Scope keeps blowing fuses...

[Cursitor Doom]

Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA

 

[Peter W.]

Did the caps fail Open or Short?

Peter Wieck
Melrose Park, PA

 

[ohg...@gmail.com]

On Saturday, October 22, 2022 at 10:26:11 AM UTC-4, Cursitor Doom wrote:

Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA

Did you do any resistance checks like for a shorted rectifier(s) or power transistor like Q1050? FWIW, installing a new fuse and waiting for smoke is a terrible idea.

 

[Liz Tuddenham]

Cursitor Doom <cd@notformail.com> wrote:

Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA

I can\'t see enough detail in that diagram to be sure of what every
component is, but as a general principle my first suspects would be any
suppression capacitors across the incoming mains; they always seem to be
the first to go and they invariably fail short-ciircuit.

If the fuse is glass bodied with a simple wire element, the state of it
after it has blown will give you some idea of the type of fault. A dead
short will blacken the inside of the glass (and may break it), a
substantial overload will blow the wire to bits and leave blobs of metal
scattered around inside the glass, a prolonged gentle overload will
leave most of the wire intact but sagged and broken in the middle. This
may give you some idea of the sort of fault you are looking for and the
most likely place to find it.


--
~ Liz Tuddenham ~
(Remove the \".invalid\"s and add \".co.uk\" to reply)
www.poppyrecords.co.uk

 

[Cursitor Doom]

On Sat, 22 Oct 2022 08:08:25 -0700 (PDT), \"Peter W.\"
<peterwieck33@gmail.com> wrote:

Did the caps fail Open or Short?

Peter Wieck
Melrose Park, P

These are X2 caps and supposed to fail short. However, for whatever
reason, they both failed open.

 

[Cursitor Doom]

On Sat, 22 Oct 2022 08:57:06 -0700 (PDT), \"ohg...@gmail.com\"
<ohger1s@gmail.com> wrote:

On Saturday, October 22, 2022 at 10:26:11 AM UTC-4, Cursitor Doom wrote:
Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA


Did you do any resistance checks like for a shorted rectifier(s) or power transistor like Q1050? FWIW, installing a new fuse and waiting for smoke is a terrible idea.

Yes, well, my options are limited due the to the *dumb* design
implementation here. Plus I can\'t probe the test points or look for
voltage drops when the power supply is constantly interrupted within
miliseconds of switch-on. Looks like I\'ll have to break out my dim
bulb tester (if I can find it).

 

[Peter W.]

I am going to vote for C1001 & C1002. Either fails short and that fuse will blow. Both fail short and that blow will be instant. That or a bad primary winding - which could have been eaten by repeated refusing without proper diagnosis.

A dim-bulb tester is a reasonable tool for this purpose as a rubber spoon makes a reasonable carving knife. Just not fine enough for any kind of accurate diagnosis.

Peter Wieck
Melrose Park, PA

 

[ehsjr]

On 10/22/2022 10:26 AM, Cursitor Doom wrote:

Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA

Thia might help:
https://www.tek.com/en/manual/2465a-and-2467-service-manual

Ed

 

[Cursitor Doom]

On Sat, 22 Oct 2022 13:44:12 -0700 (PDT), \"Peter W.\"
<peterwieck33@gmail.com> wrote:

>I am going to vote for C1001 & C1002. Either fails short and that fuse will blow. Both fail short and that blow will be instant. That or a bad primary winding - which could have been eaten by repeated refusing without proper diagnosis.

I don\'t see that as being an issue, though (the re-fusing bit) since
whatever has gone low-resistance is clearly drawing current away from
the other healthy components, no? And if said component has gone
low-res (as we assume it must) then cooking it a bit more doesn\'t
matter in the least since it\'s going to have to be replaced anyway.
The important thing is not to do it to such an extent that the PCB
becomes heat-damaged - in my submission anyway.

I\'ll check out the caps you suggested, however. Sounds like a good
call.
Thanks,

CD.

 

[Cursitor Doom]

On Sat, 22 Oct 2022 17:01:01 -0400, ehsjr <ehsjr@verizon.net> wrote:

On 10/22/2022 10:26 AM, Cursitor Doom wrote:
Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA

Thia might help:
https://www.tek.com/en/manual/2465a-and-2467-service-manual

Ed

I did actually download this yesterday and that\'s where the snippet of
the wiring diagram I posted comes from. I find, however, that service
manuals in this form are so hard to navigate. I\'d much rather have the
physical hard copy; so much more user-friendly.

 

[three_jeeps]

On Saturday, October 22, 2022 at 2:38:07 PM UTC-4, Cursitor Doom wrote:

On Sat, 22 Oct 2022 08:57:06 -0700 (PDT), \"ohg...@gmail.com\"
ohg...@gmail.com> wrote:

On Saturday, October 22, 2022 at 10:26:11 AM UTC-4, Cursitor Doom wrote:
Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA


Did you do any resistance checks like for a shorted rectifier(s) or power transistor like Q1050? FWIW, installing a new fuse and waiting for smoke is a terrible idea.
Yes, well, my options are limited due the to the *dumb* design
implementation here. Plus I can\'t probe the test points or look for
voltage drops when the power supply is constantly interrupted within
miliseconds of switch-on. Looks like I\'ll have to break out my dim
bulb tester (if I can find it).

Well, as they say, the only good short is a dead short.
I\'d start by checking all the caps and semiconductors on the line side instead of continually smoke testing it. I wouldn\'t rule out the transformer either. Shouldn\'t be too hard to find. I\'d also disconnect all the loads.
Perhaps the folks on the yahoo tek scopes group might have more targeted information - some sharp ppl there and all the do is scopes....
Good luck

 

[Phil Allison]

Cursitor Doom wrote:
-------------------------------------

Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them,

** Did both fail at the same time?

Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.

..... Phil

 

[server]

On Sat, 22 Oct 2022 18:58:49 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

Cursitor Doom wrote:
-------------------------------------
Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them,


** Did both fail at the same time?

Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.

.... Phil

Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.

--
Neural Network Software http://www.npsnn.com
JustNN Just a neural network http://www.justnn.com
EasyNN-plus More than just a neural network http://www.easynn.com
SwingNN Prediction software http://www.swingnn.com

 

[Cursitor Doom]

On Sat, 22 Oct 2022 13:44:12 -0700 (PDT), \"Peter W.\"
<peterwieck33@gmail.com> wrote:

I am going to vote for C1001 & C1002. Either fails short and that fuse will blow. Both fail short and that blow will be instant. That or a bad primary winding - which could have been eaten by repeated refusing without proper diagnosis.

A dim-bulb tester is a reasonable tool for this purpose as a rubber spoon makes a reasonable carving knife. Just not fine enough for any kind of accurate diagnosis.

Seems a tad overly pessimistic. If I powered it up via a DBT, I\'d be
able to unplug all the PSU outputs one by one until the bulb went out
(or went dim). That\'s on the alternative possibility that the fault
doesn\'t lie within the PSU board itself of course.

Peter Wieck
Melrose Park, PA

 

[Phil Allison]

st...@swingnn.com wrote:
Phil Allison
-----------------

** Did both fail at the same time?

Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.

.... Phil

Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.

** So you have no clue what a piar of Rifa 68nF supreso caps do in an off line SMPS ?

It sure as hell ain\'t smoothing...

.....Phil

 

[Peter W.]

It is already almost a certainty that the issue is in the power-supply. All that rubber spoon will tell you is that the issue is in the power-supply. At which point, you will need to troubleshoot at the component level.

Each time you apply power to what amounts to a dead-short, you may be damaging other components downline. When the second fuse blows, *STOP THERE*. Or the first, if there is any sort of associated symptoms such as heat, smoke, noise or a vaporized fuse.

Peter WIeck
Melrose Park, PA

 

[Cursitor Doom]

On Sun, 23 Oct 2022 04:54:49 -0700 (PDT), \"Peter W.\"
<peterwieck33@gmail.com> wrote:

>It is already almost a certainty that the issue is in the power-supply. All that rubber spoon will tell you is that the issue is in the power-supply. At which point, you will need to troubleshoot at the component level.

Well, I\'m inclined to agree it\'s most likely in the PSU itself.
However, if it comes down to troubleshooting at component level then
I\'m stuffed - certainly if I have to do it powered-up, because access
is very poor indeed on this model. On previous Tek scopes I\'ve worked
on, it\'s been possible to pull the board out and work on it live
without much difficulty, but the \'plumbing-in\' as it were of the
wiring on this one without some dedicated extender board is going to
be very challenging indeed.

>Each time you apply power to what amounts to a dead-short, you may be damaging other components downline. When the second fuse blows, *STOP THERE*. Or the first, if there is any sort of associated symptoms such as heat, smoke, noise or a vaporized fuse.

OK, I have! When a new fuse blew after I\'d replaced the 2 RIFAs, I
posted here and haven\'t attempted another power-up since.
Apart from the RIFAs, there\'s no sign of physical damage at all to the
other components. :-/

 

[server]

On Sun, 23 Oct 2022 04:20:47 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

st...@swingnn.com wrote:
Phil Allison
-----------------


** Did both fail at the same time?

Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.

.... Phil

Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.

** So you have no clue what a piar of Rifa 68nF supreso caps do in an off line SMPS ?

It sure as hell ain\'t smoothing...

....Phil

I was pointing out a common misunderstanding with multiple capacitors.
Rifa are alone, in parallel or in series and can all fail at the same
time as can smoothing capacitors in parallel.

--
Neural Network Software http://www.npsnn.com
JustNN Just a neural network http://www.justnn.com
EasyNN-plus More than just a neural network http://www.easynn.com
SwingNN Prediction software http://www.swingnn.com

 

[Cursitor Doom]

On Sun, 23 Oct 2022 13:31:10 +0100, steve@swingnn.com wrote:

On Sun, 23 Oct 2022 04:20:47 -0700 (PDT), Phil Allison
pallison49@gmail.com> wrote:

st...@swingnn.com wrote:
Phil Allison
-----------------


** Did both fail at the same time?

Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.

.... Phil

Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.

** So you have no clue what a piar of Rifa 68nF supreso caps do in an off line SMPS ?

It sure as hell ain\'t smoothing...

....Phil


I was pointing out a common misunderstanding with multiple capacitors.
Rifa are alone, in parallel or in series and can all fail at the same
time as can smoothing capacitors in parallel.

That\'s a bit misleading and it\'s de-railing the thread.

 

[Peter W.]

a) Do you have a VOM? And, if so, how good is it?
b) Not that testing capacitors in-situ generally accurate, but you can usually tell the difference between an open cap and a dead-shorted cap using the Ohms scale.
c) Similarly, transistors.
d) And it is possible that the RIFAs were symptoms, not the cause.

Which would be a start.

Peter Wieck
Melrose Park, PA