Tek 2465B Repair Help Please

[DC912]

My 2465B purchased about 2 months ago was working fine.  The serial number is above 51000

After reading all of the posts about the 4 electrolytic caps on the A5 board eventually leaking and damaging the board, I decided to replace them.  That was may first mistake (fixing something that wasn't broken).

I am a hobbyist, having worked on tube radios, tube communications receivers and tube amps for many years. Everything I have done in the past is point to point, and through hole.  I am still kicking myself, but somehow I convinced myself that the bar on the smd tantalums indicated the negative side because the electrolytics I use in tube gear typically have the negative indicated on the cap.  That was my second mistake (not reading the data sheet). So, I reversed polarity on all four of the replacement caps when I installed them, and when I powered up the scope, there was a "pop" and I realized pretty quickly what I did wrong. 

Cap C2965 shorted.  Caps C2011, C2331 and C2113 did not short.  I have now replaced all four of those Caps with new Caps, and C2890 looked like it burned so I replaced that cap too.  After replacing the caps, on power up, the fan runs, cycling on and off a bit, and all LEDS on the front panel light.  Pressing the A/B trigger doesn't affect the LEDS--they all stay on continuously.  There is no trace, but the scale lights work.  None of the diagnostics show on the screen.

I am going to check the low voltage test points, but if anyone can direct me to likely faults or other things to test based upon having reversed polarity on those 4 caps, and the symptoms described above, I would greatly appreciate it.  The last thing I need is another electronic doorstop.

Thanks.

[mskobier]

DC912,
    Sorry to hear you made the mistake with the tantalum caps. I've done similar things myself in the past.

One of the first things I would do is check for burned/opened traces. The traces on the board in that area are pretty small. I would refer to the schematic in the manual and start tracing the circuit. I know in one case documented on this forum where the trace was open due to corrosion was a number of components  away from the caps that where leaking. Since these traces are so small, it would be easy to burn some traces by a reversed polarity capacitor installation.

There is also a small fuse on one of the power supply boards that may have opened. Not sure which circuits it protects, as I don't have a manual handy.

Please keep us informed as to what you find.

Mitch

[SoundTech-LG]

More "repair help" needed. My 2467B seemed to be intermittently varying in intensity. Now, it is intermittently going FULL BRIGHTNESS. Seems to do this after about 1 hour on time. Turning off, and back on on "solves" this issue, then it may re-occur later once again...  nice feature. I'm afraid to walk away from it turned on for fear of screen burn.

[MarkL]

Does the screen look otherwise normal?  Is it just like someone turned the brightness (Intensity) knob all the way up and it's only the traces?  Or is it everything on the screen?

The problem is that there are a lot of subsystems that have a say on beam brightness at any given time.

Assuming it's just the traces, the intensity knob is interpreted by the processor, so there could be a problem trying to read it.  I would take a look at pin 2 on U2501 which is where it enters the MUX with other pots that are being read.  Of course while the problem is happening.

Depending what equipment you have available, you could also take a look at the output of this MUX when pin 2 is selected and enabled.


And if it's happening to everything on the screen, it's a more general problem.  In that case, maybe poke around U950 TP65 and TP66, or BLANK and BRIGHT outputs on U650.


Of course the major difficulty here may be waiting for it to happen.  One approach is to take readings on multiple suspect test points and compare the normal and problem states.

I know you don't want to burn your screen, but you could spend a little time characterizing the problem when it happens to find out what knobs or inputs have any effect.  Perhaps even the Z-axis input.

EDIT: Typo.

[oldway]

Setting the beam out of focus help to prevent burning the screen.

[SoundTech-LG]

Thanks. All good suggestions. Not sure if FOCUS is changing anything. It gets really bright (everything) at that point the INTENSITY control does nothing. My only escape is power off... power on.
The problem is, it is so very intermittent, but as I said, it appears there are intensity variations occurring all the time. I have not seen it occur after the beam SHUTDOWN activation. But it may be possible during that time as well.

[DC912]

Regarding the original post, I believe I've narrowed down the problem to the +5V Digital supply (all of the other low voltage readings are correct).  At the test point for +5V D on the main board, my DMM reads about 2 volts.  According to the schematic, the positive side of two of the caps I replaced on the A5 control board are connected to +5V D, and the reading on both of those caps is also about 2 volts.  What would be the best method of attempting to determine whether the faulty components is on the main board vs. the A5 control board?  Are there particular interconnects from the boards that I can remove such that the +5V D supply source will be present on the A5 board without having something on the main board potentially affect that voltage?  I want to try to isolate the problem to one of the boards, if possible, so that I can narrow my search for the cause.

[MarkL]

Did you check fuse F1102 on the inverter board?  Tracing back through the schematics, short circuit current for C2965 would have gone through it.  It looks like a solder-in type.

While you're there, Also check F1101 right next to it.

If the fuse didn't blow, the shorting cap could have taken out dual rectifier CR1110.

(Credit: Mitch had suggested checking fuses.)

[DC912]

I checked a glass fuse in a holder on the inverter board and a glass fuse in a holder on the regulator board when I read Mitch's reply and they were both intact. I did not see any other fuses, but I was not looking for soldered in fuses. I'll pull the boards again and have another look. Thanks.

[MarkL]

Thanks. All good suggestions. Not sure if FOCUS is changing anything. It gets really bright (everything) at that point the INTENSITY control does nothing. My only escape is power off... power on.
The problem is, it is so very intermittent, but as I said, it appears there are intensity variations occurring all the time. I have not seen it occur after the beam SHUTDOWN activation. But it may be possible during that time as well.

(Seems that we have two threads in one here...)

If you think the brightness is varying all the time, something must be intermittent all the time and then it finally fails for whatever reason.

I would start looking at some of those points I mentioned previously with a scope.  Try to find any randomly changing voltages that would correlate with the brightness changes.  A good place to start would be the test points on U950.  Expected waveforms are shown in the service manual.  If anything strange is happening it should show up there since that's driving the grid on the CRT almost directly.   Then you can work backwards to find the source.

This is one of those cases where you can troubleshoot the scope with itself, if you don't have a second scope.  From what you're saying, the fluctuations are unrelated to the trace sweep.


EDIT: A slight correction: You can troubleshoot the scope with itself, BUT you won't see the waveforms in the manual because those ARE correlated to the sweep.  You can, however, see any random fluctuations.  So... It can be done if you have no choice, but it would be better to use a second scope.

[SoundTech-LG]

Again, thanks for the tips. Do not want to hijack here, the thread title was maybe a bit wide open. I'll start a new one with a little narrower subject if neecessary.

[DC912]

Thanks guys--the 5A wire lead fuse was blown. Since I had to separate the two boards to replace the fuse, I decided to replace all of the caps on both boards too. Seems like everything is okay now. With luck, the scope is ready for another 20 years!

[Bryan]

Thanks guys--the 5A wire lead fuse was blown. Since I had to separate the two boards to replace the fuse, I decided to replace all of the caps on both boards too. Seems like everything is okay now. With luck, the scope is ready for another 20 years!

Well, not quite. the Dallas 1225 SRAM that holds the calibration data is probably on borrowed time. May want to look at replacing, but needs some careful reading on other posts and forums as you don't want to lose the calibration data on changeover.

[DC912]

Yes, thanks. I replaced that when I replaced the caps on the A5 board.
That was the only part of the A5 rebuild that I did not have to do muliple times. Think I'll stick with repairing tube radios and leave SMDs to others for awhile. Dave

[Addicted2AnalogTek]

SoundTechLG -  I don't know if this applies to your scope, but I just read something regarding the exact issue that you are having.

It was off of a pdf linked to by tautech in "Repair documents and links":
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf
 Page 49.

Hope this helps!

[Addicted2AnalogTek]

SoundTech -

I ran across this article while searching for info for tantalum cap ESR for the ol 468 and the like:

http://www.condoraudio.com/wp-content/uploads/Projects/Tektronix-2465B-Oscilloscope-Restoration-Repair.pdf

Hope it helps!!!

[SoundTech-LG]

Yes, tantalums are ALWAYS suspect. Will try to keep in mind. All the work you referenced has been completed on mine, unless there is even MORE unseen electrolyte floating around somewhere in there that I still have not noticed, corroding things up.

[Analogdevice]

Hello friends,

I've got a problem with my 2465B. The display went blank, intensity pot does not work.
Turning up the grid bias did bring the display into view, but the intensity adjustment makes no difference at all. At the VZ out is +8V, which means blank screen, this voltage should be reach +75V for maximum intensity, but there is 8V and doesn't matter in which position is intensity pot.


Any ideas?