzy trace?


@tautech
Thanks for the link.
I checked but I didn't find my part:(

[tautech]

@tautech
Thanks for the link.
I checked but I didn't find my part:(

There's a few listings for 22** knobs, some of which are out of stock however you should email them.
http://www.sphere.bc.ca/test/tek-parts/tek-knobs.html

Edit
This one?

2mV -50V/div
366-2148-01

If you search Google there's plenty of hits using the part #

[KerryW]

At those settings, I can trigger on the noise.  It is about 0.4mV P-P and shows 6 or 7 cycles.  It is not big enough to trigger the freq counter.  Shorting either BNC reduces the signal but does not eliminate it.

I suspect it is from a switching power supply in the scope.  Yours may have a bad cap.

[belzrebuth]

I think I'll make a short video showing other settings and how the scope behaves in general in order to give you a better idea.

I may have to open the scope up and visually check for bad caps.
That makes me a little uncomfortable because from what I've seen the power supply is not easily accessible.
If I start desoldering caps just to test them I could just as well replace them anyway;first step should be just to open it up and write down what caps are already there and order replacements.
I think Nichicon pw series might be a good candidate since they're known for their long life inside switching PSUs.

I'll make a video as soon as I find some time and we'll go from there.

[David Hess]

That noise has got to be from the switching power supply which uses an off-line buck preregualtor followed by a saturable core inverter.  The output capacitors are probably worn out which is a common problem with 22xx series oscilloscopes simply do to their age.

I would not remove any capacitor for testing without replacing it with a new one.

A replacement knob should not be difficult to find.

[belzrebuth]

I opened it up and looked thru the power supply grill vent.
There are five or six caps that are slightly leaking on top.Tops are slightly bulged too.
The scope definitely needs a recap.I hope that will fix my noisy trace problem.

I was a bit lazy and didn't remove the vent but I intend to do this tomorrow.
Anyone has any suggestions on what replacement caps I should buy?

I will probably order from TME.eu

[JPortici]

check the requirements for ESR on the service manual, if you can keep a simillar ESR try to increase voltage and temperature ratings (don't bother if it costs too much though, those specified were good for about 25 years)

[belzrebuth]

I tried to remove the PSU shield by unscrewing all the torx screws but I can't.
Something is holding it back!
Anyways I peeked inside and the bulging caps are branded "IC" (Illinois) and are 1000uf at 25V.
I've never encountered/used illinois caps before so I can't recognize what series they are.
They're brown and don't seem very specialised judging from their bad lettering.
I could very well be wrong on this but they seem generic.

I'm not sure about the c968 but appears to have similar size.

These are the caps that appear to be problematic.
C960
C961
C962
C963
C968
C970

I can't find any info on these other than their values on the service manual.No voltage ratings and no esr.

[belzrebuth]

Service manual states these values for the 960-63 and 68/70.

C960 840uf
C961 840uf
C962 840uf
C963 840uf
C968 840uf
C970 840uf

[David Hess]

check the requirements for ESR on the service manual, if you can keep a simillar ESR try to increase voltage and temperature ratings (don't bother if it costs too much though, those specified were good for about 25 years)

There are no requirements listed.  Finding the specifications will require the original datasheets and the part numbers may not even be available; I have a whole set I removed made by Mallory which use Tektronix house numbers.

As a practical matter, the capacitors at the output of the saturable inverter do not have stringent ripple current requirements because the inverter output is a square wave.  The capacitors at the input and output of the buck preregulator are another matter of course.

Something else which can be done is to use the capacitor list from a later 22xx oscilloscope that has a higher power power supply.  The 2235 was the base model and the 2236, 2230, and 2232 used slightly larger capacitors.