Pease help diagnosing my 2235
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Tom Miller <tmiller@...>
Just for the hell of it, take a look at the CRT shield to the case ground
near the rear of the scope.
See the photos in my last message (go to the attachments section).
Tom M
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lagunablue94c
So I know this probably is not advisable, but I did it anyway. I measured the PS ripple in the scope--with the scope. It seemed to work fine but it looks like there's way too much ripple across all rails. 150mV p-p at ~21kHz in the 100v rail:  From tektronix">pic1 50mV p-p at ~40kHz on the 30v rail:  From tektronix">pic2 +8.6v, +5.2v and -8.6v rails looked like this:  From tektronix">pic3 20mV p-p at ~10MHz. All way out of spec. I see there's a long thread going on about the same problem here: 2235 with 360 kHz 100mV P-P ringing on +100V PSU rail, so I'll have to read that when I get a chance. For the most part the scope is working fine but I can still see a little of the retrace and the line on the left hand side is still there:  From tektronix">pic4 Also you can see the waveform gets a little noisy just past the peaks. Overall though, the scope is very usable, and a helluva deal at $70.  From tektronix">pic5 I'll keep tinkering on this thing until I get it in tip top shape. Thank you all for the help, and let me know if you have any more advice. Bryan
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Tom Miller <tmiller@...>
Well, you are making some forward progress. A very fast check of a resistor
is to measure the resistor in circuit. If you get a reading larger than the
value marked on the resistor, you know it is bad.
There is no lazy way to repair this stuff and learn anything, so go ahead
and get the iron out. Make sure you get an ESD safe iron that has the tip
grounded. Failure to use an ESD safe iron is sudden death to expensive and hard
to find parts.
You could make measurements of voltages in a working circuit but for these,
you will need a good HV probe for the DMM. Also, you will need to make many
calculations to find out what you are seeing.
Someone (Kevin Wood) has made an important comment about fixing the
intensity problem first. You can burn in the screen with the brightness up so
high.
Take your ohmmeter and measure the resistance from the input BNC connector
outer shell to the ground point of the scope. Report your findings.
Good luck,
Tom
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Robert Simpson
Bryan,
toggle quoted messageShow quoted text
If you are working on non grounded devices, you must learn about differential probe techniques. join http://www.ebaman.com/index.php/home then search on differential, Tek has a great article. Otherwise you will get all kinds of screwy results. Bob
--- On Fri, 5/20/11, lagunablue94c <bryanlow70@...> wrote:
From: lagunablue94c <bryanlow70@...> Subject: [TekScopes] Re: Pease help diagnosing my 2235 To: TekScopes@... Date: Friday, May 20, 2011, 12:56 PM ---
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lagunablue94c
Okay, were getting closer. Chalk that up to user error I guess.
toggle quoted messageShow quoted text
 From tektronix">Pic1 Connected Amp ground to the banana plug jack on the front panel labeled "GND" duh...  From tektronix">Pic2 Whatever that clip is, it's not connected to chassis ground.
--- In TekScopes@..., "Tom Miller" wrote:
> > If I remember right, is there a separate ground wire on the input BNC connector(s) on these scopes? > > Tom >
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Kevin Wood G7BCS
Here are a few more pictures for reference:I would fix the intensity problem first if I were you.Running the tube as bright as it looks in those pictures for any length of time will do it no favours at all. Check over all the components in the cathode & grid drive / DC restorer and see if you can crack that. Then you can focus on the hum without worrying about burning the tube. Kevin
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lagunablue94c
--- In TekScopes@..., "Tom Miller" wrote: > > For #1, where in the CRT power supply did you measure the ripple? Do you have a bad probe ground? Are you trying to measure the audio in a floating system? > I haven't measured ripple yet. I'd need a scope for that :( The scope came four probes, three of which I believe to be good. They all give the same readings so I don't think it's a probe problem Floating system? Hmm, yes I believe it is. Could this be causing the superimposed signals problem? The amplifier in question is a simple 8w mono amp. Power input is +12v -12vdc. No ground. Here are a few more pictures for reference:  From tektronix">pic1 probe not touching  From tektronix">pic2 probe touching (+)speaker out, Amp NOT PLUGGED IN. NO VOLTAGE PRESENT. From tektronix">pic3 probing (+)speaker out. AMP PLUGGED IN, NO AUDIO PLAYING. Why would these waveforms be showing up, and if this was really a PS ripple wouldn't the waveform be smooth, and without these "kinks" in it?  From tektronix">pic4 Probing (+)speaker out, PLAYING MUSIC. Connecting ground clip to (-)speaker out has no apparent effect on waveforms. > 2. Out of spec resistors are known causes of display problems in the 2235 (and other scopes). There are some carbon comp resistors R888-R893, 510K that are known to drift high. Just lift one lead and measure them. Replace the ones that are out of spec. > Is there any way to check a resistor without breaking out the soldering iron? I was thinking about measuring voltage drops, but that would require making an assumption about current flowing. Thanks, Bryan
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Tom Miller <tmiller@...>
If I remember right, is there a separate ground wire on the input BNC
connector(s) on these scopes?
Tom
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Tom Miller <tmiller@...>
For #1, where in the CRT power supply did you measure the ripple? Do you
have a bad probe ground? Are you trying to measure the audio in a floating
system?
2. Out of spec resistors are known causes of display problems in the 2235
(and other scopes). There are some carbon comp resistors R888-R893, 510K that
are known to drift high. Just lift one lead and measure them. Replace the ones
that are out of spec.
3. The HV probe is a very useful thing to have and will save your 77 from
an oops.
4. You will only need the expensive equipment to do a full calibration for
horizontal sweep and vertical levels.
Tom
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lagunablue94c
It really is strange that I don't have any ripple visible in the
calibrator signal. The scope came with three good probes, and all of them react similarly on both channels so I don't think the problem is the probes themselves. Could the probes possibly be referenced to something other than ground, and there's a ripple in the reference voltage? Only thing that might be handy is another scope to check for ripple onthe power supply circuitry although if the ripple is not present when
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Kevin Wood G7BCS
3. The DC Restorer circuitry is not working properly. Kevin, I'mYep, that's it. You can regard it as a network that turns the Z axis signal (let's say it varies from 0-100V for the sake of argument. I'm not sure on the exact range) into an AC signal with an amplitude of 0-100v. That signal is then coupled into the cathode circuit where it's rectified again, so it appears as a -2000V to -2100V signal. C855 bypasses it to improve the transient response. So here's my dilemma: The manual lists a bunch of equipment needed to Based on the above assumptions I've ordered a Fluke 80k-6 probe to checkI wouldn't say so at this stage. The DC restorer just needs the diodes and resistor values around it checked. That can be done with the fluke. Only thing that might be handy is another scope to check for ripple on the power supply circuitry although if the ripple is not present when measuring the calibrator signal it makes me wonder if it's the ground connection in your probe that's the problem. Kevin
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lagunablue94c
Thank you all for the suggestions.
I think I'm a little in over my head here, nevertheless it is a fascinating exercise and a great learning experience. Based on what you guys have said, I'm making some assumptions:
So here's my dilemma: The manual lists a bunch of equipment needed to make adjustments, none of which I can really afford. My entire test-bench consists of a Fluke 77 DMM and a broken Tektronix 2235. Based on the above assumptions I've ordered a Fluke 80k-6 probe to check the CRT voltages and now I'm planning to get an ESR meter to check the caps in and around the PS. Is there any other piece of equipment I absolutely need to have to make this diagnosis? Thanks again, Bryan
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Greg
"lagunablue94c" <bryanlow70@...> wrote:In part make sure you understand all the controls. There are two brightness controls. The delay controls are some what confusing (to me) and take some prctice, but I am not saying your don't have an issue. What I am saying when I first got mine I though something was wrong, but it was OK and I just did not understand the operation of the 2235a. Bryan I have a 2235a and love it. I had a few issues. I can't address your specifics, but all my problems were with the two vertical attenuator's X1 and X10 (per channel). They are very delicate and subject go failure. Some times you can unplug them, pop off the snap-on plastic cover and re-solder some connection. Obviously your issues sound more in-depth, but I thought I share that with you.
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Kevin Wood G7BCS
Hi Bryan,
In addition to the excellent suggestions already made, if the cathode is OK at about -2KV I would have a careful look at the DC restorer circuit that drives the control grid of the tube. I.E. C855, CR851, 853, 854, 855 and associated components This circuit takes the Z axis signal from the intensity control incorporating the blanking pulse when the timebase retraces (a few tens of volts at a guess) and references it to the cathode voltage (-2KV if all is well) so it can control the grid of the tube relative to the cathode. I had a scope with exactly the same symptoms of excessive brightness and a couple of the diodes in the restorer circuit were to blame. While you're in the area, check the values of all the resistors. The 510k resistors in the focus chain are particularly prone to going high in value, although this doesn't appear to be your main problem there may be other resistors going faulty. As for the AC signal that's superimposed on the trace, can you estimate its' frequency? I agree that this is probably down to ripple on one of the power supply rails. Checking its' frequency relative to the two supply choppers might give you some clues - in addition to just checking the rails for ripple, of course. Kevin
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magnustoelle
G'd Day Bryan,
and welcome to the group. Just to amend to Tom Miller's excellent advice: Do not compromise on any safety aspects when doing high voltage probing. You might want to check the usual online-auctions for a SIMPSON High Voltage probe or similar. I am using a SIMPSON 00509 probe for some time now and I am happy with it. Good Luck with your troubleshooting. Magnus
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Tom Miller <tmiller@...>
Ok, here is what you need to do first.
Go to section five of the service manual, "Adjustment Procedures" and start
doing steps 1 to 6. Look at the AC ripple on each of the low voltage DC test
points. Record each of the levels. Be vary careful doing this as a slip of a
probe can make things much worse, On page 6-8, the voltages and ripple
limits are given.
Tell us what the results are. You will need a high voltage probe for your
DC meter to measure the cathode voltage correctly. It will be about 2000 volts
negative.
At first glance, it looks like the grid bias is up too high.
Regards,
Tom
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Lindberg, Steve <steve.lindberg@...>
The line going back is retrace. Check power supplies, especially around focus and intensity stuff in HV section. Make sure you don't get shocked.
Steve Lindberg
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of lagunablue94c
Sent: Wednesday, May 18, 2011 12:59 PM To: TekScopes@... Subject: [TekScopes] Pease help diagnosing my 2235
Hello everyone, 1. Intensity knob only adjusts from bright to really bright 2. Vertical line on left hand side of screen 3. Line running through waveform - looks like the electron gun moving to start position 4. Waveform is noisy 5. When measuring a low voltage signal, the signal is added to a phantom 24v sine wave 6. Delay trace shows up as a vertical line on the left side of the screen Here of some pictures of this scope in action:
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lagunablue94c
Hello everyone,
First off I'd like to thank you for having a group like this who might help a poor chump like me. I'm an electronics novice trying to learn more so I bought a 2235 for $70 with the understanding that one channel works. Well, upon initial startup only one trace showed up. After making some quick adjustments I was delighted to find traces for both channels. Unfortunately after further investigation it seems neither channel works correctly. I've searched through the Textronics troubleshooting manual, service manual, operators manual and schematic trying to understand my problems but I'm not knowledgeable enough to make a diagnosis. I'm hoping someone here could give me a little insight so here's what's wrong:
 From tektronix">pic1 Both channels measuring 500mA internal reference signal. Seems ok.  From tektronix">pic2 120vac signal. Note vertical line on left side & lighter line running through middle of trace.  From tektronix">pic3 Other end of 120vac signal. Note light horizontal line coming from end of trace.  From tektronix">pic4 Upper trace is the line-in to an audio amplifier. Lower trace is the amplified output. It's difficult to see it here, but the real signal is added to a 24v sine wave.  From tektronix">pic5 Lower trace is measuring the 500mA internal reference signal. Vertical line in upper left corner is the delay trace. I can't see anything obviously wrong under the cover. There is some discoloration of the mainboard around some of the big power resistors but that's probably to be expected. If anyone has any ideas I'd really appreciate it. Thanks, Bryan
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