Tektronix 2465BCT Test 05 Fail 44 (24) Troubleshooting (issue solved)

[KCT_99]

Hi,

I am having a nice Tek 2465BCT on my plate with an issue I am stuck with. I am hoping someone has tripped over that problem before.
The scope came with a GPIB option, for troubleshooting, I have this option board removed.

Here is a description of what I see.:

On power up the scope seem to get stuck with ADD Led on, blank CRT no display. However, if the scope is left on for a while there is a very dim display readable.

TEST 05 FAIL 40

Acknowledging this error makes it seem to complete boot process. However trace position knobs do not seem to work properly.

Entering Test Mode manually and rerunning TEST 05 now fails with 44.

Additional Tests CT TEST 86 FAIL 02 and CT TEST 87 FAIL 18 as well.

For now I am focusing on TEST 05 Failure.

Eventually I traced this issue to the A5 Controller board, Voltages for the +1.36V and -1.25V rails seems bad. Please see my measurements with Jumper 504 in normal position vs. Jumper 504 in other position. At this point I am not understanding why it seem necessary to create these voltages dynamically with the DAC and MUX U2521. What is the purpose of Jumper 504 ?
I have yet not found anything on this jumper in the service manual.

I did find this Thread with a very similar problem but there seem no solution at this time.

https://www.eevblog.com/forum/testgear/2465b-error-05-44-trouble-tracing-down-issue/msg409416/#msg409416

With Jumper 504 in other position I do get the proper rail voltages as expected, CRT brightness is very bright, but readout brightness and trace brightness are not controllable with the knobs anymore. Also trace position and cursor position does not work properly.

[stevesousa]

Page 3a-11 of the service manual, the output of the multiplexer goes to a sample and hold circuit, if your capacitors are leaking or there's a leakage path there(dirt, electrolyte, flux) it's possible that they wont do the HOLD part, but when connected using the fixed voltage of the jumper everything is fine....

[MarkL]

I couldn't find anything on J504 either.  My guess is that it generates a usable, but inexact -1.25V reference voltage for troubleshooting purposes (i.e., not to be used for normal operation).

Besides the -1.25 reference, some of the other items you mention that aren't working, such as trace position, brightness, etc., also depend on the DAC.

If you haven't checked all the voltages in table 5-1, now is the time to do that.

If those are ok, I would start with the DAC, and start by making sure Vref+ and Vref- (pins 14 and 15) are ok.

It's also possible one of the 4051 analog muxes has gone bad and is interfering with the other.  A scope on the DAC output could be informative.

[KCT_99]

Thanks for your kind response and suggestions.

I took some pictures for you, there is one X7R decoupling capacitor which has some black grime around it but I do not suspect this to be the problem. I tried to clean this off with Isopropyl and a Q-Tip. What ever it is, its not coming off easy. I will try with a tooth brush again.
Also on resistor R2014 there was a slight green discoloration on one solder joint, not sue what that could be. I tried to clean this off as well. I did notice the solder joints in this DAC area look not really appealing to me, they look dull as if they were cold joints. I am not sure if back than Tektronix used lead-free solder. The joints look actually really crappy to me.
I tried to re-solder the worst joints using tin-lead solder and rosin flux but for some reason the solder joints still look not great. I am not sure if there is some contamination preventing a decent solder flow.

Anyway, it seems I am onto something. When I poke around this DAC circuit and aggravating using cold spray, I can sort of provoke the problem to come and go intermittently. At this point I can not put my finger on it what exactly is causing this as this is getting a little weird.

I checked the DAC signals B1 to B12, they look good to me altho I can not verify the streaming logic values. But for the moment I think they are just fine.  I looked the the DAC output Pin 18 and Pin 19 (D, E), I see various analog levels as I would expect, however the overall amplitude seems unstable. I am not sure what the peak to peak amplitude should be, maybe you guys know. Sometimes I see about 1.4Vpp sometimes a lot less about 0.6Vpp or even less than that. I can observe the trace brightness, readout brightness and trace position so as delta time cursors going crazy on the CRT.  This is consistent with my understanding how the 1.36V and -1.25V rail is used in this scope. If this is unstable so is everything else.

If I manage to get it stable for the moment, I get the scope to pass all self tests. Good news.

Poking around scoping points A, B and C, I can see bad flicker noise several milli volts.

So I

Re-soldered R2016, did not impact the problem
Replaced C2010 with an electrolytic cap, did not impact the problem
Replaced R2016, did not impact the problem
Re-soldered R2012, did not impact the problem

Double checked the 10V Reference voltage at R2016, looks good.

DAC Pin 20, +5V looks good,
DAC Pin 17, -15V looks good,
DAC Pin 13, 0V looks good.


So at this point I am convinced the DAC circuit is the offender. I am leaning to replace all the passive components around the DAC chip including the pot R2010, but then I will have to recal this DAC circuit which I would like to avoid having to do.

What are the odds that the Pot went bad ?

What are the odds the DAC chip is flaky ?

As I noticed that the DAC signal amplitude (D, E) seems to increase if I put my finger on the DAC chip for a while.

[ArcticGeek]

I don't know if this is related to your problem or not, but looking at the 3rd picture in your previous post it appears to me that the redish color 10k resistor (marked 1002) is not soldered on the one end.   Maybe it just a poor quality image and/or my eyes playing tricks on me, but just thought I would point that out.

[Johnny10]

What is all the reddish brown gunk around the capacitor ends in pic 2?

[KCT_99]

Thanks for offering help everyone. 

Well, (pic 2) I don't like the gunk around this capacitor either, I tried to scrub off this crud with Isopropyl and a toothbrush, whatever this is, it is not coming off with that. I might have to scrape this off.

Yes, that one resistor did not look properly soldered to me either, I re-soldered it, but it made no difference.  Honestly I don't like how this area was lay'd out and how the routing was done. Very likely done with digital rules.

Anyway, I made some progress today and I think I found the issue. As per schematic R2012 is connected to GND and to the 20K Pot R2010. Eventually I took the 10K Resistor R2012 (the one with possibly bad solder joint) off the board and I did some ringing with my meter.

Turns out the connection from R2012 to Pot R2010 did not exist (anymore). 

I reinstalled R2012 and put a small wire from R2012 to Pot R2010. Powered up the scope and .... Problem gone. 
Scope passes all self tests. CRT is as crisp as I expect it of a Tektronix scope.

So far I have rarely seen board failures, this is a classic one. Signal traces are for my taste too thin in this area and perhaps, my guess there could have been an acid trap lingering eating the trace away over time.

The tantalum capacitor C2010 fell victim to my troubleshooting efforts, I wrongly accused this cap as been the troublemaker and replaced it with an electrolytic cap. It does not look pretty but does the job for now. But I don't want to install a tantalum there again. I might source something more suitable and make it look neat but there is nothing I can do about the ugly wire.

So I did some measurements and documented the measured voltages, this might help someone else troubleshoot this problem since this DAC problem is not covered in the Tektronix troubleshooting guide.

I believe TEST 05 failures may all originate in the DAC circuit with this board failure. I bet there is a very similar failure mode in the other thread I had linked above.

[Johnny10]

That was a good catch.

That CRT is nice and sharp.

[KCT_99]

Yeah, I love the Tektronix gear of this vintage.  I have yet to see a Tek scope with a bad tube in it !

Soja, what are the odds of a board failure ?   

[med6753]

Yeah, I love the Tektronix gear of this vintage.  I have yet to see a Tek scope with a bad tube in it !

Soja, what are the odds of a board failure ?   

Good catch. I have two 2465's. (No suffix).

Bad tube? Oh, I have unfortunately. A 465B which refused to focus and right now I have a 7904 with a potentially bad CRT and a replacement on the way. 

[stevesousa]

I would still recommend you remove the electrolytic capacitor next to the two brown and dirty ceramic capacitors (top right). It probably leaked, and that made all those components corrode.

[tggzzz]

If the OP does a little research, they will find that replacing the RIFAs,  replacing the PSU electrolytically, replacing the A5 board electrolytically (and other damaged components) is par for the course.

There are even descriptions and parts lists to be found on this forum.

[KCT_99]

If the OP does a little research, they will find that replacing the RIFAs,  replacing the PSU electrolytically, replacing the A5 board electrolytically (and other damaged components) is par for the course.

There are even descriptions and parts lists to be found on this forum.

Thanks for your suggestions, I might look into that. All of the PS caps were already replaced in this unit prior me getting involved with this. I am not sure if other parts were replaced also. I am not the first one to have a hand in this scope, besides this fine scope is unfortunately not mine. So at this point I made it pass self test, additional refurbishment and preventive replacement of parts is another story the owner of this scope will decide for me.

Would you be so kind and link to the part list you were referring to ?

[tggzzz]

If the OP does a little research, they will find that replacing the RIFAs,  replacing the PSU electrolytically, replacing the A5 board electrolytically (and other damaged components) is par for the course.

There are even descriptions and parts lists to be found on this forum.

Thanks for your suggestions, I might look into that. All of the PS caps were already replaced in this unit prior me getting involved with this. I am not sure if other parts were replaced also. I am not the first one to have a hand in this scope, besides this fine scope is unfortunately not mine. So at this point I made it pass self test, additional refurbishment and preventive replacement of parts is another story the owner of this scope will decide for me.

Would you be so kind and link to the part list you were referring to ?

I'm sure you could find it, but here are a few of my bookmarks...
https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/
https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/msg436809/#msg436809
https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/msg1128707/#msg1128707
https://www.staze.org/tektronix-2465-300mhz-4-channel-oscilloscope-repair/
http://www.condoraudio.com/wp-content/uploads/Projects/Tektronix-2465B-Oscilloscope-Restoration-Repair.pdf

No doubt there is other info that would be useful to you in that thread.

[KCT_99]

Many Thanks, for the links. Now I have some reading to do. 

[KCT_99]

Reading the teardown thread linked by tggzzz I see someone else had exactly the same issue.

https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/1200/?action=post;quote=1921619;last_msg=2444745

https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/1200/?action=post;quote=1928314;last_msg=2444745

Too bad the OP AMR Labs did not reference any Test Failure code in connection with his findings...

As suggested by stevesousa, I will replace the offending leakers and give it another go to clean this DAC area of this gunk.

Thanks anyone for offering help.

[tggzzz]

Reading the teardown thread linked by tggzzz I see someone else had exactly the same issue.

It is a standard problem. That's why a little research on your part would have revealed it.

Finding appropriate search phrases on google is a key modern skill. In this case the search phrase would have been obvious: Tektronix 2465 test 05 fail 44

[KCT_99]

Reading the teardown thread linked by tggzzz I see someone else had exactly the same issue.

It is a standard problem. That's why a little research on your part would have revealed it.

Finding appropriate search phrases on google is a key modern skill. In this case the search phrase would have been obvious: Tektronix 2465 test 05 fail 44

Well, I did do a little (re)search on this fine forum prior posting using exactly the phrase:

Tektronix 2465 test 05 fail 44

Yet it yields precisely one hit, but no solution was offered in this thread.

But thanks again, you have been a great help for me locating the problem.

[tggzzz]

Reading the teardown thread linked by tggzzz I see someone else had exactly the same issue.

It is a standard problem. That's why a little research on your part would have revealed it.

Finding appropriate search phrases on google is a key modern skill. In this case the search phrase would have been obvious: Tektronix 2465 test 05 fail 44

Well, I did do a little (re)search on this fine forum prior posting using exactly the phrase:

Tektronix 2465 test 05 fail 44

Yet it yields precisely one hit, but no solution was offered in this thread.

But thanks again, you have been a great help for me locating the problem.

You're welcome, but this forum isn't the fount of all wisdom. Google with that phrase yields many more, and also picks up that the problem occurs on many (not all) 24x5 scopes.

[peter-h]

I have Test 05 Fail 42.

It appears to be a failure of the auto trig level circuit.

Otherwise the trig level can be adjusted manually just fine.

Isn't the relevant circuit under one of the heatsinks i.e. not serviceable?

[Alfons]

Most problems with these scopes, but also with other older devices, whether oscilloscopes or other older devices, are caused by faulty electrolytic capacitors. This can also damage other parts (often resistors) that are damaged by leaking electrolyte or loose voltages. Further errors are then caused by the repair. It also happens that ICs or transistors fail, but in most cases they are still OK. After reading a lot and gaining the experience of others and repairing some broken devices myself, that is clearly the case.
And especially the Tek 2465 / 67B: 99% of the errors is found on the A5-Board power rails and in the PSU. In very rare cases, a defect can be found on the A1 board (U800).

[BravoV]

I have Test 05 Fail 42.

It appears to be a failure of the auto trig level circuit.

Otherwise the trig level can be adjusted manually just fine.

Isn't the relevant circuit under one of the heatsinks i.e. not serviceable?

Its a chip attached to the heat sink, and not serviceable.

You could try unscrew it, clean the contacts, as I read somewhere sometimes the IC's contacts get corroded overtime.

Worth a try though.

[peter-h]

Are there any pictures anywhere of what is under the heat sink?

Are there normal PCB-mounted chips, and the heat sink just sits on top of them, with thermal grease to make a contact? That is widely used on memory modules, drone power modules, etc.

If the chip is glued to the heatsink, how does it connect to the PCB in a way which allows the heatsink to be removed?

[tggzzz]

The ic is glued to the heatsink.

When you remove the four bolts, the ic plus heatsink lifts off. Clean contacts with IPA, or perhaps DeOxit. Retighten bolts in the usual way to minimise stresses.

U800 is the problematic one in 24x5 scopes. Don't touch that without very good reason!

[KCT_99]

Peter-h,

Test 05 failures are all originating in the DAC circuit.

The resulting auto trig level failure you see is only a symptom of a flaky DAC circuit. I suggest you have a good look at the DAC and surrounding components, use the findings I had, as described above, as a guide. Verify that you get 2.7pp analog voltage cloud on the output of the DAC. You want to see a stable consistent stream on various analog voltages. If you see an instability you might have the same fault I had, that is, a trace was slowly rotten away causing various Test 05 faults and associated secondary faults such as trigger issues or control panel issues.

At this point, there is no need to mess with the chips under the heatsinks. If you disturb them and you need a replacement you might find they are difficult to come by if you want to good chip. I just went through that with a 2432, where I had a half dead CCD chip and needed a replacement. The replacement chip I bought was totally dead. So I ended up having to buy a dead donator scope gambling that at least one good CCD chip would be in there.

Anyway, please look at the DAC circuit, you will find the problem right there !