Topics

Tek 7904 help!


Chuck Harris
 

Resistance measurements were originally taken with no plugins,
and using an old fashioned Tripolett or Simpson VOM... the type
is specified in the manual somewhere near the readings.

Anything else, and the already only slightly valid readings become
less so.

If your scope has aluminum can type electrolytic capacitors, they
were replaced by someone earlier in the scope's existence.

The two capacitors on the +/-15V supplies are on the schematic,
but the tantalum on the 50V supply is not... and yet it is there.

You can often figure out which voltage is putting the power supply
in tick mode by measuring the supplies while the ticking is going on.
You will see most of the supply voltages tick up into a goodly fraction
of their nominal voltage, but the supply with the shorted load will
not.

-Chuck Harris

Bill Riches via groups.io wrote:

Hi Chck and gang,
I have a later? horizontal board - no tant - 2 small can type caps.  My manual shows the tant caps.  Did check the caps and they were ok.  
Some more measurements.
1. PS disconnected completely2. Resistance measurements taken at Z axis board with Fluke DVM
    VOLTAGE TP    NORMAL    NO PLUGINS    WITH PLUGINS

    +130                    6.6K                9.5K                9.5K
    +50                        2K                 4.6K                4.2K
    +15                        90                  92                    27
     +5                         65                  49                    19
     -15                       110                216                   31
     -50                       2K                   55K                 55K
     +LAMP 5              INF                  INF                 55k
(red underlines from spell check)
3. Highlighted readings look strange.  Could I have 4 plugins with loading.  Inserting each module reduced resistance by a similar amount from 216 ohms to 31 for -15 volt source.  
4. Checked power supply voltages out of mainframe with 120 VAC.  Power about 10 watts or so.130 = 136+5 =   4.8-50= 12+50 = 25-15 = 2+15 = 10
I measured the crt anode voltage out of the PS using and old Pomona tv hv meter and the meter would kick a bit - maybe a few hundred volts..

I guess that low voltages would be low due to burst mode not allowing output filters to charge to normal value.
I am thinking PS is ok or it is shutting itself down.  Just curious why the different resistance readings when plug-ins are installed.  We had quite an electrical storm on Tuesday Evening. Wednesday morning the scope was not working.  As you can see from my QRZ.com page I have a bunch of toys and they are all working - so far.
So - 2 questions - why do plug-ins resistance read low and are my voltages close when the ps is not installed?
Just remembered I do have some old plug ins and will plug them in and see if I get the same resistance drop.
I really appreciate the help.
73
Bill, WA2DVUCape May

On Thursday, July 9, 2020, 12:04:05 AM EDT, Chuck Harris <cfharris@...> wrote:

There are two tantalum capacitors that every 7904 I have had
has blown.  One is on the +50V supply, and doesn't show up in
my schematics, and the other is on the -15V supply, if I recall
correctly.  They both are rated at too low of a voltage.
They are on the Horizontal board, which is on the left side of
chassis, as I recall.  One has a choke feeding it, and the other
a 10 ohm 1/4w resistor.

You should be suspicious of any tantalum capacitor that is rated
less than 2x the applied voltage.  6.3V caps on 5V, and 16V caps
on 15V are especially bad.  Anything on a 50V supply is a problem
because the maximum voltage a dry slug tantalum can be is 60V.

Be warned, there is a blue harmonica connector on that board
that turns to dust when you touch it.


The power supply itself is usually pretty solid.  Look
at the rest of the scope for a blown tantalum.

-Chuck Harris

Bill Riches via groups.io wrote:
  Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
73,
Bill
    On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

  When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?

Harvey


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond













Bill Riches
 

Hi Chck and gang,
I have a later? horizontal board - no tant - 2 small can type caps.  My manual shows the tant caps.  Did check the caps and they were ok.  
Some more measurements.
1. PS disconnected completely2. Resistance measurements taken at Z axis board with Fluke DVM
    VOLTAGE TP    NORMAL    NO PLUGINS    WITH PLUGINS

    +130                    6.6K                9.5K                9.5K
    +50                        2K                 4.6K                4.2K
    +15                        90                  92                    27
     +5                         65                  49                    19
     -15                       110                216                   31
     -50                       2K                   55K                 55K
     +LAMP 5              INF                  INF                 55k
(red underlines from spell check)
3. Highlighted readings look strange.  Could I have 4 plugins with loading.  Inserting each module reduced resistance by a similar amount from 216 ohms to 31 for -15 volt source.  
4. Checked power supply voltages out of mainframe with 120 VAC.  Power about 10 watts or so.130 = 136+5 =   4.8-50= 12+50 = 25-15 = 2+15 = 10
I measured the crt anode voltage out of the PS using and old Pomona tv hv meter and the meter would kick a bit - maybe a few hundred volts..

I guess that low voltages would be low due to burst mode not allowing output filters to charge to normal value.
I am thinking PS is ok or it is shutting itself down.  Just curious why the different resistance readings when plug-ins are installed.  We had quite an electrical storm on Tuesday Evening. Wednesday morning the scope was not working.  As you can see from my QRZ.com page I have a bunch of toys and they are all working - so far.
So - 2 questions - why do plug-ins resistance read low and are my voltages close when the ps is not installed?
Just remembered I do have some old plug ins and will plug them in and see if I get the same resistance drop.
I really appreciate the help.
73
Bill, WA2DVUCape May

On Thursday, July 9, 2020, 12:04:05 AM EDT, Chuck Harris <cfharris@...> wrote:

There are two tantalum capacitors that every 7904 I have had
has blown.  One is on the +50V supply, and doesn't show up in
my schematics, and the other is on the -15V supply, if I recall
correctly.  They both are rated at too low of a voltage.
They are on the Horizontal board, which is on the left side of
chassis, as I recall.  One has a choke feeding it, and the other
a 10 ohm 1/4w resistor.

You should be suspicious of any tantalum capacitor that is rated
less than 2x the applied voltage.  6.3V caps on 5V, and 16V caps
on 15V are especially bad.  Anything on a 50V supply is a problem
because the maximum voltage a dry slug tantalum can be is 60V.

Be warned, there is a blue harmonica connector on that board
that turns to dust when you touch it.


The power supply itself is usually pretty solid.  Look
at the rest of the scope for a blown tantalum.

-Chuck Harris

Bill Riches via groups.io wrote:
  Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
73,
Bill
    On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

  When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?

Harvey


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond









Chuck Harris
 

There are two tantalum capacitors that every 7904 I have had
has blown. One is on the +50V supply, and doesn't show up in
my schematics, and the other is on the -15V supply, if I recall
correctly. They both are rated at too low of a voltage.
They are on the Horizontal board, which is on the left side of
chassis, as I recall. One has a choke feeding it, and the other
a 10 ohm 1/4w resistor.

You should be suspicious of any tantalum capacitor that is rated
less than 2x the applied voltage. 6.3V caps on 5V, and 16V caps
on 15V are especially bad. Anything on a 50V supply is a problem
because the maximum voltage a dry slug tantalum can be is 60V.

Be warned, there is a blue harmonica connector on that board
that turns to dust when you touch it.


The power supply itself is usually pretty solid. Look
at the rest of the scope for a blown tantalum.

-Chuck Harris

Bill Riches via groups.io wrote:

Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
73,
Bill
On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?

Harvey


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond









Bill Riches
 

Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
73,
Bill

On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?

Harvey


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond




Harvey White
 

When I used a variac, I looked at the current draw.  If it turned out to be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran it up to the 80 to 90 volt range if the current draw was excessive.  I used the variac to check if the main bulk supply was shorted.  When it wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies draw excessive current because the switch doesn't come on (and off) until a minimum voltage, that's the reason for being sneaky in this case.  You try to avoid that overcurrent draw due to the switching circuit not working.

More of a turn it up and see what it draws, turn it right back down, then figure out why.

think that'll work?

Harvey

On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul 9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond



 

On Thu, Jul 9, 2020 at 02:10 AM, Harvey White wrote:


Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond


Harvey White
 

Comments interspaced.  While I have a 7904, I don't remember these particular problems, but these ideas might help.

On 7/8/2020 7:30 PM, Bill Riches via groups.io wrote:
My 7904 just stopped working.  I have had it for about 5 years - no problems - used several times a week.
1. No blown fuses
No blown fuses says that the bulk supply, rectifiers, and filter capictors are likely ok, but does not say anything about the bulk capacitors not being open.  Check them by looking at ripple on the bulk supply if you have another scope.
.2.  Powered with metered Sencore variac -  it draws about 10 watts after initial kick and meter pulses slightly.  I assure it is in pulse mode.
It would say that, and it makes sense.  Been there, think I've done that.

3.  High voltage warning light is on and when power is removed it goes out after a few min.
Not sure that this is significant, depending on where the HV supply is derived.  Things at this point suggest that you start disconnecting power supply and see if it works.  Note that supply may need a minimum load on one supply to regulate.  Not sure about this.
3. Removed all plugins and it still would not power up.
Plugins therefore are not the only problem.  If using a plugin in a working scope brings the supply down, then it's a plugin.

    4. Removed ps and checked all caps and diodes for shorts.

Then the PS, unless it needs a minimum load, should be ok for shorts.  If it will work without a load, look at the supply that has an adjustment.  It will be a reference supply.  Tektronix liked to use a negative supply.  That must be very close for the other supplies to be within tolerance.


5. Checked resistance readings of supply voltages with ps connected and unconnected - they are close.

I think here you need to define "close".

The supply generally ticks (in startup mode) when there's too much current drawn.  Something's got to be causing that.


I have not checked out ps with 20 volts input and using test scope yet.
Go for the test scope.  See if any supply actually gets to the desired voltage before the scope PS shuts down.  Storage scope would be good, but slow sweep and dim light can give you an idea.


I would like to rule out ps problems and wonder if any of our group would have a working ps that I could purchase.
Nope, not here but it's diagnosable... and fixable, generally.

Question is:  what supply is shutting the scope down.  IIRC, there's a resistor from each supply to a summing junction, and then that junction has to be right for the main supply (not bulk) to be running.

So your problems can be twofold, one is that there is a supply demanding too much current, another is that there's a supply overvoltage.

Since you have a variac, you might want to run the voltage up from zero, looking at the outputs of a supply and the inputs to that regulator.  Until you get to about 80 or 90 volts, you can expect the supplies to be a little under.  If any supply overshoots sufficiently, then you're messing with the overvoltage protection.  SImilarly, look at the overcurrent protection on any supply, it's generally a transistor BE junction across a sense resistor, and that sense resistor in the output of the supply just after the regulator.

Watching that voltage (either one) as the supply ticks should show you a lot.

Harvey


Any help would be appreciated.
73,
Bill, WA2DVU - (good on QRZ)Cape May, NJ609 425 8651



Bill Riches
 

My 7904 just stopped working.  I have had it for about 5 years - no problems - used several times a week.
1. No blown fuses.2.  Powered with metered Sencore variac -  it draws about 10 watts after initial kick and meter pulses slightly.  I assure it is in pulse mode.3.  High voltage warning light is on and when power is removed it goes out after a few min. 3. Removed all plugins and it still would not power up.4. Removed ps and checked all caps and diodes for shorts.5. Checked resistance readings of supply voltages with ps connected and unconnected - they are close.
I have not checked out ps with 20 volts input and using test scope yet.
I would like to rule out ps problems and wonder if any of our group would have a working ps that I could purchase.
Any help would be appreciated.
73,
Bill, WA2DVU - (good on QRZ)Cape May, NJ609 425 8651