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On Fri, 6 Apr 2018 18:09:41 -0400, you wrote:

Thanks for your informed response, Chuck. I live in Queens, New York. I
will check the green wire in the scope's cord though I would think that if
it were open the unit would still power up.

Perhaps it took out something needed.

But maybe there is safety
circuitry inside the frame that is preventing the unit from powering on
because it doesn't detect an earth ground or some other situation? I think
that might be the case because the scope was still powered on after the
short occurred.

More likely, there's a "start up" set of limits, and a "run" set of
limits".

I've fixed one of these, (actually, a 7400), and I don't remember ever
seeing anything like that kind of protection.

After I powered it down, it wouldn't power back up.
Alternatively, as you point out, there could be a "power on good" circuit
that is preventing the power on because of the compromised caps. Note that
I had removed all the plugins so the mainframe is empty.

That is good, then. It's possible that you 1) may have stressed a
tantalum (or more than one) that was ready to fail and it did. Most
of those supplies will "tick" when trying to start up, and then shut
down. This (IMHO) is that there's an excessive load on the supply.

Another possibility is that you've managed to blow something in the
bulk supply, without which, nothing much happens. Now (and I don't
remember this, but it's common in computer power supplies), if there's
an auxiliary supply that has to run to supply the main supply with
bias voltages...... if that's gone, the nothing runs. The block
diagram is your friend here, as is the theory of operation.

Harvey

As far as using the isolation transformer, I wanted to isolate the earth
ground by plugging in the scope and the UUT on the secondary side of the
transformer - it was dumb and unsafe, I won't even go there.

On Fri, Apr 6, 2018 at 5:29 PM, Chuck Harris <cfharris@...> wrote:

You don't say where in the world you are, so I have
no insight into whether you are in 120V land, or 240V
land, but....

There really isn't any place on a 7904, or its usual
plugins where you could stuff the power line current
into that would destroy anything more than the bench's
fuse, or one of the scope's ground leads... particularly
the probe's.

If you are a retired electronics engineer, put you hat
back on for a few minutes, and draw some sketches of
how the isolation transformer, and the stereo's guts
could force current into each other with what you did
when you saw the flash.

I think you will come to agree with me that the only
two parts of your 7904 that are at risk are the scope
probe's ground wire, and the green (bond) wire on the
scope's power cord.

All of the 1M vertical amplifier inputs are safe to 300V,
as are most every other input in the scope. The 50 ohm
vertical amplifiers will blow, but the damage is limited
to the plugin.

7904's are notorious for eating up tantalum electrolytics
on the +/- 15V filters on various PC boards, and plugins,
and the +50V filters on the CRT amplifier cards. They
blow when you look at them cross-eyed, or don't... when
the wind blows, or doesn't.

7904's are notorious for having a power supply that is
extremely sensitive to overload, and as such protects
itself from just about any insult you could hurl at it.

The second part of my diatribe has to do with isolation
transformers, and why you were using one.

First, your scope's measurement ground is directly
connected to your scope's case, is directly connected to
the bond wire on the third pin on the power cord. As a
result, it is never safe to use an isolation transformer
to break that bond wire, and float your scope... I hope
you weren't doing that!

Second, it is rarely necessary to use one for anything
other than switching power supply work on a DUT.

What were you expecting the isolation transformer to
isolate?

-Chuck Harris




JJ wrote:

Hello, I own a beautiful 7904 scope that I damaged when probing a

stereo

receiver. I didn't realize that my isolation transformer was earth

grounded

- forgot to remove the ground strap. So I effectively shorted out the

scope

by touching the probe's ground lead to a voltage node. The mainframe
(without any plugins installed) doesn't power up. When the event

happened,

the scope remained on. But, after shutting it off, it wouldn't come

back

on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem

since

the surge came over the ground. I have the service manual. I was just
wondering if there was an obvious place to start looking since this

could

have happened to others. I'm hoping that it's repairable - my heart is
broken. I've used this scope for over 40 years. I'm a retired 70 year

old

electronics engineer. I guess I'm just getting old!

Often ( I have heard ) the probe ground lead vaporizes in these cases
because it is the weak link.

Have you tried anything else in the outlet the isolation transformer was
plugged into? And, is there a ground fault interrupter in the path that
may not be reset.

Bob.


On 4/6/2018 3:09 PM, JJ wrote:

Thanks for your informed response, Chuck. I live in Queens, New York. I
will check the green wire in the scope's cord though I would think that if
it were open the unit would still power up. But maybe there is safety
circuitry inside the frame that is preventing the unit from powering on
because it doesn't detect an earth ground or some other situation? I think
that might be the case because the scope was still powered on after the
short occurred. After I powered it down, it wouldn't power back up.
Alternatively, as you point out, there could be a "power on good" circuit
that is preventing the power on because of the compromised caps. Note that
I had removed all the plugins so the mainframe is empty.

As far as using the isolation transformer, I wanted to isolate the earth
ground by plugging in the scope and the UUT on the secondary side of the
transformer - it was dumb and unsafe, I won't even go there.

On Fri, Apr 6, 2018 at 5:29 PM, Chuck Harris <cfharris@...> wrote:

You don't say where in the world you are, so I have

no insight into whether you are in 120V land, or 240V
land, but....

There really isn't any place on a 7904, or its usual
plugins where you could stuff the power line current
into that would destroy anything more than the bench's
fuse, or one of the scope's ground leads... particularly
the probe's.

If you are a retired electronics engineer, put you hat
back on for a few minutes, and draw some sketches of
how the isolation transformer, and the stereo's guts
could force current into each other with what you did
when you saw the flash.

I think you will come to agree with me that the only
two parts of your 7904 that are at risk are the scope
probe's ground wire, and the green (bond) wire on the
scope's power cord.

All of the 1M vertical amplifier inputs are safe to 300V,
as are most every other input in the scope. The 50 ohm
vertical amplifiers will blow, but the damage is limited
to the plugin.

7904's are notorious for eating up tantalum electrolytics
on the +/- 15V filters on various PC boards, and plugins,
and the +50V filters on the CRT amplifier cards. They
blow when you look at them cross-eyed, or don't... when
the wind blows, or doesn't.

7904's are notorious for having a power supply that is
extremely sensitive to overload, and as such protects
itself from just about any insult you could hurl at it.

The second part of my diatribe has to do with isolation
transformers, and why you were using one.

First, your scope's measurement ground is directly
connected to your scope's case, is directly connected to
the bond wire on the third pin on the power cord. As a
result, it is never safe to use an isolation transformer
to break that bond wire, and float your scope... I hope
you weren't doing that!

Second, it is rarely necessary to use one for anything
other than switching power supply work on a DUT.

What were you expecting the isolation transformer to
isolate?

-Chuck Harris




JJ wrote:

Hello, I own a beautiful 7904 scope that I damaged when probing a stereo

receiver. I didn't realize that my isolation transformer was earth
grounded
- forgot to remove the ground strap. So I effectively shorted out the
scope
by touching the probe's ground lead to a voltage node. The mainframe
(without any plugins installed) doesn't power up. When the event
happened,
the scope remained on. But, after shutting it off, it wouldn't come back
on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem since

the surge came over the ground. I have the service manual. I was just
wondering if there was an obvious place to start looking since this could
have happened to others. I'm hoping that it's repairable - my heart is
broken. I've used this scope for over 40 years. I'm a retired 70 year old
electronics engineer. I guess I'm just getting old!

Thanks for your informed response, Chuck. I live in Queens, New York. I
will check the green wire in the scope's cord though I would think that if
it were open the unit would still power up. But maybe there is safety
circuitry inside the frame that is preventing the unit from powering on
because it doesn't detect an earth ground or some other situation? I think
that might be the case because the scope was still powered on after the
short occurred. After I powered it down, it wouldn't power back up.
Alternatively, as you point out, there could be a "power on good" circuit
that is preventing the power on because of the compromised caps. Note that
I had removed all the plugins so the mainframe is empty.

As far as using the isolation transformer, I wanted to isolate the earth
ground by plugging in the scope and the UUT on the secondary side of the
transformer - it was dumb and unsafe, I won't even go there.

On Fri, Apr 6, 2018 at 5:29 PM, Chuck Harris <cfharris@...> wrote:

You don't say where in the world you are, so I have
no insight into whether you are in 120V land, or 240V
land, but....

There really isn't any place on a 7904, or its usual
plugins where you could stuff the power line current
into that would destroy anything more than the bench's
fuse, or one of the scope's ground leads... particularly
the probe's.

If you are a retired electronics engineer, put you hat
back on for a few minutes, and draw some sketches of
how the isolation transformer, and the stereo's guts
could force current into each other with what you did
when you saw the flash.

I think you will come to agree with me that the only
two parts of your 7904 that are at risk are the scope
probe's ground wire, and the green (bond) wire on the
scope's power cord.

All of the 1M vertical amplifier inputs are safe to 300V,
as are most every other input in the scope. The 50 ohm
vertical amplifiers will blow, but the damage is limited
to the plugin.

7904's are notorious for eating up tantalum electrolytics
on the +/- 15V filters on various PC boards, and plugins,
and the +50V filters on the CRT amplifier cards. They
blow when you look at them cross-eyed, or don't... when
the wind blows, or doesn't.

7904's are notorious for having a power supply that is
extremely sensitive to overload, and as such protects
itself from just about any insult you could hurl at it.

The second part of my diatribe has to do with isolation
transformers, and why you were using one.

First, your scope's measurement ground is directly
connected to your scope's case, is directly connected to
the bond wire on the third pin on the power cord. As a
result, it is never safe to use an isolation transformer
to break that bond wire, and float your scope... I hope
you weren't doing that!

Second, it is rarely necessary to use one for anything
other than switching power supply work on a DUT.

What were you expecting the isolation transformer to
isolate?

-Chuck Harris




JJ wrote:

Hello, I own a beautiful 7904 scope that I damaged when probing a stereo

receiver. I didn't realize that my isolation transformer was earth grounded
- forgot to remove the ground strap. So I effectively shorted out the scope
by touching the probe's ground lead to a voltage node. The mainframe
(without any plugins installed) doesn't power up. When the event happened,
the scope remained on. But, after shutting it off, it wouldn't come back
on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem since

the surge came over the ground. I have the service manual. I was just
wondering if there was an obvious place to start looking since this could
have happened to others. I'm hoping that it's repairable - my heart is
broken. I've used this scope for over 40 years. I'm a retired 70 year old
electronics engineer. I guess I'm just getting old!

Yes, I checked the fuse and it was good. I put in a brand new fuse in any
case. I haven't tried to trace the problem through the schematics as yet
because I thought maybe someone on the forum had encountered a similar
situation and could lead me to a solution. There are a lot of specialized
ICs in the power supply section so it could be anything. It could also be
something very simple - usually when a unit doesn't power on at all, it's
something friendly.

On Fri, Apr 6, 2018 at 6:16 PM, Dave Casey <polara413@...> wrote:

You know it's not the fuse, but have you actually checked?

On Fri, Apr 6, 2018 at 5:12 PM, JJ <jajustin@...> wrote:

Thanks for your response. The scope doesn't make that beautiful ticking
sound during power on. It doesn't do anything.

On Fri, Apr 6, 2018 at 5:38 PM, tmillermdems <tmiller11147@...>
wrote:

Your isolation transformer should be grounded. And pass the safety

ground

through to the UUT.

What should happen is the connection between the Neutral and the

Ground

must be broken. This connection is made back at your service entry

panel.

With that condition removed, it should not matter if you place a

ground

on

either the Line or the Neutral on the secondary of the isolation
transformer. This is no different than the old transformer based power
system.

Maybe you can find a working 7904 power supply somewhere. There were

many

of those scopes made.

Does the power supply make a ticking sound when power is applied? Any
visible signs of blown parts in the supply? Do you have continuity

through

the RFI filter on the scope?

You might say where you are located case someone is near you that can

help.

Regards




----- Original Message ----- From: "JJ" <jajustin@...>
To: <[email protected]>
Sent: Friday, April 06, 2018 4:23 PM
Subject: [TekScopes] 7904 Mainframe damaged


Hello, I own a beautiful 7904 scope that I damaged when probing a

stereo

receiver. I didn't realize that my isolation transformer was earth

grounded

- forgot to remove the ground strap. So I effectively shorted out the

scope

by touching the probe's ground lead to a voltage node. The mainframe
(without any plugins installed) doesn't power up. When the event

happened,

the scope remained on. But, after shutting it off, it wouldn't come

back

on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem

since

the surge came over the ground. I have the service manual. I was just
wondering if there was an obvious place to start looking since this

could

have happened to others. I'm hoping that it's repairable - my heart is
broken. I've used this scope for over 40 years. I'm a retired 70 year

old

electronics engineer. I guess I'm just getting old!

You know it's not the fuse, but have you actually checked?

On Fri, Apr 6, 2018 at 5:12 PM, JJ <jajustin@...> wrote:

Thanks for your response. The scope doesn't make that beautiful ticking
sound during power on. It doesn't do anything.

On Fri, Apr 6, 2018 at 5:38 PM, tmillermdems <tmiller11147@...>
wrote:

Your isolation transformer should be grounded. And pass the safety

ground

through to the UUT.

What should happen is the connection between the Neutral and the Ground
must be broken. This connection is made back at your service entry

panel.

With that condition removed, it should not matter if you place a ground

on

either the Line or the Neutral on the secondary of the isolation
transformer. This is no different than the old transformer based power
system.

Maybe you can find a working 7904 power supply somewhere. There were

many

of those scopes made.

Does the power supply make a ticking sound when power is applied? Any
visible signs of blown parts in the supply? Do you have continuity

through

the RFI filter on the scope?

You might say where you are located case someone is near you that can

help.

Regards




----- Original Message ----- From: "JJ" <jajustin@...>
To: <[email protected]>
Sent: Friday, April 06, 2018 4:23 PM
Subject: [TekScopes] 7904 Mainframe damaged


Hello, I own a beautiful 7904 scope that I damaged when probing a

stereo

receiver. I didn't realize that my isolation transformer was earth

grounded

- forgot to remove the ground strap. So I effectively shorted out the

scope

by touching the probe's ground lead to a voltage node. The mainframe
(without any plugins installed) doesn't power up. When the event

happened,

the scope remained on. But, after shutting it off, it wouldn't come

back

on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem since
the surge came over the ground. I have the service manual. I was just
wondering if there was an obvious place to start looking since this

could

have happened to others. I'm hoping that it's repairable - my heart is
broken. I've used this scope for over 40 years. I'm a retired 70 year

old

electronics engineer. I guess I'm just getting old!

Thanks for your response. The scope doesn't make that beautiful ticking
sound during power on. It doesn't do anything.

Thanks for your informed response, Chuck. I live in Queens, New York. I
will check the green wire in the scope's cord though I would think that if
it were open the unit would still power up. But maybe there is safety
circuitry inside the frame that is preventing the unit from powering on
because it doesn't detect an earth ground or some other situation? I think
that might be the case because the scope was still powered on after the
short occurred. After I powered it down, it wouldn't power back up.
Alternatively, as you point out, there could be a "power on good" circuit
that is preventing the power on because of the compromised caps. Note that
I had removed all the plugins so the mainframe is empty.

As far as using the isolation transformer, I wanted to isolate the earth
ground by plugging in the scope and the UUT on the secondary side of the
transformer - it was dumb and unsafe, I won't even go there.

On Fri, Apr 6, 2018 at 5:29 PM, Chuck Harris <cfharris@...> wrote:

Thanks for your response. The scope doesn't make that beautiful ticking
sound during power on. It doesn't do anything.

On Fri, Apr 6, 2018 at 5:38 PM, tmillermdems <tmiller11147@...>
wrote:

Your isolation transformer should be grounded. And pass the safety ground
through to the UUT.

What should happen is the connection between the Neutral and the Ground
must be broken. This connection is made back at your service entry panel.
With that condition removed, it should not matter if you place a ground

on

either the Line or the Neutral on the secondary of the isolation
transformer. This is no different than the old transformer based power
system.

Maybe you can find a working 7904 power supply somewhere. There were many
of those scopes made.

Does the power supply make a ticking sound when power is applied? Any
visible signs of blown parts in the supply? Do you have continuity

through

the RFI filter on the scope?

You might say where you are located case someone is near you that can

help.

Regards




----- Original Message ----- From: "JJ" <jajustin@...>
To: <[email protected]>
Sent: Friday, April 06, 2018 4:23 PM
Subject: [TekScopes] 7904 Mainframe damaged


Hello, I own a beautiful 7904 scope that I damaged when probing a stereo
receiver. I didn't realize that my isolation transformer was earth

grounded

- forgot to remove the ground strap. So I effectively shorted out the

scope

by touching the probe's ground lead to a voltage node. The mainframe
(without any plugins installed) doesn't power up. When the event

happened,

the scope remained on. But, after shutting it off, it wouldn't come back
on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem since
the surge came over the ground. I have the service manual. I was just
wondering if there was an obvious place to start looking since this could
have happened to others. I'm hoping that it's repairable - my heart is
broken. I've used this scope for over 40 years. I'm a retired 70 year old
electronics engineer. I guess I'm just getting old!

You don't say where in the world you are, so I have
no insight into whether you are in 120V land, or 240V
land, but....

There really isn't any place on a 7904, or its usual
plugins where you could stuff the power line current
into that would destroy anything more than the bench's
fuse, or one of the scope's ground leads... particularly
the probe's.

If you are a retired electronics engineer, put you hat
back on for a few minutes, and draw some sketches of
how the isolation transformer, and the stereo's guts
could force current into each other with what you did
when you saw the flash.

I think you will come to agree with me that the only
two parts of your 7904 that are at risk are the scope
probe's ground wire, and the green (bond) wire on the
scope's power cord.

All of the 1M vertical amplifier inputs are safe to 300V,
as are most every other input in the scope. The 50 ohm
vertical amplifiers will blow, but the damage is limited
to the plugin.

7904's are notorious for eating up tantalum electrolytics
on the +/- 15V filters on various PC boards, and plugins,
and the +50V filters on the CRT amplifier cards. They
blow when you look at them cross-eyed, or don't... when
the wind blows, or doesn't.

7904's are notorious for having a power supply that is
extremely sensitive to overload, and as such protects
itself from just about any insult you could hurl at it.

The second part of my diatribe has to do with isolation
transformers, and why you were using one.

First, your scope's measurement ground is directly
connected to your scope's case, is directly connected to
the bond wire on the third pin on the power cord. As a
result, it is never safe to use an isolation transformer
to break that bond wire, and float your scope... I hope
you weren't doing that!

Second, it is rarely necessary to use one for anything
other than switching power supply work on a DUT.

What were you expecting the isolation transformer to
isolate?

-Chuck Harris




JJ wrote:

Hello, I own a beautiful 7904 scope that I damaged when probing a stereo

receiver. I didn't realize that my isolation transformer was earth grounded
- forgot to remove the ground strap. So I effectively shorted out the scope
by touching the probe's ground lead to a voltage node. The mainframe
(without any plugins installed) doesn't power up. When the event happened,
the scope remained on. But, after shutting it off, it wouldn't come back
on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem since

the surge came over the ground. I have the service manual. I was just
wondering if there was an obvious place to start looking since this could
have happened to others. I'm hoping that it's repairable - my heart is
broken. I've used this scope for over 40 years. I'm a retired 70 year old
electronics engineer. I guess I'm just getting old!

I have a HV transformer with 7 windings (14 lugs for connecting to those
windings), and 5 additional lugs for taps on some of the 7 windings. The
windings generate various voltages from 5V to 3.5KV. it is used in a simple
push pull oscillator with about 96V on the primary. It uses a square ferrite
core, and it oscillates probably just above human hearing range (>20KHz). It
has a Tek part number 115-0601-00, and the following additional info: SAR
5081 on it.



Something happened to one of the middle windings that blew a dime size hole
outward causing it to short out one or more of the windings from near that
layer all the way to the outer layer where it is plainly visible.



I would like to carefully remove what I think is paper between windings but
it seems to have been impregnated with something that makes it extremely
hard.



Is there something I can soak the entire transformer in to soften up this
material whatever it is so I can start to "peel this onion" so to speak
without destroying the clues that might tell me what went wrong with it in
the first place.



Dennis Tillman W7PF

Hello, I own a beautiful 7904 scope that I damaged when probing a stereo receiver. I didn't realize that my isolation transformer was earth grounded - forgot to remove the ground strap. So I effectively shorted out the scope by touching the probe's ground lead to a voltage node. The mainframe (without any plugins installed) doesn't power up. When the event happened, the scope remained on. But, after shutting it off, it wouldn't come back on. There's no activity - just dead.

I checked the fuse but I knew that wasn't going to be the problem since the surge came over the ground. I have the service manual. I was just wondering if there was an obvious place to start looking since this could have happened to others. I'm hoping that it's repairable - my heart is broken. I've used this scope for over 40 years. I'm a retired 70 year old electronics engineer. I guess I'm just getting old!