Date   

Re: 465B strange ripple

Chuck Harris <cfharris@...>
 

Hi Raymond,

That depends on how isolated your work situation is. If you
are one of those that puts an isolation transformer on your
work bench power, and use a wooden bench, on the second floor
of a wooden floored building, you might not notice that your
scope's case is elevated 240V above ground.

I, of course, don't recommend isolating benches in that way...

-Chuck Harris

Raymond Domp Frank wrote:

On Tue, Feb 16, 2021 at 04:05 PM, Chuck Harris wrote:


Also, if one of the capacitors in the line input filter
is shorted to ground, it will elevate the case to the
AC mains frequency and voltage, and make lots of mains
frequency ripple available. You may, or may not notice,
depending on how isolated your work station is.
Assuming the 'scope's case is connected to ground (as it should be), that'd trip the earth leak fuse in (large parts of?) Europe...

Raymond






Re: 465B strange ripple

 

On Tue, Feb 16, 2021 at 06:02 PM, Stephen wrote:


I reduces it quite a bit, but it’s still there.
Forgive and ignore me if I'm revisting past stations:
AFAIK, the 465B has covers on top of the Hi-Z attenuators (with the pots/trimmers), like most of its siblings.
I assume those are in place?
Any influence if you short the output of the attenuators like point 1 c.q. point 10 on the Ch1 and Ch2 vertical amp to a chassis point close to it?
Or did you already establish the problem is further downstream?

Raymond


Re: 465B strange ripple

Stephen
 

On Tue, Feb 16, 2021 at 05:54 AM, Raymond Domp Frank wrote:


Only reading posts now and then at the moment.
Are we still talking about the few-mV ripple? Stephen, did you try and place
the case back on the 'scope to see what that does?

Raymond
Yes Raymond, I did. I reduces it quite a bit, but it’s still there. It’s very annoying when I try to calibrate it with a 1ms time marker (as per the manual). At that frequency, the trace is not steady. And that should not be...
So something is definitely not kosher...


Re: 465B strange ripple

 

Only reading posts now and then at the moment.
Are we still talking about the few-mV ripple? Stephen, did you try and place the case back on the 'scope to see what that does?

Raymond


Re: Stan Griffiths estate sale

Greg Muir
 

Ryan,

I hope you already have an "equipment clause" in your pre-nup.

Greg


Re: 465B strange ripple

Stephen
 

On Tue, Feb 16, 2021 at 05:31 AM, Harvey White wrote:


In a full wave bridge, each half of the transformer winding contributes
to the output, so the ripple should be 100 or 120 Hz, equal positive peaks.

If one diode is open, then 1/2 of the transformer does not contribute at
all, so the ripple is larger, and will be at the line frequency.

You could measure the ripple on one channel of a second scope, then look
at the bridge input (to ground) with the second channel.  Lining up the
waveforms ought to show you which transformer secondary branch is
contributing to what, and ought to indicate which one (or two) of the
bridge diodes is open.

Harvey
The thing is... there absolutely no access to the leads of the rectifiers...
I’ll try to find points where they connect on the board, and try to check the diodes...


Re: OT: Bad Epoxy? in an older Rohde & Schwarz receiver

Kurt Swanson
 

Bruce,

Years ago I was involved in an issue (not TEK or R&S) where we had a similar issue with an oscillator. Turned out a black rubber cushion that was put under the coil was becoming slightly conductive with time and temperature, and was causing the "Q" of the tuned circuit to drop. Since it was determined that it was only needed as an assembly aid, and served no other functional purpose once assembly was complete, the field repair solution was to just pull it out and discard it!

Bottom line - keep an open mind here.

Regards - Kurt


Re: 465B strange ripple

 

On Tue, Feb 16, 2021 at 05:30 PM, Raymond Domp Frank wrote:


Assuming the 'scope's case is connected to ground
That should've been "...connected to safety earth...", of course.

Raymond


Re: 465B strange ripple

Harvey White
 

In a full wave bridge, each half of the transformer winding contributes to the output, so the ripple should be 100 or 120 Hz, equal positive peaks.

If one diode is open, then 1/2 of the transformer does not contribute at all, so the ripple is larger, and will be at the line frequency.

You could measure the ripple on one channel of a second scope, then look at the bridge input (to ground) with the second channel.  Lining up the waveforms ought to show you which transformer secondary branch is contributing to what, and ought to indicate which one (or two) of the bridge diodes is open.

Harvey

On 2/16/2021 10:21 AM, Stephen wrote:
On Tue, Feb 16, 2021 at 04:05 AM, Chuck Harris wrote:

A shorted rectifier will usually have the effect of
blowing the fuse, and/or killing the filter capacitor...
that is true. And, the ripple will be severe.

I should have mentioned that an open rectifier can cause
line frequency ripple without damaging anything. It changes
the full wave rectifier into a half wave rectifier. The
supply filters are sized to filter adequately at the full wave
ripple frequency, not the half wave ripple frequency.I

Also, if one of the capacitors in the line input filter
is shorted to ground, it will elevate the case to the
AC mains frequency and voltage, and make lots of mains
frequency ripple available. You may, or may not notice,
depending on how isolated your work station is.

-Chuck Harris
Thanks Chuck,

None of the filter capacitors are shorted to ground. But I’ll double check that again shortly.

What I gather from what you’re saying is that one of the rectifiers is likely to be opened, not shorted.
However, since I can’t test any of them without removing the A4 board, which I’m not going to do, and since I have therefore no way of knowing which one is opened, I may just destroy them all with a wire cutter, keep some leads, and solder new ones on top, and hope that fixes the problem... I hate to do that, but I can’t think of any other way. Crossing fingers...

A better, less invasive solution is always welcome 🙏





Re: 465B strange ripple

 

On Tue, Feb 16, 2021 at 04:05 PM, Chuck Harris wrote:


Also, if one of the capacitors in the line input filter
is shorted to ground, it will elevate the case to the
AC mains frequency and voltage, and make lots of mains
frequency ripple available. You may, or may not notice,
depending on how isolated your work station is.
Assuming the 'scope's case is connected to ground (as it should be), that'd trip the earth leak fuse in (large parts of?) Europe...

Raymond


Re: 7104 out of focus/astigmatism

Francesco Franzini
 

The low voltage section and the PSU are perfectly fine, no ripple and everything is very stable from cold to hot, i starded to investigate on A22 HV board, voltage level are ok and on spec. but i found something strange on TP1784, voltage level (+98.6) ok but very high and variable ripple, when the scope is out of focus/astigmatism the ripple is 4 to 6 volt 2.5ms period then, after about 30 min, drop to 1 volt 2.5ms period and everything is working right.
At this point of my investigation i am pretty sure that the issue is on the HV circuit (--2400/-2265) and seems to be related to something that change its characteristic under power, i guess that i need to check avery single components for anomalies.

Any other advice?

Thank you

Francesco


Re: 465B strange ripple

Stephen
 

Non of the filter capacitors have a short to ground.


Re: 465B strange ripple

Stephen
 

On Tue, Feb 16, 2021 at 04:05 AM, Chuck Harris wrote:


A shorted rectifier will usually have the effect of
blowing the fuse, and/or killing the filter capacitor...
that is true. And, the ripple will be severe.
OT: That’s probably what happened to my 475A, but that will be for another thread when I’ll have time to finally fix it.


Re: 465B strange ripple

Stephen
 

On Tue, Feb 16, 2021 at 04:05 AM, Chuck Harris wrote:


A shorted rectifier will usually have the effect of
blowing the fuse, and/or killing the filter capacitor...
that is true. And, the ripple will be severe.

I should have mentioned that an open rectifier can cause
line frequency ripple without damaging anything. It changes
the full wave rectifier into a half wave rectifier. The
supply filters are sized to filter adequately at the full wave
ripple frequency, not the half wave ripple frequency.

Also, if one of the capacitors in the line input filter
is shorted to ground, it will elevate the case to the
AC mains frequency and voltage, and make lots of mains
frequency ripple available. You may, or may not notice,
depending on how isolated your work station is.

-Chuck Harris
Thanks Chuck,

None of the filter capacitors are shorted to ground. But I’ll double check that again shortly.

What I gather from what you’re saying is that one of the rectifiers is likely to be opened, not shorted.
However, since I can’t test any of them without removing the A4 board, which I’m not going to do, and since I have therefore no way of knowing which one is opened, I may just destroy them all with a wire cutter, keep some leads, and solder new ones on top, and hope that fixes the problem... I hate to do that, but I can’t think of any other way. Crossing fingers...

A better, less invasive solution is always welcome 🙏


Re: Cleaning Scratchy Bourns MOD pots

Dale H. Cook
 

On Mon, Feb 15, 2021 at 10:10 AM, Jean-Paul wrote:

Also advice on the best cleaner solution to use.
As one of the pros that Dick mentioned I will second him - use Caig FaderLube - it is designed for controls. Caig D-10 and G-10 are contact cleaners designed for cleaning switches, not controls.

Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcity/radios/


Re: 465B strange ripple

Chuck Harris <cfharris@...>
 

A shorted rectifier will usually have the effect of
blowing the fuse, and/or killing the filter capacitor...
that is true. And, the ripple will be severe.

I should have mentioned that an open rectifier can cause
line frequency ripple without damaging anything. It changes
the full wave rectifier into a half wave rectifier. The
supply filters are sized to filter adequately at the full wave
ripple frequency, not the half wave ripple frequency.

Also, if one of the capacitors in the line input filter
is shorted to ground, it will elevate the case to the
AC mains frequency and voltage, and make lots of mains
frequency ripple available. You may, or may not notice,
depending on how isolated your work station is.

-Chuck Harris

Stephen wrote:

On Tue, Feb 16, 2021 at 03:13 AM, Chuck Harris wrote:

INTERLEAVED

The cap bypasses the 15V unregulated supply where
it enters the transformer... C4006, a 47uf. Tek was so
concerned about that noise that they used both a choke
and a capacitor to filter it out.

The ripple in your picture shows 50 or 60 Hz, which is
atypical for a bad capacitor.... These are full wave rectified
supplies, so they should show 100 or 120Hz ripple, depending
on your mains frequency. This is more likely to be caused
by a missing shield, or a shorted rectifier.
Line frequency is 50Hz over here in Europe.
I don’t see any shield missing at all... anywhere. Everything is where is should be...

Wouldn’t a shorted rectifier have drastic consequences though?


Also, measuring the leakage of these capacitors is a
useless measurement. You want to know how good they are
at being a capacitor. ESR is the quickest way to find
that out. A good capacitor will typically have an ESR
below 1 ohm. A bad capacitor will typically have an ESR
well above 10 ohms.
Unfortunately I don’t have an ESR meter. But the two I painfully replaced (+110V and -8V caps) were tested for leakage, and they were good.

The HV section of the scope is among the safest places
in the scope. You could grab the anode lead, with
one hand, and the chassis with your other hand, and
hold it all day long without any more reaction than you
get when you touch a doorknob on a very dry day after
walking across a carpet. Annoying, but not harmful.

Now, the low voltage regulators, specifically the 110V,
or the 55V, could wreck your day.

It is not the voltage that kills you, it is the current,
and there is very little current available on the HV
supply. Tektronix made sure of that.
So my next step should be to check all the rectifiers?
Grrr... what a pain... they’re near impossible to remove without removing the A4 board...

-Chuck Harris





Re: 465B strange ripple

Stephen
 

On Tue, Feb 16, 2021 at 03:13 AM, Chuck Harris wrote:

INTERLEAVED

The cap bypasses the 15V unregulated supply where
it enters the transformer... C4006, a 47uf. Tek was so
concerned about that noise that they used both a choke
and a capacitor to filter it out.

The ripple in your picture shows 50 or 60 Hz, which is
atypical for a bad capacitor.... These are full wave rectified
supplies, so they should show 100 or 120Hz ripple, depending
on your mains frequency. This is more likely to be caused
by a missing shield, or a shorted rectifier.
Line frequency is 50Hz over here in Europe.
I don’t see any shield missing at all... anywhere. Everything is where is should be...

Wouldn’t a shorted rectifier have drastic consequences though?


Also, measuring the leakage of these capacitors is a
useless measurement. You want to know how good they are
at being a capacitor. ESR is the quickest way to find
that out. A good capacitor will typically have an ESR
below 1 ohm. A bad capacitor will typically have an ESR
well above 10 ohms.
Unfortunately I don’t have an ESR meter. But the two I painfully replaced (+110V and -8V caps) were tested for leakage, and they were good.

The HV section of the scope is among the safest places
in the scope. You could grab the anode lead, with
one hand, and the chassis with your other hand, and
hold it all day long without any more reaction than you
get when you touch a doorknob on a very dry day after
walking across a carpet. Annoying, but not harmful.

Now, the low voltage regulators, specifically the 110V,
or the 55V, could wreck your day.

It is not the voltage that kills you, it is the current,
and there is very little current available on the HV
supply. Tektronix made sure of that.
So my next step should be to check all the rectifiers?
Grrr... what a pain... they’re near impossible to remove without removing the A4 board...

-Chuck Harris


Re: 465B strange ripple

Chuck Harris <cfharris@...>
 

The cap bypasses the 15V unregulated supply where
it enters the transformer... C4006, a 47uf. Tek was so
concerned about that noise that they used both a choke
and a capacitor to filter it out.

The ripple in your picture shows 50 or 60 Hz, which is
atypical for a bad capacitor.... These are full wave rectified
supplies, so they should show 100 or 120Hz ripple, depending
on your mains frequency. This is more likely to be caused
by a missing shield, or a shorted rectifier.

Also, measuring the leakage of these capacitors is a
useless measurement. You want to know how good they are
at being a capacitor. ESR is the quickest way to find
that out. A good capacitor will typically have an ESR
below 1 ohm. A bad capacitor will typically have an ESR
well above 10 ohms.

The HV section of the scope is among the safest places
in the scope. You could grab the anode lead, with
one hand, and the chassis with your other hand, and
hold it all day long without any more reaction than you
get when you touch a doorknob on a very dry day after
walking across a carpet. Annoying, but not harmful.

Now, the low voltage regulators, specifically the 110V,
or the 55V, could wreck your day.

It is not the voltage that kills you, it is the current,
and there is very little current available on the HV
supply. Tektronix made sure of that.

-Chuck Harris

Stephen wrote:

On Mon, Feb 15, 2021 at 07:00 PM, Chuck Harris wrote:

Interleaved

As I understand it, you have checked the ripple on all of
the supplies, and it is small, and in spec.

I looked at your pictures, but there are no indications
of sweep speed.
https://groups.io/g/TekScopes/photo/260052/1?p=Created,,,50,2,0,0

https://groups.io/g/TekScopes/photo/260052/0?p=Created,,,50,2,0,0

Noticing the way the traces appear, they would seem to be
quite fast, indicating the trace ripple is far, far above
mains frequency.
Nope. Not fast at all. Both pictures above show that ripple at 20ms/div. Quite low...
The legends are at the bottom of the above images.


Have you checked the supply lead for the transistor that
drives the high voltage transformer?
No, I haven’t. Which cap would that be?

Often the bypass capacitor
will go bad, and the HV supply will pollute the inside of the
scope with high frequency RF noise that shows up on everything.
Hmmm... I try not to go near that HV if I can avoid it...

-Chuck Harris





Re: 465B strange ripple

Stephen
 

On Mon, Feb 15, 2021 at 07:00 PM, Chuck Harris wrote:

Interleaved

As I understand it, you have checked the ripple on all of
the supplies, and it is small, and in spec.

I looked at your pictures, but there are no indications
of sweep speed.
https://groups.io/g/TekScopes/photo/260052/1?p=Created,,,50,2,0,0

https://groups.io/g/TekScopes/photo/260052/0?p=Created,,,50,2,0,0

Noticing the way the traces appear, they would seem to be
quite fast, indicating the trace ripple is far, far above
mains frequency.
Nope. Not fast at all. Both pictures above show that ripple at 20ms/div. Quite low...
The legends are at the bottom of the above images.


Have you checked the supply lead for the transistor that
drives the high voltage transformer?
No, I haven’t. Which cap would that be?

Often the bypass capacitor
will go bad, and the HV supply will pollute the inside of the
scope with high frequency RF noise that shows up on everything.
Hmmm... I try not to go near that HV if I can avoid it...

-Chuck Harris


Re: Stan Griffiths estate sale

Clark Foley
 

Ryan,
Good citizen! Does your wife know where you are going to put all of that?

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