Topics

Tektronix 2215, pulsing/chirping noise, no trace.

Samuel Rey
 

Alright, I've tinkered with it some more and I feel like I am getting
close. I measured the caps and checked some for shorts (I straight up
removed the Rifa caps) and they seemed fine. The diodes also seem fine. I
decided to take a look at the Q933 MOSFET, and decided to check for a short
there, as I hadn't checked for a short after the rectifier. Lo and behold,
0 Ohms to ground at the source. Suspicious! I started checking all
components going to ground thru the FET, and none of them were shorted
after desoldering them. About halfway through this, though, I decided to
take a *short*cut. (Pardon me.) I fed in 5V limited to 1A with my PSU from
the source to ground, and checked which component got hot. And it was T933!
Unfortunately I ran out of time so I'll desolder and check the transformer
when I get back to the scope on Friday. I also suspect U920 might be dead,
so if I can buy a replacement at my local components store I'll get one
just in case.

Sam

El jue., 11 jul. 2019 a las 16:00, <tekscopegroup@...> escribió:

Most switching power supplies need a load on the output to be able to
operate normally, so just as a reference by testing the pre-reg section
with its output disconnected is most probably not a valid test. The full
wave rectifier that you mention deals only with the mains AC input to
convert it to DC for the operation of the switcher which then generates the
+43V, so I doubt that the problem is there as it would rather blow the main
fuse or would simply do nothing. But in any case have you checked the
voltage across the (big) 75uF input capacitor? If for any reason that
voltage is low or has a lot of ripple because that cap has aged and/or
dried out I would think this might prevent the pre-reg from starting. On
the output of the pre-reg there is only one rectifier diode that if I
remember correctly is labeled CR907. Check that one as well, it might be
leaky or open/shorted, and if in need of replacement has to be a high-speed
fast recovery rectifier diode.



tekscopegroup@...
 

Most switching power supplies need a load on the output to be able to operate normally, so just as a reference by testing the pre-reg section with its output disconnected is most probably not a valid test. The full wave rectifier that you mention deals only with the mains AC input to convert it to DC for the operation of the switcher which then generates the +43V, so I doubt that the problem is there as it would rather blow the main fuse or would simply do nothing. But in any case have you checked the voltage across the (big) 75uF input capacitor? If for any reason that voltage is low or has a lot of ripple because that cap has aged and/or dried out I would think this might prevent the pre-reg from starting. On the output of the pre-reg there is only one rectifier diode that if I remember correctly is labeled CR907. Check that one as well, it might be leaky or open/shorted, and if in need of replacement has to be a high-speed fast recovery rectifier diode.

Samuel Rey
 

I have seen that replacement, but I’m holding on, I’ll try to fix mine
before I buy it.

El El mié, 10 jul 2019 a las 18:10, Bob Albert via Groups.Io <bob91343=
yahoo.com@groups.io> escribió:

Good advice! One thing you didn't suggest was to replace the entire
board. I see that someone in Europe has a board for sale at a reasonable
price on ebay.
I was lucky that replacing the power FET solved my problem with that same
model oscilloscope.
And there is no 'green power LED' that I can see on mine. In fact, they
saved money by leaving it out.
Since the repair, mine has worked flawlessly; it's been a month or more
now.
It's not my favorite 'scope. The controls aren't very user friendly for
me, and the panel legends are hard to read. It has a few functions that
baffle me in terms of daily use. The delaying sweep especially is a bit
arcane. But considering how much I paid for it, I have no complaints. (I
won it in a raffle at a swap meet and I see why someone put it on the
table.)
Bob
On Wednesday, July 10, 2019, 07:13:33 AM PDT, <tekscopegroup@...>
wrote:

I doubt that you have a problem at the output of the pre-regulator that
is causing the problem and it shutting down due to an abnormal condition on
the load side. As you pointed out the scope works fine with an external +43
volts being fed into the secondary power supply. So that would point
towards a faulty component that is not allowing the prereg to start up. In
my experience repairing this section in a 2213A some years ago was that it
is absolutely necessary to check everything in that stage specially
transistors and diodes, even if in-circuit measurements do not point at the
part being faulty before you even apply power at all. You need to pull
every single transistor, diode, etc in the preregulator and test it, and
all the lower value and/or higher power resistors as well for open circuit.
Of course it goes without saying also check the power input components,
like the 75uF cap and the associated rectifier, etc. Double check the
Mosfet and driver transistor. Even if it was fine last time you checked,
you need to double check everything in one batch before ever applying
power. Check the regulator IC as well, I seem to remember it was a TL494
that sometimes just quits. Pull it out and place a socket then put in a new
one. The socket will also allow for easier future testing & replacement.
You might also have a faulty T906 switching transformer, which will be
difficult to verify, so leave that possibility aside for last when you are
sure everything else is ok and the preregulator is still not working. As a
matter of fact my scope happened to have a bad T948 switching transformer,
and the only ay to make sure it was the problem was to replace it. But I
digress as this particular trasformer is located downstream from the
preregulator circuit.

Unfortunately dealing with a switching power supply you will usually be
able to follow a trail until you hit the fault because it works in a closed
loop where everything either needs to be right, or all (or most) will be
wrong. You will have to test every active component and make sure it is not
open or shorted, leaky, etc. The fact that the fault happened while you
where using the scope would suggest that it is not related to some
component slowly going out of tolerance, but rather of something popping,
usually at start up which is the (very short) time interval of highest
stress for a switching regulator until after a few milliseconds when things
quickly settle down.

This prereg stage, as with usually most other switching power supplies is
very temperamental and will absolutely not work unless all is good, and
will also tend to blow freshly replaced components during the intermediary
"let me check if that was all that was bad" situation, and then you might
be quickly sent back to square one (or even further back) with one or more
of the higher power parts blown and/or smoking.

Again good luck with the hunt, and don't be discouraged. You will soon be
rewarded with that green power LED on the front panel coming back on a
fully working scope. And please don't even consider using some sort of
permanent external power supply as a hack to operate the scope instead of
fixing the problem.





Bob Albert
 

Good advice!  One thing you didn't suggest was to replace the entire board.  I see that someone in Europe has a board for sale at a reasonable price on ebay.
I was lucky that replacing the power FET solved my problem with that same model oscilloscope.
And there is no 'green power LED' that I can see on mine.  In fact, they saved money by leaving it out.
Since the repair, mine has worked flawlessly; it's been a month or more now.
It's not my favorite 'scope.  The controls aren't very user friendly for me, and the panel legends are hard to read.  It has a few functions that baffle me in terms of daily use.  The delaying sweep especially is a bit arcane.  But considering how much I paid for it, I have no complaints.  (I won it in a raffle at a swap meet and I see why someone put it on the table.)
Bob

On Wednesday, July 10, 2019, 07:13:33 AM PDT, <tekscopegroup@...> wrote:

I doubt that you have a problem at the output of the pre-regulator that is causing the problem and it shutting down due to an abnormal condition on the load side. As you pointed out the scope works fine with an external +43 volts being fed into the secondary power supply. So that would point towards a faulty component that is not allowing the prereg to start up. In my experience repairing this section in a 2213A some years ago was that it is absolutely necessary to check everything in that stage specially transistors and diodes, even if in-circuit measurements do not point at the part being faulty before you even apply power at all. You need to pull every single transistor, diode, etc in the preregulator and test it, and all the lower value and/or higher power resistors as well for open circuit. Of course it goes without saying also check the power input components, like the 75uF cap and the associated rectifier, etc. Double check the Mosfet and driver transistor. Even if it was fine last time you checked, you need to double check everything in one batch before ever applying power. Check the regulator IC as well,  I seem to remember it was a TL494 that sometimes just quits. Pull it out and place a socket then put in a new one. The socket will also allow for easier future testing & replacement.  You might also have a faulty T906 switching transformer, which will be difficult to verify, so leave that possibility aside for last when you are sure everything else is ok and the preregulator is still not working. As a matter of fact my scope happened to have a bad T948 switching transformer, and the only ay to make sure it was the problem was to replace it. But I digress as this particular trasformer is located downstream from the preregulator circuit.

Unfortunately dealing with a switching power supply you will usually be able to follow a trail until you hit the fault because it works in a closed loop where everything either needs to be right, or all (or most) will be wrong. You will have to test every active component and make sure it is not open or shorted, leaky, etc. The fact that the fault happened while you where using the scope would suggest that it is not related to some component slowly going out of tolerance, but rather of something popping, usually at start up which is the (very short) time interval of highest stress for a switching regulator until after a few milliseconds when things quickly settle down.

This prereg stage, as with usually most other switching power supplies is very temperamental and will absolutely not work unless all is good, and will also tend to blow freshly replaced components during the intermediary "let me check if that was all that was bad" situation, and then you might be quickly sent back to square one (or even further back) with one or more of the higher power parts blown and/or smoking.

Again good luck with the hunt, and don't be discouraged. You will soon be rewarded with that green power LED on the front panel coming back on a fully working scope. And please don't even consider using some sort of permanent external power supply as a hack to operate the scope instead of fixing the problem.

Samuel Rey
 

I am aware the faulty component is in the prereg, as I had previously ran
the board with its output disconnected and it still did the same thing.
Tekwiki has a doc on troubleshooting faulty psu for this model, and it
mentioned that if ticking is heard, the rectifier diodes are the first
thing to check. This is talking about actual diodes and not the full wave
rectifier, correct?

El El mié, 10 jul 2019 a las 16:13, <tekscopegroup@...> escribió:

I doubt that you have a problem at the output of the pre-regulator that is
causing the problem and it shutting down due to an abnormal condition on
the load side. As you pointed out the scope works fine with an external +43
volts being fed into the secondary power supply. So that would point
towards a faulty component that is not allowing the prereg to start up. In
my experience repairing this section in a 2213A some years ago was that it
is absolutely necessary to check everything in that stage specially
transistors and diodes, even if in-circuit measurements do not point at the
part being faulty before you even apply power at all. You need to pull
every single transistor, diode, etc in the preregulator and test it, and
all the lower value and/or higher power resistors as well for open circuit.
Of course it goes without saying also check the power input components,
like the 75uF cap and the associated rectifier, etc. Double check the
Mosfet and driver transistor. Even if it was fine last time you checked,
you need to double check everything in one batch before ever applying
power. Check the regulator IC as well, I seem to remember it was a TL494
that sometimes just quits. Pull it out and place a socket then put in a new
one. The socket will also allow for easier future testing & replacement.
You might also have a faulty T906 switching transformer, which will be
difficult to verify, so leave that possibility aside for last when you are
sure everything else is ok and the preregulator is still not working. As a
matter of fact my scope happened to have a bad T948 switching transformer,
and the only ay to make sure it was the problem was to replace it. But I
digress as this particular trasformer is located downstream from the
preregulator circuit.

Unfortunately dealing with a switching power supply you will usually be
able to follow a trail until you hit the fault because it works in a closed
loop where everything either needs to be right, or all (or most) will be
wrong. You will have to test every active component and make sure it is not
open or shorted, leaky, etc. The fact that the fault happened while you
where using the scope would suggest that it is not related to some
component slowly going out of tolerance, but rather of something popping,
usually at start up which is the (very short) time interval of highest
stress for a switching regulator until after a few milliseconds when things
quickly settle down.

This prereg stage, as with usually most other switching power supplies is
very temperamental and will absolutely not work unless all is good, and
will also tend to blow freshly replaced components during the intermediary
"let me check if that was all that was bad" situation, and then you might
be quickly sent back to square one (or even further back) with one or more
of the higher power parts blown and/or smoking.

Again good luck with the hunt, and don't be discouraged. You will soon be
rewarded with that green power LED on the front panel coming back on a
fully working scope. And please don't even consider using some sort of
permanent external power supply as a hack to operate the scope instead of
fixing the problem.



tekscopegroup@...
 

I doubt that you have a problem at the output of the pre-regulator that is causing the problem and it shutting down due to an abnormal condition on the load side. As you pointed out the scope works fine with an external +43 volts being fed into the secondary power supply. So that would point towards a faulty component that is not allowing the prereg to start up. In my experience repairing this section in a 2213A some years ago was that it is absolutely necessary to check everything in that stage specially transistors and diodes, even if in-circuit measurements do not point at the part being faulty before you even apply power at all. You need to pull every single transistor, diode, etc in the preregulator and test it, and all the lower value and/or higher power resistors as well for open circuit. Of course it goes without saying also check the power input components, like the 75uF cap and the associated rectifier, etc. Double check the Mosfet and driver transistor. Even if it was fine last time you checked, you need to double check everything in one batch before ever applying power. Check the regulator IC as well, I seem to remember it was a TL494 that sometimes just quits. Pull it out and place a socket then put in a new one. The socket will also allow for easier future testing & replacement. You might also have a faulty T906 switching transformer, which will be difficult to verify, so leave that possibility aside for last when you are sure everything else is ok and the preregulator is still not working. As a matter of fact my scope happened to have a bad T948 switching transformer, and the only ay to make sure it was the problem was to replace it. But I digress as this particular trasformer is located downstream from the preregulator circuit.

Unfortunately dealing with a switching power supply you will usually be able to follow a trail until you hit the fault because it works in a closed loop where everything either needs to be right, or all (or most) will be wrong. You will have to test every active component and make sure it is not open or shorted, leaky, etc. The fact that the fault happened while you where using the scope would suggest that it is not related to some component slowly going out of tolerance, but rather of something popping, usually at start up which is the (very short) time interval of highest stress for a switching regulator until after a few milliseconds when things quickly settle down.

This prereg stage, as with usually most other switching power supplies is very temperamental and will absolutely not work unless all is good, and will also tend to blow freshly replaced components during the intermediary "let me check if that was all that was bad" situation, and then you might be quickly sent back to square one (or even further back) with one or more of the higher power parts blown and/or smoking.

Again good luck with the hunt, and don't be discouraged. You will soon be rewarded with that green power LED on the front panel coming back on a fully working scope. And please don't even consider using some sort of permanent external power supply as a hack to operate the scope instead of fixing the problem.

Samuel Rey
 

I do have the service manual. However, I'm struggling to isolate the actual
faulty component. I have for now removed the rifa filter caps and will be
replacing them. Even though it didn't work, I might as well since they were
all cracked. Surprisingly not shorted but yeah, I don't want to have them
in there. So far, what the actual problem seems to be is that something
makes the prereg output excessive voltage, so it goes into the protection
mode and ticks. Almost all voltages I measure in the board are alternating
between too high and too low, with the ticking sound in between. What I
don't know is what could be causing it. Something shorted to GND?

El mar., 9 jul. 2019 a las 16:13, <tekscopegroup@...> escribió:

Good thing you where already able to isolate the fault to the
pre-regulator, which should make for a much easier fix. This should always
be one of the first steps to take with power supply problems in the 22xx
series. Now you need to check every part on that pre-reg stage, specially
all the semiconductors, but also look for possibly dried out elco caps
before you do any further power up tests as you might burn up recently
replaced components if there are still other faults that have not been
fixed. I assume you already have the full service manual, otherwise
definitively get one first before going in any deeper. Send in your updates
with any additional findings, and good luck with the troubleshooting.



tekscopegroup@...
 

Good thing you where already able to isolate the fault to the pre-regulator, which should make for a much easier fix. This should always be one of the first steps to take with power supply problems in the 22xx series. Now you need to check every part on that pre-reg stage, specially all the semiconductors, but also look for possibly dried out elco caps before you do any further power up tests as you might burn up recently replaced components if there are still other faults that have not been fixed. I assume you already have the full service manual, otherwise definitively get one first before going in any deeper. Send in your updates with any additional findings, and good luck with the troubleshooting.

Samuel Rey
 

Hi, I found this group trying to find a solution to my problem. I have a
Tek 2215 that died as I was using it. I was using it and in between
measurements (I was soldering a couple things and left the scope on) it
started making a chriping or hiccuping noise. I have isolated the issue to
the prereg board - A18, and I replaced an IRF820 MOSFET on the board, but
it didn't work. Output from the board is still 0V. If I feed 43V to the
scope, bypassing the prereg board, it turns on just fine, so I have to
diagnose that board. I am not sure how to, though. More info, and the chirp
itself, can be found at
https://www.eevblog.com/forum/repair/tektronix-2215-pulsingchirping-noise-no-trace/

Regards!