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Damian Menscher
 
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Default Tek 647 Scope CRT Circuit

Long story, but I'm getting desperate for advice....

About 10 years ago, I picked up a Tektronix 647 scope for $100 from
someone at a computer show. I thought it would be a way to learn
about electronics. It appears to be a tube/solid state hybrid. (It
has 6 tubes in it -- 5 to generate the HV for the 6th, the CRT.) So
it takes a few minutes to warm up. Then it works wonderfully for
about 1/2 hour. Then it goes into this strange mode where the screen
blanks, flashes 5 times at four-second intervals, then it comes back
to normal for a minute or two before repeating the blanking/flashing.

Looking in the manual, I discovered this has a "protection circuit"
that turns off the oscillator if the -2.2kV supply is overloaded.
This is accomplished using a "multivibrator" -- a matched pair of
transistors, one if which is normally off, the other normally on.
The behavior of blanking the screen and flashing after four seconds
is consistent with proper operation of this protection circuit being
activated. The problem is, I don't know what's activating it.

I replaced one of the tubes (5642) which was not lighting with the
others, since that seemed an obvious problem. When that didn't help,
I contacted a former Tek employee, who sold me a replacement for the
transformer in the HV circuit. (He said sometimes those fail and
cause this problem.) Next, I tried swapping the two transistors in
the protection circuit, thinking that maybe the "matched" set wasn't
so perfectly matched. Putting them in each others' sockets didn't
help, though.

The only remaining idea is to replace the rectifier tubes in the
voltage tripler circuit with solid-state diodes, in hopes that the
improved efficiency will somehow reduce the load on the circuit. But
I'd really rather not do that, since I somewhat like the idea of
"restoring" this, not just "fixing" it.

Another desperate attempt might be to simply disable the protection
circuit. It's entirely possible that there's no problem with the
scope, but only with the protection circuit. If something blows,
then I'll at least have something to fix. If nothing blows, then I
can refocus my effort on studying the protection circuit.

If anyone has any advice on what to try, it would be greatly
appreciated. I've had a lot of fun with this thing, but failing to
repair it for 10 years is starting to become a bit frustrating. I
can scan the schematics of the CRT circuit if it would help (assuming
1963 copyright date + 28 years has passed without renewal).

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-
  #2   Report Post  
NSM
 
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"Damian Menscher" wrote in message
...
| Long story, but I'm getting desperate for advice....
|
| About 10 years ago, I picked up a Tektronix 647 scope for $100 from
| someone at a computer show. I thought it would be a way to learn
| about electronics. It appears to be a tube/solid state hybrid. (It
| has 6 tubes in it -- 5 to generate the HV for the 6th, the CRT.) So
| it takes a few minutes to warm up. Then it works wonderfully for
| about 1/2 hour. Then it goes into this strange mode where the screen
| blanks, flashes 5 times at four-second intervals, then it comes back
| to normal for a minute or two before repeating the blanking/flashing.
|
| Looking in the manual, I discovered this has a "protection circuit"
| that turns off the oscillator if the -2.2kV supply is overloaded.

How is this determined? Excess current? Low voltage?

| This is accomplished using a "multivibrator" -- a matched pair of
| transistors, one if which is normally off, the other normally on.
| The behavior of blanking the screen and flashing after four seconds
| is consistent with proper operation of this protection circuit being
| activated. The problem is, I don't know what's activating it.

I would measure every resistor in the circuits - protection and HV. They do
drift.

Second choice would be capacitors.

| I replaced one of the tubes (5642) which was not lighting with the
| others, since that seemed an obvious problem. When that didn't help,
| I contacted a former Tek employee, who sold me a replacement for the
| transformer in the HV circuit. (He said sometimes those fail and
| cause this problem.) Next, I tried swapping the two transistors in
| the protection circuit, thinking that maybe the "matched" set wasn't
| so perfectly matched. Putting them in each others' sockets didn't
| help, though.
|
| The only remaining idea is to replace the rectifier tubes in the
| voltage tripler circuit with solid-state diodes, in hopes that the
| improved efficiency will somehow reduce the load on the circuit. But
| I'd really rather not do that, since I somewhat like the idea of
| "restoring" this, not just "fixing" it.

This is not a good solution. Never re-engineer until you have eliminated all
faults.

N


  #3   Report Post  
Asimov
 
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"Damian Menscher" bravely wrote to "All" (22 Dec 04 05:15:13)
--- on the heady topic of "Tek 647 Scope CRT Circuit"

DM From: Damian Menscher

DM Long story, but I'm getting desperate for advice....

DM About 10 years ago, I picked up a Tektronix 647 scope for $100 from
DM someone at a computer show. I thought it would be a way to learn
DM about electronics. It appears to be a tube/solid state hybrid. (It
DM has 6 tubes in it -- 5 to generate the HV for the 6th, the CRT.) So
DM it takes a few minutes to warm up. Then it works wonderfully for
DM about 1/2 hour. Then it goes into this strange mode where the screen
DM blanks, flashes 5 times at four-second intervals, then it comes back
DM to normal for a minute or two before repeating the blanking/flashing.

DM Looking in the manual, I discovered this has a "protection circuit"
DM that turns off the oscillator if the -2.2kV supply is overloaded.
DM This is accomplished using a "multivibrator" -- a matched pair of
DM transistors, one if which is normally off, the other normally on.
DM The behavior of blanking the screen and flashing after four seconds
DM is consistent with proper operation of this protection circuit being
DM activated. The problem is, I don't know what's activating it.

DM I replaced one of the tubes (5642) which was not lighting with the
DM others, since that seemed an obvious problem. When that didn't help,
DM I contacted a former Tek employee, who sold me a replacement for the
DM transformer in the HV circuit. (He said sometimes those fail and
DM cause this problem.) Next, I tried swapping the two transistors in
DM the protection circuit, thinking that maybe the "matched" set wasn't
DM so perfectly matched. Putting them in each others' sockets didn't
DM help, though.

DM The only remaining idea is to replace the rectifier tubes in the
DM voltage tripler circuit with solid-state diodes, in hopes that the
DM improved efficiency will somehow reduce the load on the circuit. But
DM I'd really rather not do that, since I somewhat like the idea of
DM "restoring" this, not just "fixing" it.

DM Another desperate attempt might be to simply disable the protection
DM circuit. It's entirely possible that there's no problem with the
DM scope, but only with the protection circuit. If something blows,
DM then I'll at least have something to fix. If nothing blows, then I
DM can refocus my effort on studying the protection circuit.

DM If anyone has any advice on what to try, it would be greatly
DM appreciated. I've had a lot of fun with this thing, but failing to
DM repair it for 10 years is starting to become a bit frustrating. I
DM can scan the schematics of the CRT circuit if it would help (assuming
DM 1963 copyright date + 28 years has passed without renewal).


Damian,

Increasing the efficiency of the rectifiers is apt to make the problem
worse. You really must trouble-shoot the input to the protection
circuit. Your multivibrator configuration is perhaps setup as a
Schmitt and is sensitive to a voltage trigger level. If every resistor
and capacitor checks okay in there (especially the sensing input)
then and only then should you look elsewhere.

Something is getting hot and causing the problem, as you stated this
happens after about 1/2 hour. May I suggest investing in a bottle of
freeze spray and patiently freezing a small area of circuit at a time?
Maybe it's a "resistor going open" from the normal circuit heating? If
you can't find anything the last resort will be to replace *every*
resistor in the protection circuit. It's stupid but sometimes a
shotgun approach saves your remaining grey hairs.

A*s*i*m*o*v

.... Dunno if we'll get that past the CSA und UL 'owever.

  #4   Report Post  
Jerry G.
 
Posts: n/a
Default

I would try using a heat gun and freeze spray to provoke the fault.
Start with the electrolytic caps in the power supply and HV generator.

I have never worked on this model of scope, so I cannot tell you
specifics. But, your problem seems to be thermo, thus in theory, it
should be able to be found with the technique of heating and cooling.
Jerry G.
======

  #5   Report Post  
Damian Menscher
 
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Damian Menscher wrote:
Looking in the manual, I discovered this has a "protection circuit"
that turns off the oscillator if the -2.2kV supply is overloaded.
This is accomplished using a "multivibrator" -- a matched pair of
transistors, one if which is normally off, the other normally on.
The behavior of blanking the screen and flashing after four seconds
is consistent with proper operation of this protection circuit being
activated. The problem is, I don't know what's activating it.


I've put a scan of the circuit schematic online at:
http://www.uiuc.edu/~menscher/647_schematic.jpg
Their color scheme is:
black for the circuit
blue for "normal" readings of circuit in operation
pencil for my notes as I try to understand it

The circuit description in the service manual gives a brief rundown
of the theory of operation, which I'll paraphrase. I'll use labels
from the schematic, since otherwise it's hard to keep everything
straight.

The CRT high voltage is obtained by using an oscillator to drive a
transformer. The transformer steps up the voltage, which then goes
through tube rectifiers to convert back to DC. The anode of the CRT
is increased further using a voltage tripler circuit.

The oscillator is called an "Armstrong oscillator", where the T820
primary feeds back into the base of the Q820 oscillator. It is
controlled by Q804, which acts as a shunt regulator. If Q804 conducts
heavily, the base current of Q820 will decrease, therefore lowering
the output voltage.

The conduction of Q804 is controlled by an error amplifier, Q803,
which compares the -2.2 kV cathode supply to the +100 V supply. Under
normal operation, there is a 110-microamp current through the R802
resistors, and a change in the cathode voltage will change that
current. The Q803 amplifies that change to control Q804.

Finally, there's the Q814A-Q814B multivibrator, which protects Q820
from overload conditions. It monitors the voltage on the base of
Q804, turning off the HV if that voltage drops too low. The trigger
is diodes D815 and D816 turning on. The circuit is designed to cut
the HV for 4 seconds. Then it restores it for 250 ms, and cuts it
again if the overload problem persists.

I've measured the voltage at the junction of the protection circuit,
and it does indeed drop shortly before the protection circuit kicks
in. So unless it's R810 that's failing, I doubt the problem is in
the protection circuit itself.

Tracing things back, the voltage change also occurs in front of the
error amplification transistor. Which means the problem could be
just about anywhere.

A note from a former Tek employee said "This scope has a failure mode
with the HV transformer diaelectric constant becomes lossy at the ore
frequency." On his advice, I replaced the transformer, but that
obviously didn't help.

Something in the region is getting fairly hot -- even the chassis is
hot to the touch. I haven't been able to identify exactly which part
is overheating, though Q820 is an obvious candidate (it's fairly warm,
but I'm not convinced it's overheating). I'm still attempting to
freeze parts of the circuit to identify the fault, but it hasn't led
me anywhere yet. Part of the problem is many components are hidden
from view in the HV cage, so I can't easily freeze them.

Any advice appreciated.

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-


  #6   Report Post  
Jim Adney
 
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On Wed, 22 Dec 2004 05:15:13 +0000 (UTC) Damian Menscher
wrote:

Looking in the manual, I discovered this has a "protection circuit"
that turns off the oscillator if the -2.2kV supply is overloaded.
This is accomplished using a "multivibrator" -- a matched pair of
transistors, one if which is normally off, the other normally on.
The behavior of blanking the screen and flashing after four seconds
is consistent with proper operation of this protection circuit being
activated. The problem is, I don't know what's activating it.


I don't know this scope, but here's a guess:

Look at the schematic for the protection circuit. There's probably a
resistor in the ground end of the CRT voltage multiplier thru which
all the current up the stack must pass. The voltage across this
resistor would be the thing that triggers the protection circuit.

Check the voltage at that point to make sure that it's safe to probe
in there, then clip your voltmeter across that resistor. Turn on the
scope and write down the voltage. Wait for the protection circuit to
kick in and note the voltage again.

If the voltage has gone up then either the current has increased, or
the resistance has gone up (or both.) Figure out which.

If the voltage didn't change, then either you're watching the wrong
resistor, or something else triggers the protection circuit.

It seems quite likely that you just have a resistor which has slowly
drifted up in value, but there are many possibilities and Tek usually
used good parts, so resistors that have drifted are much less of a
problem in Tek products that in more common household items. Still, it
does happen.

It could even be a solder joint that has opened up, adding its
resistance to the intended resistance.

If you have the 647 manual, it should have a good section explaining
the protection circuit, and this should help you find the problem.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------
  #7   Report Post  
Marko
 
Posts: n/a
Default

Damian,

Have you checked your supply voltages? In particular, I'd be
interested in the following:

+100
-75
+15
-15
Voltage on each end of C820 (near fuse)

Normally, the error amplifier is part of the feedback loop to regulate
the -2.2KV supply. The schematic indicates the base of Q804 should
idle around -14.3V (at minimum intensity). So, before the 22V Zener
diode D804, the collector of Q803B should be at +7.7V. Working
backwards, and assuming collecter and emitter current are equal, the
voltage at the emitter of Q803B should be 0.42V. Its base should be
about +1V and the base of Q803A should be about 1.5V. The darlington
input current should be almost zero so there should be almost zero
voltage drop across R804. So the voltage at the junction of R804 and
R800 should be about +1.5 to +2.0V. Here is the surprise.... you have
marked it as -8.5V. If this is true, then this is a clue to your
problem. The error amplifier thinks your -2.2KV is way too negative!
This could be caused by the +100 supply drooping to say 90V. That 100V
supply needs to be solid or you're going to have trouble.

Check it and report back. Try to check a couple of the other voltages
I calclulated as well. Check at initial power up and again during the
failure mode.

Incidentially, the capacitive connection into the input of your error
amplifier (through C833) it to monitor for spikes in the -2.2KV
supply. Any such spike will be amplified by 220 and could trigger the
one-shot (monostable multivibrator) and shut you down. I don't think
this is the problem - I'm just providing my explanation for that part
of the circuit.

Marko



On Wednesday, 22 Dec 2004 09:43:54 -500, "Asimov"
wrote:

"Damian Menscher" bravely wrote to "All" (22 Dec 04 05:15:13)
--- on the heady topic of "Tek 647 Scope CRT Circuit"

DM From: Damian Menscher

DM Long story, but I'm getting desperate for advice....

DM About 10 years ago, I picked up a Tektronix 647 scope for $100 from
DM someone at a computer show. I thought it would be a way to learn
DM about electronics. It appears to be a tube/solid state hybrid. (It
DM has 6 tubes in it -- 5 to generate the HV for the 6th, the CRT.) So
DM it takes a few minutes to warm up. Then it works wonderfully for
DM about 1/2 hour. Then it goes into this strange mode where the screen
DM blanks, flashes 5 times at four-second intervals, then it comes back
DM to normal for a minute or two before repeating the blanking/flashing.

DM Looking in the manual, I discovered this has a "protection circuit"
DM that turns off the oscillator if the -2.2kV supply is overloaded.
DM This is accomplished using a "multivibrator" -- a matched pair of
DM transistors, one if which is normally off, the other normally on.
DM The behavior of blanking the screen and flashing after four seconds
DM is consistent with proper operation of this protection circuit being
DM activated. The problem is, I don't know what's activating it.

DM I replaced one of the tubes (5642) which was not lighting with the
DM others, since that seemed an obvious problem. When that didn't help,
DM I contacted a former Tek employee, who sold me a replacement for the
DM transformer in the HV circuit. (He said sometimes those fail and
DM cause this problem.) Next, I tried swapping the two transistors in
DM the protection circuit, thinking that maybe the "matched" set wasn't
DM so perfectly matched. Putting them in each others' sockets didn't
DM help, though.

DM The only remaining idea is to replace the rectifier tubes in the
DM voltage tripler circuit with solid-state diodes, in hopes that the
DM improved efficiency will somehow reduce the load on the circuit. But
DM I'd really rather not do that, since I somewhat like the idea of
DM "restoring" this, not just "fixing" it.

DM Another desperate attempt might be to simply disable the protection
DM circuit. It's entirely possible that there's no problem with the
DM scope, but only with the protection circuit. If something blows,
DM then I'll at least have something to fix. If nothing blows, then I
DM can refocus my effort on studying the protection circuit.

DM If anyone has any advice on what to try, it would be greatly
DM appreciated. I've had a lot of fun with this thing, but failing to
DM repair it for 10 years is starting to become a bit frustrating. I
DM can scan the schematics of the CRT circuit if it would help (assuming
DM 1963 copyright date + 28 years has passed without renewal).


Damian,

Increasing the efficiency of the rectifiers is apt to make the problem
worse. You really must trouble-shoot the input to the protection
circuit. Your multivibrator configuration is perhaps setup as a
Schmitt and is sensitive to a voltage trigger level. If every resistor
and capacitor checks okay in there (especially the sensing input)
then and only then should you look elsewhere.

Something is getting hot and causing the problem, as you stated this
happens after about 1/2 hour. May I suggest investing in a bottle of
freeze spray and patiently freezing a small area of circuit at a time?
Maybe it's a "resistor going open" from the normal circuit heating? If
you can't find anything the last resort will be to replace *every*
resistor in the protection circuit. It's stupid but sometimes a
shotgun approach saves your remaining grey hairs.

A*s*i*m*o*v

... Dunno if we'll get that past the CSA und UL 'owever.


  #8   Report Post  
Damian Menscher
 
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Default

Marko wrote:
Have you checked your supply voltages?


Supply voltages are good, though I'll admit I haven't checked them
during a fault.

Normally, the error amplifier is part of the feedback loop to regulate
the -2.2KV supply. The schematic indicates the base of Q804 should
idle around -14.3V (at minimum intensity). So, before the 22V Zener
diode D804, the collector of Q803B should be at +7.7V. Working
backwards, and assuming collecter and emitter current are equal, the
voltage at the emitter of Q803B should be 0.42V. Its base should be
about +1V and the base of Q803A should be about 1.5V. The darlington
input current should be almost zero so there should be almost zero
voltage drop across R804. So the voltage at the junction of R804 and
R800 should be about +1.5 to +2.0V. Here is the surprise.... you have
marked it as -8.5V. If this is true, then this is a clue to your
problem. The error amplifier thinks your -2.2KV is way too negative!
This could be caused by the +100 supply drooping to say 90V. That 100V
supply needs to be solid or you're going to have trouble.


Check it and report back. Try to check a couple of the other voltages
I calclulated as well. Check at initial power up and again during the
failure mode.


I checked the voltages, and they are as you predicted. I'm not sure
why I wrote -8.5V there... this has been a project for many years, and
figuring out what I was thinking many years ago may be impossible. In
any case, it looks good now.

In fact, when I went to measure the voltages you requested during a
fault, I ran into trouble. I am no longer able to reproduce a fault.
I've tried leaving it on for many hours, in a hot room. So apparently
the problem fixed itself. I recently replaced Q804, so it's possible
that change was related. Other than that, all I can imagine is that
the electrolytic capacitors healed themselves after finally getting
some serious use (this is the first time in 10+ years it's been left
on for more than an hour or two at a time).

Given that it's working, I guess I'll just stop worrying about it. I
can always google for this thread if the fault reappears.

Thanks to you and everyone else for your help.

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-
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