HV present, but no traces displayed, on HP 'scope

[R. D. Davis]

After installing a new HV PSU in my HP 1707B oscilloscope, there are
no traces visible. Pressing the "beam finder" button doesn't cause
anything to appear. However, when the scope was powered off, I saw
something move quickly from the left side of the display to the
center, and a large, about a 1/2" by 5/16" or so, rectangle appeared,
which disintegrated into a blotch of light that faded away, so I know
that there was HV present and the tube was apparently still good. No
burning smells, no arcing sounds, no blown fuses. When leaving it on
for a few minutes, a similar rectangle blinked on the screen, very
briefly, once in a while ...not always, however. Twiddling varions
controls doesn't appear to make the rectangle appear. The brightness
was unaffected by the intensity knob.

Alas, the aforementioned no longer appears on the screen, but the LV
PSU still tests ok... hopefully the HV supply isn't gone again, as the
momentary staticy sound is no longer heard when the 'scope is powered
up.

I checked the LV power-supply, which tested mostly ok except for the
-50 supply being a little high, about -48V.

After the original flyback transformer burned out several years ago,
and I couldn't find a replacement (until last year), the 'scope ended
up sitting in a damp area of the basement. Everything inside still
looks ok, no rust, mold, mildew etc. ...just grey mold on the
(aluminum ?) outer parts of the chasis, such as the handle and trim,
which I sprayed with Lysol and used an SOS pad on.

Any ideas for first things to check? I've got another 'scope that I
can use for tests if necessary. Unfortunately, I don't have any
printed schematics or a service manual except for what's on
microfiche, which I don't have a reader for.

Any help will be greatly appreciated.

 

[Jim Yanik]

[email protected] (R. D. Davis) wrote in

After installing a new HV PSU in my HP 1707B oscilloscope, there are
no traces visible. Pressing the "beam finder" button doesn't cause
anything to appear. However, when the scope was powered off, I saw
something move quickly from the left side of the display to the
center, and a large, about a 1/2" by 5/16" or so, rectangle appeared,
which disintegrated into a blotch of light that faded away, so I know
that there was HV present and the tube was apparently still good. No
burning smells, no arcing sounds, no blown fuses. When leaving it on
for a few minutes, a similar rectangle blinked on the screen, very
briefly, once in a while ...not always, however. Twiddling varions
controls doesn't appear to make the rectangle appear. The brightness
was unaffected by the intensity knob.

Alas, the aforementioned no longer appears on the screen, but the LV
PSU still tests ok... hopefully the HV supply isn't gone again, as the
momentary staticy sound is no longer heard when the 'scope is powered
up.

I checked the LV power-supply, which tested mostly ok except for the
-50 supply being a little high, about -48V.

After the original flyback transformer burned out several years ago,
and I couldn't find a replacement (until last year), the 'scope ended
up sitting in a damp area of the basement. Everything inside still
looks ok, no rust, mold, mildew etc. ...just grey mold on the
(aluminum ?) outer parts of the chasis, such as the handle and trim,
which I sprayed with Lysol and used an SOS pad on.

Any ideas for first things to check? I've got another 'scope that I
can use for tests if necessary. Unfortunately, I don't have any
printed schematics or a service manual except for what's on
microfiche, which I don't have a reader for.

Any help will be greatly appreciated.

Check the CRT deflection connections for sweep ramp (Horizontal)and
calibrator signal on the vertical(connect the cal sig to the scope
input).If there's a ramp,the sweep and horizontal works.If you get the cal
signal,the vertical works.There's also a Z-axis (pulse;actually the sweep
gate signal)waveform to unblank the CRT,this should change in amplitude
with varying the intensity control,but it may be elevated to near cathode
potential(HV) by a "DC restorer". The CRT grid has to vary from near
cathode V(full inten) to about 100 V more negative(CRT cut off).A meter
will load this down unless you use a HV probe,or you may blow your DMM.
That staticky sound only happens if the scope has not been powered up and
charging the CRT anode capacitance (think Leyden jar).It takes quite awhile
to bleed off.

 

[Tweetldee]

[email protected] (R. D. Davis) wrote in


Check the CRT deflection connections for sweep ramp (Horizontal)and
calibrator signal on the vertical(connect the cal sig to the scope
input).If there's a ramp,the sweep and horizontal works.If you get the cal
signal,the vertical works.There's also a Z-axis (pulse;actually the sweep
gate signal)waveform to unblank the CRT,this should change in amplitude
with varying the intensity control,but it may be elevated to near cathode
potential(HV) by a "DC restorer". The CRT grid has to vary from near
cathode V(full inten) to about 100 V more negative(CRT cut off).A meter
will load this down unless you use a HV probe,or you may blow your DMM.
That staticky sound only happens if the scope has not been powered up and
charging the CRT anode capacitance (think Leyden jar).It takes quite awhile
to bleed off.

I agree with everything Jim has said, and I'd like to add one item for you
to check.
First thing that comes to mind when there appears to be good LV and HV power
supplies, is that one of the deflection circuits is out of whack. Find the
deflection plates on the CRT, disable the sweep, and check the potential
between each pair of deflection plates. The voltage between the vertical
plates should be close to zero, as should it be between the horizontal
plates (with no sweep).
If there is significant voltage across either pair of plates, you've found
the area that you need to work on. A large voltage between the plates will
drive the beam off the viewable area of the CRT; that's why you can't see
it.
Be sure you read the voltage between the plates, not plates to ground.
--
Tweetldee
Tweetldee at att dot net (Just subsitute the appropriate characters in the
address)

Never take a laxative and a sleeping pill at the same time!!

 

[R. D. Davis]

Check the CRT deflection connections for sweep ramp (Horizontal)and
calibrator signal on the vertical(connect the cal sig to the scope
input).If there's a ramp,the sweep and horizontal works.If you get the cal

Hi, thanks for the help with this. I'm taking a guess that the
connectors for the deflection plates are the four thin wires connected
mid-way towards the front of the tube, where the opening in the
shielding is. Is this correct? What voltage levels should I be looking
for?

signal,the vertical works.There's also a Z-axis (pulse;actually the sweep
gate signal)waveform to unblank the CRT,this should change in amplitude
with varying the intensity control,but it may be elevated to near cathode
potential(HV) by a "DC restorer". The CRT grid has to vary from near
cathode V(full inten) to about 100 V more negative(CRT cut off).A meter
will load this down unless you use a HV probe,or you may blow your DMM.

Ok... it looks like I'm going to have to build an HV probe, since I
don't want to light up like a Christmas tree, or have my DMM blown to
bits . Hmmm... I wonder if any of my VTVMs would withstand a
brief hit of 3kV without anything being fried? I'd better just build
the probe.

That staticky sound only happens if the scope has not been powered up and
charging the CRT anode capacitance (think Leyden jar).It takes quite awhile
to bleed off.

While I thought that was the case, it happened each time the 'scope
was powered up---when I would also see something on the screen when
switching the 'scope off; however, it doesn't happen now, and I no
longer see anything on the screen. This is why I'm wondering if
there's a problem, again, with the HV supply in addition to other
problems.. The HV supply's fuse didn't blow, so the HV oscillator
transistor should be ok (or at least not shorted), and the low-voltage
inputs to the HV module, as well as all of the outputs from the LV
power supply, are all close to what they should be. Of course, that's
not telling me if this oscillator is running or not... I'm guessing
that this will probably be a pulse approaching 30V p-p at the
oscillator's output. Does this sound about right?

 

[Jim Yanik]

[email protected] (R. D. Davis) wrote in

Hi, thanks for the help with this. I'm taking a guess that the
connectors for the deflection plates are the four thin wires connected
mid-way towards the front of the tube, where the opening in the
shielding is. Is this correct?

That sounds right.

What voltage levels should I be

looking for?

It depends.Different CRTs have different deflection sensitivities.The sweep
ramp should be a high amplitude signal(10's of Vp-p),with a DC level of
~35-200V.

Ok... it looks like I'm going to have to build an HV probe, since I
don't want to light up like a Christmas tree, or have my DMM blown to
bits . Hmmm... I wonder if any of my VTVMs would withstand a
brief hit of 3kV without anything being fried? I'd better just build
the probe.

Or get a service manual,so you can find the correct test points for each
signal.I'd recommend both.

While I thought that was the case, it happened each time the 'scope
was powered up---when I would also see something on the screen when
switching the 'scope off; however, it doesn't happen now, and I no
longer see anything on the screen. This is why I'm wondering if
there's a problem, again, with the HV supply in addition to other
problems.. The HV supply's fuse didn't blow, so the HV oscillator
transistor should be ok (or at least not shorted), and the low-voltage
inputs to the HV module, as well as all of the outputs from the LV
power supply, are all close to what they should be. Of course, that's
not telling me if this oscillator is running or not... I'm guessing
that this will probably be a pulse approaching 30V p-p at the
oscillator's output. Does this sound about right?

You can bring a scope probe close to the HV xfmr and see if it's
oscillating. TEK's HV outputs were 1500-3000 volts or more,and they also
fed doublers or triplers to develop the CRT anode voltage.TEK scopes (my
main experience in scopes) generally had CRT cathode V of about -2950Vdc
and anodes at 12-21 KV. HP scopes probably will be different.

 

[R. D. Davis]

I agree with everything Jim has said, and I'd like to add one item for you
to check.

Hi, thanks for the help with this.

First thing that comes to mind when there appears to be good LV and HV power
supplies, is that one of the deflection circuits is out of whack. Find the
deflection plates on the CRT, disable the sweep, and check the potential

Are those the four thinnish wires mid-way towards the front of the
tube, on top of the tube, where the opening in the tube's shielding
is? (labeled pins 5, 9 and 5, 9) Actually, there are five wires there.
The fifth, and middle, wire, on pin 947, appears to be connected to
the LV power supply.

It appears that there are two pairs of wires, each going to a
separate circuit board... horizontal and vertical amplifiers. I'm
guessing that the vertical amplifier, and hence vertical deflection
plates, are on the left side and the horizontal on the right (when
looking at the 'scope from the front).

Also, I see that there are some little foam spaces attached to these
wires, so, am I correct to guess that these (insulated) wires need to
be held a short distance apart? The (horizontal) wires are touching
in a few places, so I'll separate them just in case.

between each pair of deflection plates. The voltage between the vertical
plates should be close to zero, as should it be between the horizontal
plates (with no sweep).

Taking a guess about the wires , here's what I got with a DMM, when
rotating the vertical and horizontal position knobs from min. to max.:

Vertical:

2.5 Vdc to -2.5 Vdc, with beam finder depressed to sut off sweep
17 Vdc to -18.1 Vdc, with sweep

Horizontal:

-14.0 Vdc to 7.9 Vdc (14 Vdc w/x10 swp mag) with beam finder depressed
-57.9 Vdc to 8.2 Vdc (64 Vdc w/x10 swp mag) with sweep

Didn't check the waveforms yet, since I need to find some space for
this 'scope closer to my unscopmobiled Tektronix 531... not so easy to
move the Tek 531 to where this 'scope is.

If there is significant voltage across either pair of plates, you've found
the area that you need to work on. A large voltage between the plates will
drive the beam off the viewable area of the CRT; that's why you can't see
it.

Should the voltages measured be ok, since I could get near zero V readings
in between the min and max positions?

Be sure you read the voltage between the plates, not plates to ground.

Did that. Thanks.

 

[R. D. Davis]

Or get a service manual,so you can find the correct test points for each
signal.I'd recommend both.

At this point, with the HV PSU problem as well, it looks as though I'd
better start looking for a printed copy of the service/operation
manual, or a microfiche reader.

You can bring a scope probe close to the HV xfmr and see if it's
oscillating. TEK's HV outputs were 1500-3000 volts or more,and they also
fed doublers or triplers to develop the CRT anode voltage.TEK scopes (my
main experience in scopes) generally had CRT cathode V of about -2950Vdc
and anodes at 12-21 KV. HP scopes probably will be different.

After building an HV probe from ten 10M-ohm resistors, a straw,
heat-shrink tubing and electrical tape for enough insulation, I've
discovered that there is no HV at the output of the flyback
transformer, and the chopper transistor apparently isn't oscillating.
Although I've not seen the schematics, I understand that if this
transistor shorts out, the fuse in this PSU will blow, so, that
indicates to me that the transistor didn't short out. The emitter and
base have no ac or dc voltages present, and the collector has a DC
voltage of about 15 volts on it.

Under normal operating conditions, should I see any light from the Ne2
lamps on the HV PSU circuit board?

 

[Jim Yanik]

[email protected] (R. D. Davis) wrote in

At this point, with the HV PSU problem as well, it looks as though I'd
better start looking for a printed copy of the service/operation
manual, or a microfiche reader.


After building an HV probe from ten 10M-ohm resistors,

If your DMM input Z is 10 megohms,you only need use 9 10meg resistors,makes
for a simpler probe.I have a HV probe like this.Not any good for a scope HV
probe,though.

a straw,

heat-shrink tubing and electrical tape for enough insulation, I've
discovered that there is no HV at the output of the flyback
transformer, and the chopper transistor apparently isn't oscillating.
Although I've not seen the schematics, I understand that if this
transistor shorts out, the fuse in this PSU will blow, so, that
indicates to me that the transistor didn't short out. The emitter and
base have no ac or dc voltages present, and the collector has a DC
voltage of about 15 volts on it.

Maybe the HV osc xstr is -open-? Can you check the B-E and C-B junctions
with the diode check on your DMM? (scope power OFF!!!)

Under normal operating conditions, should I see any light from the Ne2
lamps on the HV PSU circuit board?

In TEK scopes,the neons are to limit G-K voltages to ~120V difference,so if
they were lit,there likely was a problem in the DC restorer(leaky diodes or
HV caps breaking down under HV).

I don't know how HP does their CRT circuits,it's been 30+ years since I
looked into a HP scope.