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Hello from new member w 2465A issues

Armin Schon
 

Hi,
I'm so glad I found this group! Some really useful content here.


Recently I inherited an Tek 2465A scope, the same type I used in university a lot, good old times. Except for a broken sweep fine adjust knob (any idea where I can get a replacement?) it seemed fine. It worked for 5 minutes and then within seconds two capacitors in the power supply literally went up in white smoke. I haven't opened it yet, but they must be the main rectifier caps, by the bang of it. Got myself a neat little project for a weekend or two ;-)


Thankful for any advice on these two topics!


Armin

 

I am not sure if they can self destruct with a bang but the paper X
and Y capacitors at the AC input are known to destructively fail and I
think they can damage the input current limiter. These things are
relatively easy to diagnose and repair.

On 24 Jan 2016 12:54:06 -0800, you wrote:

Hi,
I'm so glad I found this group! Some really useful content here.

Recently I inherited an Tek 2465A scope, the same type I used in university a lot, good old times. Except for a broken sweep fine adjust knob (any idea where I can get a replacement?) it seemed fine. It worked for 5 minutes and then within seconds two capacitors in the power supply literally went up in white smoke. I haven't opened it yet, but they must be the main rectifier caps, by the bang of it. Got myself a neat little project for a weekend or two ;-)

Thankful for any advice on these two topics!

Armin

Siggi
 

Here's a picture of the culprit capacitors from my 2430 <
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1635890616>.
Mine gave a single loud BANG, but didn't blow anything else, and the scope
continued working afterwards. It seems they more commonly go as you
describe.

On Sun, 24 Jan 2016 at 16:13 David @DWH [TekScopes] <
TekScopes@...> wrote:



I am not sure if they can self destruct with a bang but the paper X
and Y capacitors at the AC input are known to destructively fail and I
think they can damage the input current limiter. These things are
relatively easy to diagnose and repair.



On 24 Jan 2016 12:54:06 -0800, you wrote:

Hi,
I'm so glad I found this group! Some really useful content here.

Recently I inherited an Tek 2465A scope, the same type I used in
university a lot, good old times. Except for a broken sweep fine adjust
knob (any idea where I can get a replacement?) it seemed fine. It worked
for 5 minutes and then within seconds two capacitors in the power supply
literally went up in white smoke. I haven't opened it yet, but they must be
the main rectifier caps, by the bang of it. Got myself a neat little
project for a weekend or two ;-)

Thankful for any advice on these two topics!

Armin


[Non-text portions of this message have been removed]

Armin Schon
 

Hi David,

You are right, two large paper caps on the AC board have burst open - those were the bangs I heard. The main rectifier caps (in the black plastic holders) seem to be okay. I'll need to research the current limiter you mentioned. In your experience, should I replace all the capacitors like some people have advocated on Youtube or just the two broken ones?

 

The oscilloscope does not need the AC line capacitors to operate so I
would remove them for now and verify that there are not other
problems.

The inrush current limiter, RT1016, is *after* the capacitors so check
the condition of the fuse.

On 25 Jan 2016 06:46:42 -0800, you wrote:

Hi David,

You are right, two large paper caps on the AC board have burst open - those were the bangs I heard. The main rectifier caps (in the black plastic holders) seem to be okay. I'll need to research the current limiter you mentioned. In your experience, should I replace all the capacitors like some people have advocated on Youtube or just the two broken ones?

Ed Breya
 

Those paper AC line X-caps have been the subject of much discussion here and on the hpagilent group. They are the ones potted in amber colored epoxy, usually WIMA or RIFA brand. The bottom line is that they should be eliminated whenever encountered - even if not already failed - and replaced by proper X-rated plastic-dielectric caps. This is especially important in high line voltage (~240V) countries, where the caps are operating near their limit.

These are also used inside of some packaged line filter assemblies, and known to have caused internal failures there.

Ed

Tothwolf
 

On Mon, 25 Jan 2016, edbreya@... [TekScopes] wrote:

Those paper AC line X-caps have been the subject of much discussion here and on the hpagilent group. They are the ones potted in amber colored epoxy, usually WIMA or RIFA brand. The bottom line is that they should be eliminated whenever encountered - even if not already failed - and replaced by proper X-rated plastic-dielectric caps. This is especially important in high line voltage (~240V) countries, where the caps are operating near their limit.

These are also used inside of some packaged line filter assemblies, and known to have caused internal failures there.
Rifa brand, not Wima. Wima has never made those translucent yellow X and Y type safety capacitors. Kemet now owns Rifa and continues to make these paper insulated safety capacitors, however I will not use them and generally replace them with Epcos film types of the same safety class (X or Y, it matters).

Ed Breya
 

I remember there were at least two brands that made those or similar caps. I looked up WIMA and found this example:

http://www.wima.com/EN/mp3x2.htm http://www.wima.com/EN/mp3x2.htm

They look a little different, but I think they're the kind I recall seeing in the past. I don't remember if I had seen any failures of this brand, but I assumed that they're close enough to the same as the RIFAs, which I've seen fail a few times - perhaps because they're more common.

Regardless of what the makers say in the specs, much experience here indicates that very messy catastrophic failures can occur with paper X-caps, with bangs, meltdowns, and noxious fumes - but perhaps no fires, which is the main point. The X-rated caps are for across-line use, so don't have shock hazard issues. The Y-rated are for line to ground, so could. I don't recall ever seeing this style of cap in a Y-application, only X, although it looks like they're available. I wouldn't trust them for any use - I've purged my stock of these long ago.

Ed

Armin Schon
 

Opened the scope in the meantime and those are the ones, exactly. I guess they can be replaced by other types, don't have to be paper caps, or do they?

Armin Schon
 

Thanks, will get me some of those then.

Armin Schon
 

After confirming that C1016 and C018 have blown up I wonder why they would fail within seconds of each other. Is that to be expected they way this circuit works, or does it point to the failure of another component, which cause the caps to blow out?

Ed Breya
 

I haven't been following this story, but if the blown caps are the main raw DC filters after the rectifiers, you should always check the rectifiers or bridge before turning it back on with fresh caps. A shorted rectifier may quickly ruin a new cap.


If you have a curve tracer, you can run up the AC input to the PS with it and get a good idea of what's going on.


Ed

 

The OEM caps C1016 and C1018 are made of metalized paper with plastic cases.  The cases eventually crack and become susceptible to moisture intrusion which permeates the paper, creates leakage, which creates heat.  My guess is the scope was recently submitted to high humidity without being powered.  Once power was applied both of the old caps heated rapidly and failed explosively.
The modern solution is to use plastic film caps of similar ratings.  The modern plastics are less likely to crack and if it does, the plastic film is not so susceptible to moisture intrusion.  My advice is to always replace these caps as part of the "recapping" of  24X5 series scopes. 


From: "armin.schon@... [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Tuesday, January 26, 2016 11:48 AM
Subject: [TekScopes] Re: Hello from new member w 2465A issues

  After confirming that C1016 and C018 have blown up I wonder why they would fail within seconds of each other. Is that to be expected they way this circuit works, or does it point to the failure of another component, which cause the caps to blow out?



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[Non-text portions of this message have been removed]

Ed Breya
 

Please disregard my previous post about raw DC filter caps. I commented earlier on the paper caps (#124531), and didn't realize this discussion was still about them. Duh.


Ed

 

X class capacitors are for line to neutral applications and are flame
retardant so failure cannot cause a fire.

Y class capacitors are for line or neutral to ground applications and
in addition to be flame retardant, they must have a self healing
dielectric and be fusible.

Additional requirements for X and Y class capacitors involve
withstanding high voltage overload.

The requirements for Y class capacitors are more strict because
failure has the potential to expose a ground connected chassis to high
voltage. An X class capacitor may be replaced by a Y class capacitor
but Y class capacitors are usually much smaller because of regulatory
limits on ground current leakage. This gets interesting when you have
enough loads connected to one circuit that the combined leakage
current from the line to ground through the Y class safety capacitors
is enough to trip any ground fault interrupter.

The use of paper dielectrics in safety capacitors comes about because
it is easier to make a paper dielectric capacitor than a plastic film
or ceramic capacitor which meets the safety requirements. I think
this has to do with paper (cellulose) not melting and paper not
releasing as much conductive carbon when vaporized. As we know now
though, paper safety capacitors also have a long term reliability
issue.

These capacitors are only included for conductive EMI suppression so
the oscilloscope will operate without them.

I had to take another look at the 2465A schematics to find the Y
capacitors; they are on the *other* side of the input rectifier and
are labeled C1020 and C1050. So if these were the ones that shorted,
the rectifier could have been damaged like Ed suggested and the
rectifier labeled CR1011 needs to be checked.

Failure of the other safety capacitors, C1016 and C1018, should not
have affected the input rectifier but you might want to check it
anyway. If these capacitors failed, then series resistors R1016 and
R1018 certainly failed as well.

As far as replacing these capacitors, the dielectric is not important.
Plastic and ceramic will work just as well as paper as long as the
safety class and voltage rating is as good or better than the original
design requires.

On 25 Jan 2016 16:49:42 -0800, you wrote:

Opened the scope in the meantime and those are the ones, exactly. I guess they can be replaced by other types, don't have to be paper caps, or do they?

Armin Schon
 

The obvious failure came from C1016 and C1018, both of whom are visually destroyed. R1016 and R1018 are okay, so is the rectifier CR1011. It's a mystery... Anyhow, I have ordered a full set of cap replacements for both power supply boards. We will know more once I'll turn it on again..

BTW, since it's open anyhow, should I replace the lithium battery as well? It can not possibly still be working well after 30 years, or can it?

mosaicmerc
 

Armin Schon
 

Thanks for the explanation, which could be just spot on. The scope was stored for years in a climatized lab without being used before I received it as a present and took it home into the man cave, where it waited a month or so in a very non-regulated environment before I actually switched it on...

 

I would definitely replace the lithium battery. You need to plan the process before beginning. Or risk losing memory contents. Several cautions: 1. connect an auxiliary battery so when you replace the old one there wont be a period with no battery. 2. take special care to not short any of the battery voltages to ground, even temporarily. Otherwise, you will dump memory contents.


Here is what I did. Built battery pack of 2 AA batteries with a 100 Ohm resistor in series (the series resistor avoids high currents if the lithium battery does not have the same voltage as the battery pack.). Soldered the battery pack (and resistor) to the diodes that connect directly to the lithium battery. Removed the old battery by heating one pin at a time (consider just cutting the old leads so you can lift the battery out and then unsolder the leads separately). Cleaned the board being careful to not break connection with the auxiliary battery pack. Installed the new battery. Removed the auxiliary battery pack. When I powered the scope it worked just as before.

honolulusnowwhite
 

Make sure you use a soldering iron which is not grounded, or at least make sure the board is not connected to ground if the iron is.

I used a lab power supply to provide 3.6V while the battery was being replaced.


Patrick Wong AK6C
---In TekScopes@..., <machineguy59@...> wrote :

I would definitely replace the lithium battery. You need to plan the process before beginning. Or risk losing memory contents. Several cautions: 1. connect an auxiliary battery so when you replace the old one there wont be a period with no battery. 2. take special care to not short any of the battery voltages to ground, even temporarily. Otherwise, you will dump memory contents...