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Another A5 board repair attemp - help needed


Rogerio O
 

Dear all,
I bought a Tek 2465BCT at Ebay.
The scope was damaged during shipping and two control knobs had their shafts broken.
While opening the unit to replace the pots, I used the opportunity to take a look at the A5 board, and as usual, the SMD electrolytic caps had leaked.
Fortunately the corrosion seemed light no damage to the traces and just one pad lost.
I unsoldered the components around the DAC and the 74HCxxx IC's washed the board with water let it dry and soldered the components back.
I had to replace some smd caps and resistors, as well as two 74HC IC's.
I checked the traces for continuity and found no open traces so I was very confident the repair attempt would be successful.
I turned the scope on, heard the "clicks" and the leds flickered as usual but to my surprise, there was jus a "square" four div wide.
The intensity, position and readout controls do not work, but the horizontal seems to work.
So I do not know if there if the problem is on the A5 board or the infamous U800 has also failed.
I have uploaded the photo of the board and the screen I see after the boot in the album A5-repair attempt.
Would a broken A5 board produce a screen like the one I am seeing?

Thank you for any ideas.
Roger


Siggi
 

On Thu, Nov 5, 2020 at 6:42 PM Rogerio O <rodd414@gmail.com> wrote:

Would a broken A5 board produce a screen like the one I am seeing?
Is your beamfinder switch perhaps stuck? This looks exactly like my 2467
did before I re-animated the beamfinder switch.


satbeginner
 

Clean the Beamfind switch first, then try again.

Leo


satbeginner
 

Btw, here's the link to your album:

https://groups.io/g/TekScopes/album?id=256231

Saludos, Leo


Rogerio O
 

Thank you for all the replies.
You were right, the beam finder button was stuck and I was unfamiliar with this image.
Sorry for wasting your time on this trivial issue.

But I have fixed this and I can see that the scope has problems.
There is no horizontal trace, just a series of dots and a message on the top left corner of the screen saying A1 voltage and some number.
Intensity and position controls does not have any effect on the display.
Vertical gain has also no effect.
Focus and Horizontal time/div are working.
I have uploaded a photo of this display ( https://groups.io/g/TekScopes/album?id=256231)
Any hints on how to proceed with the troubleshooting?
Thank you


satbeginner
 

Make sure the triggering is set for auto, even select line trigger just to force the scope to trigger.

Check for the trigger light being on.
Select one channel, eg 1, and try to adjust vertical position.

Let us know what you see.
Pictures are a great help.

Succes,

Leo


Siggi
 

On Fri, Nov 6, 2020 at 6:43 AM Rogerio O <rodd414@gmail.com> wrote:

Thank you for all the replies.
You were right, the beam finder button was stuck and I was unfamiliar with
this image.
Sorry for wasting your time on this trivial issue.
Don't sweat it - we've all been there. I spent hours trying to suss my
2467, where the beam finder was simply stuck :).


But I have fixed this and I can see that the scope has problems.
There is no horizontal trace, just a series of dots and a message on the
top left corner of the screen saying A1 voltage and some number.
Intensity and position controls does not have any effect on the display.
Vertical gain has also no effect.
Focus and Horizontal time/div are working.
I have uploaded a photo of this display (
https://groups.io/g/TekScopes/album?id=256231)
Any hints on how to proceed with the troubleshooting?
Everything you're seeing could be explained by the intensity control being
at zero, whether due to the pot itself, or due to a fault on the A5 board.
If the intensity is set way down, you wouldn't see a sweep.
A fairly direct way to diagnose this would be to measure the DIR signal on
the A5 board, see where it's at, and whether it responds to the intensity
pot.

Also, as Leo (satbeginner) says, set the scope up for where it's definitely
sweeping:
CH1/2 coupling at GND.
Trigger in AUTO LVL.
Reasonable sweep speed (100us/div, perhaps).

The TRIG'D light is also a handy diagnostic aid. If you input a signal to
the scope and get it to trigger on it, that indicator will light up whether
or not you see the sweep.
So, hook a probe up to CH1 and attach it to the calibrator, then set AUTO
LVL triggering for CH1 and see whether your TRIG'D indicator lights up. If
so, you almost certainly have a sweep, you just don't see it.


Rogerio O
 

This time I got the error message test 4 fail2 (but still no trace), but I guess it is not the main problem for the moment.
I tried to check the +1.36V and the -1.25V as suggested in other trend, and they are VERY off.
The positive voltage is 0.258V (measured @ C2420) and the negative voltage is -0.234V (measured @ R2421.
The +10V Ref to U2101 is ok.
I used a hot air gun to desolder the smd parts, including the DAC.
Is it possible that I have fried the chip?
I am checking the voltages around Q2320, U2420A/D and try to figure out how this circuit works, since it is where these voltages are being generated.


Siggi
 

Hey Roger,

If your DAC's off, then all bets are off, and this could well be your sole
problem, so kudos on measuring the +1.36V and the -1.25V references.
The DAC's reference is a current, not a voltage, so it's practically
impossible to figure out what's up there by measuring voltages. You should
have very close to zero volts between pins 14 and 15 of the DAC though.
Likely you simply have a bad trace or a bad component in the DAC's
reference circuit, and the best way to check is by continuity and
resistance measurements. I think if you measure R2010/2011/2012/2013, and
how they connect up to pins 14 and 15, you'll find that there's either an
open resistor, or an open trace somewhere. The best way to do this is to
measure from the exposed contacts on the resistors and to the pins on the
IC.

You're on the right track - good luck,
Siggi

On Fri, Nov 6, 2020 at 12:03 PM Rogerio O <rodd414@gmail.com> wrote:

This time I got the error message test 4 fail2 (but still no trace), but I
guess it is not the main problem for the moment.
I tried to check the +1.36V and the -1.25V as suggested in other trend,
and they are VERY off.
The positive voltage is 0.258V (measured @ C2420) and the negative voltage
is -0.234V (measured @ R2421.
The +10V Ref to U2101 is ok.
I used a hot air gun to desolder the smd parts, including the DAC.
Is it possible that I have fried the chip?
I am checking the voltages around Q2320, U2420A/D and try to figure out
how this circuit works, since it is where these voltages are being
generated.







Rogerio O
 

Progress!
I had already replaced R2012 but forgot to check R2013, which was open!
Both have been replaced with TE RN73C2B10KBTDF from Mouser (10K, 0.1%, 10ppm).
Now the scope boots with TEST4 FAIL2 but after pressing A/B TRIG the horizontal lines is visible and the vertical position control work.
The next step is to find what is causing four dotted lines to appear near the top and bottom of the display.
I have uploaded a photo of the display to the album related to this thread (https://groups.io/g/TekScopes/album?id=256231).
Are they related to the error?
Thank you all for the help.
Roger


Siggi
 

Hey Roger,

Outstanding, congrats on getting the traces going!
TEST04 means your calibration data is gone, and you'll have to re-calibrate
the scope. See pages 6-11/6-12 in the service manual. It's not the worst
thing in the world, as the scope probably could have done with a
calibration anyway.
I believe that the dots are unrelated to the calibration error, they're
likely there because your grid bias is too high (as Leo, satbeginner had
mentioned). This means you see the portions of the readout where the beam
should be blank. This probably also means that someone was in there before
you, but they only went so far as to tweak the grid bias.

Siggi

On Fri, Nov 6, 2020 at 1:30 PM Rogerio O <rodd414@gmail.com> wrote:

Progress!
I had already replaced R2012 but forgot to check R2013, which was open!
Both have been replaced with TE RN73C2B10KBTDF from Mouser (10K, 0.1%,
10ppm).
Now the scope boots with TEST4 FAIL2 but after pressing A/B TRIG the
horizontal lines is visible and the vertical position control work.
The next step is to find what is causing four dotted lines to appear near
the top and bottom of the display.
I have uploaded a photo of the display to the album related to this thread
(https://groups.io/g/TekScopes/album?id=256231).
Are they related to the error?
Thank you all for the help.
Roger






tekscopegroup@...
 

On Fri, Nov 6, 2020 at 09:03 AM, Rogerio O wrote:


This time I got the error message test 4 fail2 (but still no trace), but I
guess it is not the main problem for the moment.
I tried to check the +1.36V and the -1.25V as suggested in other trend, and
they are VERY off.
From my own experience fixing a 2465B by dealing with the A5 board issues, if the DAC voltages are way off or not even present you will most likely not get a visible trace at all because for one the brightness setting depends on the DAC voltage references. I ended up having to replace several resistors, notably both of the 10K 0.1% ones and a few others, this aside from the obvious leaking caps. I used common leaded 1/4w 5% resistors tack soldered in place of the damaged SMD ones as a temporary test and managed to get the DAC voltages back in the ballpark and so the trace came back as well. And if you are lucky you might be greeted by a error free screen as well. Calibration will obviously still be off with those temporary resistors but at least you can get things working again and make sure the incorrect DAC voltages where your main problem related to the no trace condition. After getting these results only then went ahead and ordered all the required correct parts from Mouser. In some extreme cases the adjustment trimmer next to the DAC chip will also be damaged, but leave that for last and if possible try not to mess with it. The idea is to leave the current adjustment values to whatever they where, and BTW they are not supposed to be EXACTLY at +1.36 or -1.25 but rather show the values to whatever they where adjusted to during the last cal, usually close to those reference values, but not necessarily exactly that.

In my case the faulty resistors where either totally open or way off value (usually up) , so almost all the faulty ones where easy to find without desoldering them. And again if at all possible refrain from making any adjustments to the DAC as you might loose whatever little calibration is left. If you mess with the DAC voltage adjustment trimmer all bets are off and a full cal will indeed be required. Of course the best action would be to fully cal the scope once its back to fully working, but some of us don't have the necessary instrumentation or just don't want to bother with it, or have access to a qualified workshop that can do this, or simply do not need our scope to be absolutely on the money as we are only hobbyist and able to tolerate slight variances.

Also worth mentioning in case you did not yet do so, to ensure that you got all the leaked electrolyte off the board, make sure to wash and gently scrub off the complete top side with water and -dish soap- and use a soft toothbrush. This is the only sure way of getting rid of all the electrolyte still left on the board (IP Alcohol will not work). If you still have any hidden electrolyte left it might cause unwanted conductive paths around affected components and throwing off voltages and lead you to a merry go round while trying to troubleshoot the components. So making sure to start diagnostics and troubleshooting with an absolutely clean board is paramount.


Rogerio O
 

Hi folks I thought I have made a post relating the progress of my work yesterday, but it seems it was not "recorded" so I will try to repeat it.

Thank you all for the replies, specially Siggi for the continued support.

Since I will have to calibrate the scope anyway, my next move was to replace the NVRAM.
There is an excellent article showing how to use currently available chips to replace the Dallas DS1225, which I have uploaded to the files section.
I decide to use the Ramtron FM16W08 chip since it has no internal battery and would retain its contents for many decades.
The chip comes in SOIC version only so I had to use an adapter to install it in the DIL scope socket.

Next decided to try to use some image files available in this forum to see if I could get rid of the TEST4 FAIL2 error.
Since I do not have those fancy $500 up programmers the next challenge was to find out how to program the FRAM using one of the cheap programmers I have the TL866A or the GQ-4x.
Fortunately after some time digging the internet I found that it is possible to use the TL866A with an custom adapter to program the FM16W08 and I have uploaded a photo of the article (sorry for that) and the adapter I have built if anyone in the future find this trend useful.

I have tested several images and none of them worked so i copied the contents of the scope NVRAM to the new chip and decided to start the calibration process.

There is a video on the internet suggesting that the calibration process should start with CAL08 (CRT adjustment) so I tried that.
However the scope is behaving in a strange way (the hole display flickers) it was impossible to find the dot required to adjust the grid bias.
The good news is that I have found the I could make the horizontal "dashed" traces disappear changing grid bias (as suggested by Siggi).
The bad news is that I notice that there is a 50OHM OVERLOAD error that I have not noticed before.
So my "new" questions are:
1 - Should I start adjusting the DAC before starting the calibration steps? (I found the instructions confusing)
2 - What is causing the display flicker when the scope is set to CAL08?
3 - What are the possible causes for the 50 OHM OVERLOAD?

Roger


Rogerio O
 

Bad news.
I tried to adjust the DAC and realized that I could not do it.
The maximum positive voltage that I can get is +1.18V while the max absolute value negative voltage is -1.019V.
As I have said before, the +10.0 ref voltage is right on spot.
I will check the values of al the resistors again, since I might have skipped one.
Any idea of what may be causing the absolute value of both voltages to be low?
thank you all,
Roger


Siggi
 

Hey Roger,

the DAC takes a reference current as input. It effectively multiplies that
current, then diverts it to the two complementary outputs according to a
programmed ratio. If the reference input current is low, you'd see low
output voltages from the DAC. If the DAC is damaged, you might see low
outputs (though that seems unlikely to me). The only other reason I can
think of that would cause low voltage outputs from the DAC is if the load
resistors (R2520/R2521) were bad.
How are your +1.36/-1.25V reference voltages doing? They should be a very
good indication of where the DACs reference current is at, as they're (I
believe) set by programming a fixed code to the DAC.
If they are spot on, then you need to look at whichever DAC output goes to
J119, pin 13. Looks like it's DLY REF 1 (see schematic <5> K9). From the
DACs you go through MUXen to S&H circuits. The MUXen have been known to go
bad, and the capacitors in the S&H have been known to go leaky.

Siggi

On Tue, Nov 10, 2020 at 2:03 PM Rogerio O <rodd414@gmail.com> wrote:

Bad news.
I tried to adjust the DAC and realized that I could not do it.
The maximum positive voltage that I can get is +1.18V while the max
absolute value negative voltage is -1.019V.
As I have said before, the +10.0 ref voltage is right on spot.
I will check the values of al the resistors again, since I might have
skipped one.
Any idea of what may be causing the absolute value of both voltages to be
low?
thank you all,
Roger






Rogerio O
 

Thank you Siggi.
"How are your +1.36/-1.25V reference voltages doing?"
They have the values I have mentioned earlier +1.18V and -1.019V.
I have already measured R2520/R2521 (in circuit) and they seem to be fine.
This circuit is too complex to my basic electronics.
Could you outline a "basic" troubleshooting sequence based on this information?
Thank you


Siggi
 

On Tue, Nov 10, 2020 at 7:10 PM Rogerio O <rodd414@gmail.com> wrote:Thank
you Siggi.

"How are your +1.36/-1.25V reference voltages doing?"
They have the values I have mentioned earlier +1.18V and -1.019V.
You had this working right at some point though? I suggest looking for more
trace or component damage, or a marginal solder joint (we've all gone done
a cold joint:) ).


I have already measured R2520/R2521 (in circuit) and they seem to be fine.
This circuit is too complex to my basic electronics.
Could you outline a "basic" troubleshooting sequence based on this
information?

Unfortunately there isn't much you can measure there except continuity and
resistances, as this is ultimately to derive a reference current.

The voltage measurements that make sense are to
- measure between pins 14 and 15 of U2101 - this should be very close to
zero.
- measure the voltage drops over the various resistors and compute the
currents
going through them.

You want to sum up to a current of 1mA going into pin 14.
Note that if you find a voltage drop of zero volts over any of those
resistors, that means you have an open circuit at one end of that resistor.

As an aside, you had electrolyte corrosion of those components. How did you
clean the board? Did you neutralize or wash off the electrolyte?
If not, then possibly that's your problem. The electrolyte may keep eating
at your traces, and it may also be conductive - which may mess up your
reference current.
At minimum you want to wash the board with water, ideally demineralized
water. I hear using dish soap or other mild detergent is a good idea (I've
used Simple Green myself).
Isopropyl alcohol alone is a poor cleaner for the electrolyte I understand
- though I'm certainly no expert <cue the chemists in the group :)>.


Chuck Harris
 

There is only one time and place where the DAC should be adjusted,
and that is in the POWER SUPPLIES AND DAC REFERENCE ADJUSTMENT section
of the calibration routine. Don't cut corners, you will be disappointed.

The adjustment is controlled by the cursor knob, and is done by
measuring the voltage when the knob is fully CCW, and then adjusting
it an appropriate amount when it is fully CW.

I wrote a simple program that runs on a TI84 calculator to make the
adjustment easy.

-Chuck Harris

Rogerio O wrote:

Bad news.
I tried to adjust the DAC and realized that I could not do it.
The maximum positive voltage that I can get is +1.18V while the max absolute value negative voltage is -1.019V.
As I have said before, the +10.0 ref voltage is right on spot.
I will check the values of al the resistors again, since I might have skipped one.
Any idea of what may be causing the absolute value of both voltages to be low?
thank you all,
Roger






Rogerio O
 

Thanks Siggi and Chuck for the replies.
I will follow Siggi's suggestion of measuring the voltages around the DAC
Just as a caution I have ordered all the parts of this section of the board from Mouser in case I find another out of speck resistor (I can't find them here).
Since I it will take week before they arrive , I will have plenty of time to troubleshoot.
I read in another thread (that I can't locate anymore) of a person where the problem was located around U2420D/Q2320.
I haven't touched these parts yet.
Is there a chance they are the cause?
Roger


Siggi
 

Hey Roger,

This message (https://groups.io/g/TekScopes/message/118061) has a
discussion of how the 1.36/-1.25V references are developed. This does also
involve a MUX and a S&H, so those are also suspect.

A way to assess the individual S&H for leakage is to probe on the input of
each 4051(?) MUX, then probe on a given S&H output, and look at the traces
together. You'd expect the output to track one of the levels that the input
takes (it continuously varies) and to not droop much until the next time
it's refreshed. Note that the MUX outputs are pretty high impedance, so you
should expect even your 10M probe to load them and to cause some droop.

Siggi

On Wed, Nov 11, 2020 at 6:58 AM Rogerio O <rodd414@gmail.com> wrote:

Thanks Siggi and Chuck for the replies.
I will follow Siggi's suggestion of measuring the voltages around the DAC
Just as a caution I have ordered all the parts of this section of the
board from Mouser in case I find another out of speck resistor (I can't
find them here).
Since I it will take week before they arrive , I will have plenty of time
to troubleshoot.
I read in another thread (that I can't locate anymore) of a person where
the problem was located around U2420D/Q2320.
I haven't touched these parts yet.
Is there a chance they are the cause?
Roger