Topics

7904 Mainframe damaged

JJ
 

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!

Chuck Harris
 

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!

 

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: <TekScopes@groups.io>
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!

JJ
 

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!



JJ
 

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: <TekScopes@groups.io>
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!







Dave Casey
 

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: <TekScopes@groups.io>
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!









Chuck Harris
 

I think most probably one of two things happened:

1) The accidental grounding blew a fuse that is in
common with both your scope, and the dut.

or

2) a tantalum capacitor that thought it would make your
life interesting to short after you had that little
accident.

The scope has no knowledge of the integrity of its safety
bonding (green) wire.

Check the power socket your scope plugs into for power.

The 7904 has a permanent power cord, so it is a little
hard to check, but check to see that AC power exists on
the inside of the scope. Check the scope's fuse with
an ohmmeter out on the bench.

-Chuck Harris

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:

Chuck Harris
 

Being totally dead is great news. Go hunting
for where the AC power goes away.

-Chuck Harris

JJ wrote:

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

JJ
 

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: <TekScopes@groups.io>
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!











JJ
 

Right, I haven't probed inside as yet because it wasn't friendly to keep
the power cord plugged in when the panel is removed. But, you're right I
have to go there.

On Fri, Apr 6, 2018 at 6:25 PM, John Justin <jajustin@...> wrote:

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: <TekScopes@groups.io>
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!











bobh@joba.com
 

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!



JJ
 

Hi Bob, I plugged the scope directly into a different wall outlet - no
glory. Looks like something internal. I'm probing the AC section.

Thx
John

On Fri, Apr 6, 2018 at 7:03 PM, Robert Hay <bobh@...> wrote:

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!






Harvey White
 

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!




Harvey White
 

On Fri, 6 Apr 2018 18:25:50 -0400, you wrote:

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.
Silly question, and perhaps easy to fix: could it be the switch?
Harvey



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: <TekScopes@groups.io>
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!












Brad Thompson <brad.thompson@...>
 

On 4/6/2018 7:21 PM, JJ wrote:
Hi Bob, I plugged the scope directly into a different wall outlet - no
glory. Looks like something internal. I'm probing the AC section.
Hello--

I'm unfamiliar with the 7904's internals, but is there an AC filter module
located between the incoming power cord and the power-supply section?
I'm picturing a blown winding on a common-mode choke....

HTH, and 73--

Brad AA1IP

JJ
 

Harvey, you're right. In looking through the maintenance manual and
schematics, there is quite a bit of protection circuitry throughout the
power supply section. I think I narrowed the problem down to the "power
supply inverter board". I followed the debug procedure and it directs me
there. On that board, there is a fast blow fuse! They indicate to check a
few SCRs, zeners, bias diodes, and transistors on that board. I need to
pull the power supply out to get to that board. So, I'm reading through the
precautions so I don't electrocute myself. Luckily I have a Tek 576 Curve
Tracer to test out those 6 components if necessary.

This scope is one complex beast!

I also noticed if I let the scope sit powered off for a few minutes and
then power on I hear a noise inside the unit - so power is getting in. If I
don't wait long after I power off, and then power on, I don't hear the
noise.

On Fri, Apr 6, 2018 at 7:28 PM, Harvey White <madyn@...> wrote:

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!







JJ
 

Hi Brad, yes, there's a full wave rectifier followed by a fast blow fuse
that feeds a choke on the "power supply inverter board". Everything seems
to point to the problem being on this board. hope that it;s the fuse. The
board is located inside a high voltage cage with other boards.

On Fri, Apr 6, 2018 at 8:18 PM, Brad Thompson <brad.thompson@...>
wrote:

On 4/6/2018 7:21 PM, JJ wrote:

Hi Bob, I plugged the scope directly into a different wall outlet - no
glory. Looks like something internal. I'm probing the AC section.
Hello--

I'm unfamiliar with the 7904's internals, but is there an AC filter module
located between the incoming power cord and the power-supply section?
I'm picturing a blown winding on a common-mode choke....

HTH, and 73--

Brad AA1IP



 

Remember that the 7904 has a hidden fuse in the internal mains filter. Check that mains is getting to the bridge rectifier.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of JJ
Sent: 06 April 2018 23:26
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7904 Mainframe damaged

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.

JJ
 

Thanks David, I'm going to pull the power supply section out of the back of
the unit today. I'll look for it and probe along the rectifier filter path.

On Sat, Apr 7, 2018 at 5:47 AM, David C. Partridge <
@perdrix> wrote:

Remember that the 7904 has a hidden fuse in the internal mains filter.
Check that mains is getting to the bridge rectifier.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of JJ
Sent: 06 April 2018 23:26
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7904 Mainframe damaged

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.




 

Be warned, that mains filter is made of discrete components and is a real swine to get to as it is fastened to the inside of the rear heatsink.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of JJ
Sent: 07 April 2018 12:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7904 Mainframe damaged

Thanks David, I'm going to pull the power supply section out of the back of the unit today. I'll look for it and probe along the rectifier filter path.

On Sat, Apr 7, 2018 at 5:47 AM, David C. Partridge < @perdrix> wrote:

Remember that the 7904 has a hidden fuse in the internal mains filter.
Check that mains is getting to the bridge rectifier.

David